Yes, it is easy to penetrate the atmosphere quickly, and burn up like a meteor. The problem is to enter slowly. You can do that too, but it would take a HUGE amount of fuel with ordinary rockets. You can do it with aerobraking, including a surprisingly slow re-entry with an orbital airship; and there are some other ideas that may be possible in the not too distant future, such as a space elevator, or spinning “skyhooks”.
To see why it is so difficult with ordinary rockets, here is a quick refresher on orbits. If you could throw a ball from above our atmosphere, gravity still pulls it down in the same way as on Earth, If you throw it fast enough, what happens is that it gets beyond the horizon before it can hit the Earth, and Earth’s gravity continues to pull it around into a curve until it gets back to its starting point. That’s how satellites such as the ISS stay in orbit, and that’s why it is often called free-fall.
To skim the Earth’s atmosphere in orbit, your spacecraft has to travel at least as fast as 7.8 km / second, or about 17,500 mph. The Earth itself, with its atmosphere, is spinning eastward below you, at around 1,000 mph. So, you can reduce your re-entry speed by orbiting in the same direction that the Earth spins. However that only helps a bit. Your spacecraft still has to travel at 16,500 mph relative to our atmosphere to stay in orbit.
If you slow down by a tiny amount below that speed, even by just a few hundred miles per hour, as you skim the atmosphere, you will fall too far towards Earth before you complete your orbit. You will hit the atmosphere at thousands of miles per hour, and will re-enter in a fiery descent. At that point you depend on your aeroshell to protect you from the heat.
As he says there, he took over manually and “flew it like a helicopter” on out to the West to find a good site. That’s something you can do using rocket thrust in the weak lunar gravity. To do a hover like that against full Earth gravity would take a lot of fuel. So much so that when fully fueled to land, your spaceship would need to be nearly as large as those rockets you see in televised launches that take our spacecraft into orbit. Huge!
If you could use fuel with a high power density, such as antimatter, which lets you convert matter directly into energy, or perhaps fuel for nuclear fusion, it would be easy. Your spaceship just needs to thrust continually at 1 g away from Earth to maintain position as it slows down, and then it can lower itself down to the surface, as slowly or as quickly as your pilot wishes.
But we don’t have such fuels yet. So is there any other way to do it?
If we had that, your spaceship could dock at geostationary orbit, which is easy to do, so long as you make sure that you orbit the earth in the same direction that it spins, once every 24 hours. That way your orbit keeps you stationary above the same point on Earth all the time, and so stationary relative to the top of the elevator. So then you can just dock slowly much as they do for the ISS.
Once you’ve done that, then you would get out of your spacecraft, or attach your spacecraft to a lift, and just slowly travel down the elevator at, say, 200 mph, or whatever speed you find comfortable and safe, until you reach the Earth’s surface. You’d feel gravity gradually increase from zero g to full g as you descend.
Arthur C. Clarke’s science fiction story The Fountains of Paradise is based around this idea.
The space elevator is a mega engineering project, with many issues to sort out before it can be built. However, as well as that, at the moment there is one issue that is a deal breaker for it. It requires cables able to hold up their own weight for distances of thousands of kilometers. Present day materials such as Kevlar and the somewhat stronger Zylon can only hold up a few hundred kilometers. Carbon nanotubes made in a laboratory are strong enough for a space elevator, but when they are made into larger cables in practice they are weaker than Kevlar because of flaws. It would be possible to build a space elevator on the Moon with its weaker gravity, using Kevlar or Zylon, but not on the Earth. More on this later, but meanwhile let’s look at other ideas.
You might perhaps wonder, what about building it from below? The earliest idea for a space elevator is from Konstantin Tsiolkovsky, one of the founding fathers of the theory of rocketry, and his idea was inspired by the Eiffel tower.
But it turns out that none of our materials have anything like the compressive strength needed to hold up a thousands of kilometers tall tower from below. It would just collapse under its own weight.
So then, what about building it just to the top of the atmosphere? Might that be useful?
Well first, how high could a building be? Well at least as high as a mountain, if you have enough materials for it. The tallest mountain as measured from its base is Mauna Kea in Hawaii which rises 10.205 kilometers above the sea floor.
This paper works out the theoretical maximum height of a mountain,not taking into account any geological considerations such as how it could get to be so high or the depth of the crust supporting it. As an example at the end, they work out that a mountain made of granite with a base of around 1,000 km could be as much as 45 km high. But they say it could be much higher if the mountain gets rapidly steeper, as it gets higher, like the Eiffel tower.
The highest hyperbuilding structure to be designed by an architect is the X-Seed 4000 for Japan, which uses an Eiffel tower like construction, and would reach a height of 4 km with a 6 km diameter base, making it 224 meters higher than the similarly shaped Mount Fuji. It was designed by Peter Neville for the Taisei Corporation. The idea was to build it in Tokyo bay.
Space engineer Ben Quine from Ottawa in Canada has devised an idea for a twelve miles high building, using inflated Kevlar tubes.
Perhaps a tower as high as tens of kilometers high might be feasible, and if so, may be useful for launching to orbit, because it reduces the amount of drag.
It’s not so much the atmospheric drag which was small, for the Saturn V it was about 48 m/s delta v lost to the atmospheric drag. It’s the gravity drag due to launching vertically to avoid the thicker atmosphere, but also because it has to lift the weight of the rocket - it’s far more efficient to accelerate horizontally. If they can tip the rocket over to travel horizontally sooner it would reduce the effecdtive delta v to orbit. The Saturn V lost around 1.743 km / sec due to gravity drag (from Robert Braeunig’s Saturn V launch simulation)
But it’s not nearly high enough to help much with the landing. It’s still stationary relative to the Earth’s surface. That’s okay for a tower that goes up to geostationary orbit, but not much use if it doesn’t even reach LEO.
How can you dock with the top of a building when you are traveling at a relative speed of 16,500 mph? You have the same problem that you’d need huge amounts of fuel to land on it with a conventional rocket. So let’s look at that in a bit more detail now.
You could use a large rocket refueled from orbit of course, to slowly lower your spacecraft through the atmosphere. This would require a lot of fuel though because of the Earth’s gravity.
It’s easy to carry enough fuel to land on the Moon, in a slow controlled de-orbit. The lunar module descent stage had a total mass of 15.2 tons, and of that, 8.355 tons was propellant (these figures varied depending on the mission). So around 45% was payload, including as the payload there, the lander itself, the ascent stage, its fuel, and the crew, as well as the lunar rover and any equipment to be left on the surface.
By way of example only 4% of the Saturn V rocket used to launch the Apollo missions was payload, so 96% of the launch mass is either burnt or discarded on the way to orbit. For the Ariane 5 (European heavy lift rocket), the payload fraction is 2.5%, and it’s similar (slightly less) for the Soyuz 2 used to launch the crewed Soyuz MS.
For the Space Shuttle, only 1% was payload because most of the mass put into orbit was the shuttle itself which was returned to Earth. For more on this, see The Tyranny of the Rocket Equation and for some more example figures, this Payload fraction table
If you don’t have an atmosphere for aerobraking, it takes about the same amount of fuel and hardware to get something into orbit as to de-orbit it. So for instance, since it takes 312 tons total mass to launch the new Soyuz MS with a crew of three into orbit, you’d need that much mass in orbit already to get them back. But each launch can only carry a payload of 7.08 tons. That makes it 44 launches before you have enough mass up there to return your crew of three safely without aerobraking.
You can improve those figures by using a heavy launch vehicle but it would still take eleven launches of the Delta IV Heavy, the highest capacity launch vehicle in production, with a payload of 28.79 tons. It would take six launches of SpaceX’s Falcon Heavy when it is ready, with a payload to LEO of 54.4 tons.
You could do it more easily with the Saturn V. This had a payload to orbit of 140 tons (after boosts in payload capacity for the last two missions). So three launches would be more than enough at least in terms of the total mass. It would also take three launches of the new Space Launch System when ready.
Back in the 1960s NASA studied even larger rockets, the NOVA series, with an eye to a mission to Mars. These never flew, but would have been able to send many hundreds of tons into LEO in a single flight.
So, that would be the situation, for someone living on a planet with Earth gravity and no atmosphere, or very little atmosphere. They could send robotic spacecraft into orbit early on. Returning their citizens safely from orbit would be tough, but not impossible, if they build something like the Saturn V or even more so, the Saturn V-4X(U). However it’s no wonder that we use our atmosphere to slow down our spaceships for re-entry.
One thing that could change all this, is if you don’t have to carry the fuel on board the spaceship. If it is beamed to the spaceship from elsewhere, say from Earth, then you don’t need to use fuel to carry more fuel, and then it becomes much more feasible to land by hovering in the atmosphere. Your spacecraft also becomes much less bulky because it doesn’t have to contain all the fuel.
This is a neat idea, but so far has only been tried in small scale demos, raising models a hundred feet or so on laser beams
If you can send a spacecraft into orbit that way you can also return it from orbit the same way if you want to. However we don’t have any spacecraft yet that can do this.
This is the way it is done today, to use the upper atmosphere as a brake, then slowly parachute to the surface or glide down in the lower atmosphere. How easy that is to do depends on the spacecraft.
If it is a heavy one like the Space Shuttle (now retired of course) then it can only slow down deep in the upper atmosphere, where it is dense. So it gets very hot. That’s why the Space Shuttle had to have ceramic tiles able to withstand temperatures up to 3,000 °F (1,650 °C)
Skylon is a plane being developed by the British company Reaction Engines with funding from the UK government and ESA. It will be able to fly to orbit from a conventional runway (though reinforced to carry the extra weight of all the fuel), return back to Earth, and then take off again within a couple of days with a crew of 200 to assist.
Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit. So it slows down in the atmosphere at higher altitudes on the way down.
What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, its ballistic coefficient. Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the other stages of its flight. However it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly.
Modern planes have “stressed skin” structures, where the skin of the plane itself takes up all, or most of the external load from the wings, tail, other stabilizing structures and heavy components such as the engine (See fuselage for details). But the Skylon uses a structure much more like a zeppelin or a small plane. It’s girder-like with a thin glass ceramic outermost shell, which is just a heat resistant covering and doesn’t take any stress at all.
https://youtu.be/3H7BeyQOnPk
This approach of reducing the density of the spacecraft to lower its re-entry temperature is taken much further with the plans of JP Aerospace.
Their kilometer scale orbital airship is filled mainly with hydrogen. It’s not only lower density than a plane, and the Skylon; it’s also much lower density even than a normal airship. It only operates above 140,000 feet and is balanced for the upper atmosphere. It also has a huge cross section which it presents to the atmosphere.
This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down just through friction in the very tenuous upper atmosphere. By the time it gets to levels dense enough to heat the skin significantly it’s already slowed down hugely, so the temperature of the skin during re-entry is much less of a problem.
The most controversial part of their work is the idea of slowly accelerating to orbit. The idea of an airship re-entering from orbit is much less controversial.
It has no internal girders. Its outer shell covers an interior of many large bags of hydrogen to give it rigidity and to stop the gas bunching up at its nose. It also has inflatable trusses, with nitrogen filling the gaps in between these components. The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks.
It is balanced to float at 200,000 feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider on the way down. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry.
So what keeps it up is partly aerodynamic lift and partly buoyancy. To start with it’s mainly aerodynamic, as it slows down on its long glide through the upper atmosphere. The aerodynamic effects keep it higher in the atmosphere for longer, and so keep it cooler on the way down. As it slows down to a halt in the atmosphere, it’s finally kept up by buoyancy.
Many details of the design are given in their Floating to Space: The Airship to Orbit Program. They don’t actually give expected skin temperatures. But the design uses nylon rip-stop polyethylene (page 111) which suggests that they expect external skin temperatures well below 100 °C (212 °F) for continuous use.
(Most commercial grade Polyethylene starts to soften at 60 °C (140 °F) and has a maximum continuous use temperature of 65 °C (149 °F), High Temperature Polyethylene can retain its properties up to 100 °C (212 °F) )
On page 109 they say
The idea started off as a US Airforce contract for a near space reconnaissance airship. But the US canceled the contract in 2004 or 2005 after first persuading them to attempt to launch one of their prototypes for a lower atmosphere airship in a 50 mph wind (which would count as a “strong gale”). It was only rated as sturdy enough for launch in a 2 mph wind at the time (an airship is particularly vulnerable in the short time it takes to launch it from the ground). They did this with some reluctance - and it blew apart in the strong winds, causing some minor injuries. The inventor himself sustained three broken ribs. That was enough for the US Airforce to cancel the contract.
JP Aerospace have now solved the problem and can launch their lower atmosphere V shaped airships in any wind conditions. You can read their account of this story here. It’s now a civilian company entirely self financed, and they are not interested in any more such contracts, naturally enough.
It’s probably going to take them a fair while, with this approach, maybe decades but it’s interesting: “watch this spot”.
You might wonder what happens if the airship is hit by a meteorite or orbital debris. From page 112 of the book:
The JP Aerospace orbital airships are so lightweight they could never survive at ground level. The slightest wind would tear them apart. So in their plan, they have conventional airships that take passengers up to a docking station in the upper atmosphere, the “Dark Sky Station”, where they then transfer to the orbital airships that then slowly accelerate to orbit over several days.
This idea has been much criticized by physicists and they themselves say they don’t know if it is possible, but that it is interesting trying for what they learn along the way whether it’s possible or not.
This is the most controversial part of their idea, the accelerating to orbit. So, let’s look at it briefly. It’s a self contained section - if you aren’t interested, skip to the next section:
The key to the way it is supposed to work is that at hypersonic speeds the shock wave can be pushed back against the body which then adds to the lift. It sounds a bit like lifting yourself by your own boot straps, but it works out when you do the physics.
The denser air below the airship gives it lift through buoyancy. With something as lightweight as an orbital airship you get a lot of extra lift through the buoyancy as a result of resting its weight on the shock wave.
This increases the lift to drag ratio and so means you need less fuel to keep it aloft. This idea of resting some of the aircraft’s weight on its shockwave is the idea of a hypersonic waverider. It is standard in hypersonic flight and makes a big difference to the lift to drag ratio for planes that fly at those speeds. You don’t get it at normal supersonic speeds because the shockwave doesn’t touch the plane’s body.
It has solar panels over its vast upper surface to generate power, and uses these to power “dirty ion thrusters”. This is an innovative engine they are developing, the “symphony engine”. It burns the fuel like a normal rocket to generate a plasma (a flame is a plasma), but that means that there are ions in the flame, so then it accelerates the ions using electrical fields. These then drag the rest of the exhaust fuel with them, which is why they call it a “dirty ion thruster” as the exhaust is a mix of ions and neutral atoms.
These let you accelerate with a much higher exhaust velocity than a normal rocket, and so, with a small total amount of fuel, so long as you have plenty of power. It would have no shortage of power with such a large area of solar panels. However it only works for low thrust motors so wouldn’t do for a normal rocket engine.
However it takes a long time to get to orbital velocity, so the drag has to be very low or you lose the advantage of the more efficient rocket. To have less drag and still fly, the lift to drag ratio has to be very high.
He says his airhsips are buoyant up to 200,000 feet, with his “dark sky station” at 140,000 feet.
Based on a buoyancy calculation by James Fincannon, I came to the conclusion that:
Anyway it is either that, or radically change the design, make it buoyant up to 113,000 feet, and the “Dark sky station” perhaps at 60,000 feet (a height where you can survive with only an oxygen mask).
JP say that it would take three days to get to orbit. Putting in three days in seconds into the right hand side of that formula:
But a hypersonic waverider with nearly all the weight of the airship offset by buoyancy is in very unfamiliar territory. Could it have a lift to drag ratio as high as 300: 1?
That makes it more like a commercial jet, or the lunar module taking off from the Moon than a normal launch from Earth.
For more details of the calculations: Can JP Aerospace's Future Giant Airships Slowly Accelerate To Orbit? Looking At The Numbers
See also my Can giant airships accelerate to orbit? where I talk about some of the issues about its feasibility from physics and ask if they can be overcome.
For more background, see my "Can Giant Airships Slowly Accelerate to Orbit Over Several Days"
There are several other places in our solar system with thick atmospheres like Earth, including Venus, and Saturn’s moon Titan. Mars also has a very thin atmosphere. The gas giants have thick atmospheres too (with no solid surface).
JP Aerospace hope the same idea can be used for Venus, with a high altitude staging post again, this time of course in the Venus atmosphere. The aim wouldn’t be to land on the surface, which is incredibly hot and high pressure, but to go down to the Venusian cloud tops to study them and perhaps build habitats there.
Perhaps they could use it for Mars too. The atmosphere of Mars is so thin that you could land an orbital airship like this on the surface. The strongest winds on Mars would only barely move an autumn leaf, fast though they are.
If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship. Northrop group’s “VAMP” project to study the Venus atmosphere uses an airship design like JP Aerospace, and they would inflate it outside of the atmosphere, so again that’s very like the JP Aerospace idea. It enters the Venus atmosphere already inflated, and because it is so large (55 meters in diameter) and low density, it doesn’t need an aeroshell.
However, unlike the JP Aerospace design, it’s able to fly in an Earth pressure atmosphere, so it’s not nearly as low density as an orbital airship. It still gets quite hot during the descent.
https://youtu.be/0EjgoEFiKko
The first tests of VAMP would use the Earth’s atmosphere. So it could also be used for Earth re-entry. It might be useful for surveillance, photographing the Earth from above, and also for scientific studies of the upper atmosphere.
The same ideas could also be used for Titan - a moon of Jupiter with an extremely cold atmosphere at -180 °C, but it’s also dense, with the same pressure as Earth’s. This means that humans could go out of doors there, without needing a pressurized spacesuit. Of course they would need protection from the extreme cold and they would need air to breathe, so you are talking about warm clothing, as for Antarctica, perhaps heated clothes, and an oxygen mask.
However, they could get the oxygen to breathe by splitting water ice from Titan, and then burn the methane from Titan in that oxygen, a process that, rather neatly, creates an excess of energy which could then split more water, generating more oxygen for heat, and also for the colonists to breathe. The habitats could be built like many modern Antarctic bases, on legs to hold them above the cold surface. Since the air pressure is the same inside and out, the air could be kept in using double doors in a building of normal construction, again like an Antarctic base. See Let's Colonize Titan, and there are more details in their Beyond Earth: Our Path to a New Home in the Planets.
The normal way to re-enter Earth’s atmosphere at present is to use an aeroshell. This absorbs most of the heat, all the way through the early stages of re-entry, until the spacecraft is traveling slowly enough to drop the aeroshell and deploy parachutes. The spacecraft hits the atmosphere at many kilometers per second, so there is a lot of heat to dissipate. The main methods they use to keep the temperatures within reasonable bounds are:
You might wonder why the astronauts returned from the Moon at such a high speed. After all, it orbits the Earth at a speed of only 1.02 km / sec relative to Earth.
The reason is that if you start higher in the Earth’s gravitational well, and drop down to LEO, you accelerate all the way, and actually end up faster than if you started off in LEO originally. If you come back in a transfer from the Moon you hit the atmosphere at 11.1 km / second. There’s an online transfer orbit calculator here. Starting from a higher orbit just makes things worse for you.
The one thing you can do to help with re-entry speed is to orbit the Earth in the same direction that it spins. The Earth’s surface (and so its atmosphere too) moves at 460 meters per second towards the East, or about 1,000 miles per hour) because of its rotation (it’s spinning towards the rising sun).
So if your satellite is orbiting in the same direction as the Earth in an equatorial orbit, West to East, it has 0.92 km / sec less delta v relative to the atmosphere than if it orbits in the opposite direction. This makes re-entry just a little easier.
An orbit in this direction also makes the launch easier. You need around 0.92 km / sec less delta v to get into orbit if you launch from West to East. That’s quite a huge saving in fuel, which is why all the US launches are from Florida, launching over the Atlantic, and why all the Soyuz launches from Russia are from West to East too
But you can achieve a much gentler re-entry using a ballute - a cross between a balloon and a parachute. It works like an aeroshell but decelerates much higher in the atmosphere
Then there’s an idea from 1966 to return a human being from orbit, in an emergency, using a balloon to dissipate some of the heat, though the seat for the astronaut acts as an aeroshell to dissipate most of it. It combines some of the approaches of the previous ideas. You sit inside the balloon which is filled with breathable air to keep you alive during the re-entry. It’s never actually been tested in space.
This is another idea originally developed for Gemini in the early 1960s. For a while, before they settled on the familiar parachutes, the engineers thought that after the fiery stage of re-entry, the capsules would glide down to Earth beneath a parasail or paraglider. Those tests were quite promising, though they ran into many issues, for instance getting the glider to unfold. Eventually this line of research ended in 1964, when they changed to the parachutes as used by Apollo. The Russians also used parachutes for the Soyuz flights. For details of the paraglider research, see: Coming Home
Anyway at around the same time, in 1960, the engineers came up with the idea of using the same paraglider approach to go all the way from orbit, right down to the surface, without an aeroshell. This was the idea of the inflatable paraglider (Rogallo wing), called “FIRST” (Fabrication of Inflatable Re-entry Structures for Test), another idea for a space self-rescue system for astronauts.
It could be folded up into a small cylindrical package which would be kept docked to a space station, much as our modern Soyuz TMA is. In an emergency, the crew enter this cylinder, and separate. The paraglider then inflates and deploys. It would re-enter at an angle of 1 degree, with the paraglider angle of attack of 70 degrees. They found that deceleration would not exceed two g’s, and that there would be minimal heating because of the way it glides down to the surface. It would approach the speed of sound at 43 km altitude, and from there it would be able to glide 345 km horizontally before eventually landing. Details here: FIRST Re-Entry Glider
The Spaceship-One uses a different idea for re-entry. It changes its wings to the “feather” position which also tilts the spacecraft so that it presents as much surface area as possible to its direction of flight, so slowing it down. This is only for a sub-orbital hop at present. The first demonstration of the feather system was in 2011.
https://youtu.be/Lqhzlq7UReAThe Virgin Galactica crash in 2014 was a result of the pilot accidentally unlocking the feathering system too soon. It then deployed by itself and changed the shape of the rocket far too early, when it still needed to be streamlined.
What about returning a final stage? That also is low mass and it presents a large cross section if you fly it backwards, rocket motors first, with supersonic retropropulsion. So you’d think it might be reasonably easy to do.
First, some background. Every time a spaceship goes into orbit, it needs a final stage, a thin container full of fuel which is burnt right at the end, to get it to orbital velocity. It has to do that, because the spaceship itself is far too small to have enough fuel to get to orbit by itself, even with the help of the first (and sometimes second) stage.
It then discards the final stage, which normally orbits Earth a few times and finally falls back to Earth (apart from interplanetary missions and missions to the Moon, which often use a more powerful final stage, for instance nearly every mission to Mars also sends a final stage in the general direction of Mars too).
So, could a final stage be returned to Earth in the same way that SpaceX have returned the Falcon first stage? Well, when SpaceX returns the first stage of the Falcon 9, it slows down partly through friction in the upper atmosphere. The landing legs alone reduce its terminal velocity by a factor of two. It also has a burn in orbit and another burn just before it reaches the barge. You can see the first stage at the beginning and end of this movie (most of it is for the second stage). That may seem rather similar, but it only has to shed one kilometer per second of delta v, and much of that is done with the two burns.
https://www.youtube.com/watch?v=XdtK0poWp20Elon Musk has said he plans to re-use the final stage in the future, though it’s probably not going to work for the current Falcon 9. Here he answers the question: “Any plans for a reusable second stage?”
https://youtu.be/-pQ8M9mTw0M?t=782
The basic idea of all these designs is that the lighter it is, or the greater the cross sectional area it presents to the atmosphere, then the higher it is when it slows down, and so the lower the temperature of its skin during re-entry. What matters is the mass per cross sectional area - or more precisely, the ballistic coefficient (which is complex to calculate).
If the spaceship can use a glide to stay high in the atmosphere, this also helps. It also helps if it can use retropropulsion to reduce its velocity before it enters the atmosphere, and as it descends. Then, if it can radiate or absorb heat or ablate, for instance with an aeroshell, or use active cooling (perhaps in the future), this also helps.
Another way around it is to use a space elevator. Though we can’t build a space elevator quite yet, there are other things we could do right away that are rather similar. But first let’s look a bit closer at why we can’t build a space elevator quite yet.
The main problem with building a space elevator is that it has to be so tall, because geostationary orbit is very high, around 35,786 km above the Earth’s surface. That’s a long length of cable to hold up against gravity.
It’s a little better than that figure might suggest, because gravity would get less, rapidly, as you go up the cable (by inverse square law). Also the cable is slowly spinning around the Earth at the centre, leading to a centrifugal force (strictly speaking, a “fictitious force” in the rotating rest frame) which counteracts gravity. At geostationary orbit you are in free fall and those two forces balance each other out, so your effective weight, and the weight of that section of the cable, is zero.
After taking that into account, it turns out that the material needs to be strong enough to hold up 4,960 km of its own weight under full gravity (result given by Arthur C. Clarke). It will then be able to hold up the full length of the space elevator in the Earth’s varying gravity all the way out to GEO.
But our current cables come a bit short of that. Kevlar is one of our best materials for this job, and under full gravity, it can only hold up 255 km of its own weight before snapping.
You can improve this situation to some extent by tapering. This is easiest to explain assuming uniform gravity, and let’s use 200 km sections of Kevlar for simplicity of the calculations, and a safety margin:
Calculation indented to make it easy to skip:
You can make the figures better if you use a continuous taper instead of steps of 100 km each, but it’s still impractical. You can find out the details in the published literature on the subject. For a tapered Kevlar space elevator you’d need the top of the cable to be 260 million times wider than the bottom. For instance, to suspend one millimeter diameter of cable to the ground, you need it to be 260 kilometers in diameter at GEO. (Figures for this and next paragraph from introduction to this paper)
For more on this see Arthur C. Clarke’s 1976 address to the IAU in Munich: The Space Elevator: ‘Thought experiment’ or key to the universe? (part 2) (see also Part 1 and Part 3). You may not know that he trained as a physicist with a first class degree in maths and physics before he became a science fiction writer.
We don’t quite have that technology yet. If only we had perfect carbon nanotubes, they would do the trick, with a taper ratio of 1.9, so the top of the cable only needs to be a little under twice the diameter of the bottom. For a while scientists were quite optimistic about this.
https://youtu.be/4XDJC64tDR0It turns out that if there is just a single atom out of place, they lose their strength. Sadly, our technology isn’t up to the task of making perfect carbon nanotubes with not an atom out of place, long enough to join together to make cables thousands of kilometers long. Also, with nanotubes so sensitive to damage, there’s the problem of what happens if you get defects introduced as a result of micrometeorites and other wear and tear.
Other materials also just aren’t strong enough yet. Kevlar has a breaking height of 255 km. There are a few other promising ones, Zylon (polybenzoxazole fiber) has a breaking height of 379 km (from page 14 of this study). But we’ve got a long way to go.
So it remains an idea at present. Some enthusiasts think it may be practical in the near future, perhaps even just a decade or two away.
Though we can’t build a space elevator on Earth yet with present day materials, we could build a space elevator from the Moon. It would be around 56,000 km from the Moon to the L1 position between the Moon and Earth, which is a point where the gravitational forces balance.
You can also use a skyhook. It’s very like the space elevator, but you construct it downwards from a rather lower orbit than geostationary orbit. The bottom end can’t be attached to anything so it just dangles in our atmosphere, traveling around, much faster than our Earth rotates. That’s why it is called a skyhook. You could then just fly up to the bottom end of the skyhook, and if you fly fast enough you could keep pace with it in our atmosphere, and attach yourself to it. Then once you’ve done that, you can travel up the cable much as you do for the space elevator, until you are in orbit.
It’s easiest to construct a skyhook, if you have a handy moon or asteroid to attach it to. It’s a bit like the idea of the lunar elevator but constructed on a much smaller moon. If we had an extra moon close to Earth it might be very useful. We don’t have one, but Mars does, two of them. Here is an early concept study to use two skyhooks for Mars from a 1985 paper (described on page 70 of the Tethers in Space handbook):
The idea is - if you build a 6,100 km long skyhook type tether outwards from Deimos then it can throw objects out with escape velocity and also catch incoming spacecraft from elsewhere in the solar system to a gentle rendezvous that only needs docking thrusters, like docking with the ISS.
Then if you build a 2,960 km long skyhook tether inwards from Deimos and a 940 km tether outwards from Phobos, then it turns out that if you drop a spaceship off the Deimos tether, then it is traveling at just the right velocity to send you in a transfer orbit to the Phobos tether. Your transfer vehicle ends up stationary next to the Phobos tether when it gets there, with plenty of time to dock, much as spacecraft do with the ISS. Then you can just go down that tether to Phobos. All this requires no acceleration and no rocket fuel, apart from maneuvering thrusters for the docking itself. It works just as well in the opposite direction, drop your spaceship off the Phobos tether and the tether’s extra velocity will boost it to the Deimos one where it will come to rest with plenty of time to dock there.
Once you are on Phobos, a 1,160 km tether extended downwards towards Mars can be used to drop materials down to an elliptical orbit which reaches down to Low Mars Orbit at a height of 375 km when closest to Mars. Hop David found that slightly longer 1400 kilometer long tether could put it into an atmosphere grazing orbit so that you can use the Mars atmosphere for aerobraking to circularize its orbit.
The total mass for all these tethers if made from Kevlar is not that great considering what it does. You need between 5,000 and 90,000 tons if designed to handle a payload of 20,000 tons.
You could also bypass Deimos and just have a single tether from Phobos upwards to reach escape velocity right away.
The mass of Deimos and Phobos is so great that you could run this system for decades with no noticeable effects on their orbits. They act as a “momentum bank” so you wouldn’t need to be careful about always balancing movement of materials outwards with movement inwards, or using other methods to keep reboosting the tether.
Hop David has explored Phobos tethers in a lot more detail. He finds that a much shorter 7 km tether extending from Phobos towards Mars would let spacecraft dock with its L1 position where the gravity of Phobos is balanced with the gravity of Mars. It’s a tiny amount of gravity, only a ninetieth of Earth’s (0.11 m/sec²), but that way you could land without throwing up a cloud of dust, and also perhaps, without contaminating the surface of Phobos with rocket exhausts, which might well be useful.
Then, a very long 1,400 km tether grazing the Mars atmosphere would mean you only need 570 meters per second delta v to land on Mars. See Lower Phobos Tether and his General template for space elevators.
We don't have any handy moons like that for Earth, but it turns out that there’s a better way to do it here. Instead of a simple skyhook, you use a “spinning skyhook” or rotovator, often called a “momentum exchange tether”. This spins in such a way that the end closest to the Earth moves backwards relative to its orbit around Earth. - this animation shows the idea:
As the tether orbits Earth, it spins, and if you arrange the rotation rate carefully you can arrange it so that the tip closest to Earth moves backwards at just the right speed to be completely stationary above Earth, just for a short while in each spin. You can never manage a stationary tip like that with a a skyhook.
That could be used to take a spacecraft traveling at faster than orbital velocity, (perhaps in a transfer orbit from the Moon or GEO), and slow it down so it is stationary, and momentarily hovering above the Earth’s atmosphere. Once you’ve done that then you can let it go gently, from the tip of the tether, and it falls down vertically, with a parachute perhaps. In the other direction, you could use the same approach to lift a payload, for instance, attached to a balloon, and attach it to the bottom of the space tether when stationary relative to Earth, and the tether would boost it into orbit, or send it to the Moon or in a transfer orbit to GEO all in one go.
A momentum exchange tether doesn’t have to be as long as the space elevator to do this. You could have a shorter tether which spins more rapidly, in an orbit closer to Earth. That makes it a less massive construction.
It also means you get into orbit quickly. With the space elevator, it could take quite a while to get there. At 200 mph it would take seven and a half days to go up all the way to GEO, and at 2,000 mph it would take nearly 18 hours.
The space elevator can be thought of as a rotovator that rotates once a day and so can have one end fixed permanently to Earth.
This video shows the idea of launching a rocket vertically to meet the bottom end of a rotovator that is rotating at just the right speed to be stationary relative to Earth’s atmosphere. That’s a far easier thing to do than to launch a rocket to orbit.
https://www.youtube.com/watch?v=Ox_-3S3AMxkHowever we don’t have the materials to do that yet either. Though the tether is shorter, it also has to spin more rapidly. You still need very strong materials, nearly as strong as for the space elevator.
Let’s try a couple of examples to give the idea, as usual, with the calculation indented to make it easy to skip:
If some day we do have space elevator materials, then these tethers would give a great way of achieving a similar effect for much less total mass, and also, because the tether is shorter, it’s a way of getting into orbit more quickly than for the space elevator. But until we have those materials, we need to use a “watered down version” which is not able to hover stationary over the Earth’s surface. However, it would still be enough to make a difference. Normally, you need to travel at about Mach 20-25 to go into low Earth orbit (depending on how high the orbit is). The launch assist tether reduces that to Mach 12 or less.
https://youtu.be/CWLzGGKV0jIFor more details, see Launch Assist Tethers. You could use the same process in reverse to de-orbit a spacecraft to Mach 12 in the upper atmosphere, and then it glides down from there.
So this would give you a way to slow down your orbiting spacecraft to Mach 12 instead of Mach 20 upwards, which would be quite a plus.
That’s still very fast. It’s four times the speed of the Lockheed SR-71 Blackbird supersonic spy plane, or Virgin Galactica’s SpaceShipOne (Mach 3.09), and more than twice the speed of the ESA’s projected Mach 5 aircraft:
They have other ideas in the paper such as a “two stage tether” where you have a smaller tether spinning around the tip of a larger tether. This reduces the total tether mass, rather like the idea of a two stage rocket launch, for a given tether tip velocity.
The tether loses speed, dropping into a lower orbit to compensate, every time it lifts a spaceplane out of the atmosphere. But it can get back into position in between those gravity assists, by using solar power to send electric current along the tether. This uses the Earth's magnetic field for a motor to accelerate back into orbit.
Or, rather elegantly, it can also do it by de-orbiting a similar payload. For instance, it could de-orbit a spacecraft returning to Earth, or materials exported to Earth from the Moon or asteroids. This was all fully worked out in the plans for the Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.
Hoyt has suggested that we could use the same approach for exports from the Moon. Our materials are already easily strong enough to make a rotovator which can be stationary relative to the Moon, for a short while each time it briefly touches the lunar surface.
The Moon is high in the Earth’s gravitational well, so if we had two such tethers, one orbiting the Moon and one orbiting the Earth, the whole thing could be powered by movement of materials “downwards” from the Moon to Earth. It’s rather like the way movement of water downhill powers a waterwheel. For more details, a diagram and cites, see the Exporting materials from the Moon section in my “Case for Moon” book, available to read free online, or for kindle.
This cisulnar transport system is not nearly as massive as a space elevator, with a total mass less than thirty times a typical payload, so there is much less construction needed. That’s few enough payloads, so that it would soon pay for itself with frequent transport to and from the Moon. In his scheme the rotovator in Earth orbit just slows down payloads to LEO speeds, for instance to deliver ice from the Moon as fuel to use in LEO. But the materials could also be returned to a HASTOL type tether and into the Earth’s atmosphere easily enough.
Robert Hoyt and the science fiction writer Robert Forward are hoping to actually build orbital tethers some day, and have founded a small company Tethers Unlimited to do it.
https://youtu.be/pCAEFocoVdM
You can also have "free skyhooks" orbiting in interplanetary space, as in tthis suggestion by Hans Moravec from 1986. These just spin on their own axis in free space, nowhere near any planet. He describes a 20,000 ton skyhook made of Kevlar designed to boost a payload of 50 tons (same mass as the Space Shuttle).
Within those parameters, it could be anything from 100 km long, spinning once every 4 minutes to accelerate the payloads at 8 g, or 20,000 km long, spinning once every 12 hours, to accelerate them at 1/25 of a g.
The idea is to cause materials to levitate by accelerating them inside an evacuated tube. You could use a belt or a continuous chain, accelerated magnetically much as you accelerate a maglev train. The materials inside the track go around a circular track at the end and return the way they came so making a continuous loop.
Then you accelerate passengers and cargo to orbit along this levitated track, again using maglev.
Loftstrom estimates that you could build a launch loop for $10 billion able to send 40,000 metric tons to orbit per year. He estimates a similar payload cost per ton to the space elevator of $300 per ton. But it wouldn’t be used for landing.
Yunitskiy had the idea of building a levitating loop all the way around the equator, using a similar construction to the launch loop. I don’t understand his idea in detail - most of his writings on it are in Russian. But his basic idea is to first build an overpass all the way around the Earth at the equator which would already be useful for transport from place to place on the Earth.
Then, he would have an evacuated tube in it, attached to the overpass and clamped to it, evacuated to a hard vacuum. Inside this, he has a magnetically levitated inner ring, the “rotor”, which also continues all the way around the equator, which is a sort of belt or chain. Both rings are segmented with flexible joints that can be stretched. He then accelerates the inner ring. He keeps the whole thing clamped to the ground until the inner ring is traveling at faster than orbital velocity at sea level. He then releases the clamps and because it is moving so fast, this causes it to lift up off the ground, all round the world at once, levitated as for the launch loop until eventually it reaches orbit.
His system is much more complex than this description, and for some reason, it has two rings running in opposite directions inside. It’s summarized in wikipedia here. His video about it is here. I’m not sure how well it would work but it has some interesting points in it which is why I thought it was worth sharing.
So that then leads naturally to Paul Birch’s idea of orbital rings.
This is a fun idea. It doesn’t get as much attention in science fiction as the space elevator for some reason. Unlike a space elevator, you can build it already with present day materials, not just on the Moon, or in free space, but as a way to get into orbit from Earth as well. But it is likely to be very expensive to build it in the first place, unless you have a low cost way to get materials to orbit already.
The first orbital ring is very very expensive. But once you’ve built it, you can then send up more materials to expand it easily for a very low cost.
As with the space elevator you just go up to the platform above the upper atmosphere in an elevator. Then in the opposite direction, once you have an orbital ring you can just travel down to the surface in an elevator.
https://youtu.be/MQLDwY-LT_o?list=PLIIOUpOge0LtW77TNvgrWWu5OC3EOwqxQIt’s rather similar to the sky towers idea. You are stationary above the Earth, and experiencing nearly full g as you are not in free fall - so you still need some way to get to and from orbit from the ring, but you could use maglev to accelerate along the ring to get to orbital speed.
To build this structure, what you need is a wire that circles the Earth, at orbital heights and moving around the Earth at orbital velocity. It would be very thin to start with; this doesn’t matter. It stays in orbit just through its own velocity, like an orbiting satellite. Run a current through it and then you can have objects levitating on it much like a maglev train. But in the case of a maglev train, the track stays still and the train travels around it at up to hundreds of miles an hour. Here the track travels around the world at its orbital velocity of 7.8 km / sec (if you build it in LEO), and your levitated platform travels along the track in the opposite direction nearly as fast relative to the track, racing in order to stay still as it were.
At 16,500 mph relative to the track, it’s traveling more than twenty seven times faster than the fastest maglev train (603 km/h) and also many times faster than a hypervelocity bullet. But there are no earthquakes, no weather, and its in the hard vacuum of space. If you do that, your platform will be stationary relative to the ground.
So now you can lower a cable down from your magnetically levitated platform to the ground. If you do that just anywhere then there is no way the orbital ring will support it, but you can arrange to have a kink like this in your ring to support it from:
Now the orbital ring consists of two elliptical segments (the diagram is hugely exaggerated in scale; they would both be close to circular) and then the kink gives an upward force that let’s suspend your skyhook, or “Jacob’s ladder” as Paul Birch called it when he proposed the idea in a series of three articles for the British Interplanetary Society.
As soon as you have the first one built, however small, you can then bring up materials along the cable and use it to make your orbital ring more massive, or build new ones. You can build an orbital ring like this at any height. You can also build it at any angle too. It doesn’t have to orbit above the equator, so you can build it along a slanting orbit like the one the ISS follows. You can arrange the balance of forces on the whole system, including the weight of the skyhooks, so that it slowly precesses, and if you do that carefully you can get it to precess once every 24 hours, which keeps it stationary relative to Earth.
So in this way you can actually build elevators up into space from anywhere on the Earth to a platform hovering at the same height as LEO, but stationary relative to the Earth. You wouldn’t be at orbital velocity yet when you get there, you’d just be above the atmosphere and stationary relative to the Earth looking down. But you can then have, say, a maglev train that accelerates along a track along the orbital ring, until you reach orbital velocity.
It’s safe also. The ladders into space are stationary relative to the Earth, so would just fall down vertically into an area a few kilometers long and a few hundred meters wide if they broke. As for the orbital rings themselves, if damaged, they would actually “fall” upwards into a higher orbit. You can read the techy details here: Orbital Rings-I, II, III.
You can build orbital rings around the Moon too. And though orbital rings around Earth would be so expensive to build with rockets, perhaps a trillion dollars (in his 1980s calculation), the cost would go down a lot if you could get materials from the Moon.
It would be a huge asset once built, as the cost per kilogram to send materials into orbit would be only cents. It’s a “bootstrapping problem”. If only we had an orbital ring already, even a small one, it would be very easy and low cost to build lots of orbital rings. But how do we get started? See also the wikipedia article Orbital ring
If one of the other techniques mentioned already gives you a very low cost way to get materials to low Earth orbit (such as skylon, orbital airship, or launch loop) or from the Moon (Hoyt’s cislunar tether transport system), then perhaps we can build an orbital ring as well, for a reasonable cost.
Check out my book for more surprising astronomy answers like this:
Simple Questions - Surprising Answers - In Astronomy,
It is a compilation of some of my quora answers . I’ve just added this answer as the last one in the book.
To see why it is so difficult with ordinary rockets, here is a quick refresher on orbits. If you could throw a ball from above our atmosphere, gravity still pulls it down in the same way as on Earth, If you throw it fast enough, what happens is that it gets beyond the horizon before it can hit the Earth, and Earth’s gravity continues to pull it around into a curve until it gets back to its starting point. That’s how satellites such as the ISS stay in orbit, and that’s why it is often called free-fall.
To skim the Earth’s atmosphere in orbit, your spacecraft has to travel at least as fast as 7.8 km / second, or about 17,500 mph. The Earth itself, with its atmosphere, is spinning eastward below you, at around 1,000 mph. So, you can reduce your re-entry speed by orbiting in the same direction that the Earth spins. However that only helps a bit. Your spacecraft still has to travel at 16,500 mph relative to our atmosphere to stay in orbit.
If you slow down by a tiny amount below that speed, even by just a few hundred miles per hour, as you skim the atmosphere, you will fall too far towards Earth before you complete your orbit. You will hit the atmosphere at thousands of miles per hour, and will re-enter in a fiery descent. At that point you depend on your aeroshell to protect you from the heat.
But couldn’t you just hover above the atmosphere much in the same way that the lunar module pilots hovered above the Moon’s surface? You may remember that the Apollo 11 landing module nearly ran out of fuel as they were landing, because the computer was taking them to the steep slopes of a crater with rocks the size of automobiles. Here is Neil Armstrong narrating his landing:Example:
On December 11, 2015, the Soyuz TMA 17M mission started from a low Earth orbit at an altitude of 416.7 km, and re-entered with a change in speed of 128 meters per second or 286 miles per hour.
As he says there, he took over manually and “flew it like a helicopter” on out to the West to find a good site. That’s something you can do using rocket thrust in the weak lunar gravity. To do a hover like that against full Earth gravity would take a lot of fuel. So much so that when fully fueled to land, your spaceship would need to be nearly as large as those rockets you see in televised launches that take our spacecraft into orbit. Huge!
If you could use fuel with a high power density, such as antimatter, which lets you convert matter directly into energy, or perhaps fuel for nuclear fusion, it would be easy. Your spaceship just needs to thrust continually at 1 g away from Earth to maintain position as it slows down, and then it can lower itself down to the surface, as slowly or as quickly as your pilot wishes.
But we don’t have such fuels yet. So is there any other way to do it?
SPACE ELEVATOR
We could lower a spacecraft slowly through our atmosphere with a space elevator. This is basically a giant lift, with the top a long way above our atmosphere, extending beyond geostationary orbit, and usually reaching to the ground at the equator. It is held in place, and tensioned, by a counterweight above geostationary orbit. This video gives an idea of how it works - and it also gives a refresher on orbits at the beginning.If we had that, your spaceship could dock at geostationary orbit, which is easy to do, so long as you make sure that you orbit the earth in the same direction that it spins, once every 24 hours. That way your orbit keeps you stationary above the same point on Earth all the time, and so stationary relative to the top of the elevator. So then you can just dock slowly much as they do for the ISS.
Once you’ve done that, then you would get out of your spacecraft, or attach your spacecraft to a lift, and just slowly travel down the elevator at, say, 200 mph, or whatever speed you find comfortable and safe, until you reach the Earth’s surface. You’d feel gravity gradually increase from zero g to full g as you descend.
Arthur C. Clarke’s science fiction story The Fountains of Paradise is based around this idea.
The space elevator is a mega engineering project, with many issues to sort out before it can be built. However, as well as that, at the moment there is one issue that is a deal breaker for it. It requires cables able to hold up their own weight for distances of thousands of kilometers. Present day materials such as Kevlar and the somewhat stronger Zylon can only hold up a few hundred kilometers. Carbon nanotubes made in a laboratory are strong enough for a space elevator, but when they are made into larger cables in practice they are weaker than Kevlar because of flaws. It would be possible to build a space elevator on the Moon with its weaker gravity, using Kevlar or Zylon, but not on the Earth. More on this later, but meanwhile let’s look at other ideas.
COULD WE BUILD THE SPACE ELEVATOR FROM BELOW?
You might perhaps wonder, what about building it from below? The earliest idea for a space elevator is from Konstantin Tsiolkovsky, one of the founding fathers of the theory of rocketry, and his idea was inspired by the Eiffel tower.
Konstantin Tsiolkovsky - Wikipedia on a Soviet postage stampHe had the idea of building a space elevator from below, inspired by the Eiffel tower. But it turns out that we can’t do this, materials just aren’t strong enough to hold up the compressive weight of the tower.
But it turns out that none of our materials have anything like the compressive strength needed to hold up a thousands of kilometers tall tower from below. It would just collapse under its own weight.
So then, what about building it just to the top of the atmosphere? Might that be useful?
Well first, how high could a building be? Well at least as high as a mountain, if you have enough materials for it. The tallest mountain as measured from its base is Mauna Kea in Hawaii which rises 10.205 kilometers above the sea floor.
This paper works out the theoretical maximum height of a mountain,not taking into account any geological considerations such as how it could get to be so high or the depth of the crust supporting it. As an example at the end, they work out that a mountain made of granite with a base of around 1,000 km could be as much as 45 km high. But they say it could be much higher if the mountain gets rapidly steeper, as it gets higher, like the Eiffel tower.
The highest hyperbuilding structure to be designed by an architect is the X-Seed 4000 for Japan, which uses an Eiffel tower like construction, and would reach a height of 4 km with a 6 km diameter base, making it 224 meters higher than the similarly shaped Mount Fuji. It was designed by Peter Neville for the Taisei Corporation. The idea was to build it in Tokyo bay.
More images here (page is in Arabic)
Mount Fuji is similar in shape and 224 meters lowerWilliam Baker who worked on the designs for Burj Kalifa, the tallest building in the world, says that the same buttressed core approach could be used to make much taller buildings, quoted in this article in Business Insider: “Someday We Could Build A Skyscraper Taller Than Mount Everest”. He suggests that if made of light enough materials and with a wide enough base, buildings could be built to be higher than mountains and reach heights of tens of kilometers.
Space engineer Ben Quine from Ottawa in Canada has devised an idea for a twelve miles high building, using inflated Kevlar tubes.
See: Going up Canadian patents an 'elevator' to space.The patent itself is short on details however, just covering a method for attaching an elevator car to such a tower. Can you prevent a cascading collapse of the whole thing downwards? How does it respond to wind stress? What about icing? Would it help much anyway to launch from above the thickest part of the atmosphere? See: This insane 12-mile-tall 'space elevator' concept is driving engineers crazy
Perhaps a tower as high as tens of kilometers high might be feasible, and if so, may be useful for launching to orbit, because it reduces the amount of drag.
It’s not so much the atmospheric drag which was small, for the Saturn V it was about 48 m/s delta v lost to the atmospheric drag. It’s the gravity drag due to launching vertically to avoid the thicker atmosphere, but also because it has to lift the weight of the rocket - it’s far more efficient to accelerate horizontally. If they can tip the rocket over to travel horizontally sooner it would reduce the effecdtive delta v to orbit. The Saturn V lost around 1.743 km / sec due to gravity drag (from Robert Braeunig’s Saturn V launch simulation)
But it’s not nearly high enough to help much with the landing. It’s still stationary relative to the Earth’s surface. That’s okay for a tower that goes up to geostationary orbit, but not much use if it doesn’t even reach LEO.
How can you dock with the top of a building when you are traveling at a relative speed of 16,500 mph? You have the same problem that you’d need huge amounts of fuel to land on it with a conventional rocket. So let’s look at that in a bit more detail now.
TRADITIONAL ROCKET
You could use a large rocket refueled from orbit of course, to slowly lower your spacecraft through the atmosphere. This would require a lot of fuel though because of the Earth’s gravity.
It’s easy to carry enough fuel to land on the Moon, in a slow controlled de-orbit. The lunar module descent stage had a total mass of 15.2 tons, and of that, 8.355 tons was propellant (these figures varied depending on the mission). So around 45% was payload, including as the payload there, the lander itself, the ascent stage, its fuel, and the crew, as well as the lunar rover and any equipment to be left on the surface.
Apollo 11 lunar module. The fuel for the landing was only 55% of its total mass.The ascent stage was even more efficient with less that 50% of its mass consisting of fuel.
The ascent stage for Apollo 9’s “Spider” lunar module, seen from below, showing the ascent nozzle. This was a test in LEO as you can see from the Earth behind in the photo.If Earth had as little gravity as the Moon it would be easy to get into orbit and back again and we wouldn’t need to use the atmosphere at all. However, though the gravity is only six times greater on Earth, we need far more than six times the amount of fuel because of the way rockets work. For every few tons of fuel at the end of the journey, you may many extra tons early on, which is just fuel to accelerate fuel.
The lunar module’s ascent stage had a total mass of 4.76 tons. Of that, the crew was 144 kg and the propellant was 2.375 tons. The fuel amounted to less than 50% of its total mass. Of the original 15.2 tons of the fully loaded and fueled lunar module, 2.405 tons did the round trip all the way back to orbit. That’s about 15.8%. (Some of the payload of course was left on the surface such as the lunar rover in later missions, and the experiments).
By way of example only 4% of the Saturn V rocket used to launch the Apollo missions was payload, so 96% of the launch mass is either burnt or discarded on the way to orbit. For the Ariane 5 (European heavy lift rocket), the payload fraction is 2.5%, and it’s similar (slightly less) for the Soyuz 2 used to launch the crewed Soyuz MS.
For the Space Shuttle, only 1% was payload because most of the mass put into orbit was the shuttle itself which was returned to Earth. For more on this, see The Tyranny of the Rocket Equation and for some more example figures, this Payload fraction table
If you don’t have an atmosphere for aerobraking, it takes about the same amount of fuel and hardware to get something into orbit as to de-orbit it. So for instance, since it takes 312 tons total mass to launch the new Soyuz MS with a crew of three into orbit, you’d need that much mass in orbit already to get them back. But each launch can only carry a payload of 7.08 tons. That makes it 44 launches before you have enough mass up there to return your crew of three safely without aerobraking.
You can improve those figures by using a heavy launch vehicle but it would still take eleven launches of the Delta IV Heavy, the highest capacity launch vehicle in production, with a payload of 28.79 tons. It would take six launches of SpaceX’s Falcon Heavy when it is ready, with a payload to LEO of 54.4 tons.
You could do it more easily with the Saturn V. This had a payload to orbit of 140 tons (after boosts in payload capacity for the last two missions). So three launches would be more than enough at least in terms of the total mass. It would also take three launches of the new Space Launch System when ready.
Back in the 1960s NASA studied even larger rockets, the NOVA series, with an eye to a mission to Mars. These never flew, but would have been able to send many hundreds of tons into LEO in a single flight.
Nova - studied from 1959 to 1962. Finally cancelled 1964. Figures show payload to LEO in metric tons. Image © Mark WadeLater on they explored ideas for modifying the Saturn V for a Mars mission. The Saturn V-4X(U), designed but never built, could have sent 527.6 tons to a 486 km orbit at 28 degrees. That would be much more than enough to get the entire mass of a Soyuz 2 fully loaded with fuel + payload to LEO in a single launch.
So, that would be the situation, for someone living on a planet with Earth gravity and no atmosphere, or very little atmosphere. They could send robotic spacecraft into orbit early on. Returning their citizens safely from orbit would be tough, but not impossible, if they build something like the Saturn V or even more so, the Saturn V-4X(U). However it’s no wonder that we use our atmosphere to slow down our spaceships for re-entry.
One thing that could change all this, is if you don’t have to carry the fuel on board the spaceship. If it is beamed to the spaceship from elsewhere, say from Earth, then you don’t need to use fuel to carry more fuel, and then it becomes much more feasible to land by hovering in the atmosphere. Your spacecraft also becomes much less bulky because it doesn’t have to contain all the fuel.
FLYING TO ORBIT AND BACK ON A BEAM OF LIGHT
This is a neat idea, but so far has only been tried in small scale demos, raising models a hundred feet or so on laser beams
Who knows, it might become the standard way to get into space some time in the future, but it is a long way from achieving that potential right now. A related idea is a mixed system with laser or microwave supplying energy, to heat up propellant on the rocket itself, which requires much less fuel, as you don’t need to “burn” the propellant to supply energy to fire the rest of the propellant out of the rocket exha ust. See Laser Propulsion Could Beam Rockets into Space, and Jordin Kare's talks to the Space Show.
Lightcraft
If you can send a spacecraft into orbit that way you can also return it from orbit the same way if you want to. However we don’t have any spacecraft yet that can do this.
USING THE UPPER ATMOSPHERE AS A BRAKE - SPACE SHUTTLE
This is the way it is done today, to use the upper atmosphere as a brake, then slowly parachute to the surface or glide down in the lower atmosphere. How easy that is to do depends on the spacecraft.
If it is a heavy one like the Space Shuttle (now retired of course) then it can only slow down deep in the upper atmosphere, where it is dense. So it gets very hot. That’s why the Space Shuttle had to have ceramic tiles able to withstand temperatures up to 3,000 °F (1,650 °C)
Space Shuttle Enterprise banking on its second approach and landing test, during early flight tests.
NASA artwork for Space Shuttle re-entry - it’s high density, so can only slow down deep in the upper atmosphere, and gets very hot during that stage of its flight
LOWER TEMPERATURE REENTRY - SKYLON
Skylon is a plane being developed by the British company Reaction Engines with funding from the UK government and ESA. It will be able to fly to orbit from a conventional runway (though reinforced to carry the extra weight of all the fuel), return back to Earth, and then take off again within a couple of days with a crew of 200 to assist.
Its design is much lower in density than the space shuttle, once it has used up its fuel to get into orbit. So it slows down in the atmosphere at higher altitudes on the way down.
What really matters is the mass per cross sectional area it presents to the atmosphere or more exactly, its ballistic coefficient. Skylon could slow down even higher in the atmosphere if it presented a large blunt face like an aeroshell, but it has to be streamlined for the other stages of its flight. However it is also able to compensate for that to some extent by steering during the early part of the flight to slow down more quickly.
Skylon (future design being developed by UK / ESA). It flies to orbit from a normal length runway, reinforced to take the weight of fuel on lift off and may fly in the 2020s. It is heavy when it takes off, but during the landing, having used up most of its fuel, it is low density and so slows down much higher in the atmosphere than the Space ShuttleAs a result, it will reach lower temperatures than the Space Shuttle on re-entry though higher than a supersonic jet at Mach 3. Here are a few figures for skin temperatures for comparison, hottest first. These are the figures for the hottest parts of the spacecraft or plane:
- Space Shuttle re-entry: 1,650 °C (3000 °F)
- Skylon re-entry: 830 °C (1520 °F) - see section 4.3 of this paper
- US SR-71 Blackbird supersonic spy plane which flew at Mach 3, - external temperatures for the titanium around its cockpit windows of 232 °C (450 °F) - see (page 18 of this report)
- Concorde 153°C (307°F) when flying at Mach 2.2
SKYLON’S ZEPPELIN-LIKE TRUSS CONSTRUCTION, WITH A REINFORCED GLASS CERAMIC AEROSHELL
Modern planes have “stressed skin” structures, where the skin of the plane itself takes up all, or most of the external load from the wings, tail, other stabilizing structures and heavy components such as the engine (See fuselage for details). But the Skylon uses a structure much more like a zeppelin or a small plane. It’s girder-like with a thin glass ceramic outermost shell, which is just a heat resistant covering and doesn’t take any stress at all.
Structure of the Skylon - internal truss framework made from carbon fibre reinforced plastic composite held together with Kevlar ties. It has aluminium propellant tanks suspended inside it. Covering that, it has a thin outer aeroshell of a high temperature silicon carbide fibre reinforced glass ceramic material. For details see page 2 of this reportThis ceramic outer skin is black, which is why Skylon is shown that colour in most of the artist renderings. This is an animation to show the concept for a mission to orbit, and back, by Reaction Engines who developed the idea. Re-entry starts about seven minutes into the video
https://youtu.be/3H7BeyQOnPk
VERY LOW TEMPERATURE RE-ENTRY - ORBITAL AIRSHIPS
This approach of reducing the density of the spacecraft to lower its re-entry temperature is taken much further with the plans of JP Aerospace.
Their kilometer scale orbital airship is filled mainly with hydrogen. It’s not only lower density than a plane, and the Skylon; it’s also much lower density even than a normal airship. It only operates above 140,000 feet and is balanced for the upper atmosphere. It also has a huge cross section which it presents to the atmosphere.
This spaceship design consists of a near vacuum of hydrogen floating in a near vacuum of normal air. If they succeed in building it, then it will be able to slow down just through friction in the very tenuous upper atmosphere. By the time it gets to levels dense enough to heat the skin significantly it’s already slowed down hugely, so the temperature of the skin during re-entry is much less of a problem.
JP Aerospace orbital airship - kilometer scale, very very low density - this has the least temperature of all during re-entry, If it works out, the cost per kilogram to get cargo and passengers to orbit would be far less even than the space elevator ($500 per kilogram to GEO for the space elevator, and JP Aerospace’s estimate for orbital airships is $310 per ton to GEO, so 31 cents per kilogram), and it has much less development cost. It would be a leisurely journey as you would get there slowly over several days.Although it may not look it, its huge V shape is designed to be aerodynamic at hypersonic speeds in the near vacuum upper atmosphere. They have done modeling, calculations and wind tunnel tests with scale models to test this.
The most controversial part of their work is the idea of slowly accelerating to orbit. The idea of an airship re-entering from orbit is much less controversial.
ORBITAL AIRSHIP CONSTRUCTION
It has no internal girders. Its outer shell covers an interior of many large bags of hydrogen to give it rigidity and to stop the gas bunching up at its nose. It also has inflatable trusses, with nitrogen filling the gaps in between these components. The nitrogen is vented if necessary and then replaced from liquid nitrogen tanks.
It is balanced to float at 200,000 feet altitude in the atmosphere. But since it is aerodynamic, it also behaves like a glider on the way down. It doesn't look much like a glider to our eyes perhaps, but that big voluminous V shape makes a great glider in the very tenuous upper atmosphere during re-entry.
So what keeps it up is partly aerodynamic lift and partly buoyancy. To start with it’s mainly aerodynamic, as it slows down on its long glide through the upper atmosphere. The aerodynamic effects keep it higher in the atmosphere for longer, and so keep it cooler on the way down. As it slows down to a halt in the atmosphere, it’s finally kept up by buoyancy.
Many details of the design are given in their Floating to Space: The Airship to Orbit Program. They don’t actually give expected skin temperatures. But the design uses nylon rip-stop polyethylene (page 111) which suggests that they expect external skin temperatures well below 100 °C (212 °F) for continuous use.
(Most commercial grade Polyethylene starts to soften at 60 °C (140 °F) and has a maximum continuous use temperature of 65 °C (149 °F), High Temperature Polyethylene can retain its properties up to 100 °C (212 °F) )
On page 109 they say
"By losing velocity before it reaches the lower thicker atmosphere, the reentry temperatures are radically lower.... This makes reentry as safe as the climb to orbit"It’s an interesting company - in Sacramento, California, JP Aerospace, America's OTHER Space Program. Their idea is that they don’t do any big expensive “succeed or go bankrupt” type tests like SpaceX did in their early years. Instead every stage along the way pays for itself. At present they pay for the tests through pongsats and other ways to lift material to the edge of space. Their tests involve high altitude balloons, and V shaped airships rated for the lower atmosphere. They have also tested a high altitude balloon based airship design.
JP Aerospace hold the altitude record for an airship, propeller driven, remotely controlled from the ground, and flying at a height of 95,000 feet above sea level.Later on they plan a “dark sky” station at the edge of space which will be of a lot of interest for itself both scientifically and for tourists. It gets the name because at that height the sky will be dark even in daytime, as for the Moon. Next, they plan small airships doing test hypersonic glides back to Earth. Finally they do test flights to orbit with smaller airships, then the first human pilots to orbit, and then huge orbital airships with passengers and cargo.
The idea started off as a US Airforce contract for a near space reconnaissance airship. But the US canceled the contract in 2004 or 2005 after first persuading them to attempt to launch one of their prototypes for a lower atmosphere airship in a 50 mph wind (which would count as a “strong gale”). It was only rated as sturdy enough for launch in a 2 mph wind at the time (an airship is particularly vulnerable in the short time it takes to launch it from the ground). They did this with some reluctance - and it blew apart in the strong winds, causing some minor injuries. The inventor himself sustained three broken ribs. That was enough for the US Airforce to cancel the contract.
JP Aerospace have now solved the problem and can launch their lower atmosphere V shaped airships in any wind conditions. You can read their account of this story here. It’s now a civilian company entirely self financed, and they are not interested in any more such contracts, naturally enough.
It’s probably going to take them a fair while, with this approach, maybe decades but it’s interesting: “watch this spot”.
You might wonder what happens if the airship is hit by a meteorite or orbital debris. From page 112 of the book:
"One of the most common questions asked about ATO is about meteorites. "What happens if a meteor popped the airship?" The answer is very little would happen. A balloon pops because the inside is at a higher pressure than the air on the outside. The inner cells of the airship are "zero pressure balloons". ... There is no difference in pressure to create a bursting force. All a meteorite would do is to make a hole. The gas would leak out staggeringly slowly... "To find out more about this see their book Floating to Space: The Airship to Orbit Program
The JP Aerospace orbital airships are so lightweight they could never survive at ground level. The slightest wind would tear them apart. So in their plan, they have conventional airships that take passengers up to a docking station in the upper atmosphere, the “Dark Sky Station”, where they then transfer to the orbital airships that then slowly accelerate to orbit over several days.
This idea has been much criticized by physicists and they themselves say they don’t know if it is possible, but that it is interesting trying for what they learn along the way whether it’s possible or not.
HOW THE JP AEROSPACE AIRSHIP WOULD ACCELERATE TO ORBIT
This is the most controversial part of their idea, the accelerating to orbit. So, let’s look at it briefly. It’s a self contained section - if you aren’t interested, skip to the next section:
ATMOSPHERIC RE-ENTRY FOR VENUS AND TITAN
The key to the way it is supposed to work is that at hypersonic speeds the shock wave can be pushed back against the body which then adds to the lift. It sounds a bit like lifting yourself by your own boot straps, but it works out when you do the physics.
The denser air below the airship gives it lift through buoyancy. With something as lightweight as an orbital airship you get a lot of extra lift through the buoyancy as a result of resting its weight on the shock wave.
This increases the lift to drag ratio and so means you need less fuel to keep it aloft. This idea of resting some of the aircraft’s weight on its shockwave is the idea of a hypersonic waverider. It is standard in hypersonic flight and makes a big difference to the lift to drag ratio for planes that fly at those speeds. You don’t get it at normal supersonic speeds because the shockwave doesn’t touch the plane’s body.
It has solar panels over its vast upper surface to generate power, and uses these to power “dirty ion thrusters”. This is an innovative engine they are developing, the “symphony engine”. It burns the fuel like a normal rocket to generate a plasma (a flame is a plasma), but that means that there are ions in the flame, so then it accelerates the ions using electrical fields. These then drag the rest of the exhaust fuel with them, which is why they call it a “dirty ion thruster” as the exhaust is a mix of ions and neutral atoms.
These let you accelerate with a much higher exhaust velocity than a normal rocket, and so, with a small total amount of fuel, so long as you have plenty of power. It would have no shortage of power with such a large area of solar panels. However it only works for low thrust motors so wouldn’t do for a normal rocket engine.
However it takes a long time to get to orbital velocity, so the drag has to be very low or you lose the advantage of the more efficient rocket. To have less drag and still fly, the lift to drag ratio has to be very high.
He says his airhsips are buoyant up to 200,000 feet, with his “dark sky station” at 140,000 feet.
Based on a buoyancy calculation by James Fincannon, I came to the conclusion that:
- You have to read “feet” as “meters” in his figures for them to work. This makes the airship over three times longer and the lift is getting on for 30 times more, a big difference.
Anyway it is either that, or radically change the design, make it buoyant up to 113,000 feet, and the “Dark sky station” perhaps at 60,000 feet (a height where you can survive with only an oxygen mask).
- If it is buoyant at 200,000 feet, the skin must be micron thick for his transatmospheric airship at 1200 meters (instead of feet) long and 0.55 microns thick for his orbital airship which then becomes a 6 km long airship.
- A small overpressure from a shockwave of just 0.0366 millibars can support the airship's weight
- The time to orbit depends on the lift to drag ratio, based on some straightforward assumptions.
7,844 * L / 9.80655 secsThat’s assuming a lift to drag ratio of L and a target orbital velocity of 7,844 meters per second.
- Also, from the time to orbit, you can work backwards to find the lift to drag ratio, assuming at most 50% of the thrust is used to offset drag.
L = t * 9.80655 / 7,844There t is the time to get to orbit in seconds.
JP say that it would take three days to get to orbit. Putting in three days in seconds into the right hand side of that formula:
- JP seems to be targeting a lift to drag ratio of over 300.
But a hypersonic waverider with nearly all the weight of the airship offset by buoyancy is in very unfamiliar territory. Could it have a lift to drag ratio as high as 300: 1?
- His 2000 ISP engine, if he can achieve that, needs only a bit over 50% of the mass for fuel, no matter how long or short the time to orbit, assuming at most 50% of the thrust is used to overcome drag.
Techy aside: this is a clever trick which is used to take account of drag in rocket launches, and is why they talk about orbital velocity as requiring a delta v of around 9 rather than 7 km/sec, for instance the Saturn V had a delta v to orbit of around 9.194 km / sec delta v most of which is due to “gravity drag” - the effect of accelerating vertically instead of the more efficient use of fuel to accelerate horizontally.So, assuming a thrust to drag ratio of 2, just input double the delta v to orbit ( which we are taking as 7,844 m/s), or 15,688 m/s into the ideal rocket equation, assuming an isp of 2000, mass of, say 100 tons. It turns out that the dry mass you get to orbit is 44.9 tons. So you need 55.1 tons of fuel. Here is an ideal rocket equation calculator to check the calculations.
That makes it more like a commercial jet, or the lunar module taking off from the Moon than a normal launch from Earth.
For more details of the calculations: Can JP Aerospace's Future Giant Airships Slowly Accelerate To Orbit? Looking At The Numbers
See also my Can giant airships accelerate to orbit? where I talk about some of the issues about its feasibility from physics and ask if they can be overcome.
For more background, see my "Can Giant Airships Slowly Accelerate to Orbit Over Several Days"
ATMOSPHERIC RE-ENTRY FOR VENUS AND TITAN
There are several other places in our solar system with thick atmospheres like Earth, including Venus, and Saturn’s moon Titan. Mars also has a very thin atmosphere. The gas giants have thick atmospheres too (with no solid surface).
JP Aerospace hope the same idea can be used for Venus, with a high altitude staging post again, this time of course in the Venus atmosphere. The aim wouldn’t be to land on the surface, which is incredibly hot and high pressure, but to go down to the Venusian cloud tops to study them and perhaps build habitats there.
Perhaps they could use it for Mars too. The atmosphere of Mars is so thin that you could land an orbital airship like this on the surface. The strongest winds on Mars would only barely move an autumn leaf, fast though they are.
VAMP ORBITAL AIRSHIP RE-ENTRY - FOR VENUS AND TITAN, ALSO EARTH
If you want to fly all the way down to ground level on Earth in one go, then you need a more massive airship. Northrop group’s “VAMP” project to study the Venus atmosphere uses an airship design like JP Aerospace, and they would inflate it outside of the atmosphere, so again that’s very like the JP Aerospace idea. It enters the Venus atmosphere already inflated, and because it is so large (55 meters in diameter) and low density, it doesn’t need an aeroshell.
However, unlike the JP Aerospace design, it’s able to fly in an Earth pressure atmosphere, so it’s not nearly as low density as an orbital airship. It still gets quite hot during the descent.
https://youtu.be/0EjgoEFiKko
It inflates before it enters the atmosphere (see patent for details), and rather similarly to the JP Aerospace idea it decelerates slowly in the upper atmosphere, so generating much less heat, because of its low ballistic coefficient. So it doesn’t need an aeroshell, though because its designed to operate right down to the equivalent of ground level on Earth, its denser and its outer envelope is reinforced to withstand up to 1,200 °C (2,192 °F) along leading edgesIt would only descend as far as the Venus upper atmosphere, at the cloud tops, where temperatures and pressures are the same as for Earth. The cloud tops also have natural protection from cosmic radiation, and nearly all the ingredients for life. Indeed there are suggestions that it could be a good place for humans to settle outside of Earth. See my Will We Build Colonies That Float Over Venus Like Buckminster Fuller's "Cloud Nine?". Some astrobiologists think there may be life in the upper Venus atmosphere, already, which could have migrated there long ago when Venus was more habitable. The Russians are interested to search for this life, and may include an unmanned aerial vehicle, possibly VAMP in their Venera D mission to Venus in the mid 2020s.
They hope it can be used for Venus, and also Titan, and possibly Mars.
The first tests of VAMP would use the Earth’s atmosphere. So it could also be used for Earth re-entry. It might be useful for surveillance, photographing the Earth from above, and also for scientific studies of the upper atmosphere.
The same ideas could also be used for Titan - a moon of Jupiter with an extremely cold atmosphere at -180 °C, but it’s also dense, with the same pressure as Earth’s. This means that humans could go out of doors there, without needing a pressurized spacesuit. Of course they would need protection from the extreme cold and they would need air to breathe, so you are talking about warm clothing, as for Antarctica, perhaps heated clothes, and an oxygen mask.
However, they could get the oxygen to breathe by splitting water ice from Titan, and then burn the methane from Titan in that oxygen, a process that, rather neatly, creates an excess of energy which could then split more water, generating more oxygen for heat, and also for the colonists to breathe. The habitats could be built like many modern Antarctic bases, on legs to hold them above the cold surface. Since the air pressure is the same inside and out, the air could be kept in using double doors in a building of normal construction, again like an Antarctic base. See Let's Colonize Titan, and there are more details in their Beyond Earth: Our Path to a New Home in the Planets.
VAMP flying over Titan to sample and explore the upper atmosphere - Titan’s atmosphere is similar in pressure to Earth’s at ground level, though much colder, so you have similar methods for re-entry for Titan and for Earth. Though its gravity is much less - indeed a human falling from a plane or aerostat on Titan would easily survive the landing without a parachute.So, let’s now look at the way re-entry is done at present, using aeroshells. Can this be improved on?
AEROSHELL - HIGH TEMPERATURES
The normal way to re-enter Earth’s atmosphere at present is to use an aeroshell. This absorbs most of the heat, all the way through the early stages of re-entry, until the spacecraft is traveling slowly enough to drop the aeroshell and deploy parachutes. The spacecraft hits the atmosphere at many kilometers per second, so there is a lot of heat to dissipate. The main methods they use to keep the temperatures within reasonable bounds are:
Passive heat sink
. Used in early spacecraft designs but requires a much heavier aeroshell. It heats up during the fiery part of the descent, then radiates the heat away afterwards, through the rest of the descent.Ablative aeroshell
- the atmosphere ablates away part of the heat shield removing heat right away. This is used for most modern spacecraft. The aeroshell itself still heats up a fair bit (thermal soak), as for the passive heat sink, but is insulated from the interior, which is kept cool.Actively cooled with a refrigerant
. This is a possible design idea for the future, but is not actually used yet. See Atmospheric entry - Thermal Protection Systems.
Artist’s rendering for Apollo command module re-entry. Temperatures reached 5,000 °F on the outside of the capsule, or around 2,760 °C. How the Apollo Spacecraft Worked (and old Apollo Flight Tests fact sheet )Re-entry speeds are
From LEO
, Soyuz TMA re-enters at 7.591 km / sec .From the Moon
, around 11 km / sec. The fastest was Apollo 10 with a re-entry speed of 35,314 feet per sec, or 11.0685 km / sec.From Mars
- the best trajectories have re-entry velocities similar to the Moon. 11.5 - 12.5 km / sec.Fastest proposed re-entry, from Mars, returning via Venus
, Dennis Tito’s Inspiration Mars would have re-entered at 14.18 km / sec - they might be able to use denser versions of existing heat shield materials. They could also use an initial pass through the atmosphere to bleed off velocity. However, to do that requires it to skim the atmosphere to very high precision at an altitude accurate to within a few kilometers, for this first pass.
The reason their return velocity was so fast is because they planned to use a flyby of Venus on the way back to shorten the total journey time to 500 days. There are many ways to do a return trip to Mars, and some missions that do flybys of Venus on the way out or the way back have low re-entry velocities for Earth but they tend to be longer missions.
WHY IS RE-ENTRY SO MUCH FASTER FROM A HIGHER ORBIT SUCH AS THE MOON?
You might wonder why the astronauts returned from the Moon at such a high speed. After all, it orbits the Earth at a speed of only 1.02 km / sec relative to Earth.
The reason is that if you start higher in the Earth’s gravitational well, and drop down to LEO, you accelerate all the way, and actually end up faster than if you started off in LEO originally. If you come back in a transfer from the Moon you hit the atmosphere at 11.1 km / second. There’s an online transfer orbit calculator here. Starting from a higher orbit just makes things worse for you.
The one thing you can do to help with re-entry speed is to orbit the Earth in the same direction that it spins. The Earth’s surface (and so its atmosphere too) moves at 460 meters per second towards the East, or about 1,000 miles per hour) because of its rotation (it’s spinning towards the rising sun).
So if your satellite is orbiting in the same direction as the Earth in an equatorial orbit, West to East, it has 0.92 km / sec less delta v relative to the atmosphere than if it orbits in the opposite direction. This makes re-entry just a little easier.
An orbit in this direction also makes the launch easier. You need around 0.92 km / sec less delta v to get into orbit if you launch from West to East. That’s quite a huge saving in fuel, which is why all the US launches are from Florida, launching over the Atlantic, and why all the Soyuz launches from Russia are from West to East too
Shows the direction of the launches of Soyuz from West to EastThis is why it was such a major gaff for the Gravity film when it showed all the orbital debris orbiting Earth from East to West, as Neil deGrasse Tyson tweeted.
BALLUTE - INFLATABLE AEROSHELL
But you can achieve a much gentler re-entry using a ballute - a cross between a balloon and a parachute. It works like an aeroshell but decelerates much higher in the atmosphere
BalluteIn the first ever re-entry test of a ballute by ESA for instance, the maximum re-entry temperature on its skin was 200 °C (392 °F).
Artist’s impression of the ESA Ballute
Graph from this paper. Measured temperature reached a maximum of 200 °C (392 °F).
BALLOON RETURN FROM ORBIT
Then there’s an idea from 1966 to return a human being from orbit, in an emergency, using a balloon to dissipate some of the heat, though the seat for the astronaut acts as an aeroshell to dissipate most of it. It combines some of the approaches of the previous ideas. You sit inside the balloon which is filled with breathable air to keep you alive during the re-entry. It’s never actually been tested in space.
SAVERThe space engineers in the early 1960s explored many other such ideas detailed here: Rescue. Some seem rather hair-raising including the Paracone - the astronaut just sits in a seat, with their back towards the Earth, and aims towards the centre of a large continent, as its margin of error is 600 kilometers. When it re-enters, then a large inflatable aeroshell deploys with a crushable cone. There is no parachute - it relies on the aeroshell crushing during landing to protect the astronaut.
Paracone. The astronaut has an inflatable aeroshell stowed away in the seat. During re-entry this deploys. They have no parachute - the aeroshell just falls through the atmosphere at its terminal velocity of 42 km / hour (26 miles per hour) and its nose crumples when it hits the ground. It’s a bit like a deliberate slow car crash into a brick wall with a shock absorbing crumple zone.Another similarly hair-raising idea is MOOSE (Manned Orbital Operations Safety Equipment). It’s a polyester (PET) film bag, with a flexible quarter inch thick ablative heat shield on the back, which you climb inside, and then fill with polyurethane foam. Unlike the Paracone, you do have a parachute as well, for the landing. Here is how it works:
PARAGLIDING FROM ORBIT
This is another idea originally developed for Gemini in the early 1960s. For a while, before they settled on the familiar parachutes, the engineers thought that after the fiery stage of re-entry, the capsules would glide down to Earth beneath a parasail or paraglider. Those tests were quite promising, though they ran into many issues, for instance getting the glider to unfold. Eventually this line of research ended in 1964, when they changed to the parachutes as used by Apollo. The Russians also used parachutes for the Soyuz flights. For details of the paraglider research, see: Coming Home
Anyway at around the same time, in 1960, the engineers came up with the idea of using the same paraglider approach to go all the way from orbit, right down to the surface, without an aeroshell. This was the idea of the inflatable paraglider (Rogallo wing), called “FIRST” (Fabrication of Inflatable Re-entry Structures for Test), another idea for a space self-rescue system for astronauts.
It could be folded up into a small cylindrical package which would be kept docked to a space station, much as our modern Soyuz TMA is. In an emergency, the crew enter this cylinder, and separate. The paraglider then inflates and deploys. It would re-enter at an angle of 1 degree, with the paraglider angle of attack of 70 degrees. They found that deceleration would not exceed two g’s, and that there would be minimal heating because of the way it glides down to the surface. It would approach the speed of sound at 43 km altitude, and from there it would be able to glide 345 km horizontally before eventually landing. Details here: FIRST Re-Entry Glider
CHANGING SHAPE FOR RE-ENTRY
The Spaceship-One uses a different idea for re-entry. It changes its wings to the “feather” position which also tilts the spacecraft so that it presents as much surface area as possible to its direction of flight, so slowing it down. This is only for a sub-orbital hop at present. The first demonstration of the feather system was in 2011.
https://youtu.be/Lqhzlq7UReAThe Virgin Galactica crash in 2014 was a result of the pilot accidentally unlocking the feathering system too soon. It then deployed by itself and changed the shape of the rocket far too early, when it still needed to be streamlined.
RETURNING THE FINAL STAGE OF A CONVENTIONAL ROCKET
What about returning a final stage? That also is low mass and it presents a large cross section if you fly it backwards, rocket motors first, with supersonic retropropulsion. So you’d think it might be reasonably easy to do.
First, some background. Every time a spaceship goes into orbit, it needs a final stage, a thin container full of fuel which is burnt right at the end, to get it to orbital velocity. It has to do that, because the spaceship itself is far too small to have enough fuel to get to orbit by itself, even with the help of the first (and sometimes second) stage.
It then discards the final stage, which normally orbits Earth a few times and finally falls back to Earth (apart from interplanetary missions and missions to the Moon, which often use a more powerful final stage, for instance nearly every mission to Mars also sends a final stage in the general direction of Mars too).
So, could a final stage be returned to Earth in the same way that SpaceX have returned the Falcon first stage? Well, when SpaceX returns the first stage of the Falcon 9, it slows down partly through friction in the upper atmosphere. The landing legs alone reduce its terminal velocity by a factor of two. It also has a burn in orbit and another burn just before it reaches the barge. You can see the first stage at the beginning and end of this movie (most of it is for the second stage). That may seem rather similar, but it only has to shed one kilometer per second of delta v, and much of that is done with the two burns.
https://www.youtube.com/watch?v=XdtK0poWp20Elon Musk has said he plans to re-use the final stage in the future, though it’s probably not going to work for the current Falcon 9. Here he answers the question: “Any plans for a reusable second stage?”
https://youtu.be/-pQ8M9mTw0M?t=782
“The next generation vehicles after the Falcon architecture will be designed for full reusability.”
“I don't expect the Falcon 9 to have a reusable upper stage, just because with a kerosene-based system, the specific impulse isn't really high enough to do that, and a lot of the missions we do for commercial satellite deployment are geostationary missions. So, we're really going very far out. These are high delta-velocity missions, so to try to get something back from that is really difficult. But, with the next generation of vehicles, which is going to be a sub cooled methane oxygen system where the propellants are cooled closed to their freezing temperature to reduce their density, we could definitely do full reusability”
BASIC IDEA - LIGHTER FOR LOWER RE-ENTRY TEMPERATURES
The basic idea of all these designs is that the lighter it is, or the greater the cross sectional area it presents to the atmosphere, then the higher it is when it slows down, and so the lower the temperature of its skin during re-entry. What matters is the mass per cross sectional area - or more precisely, the ballistic coefficient (which is complex to calculate).
If the spaceship can use a glide to stay high in the atmosphere, this also helps. It also helps if it can use retropropulsion to reduce its velocity before it enters the atmosphere, and as it descends. Then, if it can radiate or absorb heat or ablate, for instance with an aeroshell, or use active cooling (perhaps in the future), this also helps.
Another way around it is to use a space elevator. Though we can’t build a space elevator quite yet, there are other things we could do right away that are rather similar. But first let’s look a bit closer at why we can’t build a space elevator quite yet.
MATERIALS FOR THE SPACE ELEVATOR
The main problem with building a space elevator is that it has to be so tall, because geostationary orbit is very high, around 35,786 km above the Earth’s surface. That’s a long length of cable to hold up against gravity.
It’s a little better than that figure might suggest, because gravity would get less, rapidly, as you go up the cable (by inverse square law). Also the cable is slowly spinning around the Earth at the centre, leading to a centrifugal force (strictly speaking, a “fictitious force” in the rotating rest frame) which counteracts gravity. At geostationary orbit you are in free fall and those two forces balance each other out, so your effective weight, and the weight of that section of the cable, is zero.
After taking that into account, it turns out that the material needs to be strong enough to hold up 4,960 km of its own weight under full gravity (result given by Arthur C. Clarke). It will then be able to hold up the full length of the space elevator in the Earth’s varying gravity all the way out to GEO.
But our current cables come a bit short of that. Kevlar is one of our best materials for this job, and under full gravity, it can only hold up 255 km of its own weight before snapping.
TAPERING
You can improve this situation to some extent by tapering. This is easiest to explain assuming uniform gravity, and let’s use 200 km sections of Kevlar for simplicity of the calculations, and a safety margin:
- The bottom 200 km of Kevlar can hold up itself.
- Now we add two strands of Kevlar each 100 km long side by side, attached to the top of that first strand. Each of these can hold up itself plus an extra 100 km. Together they hold up themselves plus the bottom 200 km.
- Now we have the mass of 400 km of Kevlar, though only spanning 300 km of height. So now we need four strands of 100 km Kevlar to hold up all that plus themselves, which then is enough to make the next 100 km of the space elevator.
- The next 100 km requires 8 strands, then 16 and so on.
Calculation indented to make it easy to skip:
The general formula is that for (n+1)It’s so close, yet so far! That increase in length from 1,000 km to 5,000 km is enough to turn something that’s reasonably practical to totally impossible. If only Kevlar was five times stronger, then it would be easy !\times 100 km of elevator you need(2^n-1) *100 km of cable.
So you could span 1,000 km without much trouble, as that’s just100\times (2^9-1) or 51,100 km of cable.
However, to span 5,000 km with Kevlar under uniform gravity, you need100\times (2^49-1) km of cable, or around 56,000,000,000,000,000 km.
Actually you’d need a lot more than that, as the 4,960 km under full gravity corresponds to 35,768 km under variable gravity. It would also be a lot more than seven times the amount, as most of the extra mass in this construction is right at the top near the hub. But with so many zeroes already, who cares?
Not practical.
You can make the figures better if you use a continuous taper instead of steps of 100 km each, but it’s still impractical. You can find out the details in the published literature on the subject. For a tapered Kevlar space elevator you’d need the top of the cable to be 260 million times wider than the bottom. For instance, to suspend one millimeter diameter of cable to the ground, you need it to be 260 kilometers in diameter at GEO. (Figures for this and next paragraph from introduction to this paper)
For more on this see Arthur C. Clarke’s 1976 address to the IAU in Munich: The Space Elevator: ‘Thought experiment’ or key to the universe? (part 2) (see also Part 1 and Part 3). You may not know that he trained as a physicist with a first class degree in maths and physics before he became a science fiction writer.
FLAWED CARBON NANOTUBES
We don’t quite have that technology yet. If only we had perfect carbon nanotubes, they would do the trick, with a taper ratio of 1.9, so the top of the cable only needs to be a little under twice the diameter of the bottom. For a while scientists were quite optimistic about this.
Scanning electron micrograph of a “chiral” or spiral pattern carbon nanotube - one of several types. If we could make perfect nanotubes and combine them to make cables kilometers long we could use them to build a space elevator.It seemed so promising, as perfect carbon nanotubes measured in the laboratory have a breaking height of 2,200 km, but all the carbon nanotubes constructed for practical applications have only a hundredth of their theoretical strength. They are actually weaker than Kevlar (1 GPa compared to 3.6 GPa for Kevlar). We do actually have carbon nanotube fibres, with Rice University pioneering the process that made them possible, with closely packed, aligned carbon nanotubes. These are strong, flexible, and conduct electricity well. They just aren’t quite space elevator material yet.
https://youtu.be/4XDJC64tDR0It turns out that if there is just a single atom out of place, they lose their strength. Sadly, our technology isn’t up to the task of making perfect carbon nanotubes with not an atom out of place, long enough to join together to make cables thousands of kilometers long. Also, with nanotubes so sensitive to damage, there’s the problem of what happens if you get defects introduced as a result of micrometeorites and other wear and tear.
Other materials also just aren’t strong enough yet. Kevlar has a breaking height of 255 km. There are a few other promising ones, Zylon (polybenzoxazole fiber) has a breaking height of 379 km (from page 14 of this study). But we’ve got a long way to go.
So it remains an idea at present. Some enthusiasts think it may be practical in the near future, perhaps even just a decade or two away.
Though we can’t build a space elevator on Earth yet with present day materials, we could build a space elevator from the Moon. It would be around 56,000 km from the Moon to the L1 position between the Moon and Earth, which is a point where the gravitational forces balance.
Diagram from this studyFor more about the lunar space elevator see Lunar space elevator and see Hop David’s Beanstalks, Elevators, Clarke Towers
SKYHOOKS
You can also use a skyhook. It’s very like the space elevator, but you construct it downwards from a rather lower orbit than geostationary orbit. The bottom end can’t be attached to anything so it just dangles in our atmosphere, traveling around, much faster than our Earth rotates. That’s why it is called a skyhook. You could then just fly up to the bottom end of the skyhook, and if you fly fast enough you could keep pace with it in our atmosphere, and attach yourself to it. Then once you’ve done that, you can travel up the cable much as you do for the space elevator, until you are in orbit.
USING SKYHOOKS IF YOU HAVE A HANDY MOON
It’s easiest to construct a skyhook, if you have a handy moon or asteroid to attach it to. It’s a bit like the idea of the lunar elevator but constructed on a much smaller moon. If we had an extra moon close to Earth it might be very useful. We don’t have one, but Mars does, two of them. Here is an early concept study to use two skyhooks for Mars from a 1985 paper (described on page 70 of the Tethers in Space handbook):
The idea is - if you build a 6,100 km long skyhook type tether outwards from Deimos then it can throw objects out with escape velocity and also catch incoming spacecraft from elsewhere in the solar system to a gentle rendezvous that only needs docking thrusters, like docking with the ISS.
Then if you build a 2,960 km long skyhook tether inwards from Deimos and a 940 km tether outwards from Phobos, then it turns out that if you drop a spaceship off the Deimos tether, then it is traveling at just the right velocity to send you in a transfer orbit to the Phobos tether. Your transfer vehicle ends up stationary next to the Phobos tether when it gets there, with plenty of time to dock, much as spacecraft do with the ISS. Then you can just go down that tether to Phobos. All this requires no acceleration and no rocket fuel, apart from maneuvering thrusters for the docking itself. It works just as well in the opposite direction, drop your spaceship off the Phobos tether and the tether’s extra velocity will boost it to the Deimos one where it will come to rest with plenty of time to dock there.
Once you are on Phobos, a 1,160 km tether extended downwards towards Mars can be used to drop materials down to an elliptical orbit which reaches down to Low Mars Orbit at a height of 375 km when closest to Mars. Hop David found that slightly longer 1400 kilometer long tether could put it into an atmosphere grazing orbit so that you can use the Mars atmosphere for aerobraking to circularize its orbit.
The total mass for all these tethers if made from Kevlar is not that great considering what it does. You need between 5,000 and 90,000 tons if designed to handle a payload of 20,000 tons.
You could also bypass Deimos and just have a single tether from Phobos upwards to reach escape velocity right away.
The mass of Deimos and Phobos is so great that you could run this system for decades with no noticeable effects on their orbits. They act as a “momentum bank” so you wouldn’t need to be careful about always balancing movement of materials outwards with movement inwards, or using other methods to keep reboosting the tether.
Hop David has explored Phobos tethers in a lot more detail. He finds that a much shorter 7 km tether extending from Phobos towards Mars would let spacecraft dock with its L1 position where the gravity of Phobos is balanced with the gravity of Mars. It’s a tiny amount of gravity, only a ninetieth of Earth’s (0.11 m/sec²), but that way you could land without throwing up a cloud of dust, and also perhaps, without contaminating the surface of Phobos with rocket exhausts, which might well be useful.
Then, a very long 1,400 km tether grazing the Mars atmosphere would mean you only need 570 meters per second delta v to land on Mars. See Lower Phobos Tether and his General template for space elevators.
ROTOVATORS - SPINNING “SKYHOOKS”
We don't have any handy moons like that for Earth, but it turns out that there’s a better way to do it here. Instead of a simple skyhook, you use a “spinning skyhook” or rotovator, often called a “momentum exchange tether”. This spins in such a way that the end closest to the Earth moves backwards relative to its orbit around Earth. - this animation shows the idea:
Cycloid - zortig (wikipedia)The line there represents half of a tether. A space tether should actually span the full diameter of the circle, but it’s the same idea.
As the tether orbits Earth, it spins, and if you arrange the rotation rate carefully you can arrange it so that the tip closest to Earth moves backwards at just the right speed to be completely stationary above Earth, just for a short while in each spin. You can never manage a stationary tip like that with a a skyhook.
That could be used to take a spacecraft traveling at faster than orbital velocity, (perhaps in a transfer orbit from the Moon or GEO), and slow it down so it is stationary, and momentarily hovering above the Earth’s atmosphere. Once you’ve done that then you can let it go gently, from the tip of the tether, and it falls down vertically, with a parachute perhaps. In the other direction, you could use the same approach to lift a payload, for instance, attached to a balloon, and attach it to the bottom of the space tether when stationary relative to Earth, and the tether would boost it into orbit, or send it to the Moon or in a transfer orbit to GEO all in one go.
SHORTER TETHER THAN A SPACE ELEVATOR
A momentum exchange tether doesn’t have to be as long as the space elevator to do this. You could have a shorter tether which spins more rapidly, in an orbit closer to Earth. That makes it a less massive construction.
It also means you get into orbit quickly. With the space elevator, it could take quite a while to get there. At 200 mph it would take seven and a half days to go up all the way to GEO, and at 2,000 mph it would take nearly 18 hours.
The space elevator can be thought of as a rotovator that rotates once a day and so can have one end fixed permanently to Earth.
This video shows the idea of launching a rocket vertically to meet the bottom end of a rotovator that is rotating at just the right speed to be stationary relative to Earth’s atmosphere. That’s a far easier thing to do than to launch a rocket to orbit.
https://www.youtube.com/watch?v=Ox_-3S3AMxkHowever we don’t have the materials to do that yet either. Though the tether is shorter, it also has to spin more rapidly. You still need very strong materials, nearly as strong as for the space elevator.
Let’s try a couple of examples to give the idea, as usual, with the calculation indented to make it easy to skip:
This calculation is for untapered tethers only, and measurements are from centre to tip rather than tip to tip, and with the tethers skimming the atmosphere at 80 km.So the results for spinning untapered tethers with their tip stationary momentarily over the Earth’s surface at an altitude of 80 km, and using the length of tether from center to tip are:
To take an example, suppose you constructed
a 1,000 km long tether
(2,000 km tip to tip) spinning in space, with the center at an altitude of 1,080 km, and the lower tip skimming our atmosphere at 80 km. By this online orbit calculator tool its orbital speed is about 7.314 km / sec . The Earth’s surface moves eastward at 0.46 km / sec. Difference is 6.854 km / sec, So, using this tool SpinCalc it rotates at 0.06545 rotations per minute, relative to its centre. But that’s in a rotating rest frame so we have to correct for that; a small adjustment.
It does one rotation every 15.279 minutes. It orbits Earth every 106.68 minutes, so actually it does 6.982 rotations in each orbit. Then add an extra rotation for circling the Earth so in a stationary rest frame it is spinning 7.982 times every 106.68 minutes or at 0.07482 rpm. Now we can go back to SpinCalc and find it generates around 6.26 gs of artificial gravity at the tip. It does one rotation every 13.333 minutes. It’s around 3.13 gs half way between the tip and the centre, and that’s the average (the acceleration is?^2r for fixed angular velocity ? and so increases linearly from 0 at the centre).
You have to add 1 g for Earth’s gravity when it is hanging downwards towards Earth. So, although the cable length from the centre is only 1,000 km, it’s equivalent to holding up about 4,130 km of its own weight (that’s (1+3.13)*1000 km). In this calculation I haven’t taken account of the difference in gravity between 80 and 1,080 km above the surface of Earth which is quite considerable. It’s got only 73.2% of Earth’s gravity acting downwards at the centre of the tether (same as the accelerationv^2/r = 7.1795 meters per second, compared to full g of 9.807).
So, an untapered tether needs to hold up the equivalent of between 3,862 km
and 4,130 km of cable under 1 g
. (there, 3,862 is 3.130 km + 73.2% of 1,000 km)
That’s a saving of getting on for 1,000 km in the amount of cable the material has to be able to hold under its own weight in full g, over the nearly 5,000 km figure, for a complete space elevator. So the 1,000 km is better than the space elevator in this respect.
Let’s try a
shorter tether of 100 km
, say. By the orbit calculator its velocity at the center (at a height of 180 km) is 7.8 kilometers / sec, or relative to the surface, 7.4 km / sec.
It has an average of 56.84 gs (and 0.7 revolutions per second) when you do the calculation, okay for water and fuel, but no good for humans. Add 1 g when closest to Earth, and you work out that it needs to hold up the equivalent of
around 5,780 km of cable under 1 g
(and only very slightly less once you take account of varying gravity).
So the 100 km tether is actually worse than the full length space elevator. It’s not really worth taking account of the varying gravity or the rotating rest frame in this case, because for such a short tether the effects will be quite small.
1,000 km
holds up the equivalent in weight ofbetween 3,862 km
and 4,130 km of cable under 1 g
100 km
holds up the equivalent in weight ofaround 5,780 km of cable under 1 g
(not taking account of varying Earth gravity along the length of the tether or the extra revolution of the tether caused by its orbit around Earth, as these are small effects in this case)
The general pattern is that you need the strongest materials for very short tethers. You also need very strong materials for a tether all the way to GEO. In between, you find a sweet spot where the tethers don’t need to be quite so strong, but still they aren’t far off the strength you need for a space elevator.To answer these questions requires much more complex calculations. Not only do you have to take account of the inverse square law variation of Earth’s gravity along the cable; you also have to take account of tapering, and then turn it into a minimization problem. I haven’t found these figures anywhere yet. If anyone reading this knows of anyone who has worked this out, do say in the comments, thanks!
- This calculation is for untapered tethers.
What is the mass of a tapered tether
for each tether size and breaking height?Which is the minimal mass tapered tether for a given material with a particular breaking height
. For instance, what is the minimum mass for a Zylon tapered tether, breaking height 379 km, with the tip momentarily Earth stationary when closest to Earth, and how long is it?Does it make a difference if you require a realistic maximum g tolerance
at the tip for the cargo and for passengers?What strength of material could make this type of tether feasible
, and is it significantly less strong than for a space elevator?
If some day we do have space elevator materials, then these tethers would give a great way of achieving a similar effect for much less total mass, and also, because the tether is shorter, it’s a way of getting into orbit more quickly than for the space elevator. But until we have those materials, we need to use a “watered down version” which is not able to hover stationary over the Earth’s surface. However, it would still be enough to make a difference. Normally, you need to travel at about Mach 20-25 to go into low Earth orbit (depending on how high the orbit is). The launch assist tether reduces that to Mach 12 or less.
This shows rendezvous of tether tip with a hypersonic plane lifting a payload from it which will then be accelerated into orbit. Image credit Tethers Unlimited. See also: " Disruptive TechnologyThis video explains it well:
See also : Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System..
https://youtu.be/CWLzGGKV0jIFor more details, see Launch Assist Tethers. You could use the same process in reverse to de-orbit a spacecraft to Mach 12 in the upper atmosphere, and then it glides down from there.
So this would give you a way to slow down your orbiting spacecraft to Mach 12 instead of Mach 20 upwards, which would be quite a plus.
That’s still very fast. It’s four times the speed of the Lockheed SR-71 Blackbird supersonic spy plane, or Virgin Galactica’s SpaceShipOne (Mach 3.09), and more than twice the speed of the ESA’s projected Mach 5 aircraft:
ESA invests in 4,000mph hybrid rocket jet engine firmSo you still need something that is much more like a space plane than a normal hypersonic aircraft. But the big advantage is that a Mach 12 or lower space plane is much easier to make re-usable than a Mach 20 - 25 one.
They have other ideas in the paper such as a “two stage tether” where you have a smaller tether spinning around the tip of a larger tether. This reduces the total tether mass, rather like the idea of a two stage rocket launch, for a given tether tip velocity.
Two stage tether idea from their paper. A shorter tether spinning around the tip of the longer tether leading to a greater tip velocity - so slower speed in the upper atmosphere - for less total mass. It’s got some parallels with the two stage rocket propulsion ideas.
REBOOSTING THE TETHER AFTER THE GRAVITATIONAL ASSIST, USING SOLAR POWER, OR BY DROPPING MATERIALS INTO EARTH’S GRAVITATIONAL WELL
The tether loses speed, dropping into a lower orbit to compensate, every time it lifts a spaceplane out of the atmosphere. But it can get back into position in between those gravity assists, by using solar power to send electric current along the tether. This uses the Earth's magnetic field for a motor to accelerate back into orbit.
Or, rather elegantly, it can also do it by de-orbiting a similar payload. For instance, it could de-orbit a spacecraft returning to Earth, or materials exported to Earth from the Moon or asteroids. This was all fully worked out in the plans for the Hypersonic Airplane Space Tether Orbital Launch (HASTOL) System.
USING THE SAME IDEA OF DROPPING MATERIALS INTO A GRAVITATIONAL WELL TO POWER A TETHER TRANSPORTATION SYSTEM TO AND FROM THE MOON
Hoyt has suggested that we could use the same approach for exports from the Moon. Our materials are already easily strong enough to make a rotovator which can be stationary relative to the Moon, for a short while each time it briefly touches the lunar surface.
The Moon is high in the Earth’s gravitational well, so if we had two such tethers, one orbiting the Moon and one orbiting the Earth, the whole thing could be powered by movement of materials “downwards” from the Moon to Earth. It’s rather like the way movement of water downhill powers a waterwheel. For more details, a diagram and cites, see the Exporting materials from the Moon section in my “Case for Moon” book, available to read free online, or for kindle.
This cisulnar transport system is not nearly as massive as a space elevator, with a total mass less than thirty times a typical payload, so there is much less construction needed. That’s few enough payloads, so that it would soon pay for itself with frequent transport to and from the Moon. In his scheme the rotovator in Earth orbit just slows down payloads to LEO speeds, for instance to deliver ice from the Moon as fuel to use in LEO. But the materials could also be returned to a HASTOL type tether and into the Earth’s atmosphere easily enough.
Robert Hoyt and the science fiction writer Robert Forward are hoping to actually build orbital tethers some day, and have founded a small company Tethers Unlimited to do it.
https://youtu.be/pCAEFocoVdM
USING SPINNING “FREE SKYHOOKS” TO EXPORT MATERIALS THROUGHOUT THE SOLAR SYSTEM
You can also have "free skyhooks" orbiting in interplanetary space, as in tthis suggestion by Hans Moravec from 1986. These just spin on their own axis in free space, nowhere near any planet. He describes a 20,000 ton skyhook made of Kevlar designed to boost a payload of 50 tons (same mass as the Space Shuttle).
Within those parameters, it could be anything from 100 km long, spinning once every 4 minutes to accelerate the payloads at 8 g, or 20,000 km long, spinning once every 12 hours, to accelerate them at 1/25 of a g.
“Eight such skyhooks could span the solar system. One would be needed in the orbit of Mercury, one halfway between Mercury and Venus, one each in the orbits of Venus, Earth and Mars, one in the asteroids, one by Jupiter, and a final one at Uranus.Just as for Hoyt’s cislunar tether, the whole thing could be powered by materials flowing down the gravitational well. In this case the gravitational well of the Sun itself.
Each provides enough delta v to get a payload to the next one, and Uranus' accelerates to solar escape velocity. The trip from Mercury to Earth takes less than a year, as does Earth to Mars. An extra year and a half is needed to reach the asteroids, and the outer planet part of the journey takes decades."
This is from his Orbital Bridges. He also gives some figures for lengths and cross sectional areas for various free skyhooks to catch a Venus-Earth transfer and accelerate it to Earth-Mars transfer orbit here: Free Space Skyhooks
LAUNCH LOOP - ANOTHER WAY TO GET TO ORBIT - BUT NOT BACK
The idea is to cause materials to levitate by accelerating them inside an evacuated tube. You could use a belt or a continuous chain, accelerated magnetically much as you accelerate a maglev train. The materials inside the track go around a circular track at the end and return the way they came so making a continuous loop.
Then you accelerate passengers and cargo to orbit along this levitated track, again using maglev.
Loftstrom estimates that you could build a launch loop for $10 billion able to send 40,000 metric tons to orbit per year. He estimates a similar payload cost per ton to the space elevator of $300 per ton. But it wouldn’t be used for landing.
YUNITSKIY’S IDEA - BUILD A SIMILAR LEVITATING LOOP ROUND THE WORLD
Yunitskiy had the idea of building a levitating loop all the way around the equator, using a similar construction to the launch loop. I don’t understand his idea in detail - most of his writings on it are in Russian. But his basic idea is to first build an overpass all the way around the Earth at the equator which would already be useful for transport from place to place on the Earth.
Then, he would have an evacuated tube in it, attached to the overpass and clamped to it, evacuated to a hard vacuum. Inside this, he has a magnetically levitated inner ring, the “rotor”, which also continues all the way around the equator, which is a sort of belt or chain. Both rings are segmented with flexible joints that can be stretched. He then accelerates the inner ring. He keeps the whole thing clamped to the ground until the inner ring is traveling at faster than orbital velocity at sea level. He then releases the clamps and because it is moving so fast, this causes it to lift up off the ground, all round the world at once, levitated as for the launch loop until eventually it reaches orbit.
This shows it in action starting to rise up into the sky. The pipe above it is evacuated and inside it is a “rotor” - a continuous chain or belt that orbits the Earth, magnetically levitated inside the tube and traveling through it at more than the ground level orbital velocity of thousands of miles an hour. Because it is moving so fast, this causes it to levitate above the Earth, raising the tube that encloses it with it. By continuing to accelerate the rotor inside the tube, the entire ring rises up into the sky until it reaches the height of Low Earth Orbit, carrying everything attached to it.His idea is to use it to raise payload to orbit by attaching payloads to the ring, cause it to rise up to orbit, where it is then used to accelerate and release payloads and then take incoming payloads in orbit and de-orbit them in the same way by reversing the entire process to lower the ring back to Earth.
Artist’s impression from here Unitsky String Technologies - Site News
His system is much more complex than this description, and for some reason, it has two rings running in opposite directions inside. It’s summarized in wikipedia here. His video about it is here. I’m not sure how well it would work but it has some interesting points in it which is why I thought it was worth sharing.
So that then leads naturally to Paul Birch’s idea of orbital rings.
ORBITAL RINGS
This is a fun idea. It doesn’t get as much attention in science fiction as the space elevator for some reason. Unlike a space elevator, you can build it already with present day materials, not just on the Moon, or in free space, but as a way to get into orbit from Earth as well. But it is likely to be very expensive to build it in the first place, unless you have a low cost way to get materials to orbit already.
The first orbital ring is very very expensive. But once you’ve built it, you can then send up more materials to expand it easily for a very low cost.
As with the space elevator you just go up to the platform above the upper atmosphere in an elevator. Then in the opposite direction, once you have an orbital ring you can just travel down to the surface in an elevator.
https://youtu.be/MQLDwY-LT_o?list=PLIIOUpOge0LtW77TNvgrWWu5OC3EOwqxQIt’s rather similar to the sky towers idea. You are stationary above the Earth, and experiencing nearly full g as you are not in free fall - so you still need some way to get to and from orbit from the ring, but you could use maglev to accelerate along the ring to get to orbital speed.
To build this structure, what you need is a wire that circles the Earth, at orbital heights and moving around the Earth at orbital velocity. It would be very thin to start with; this doesn’t matter. It stays in orbit just through its own velocity, like an orbiting satellite. Run a current through it and then you can have objects levitating on it much like a maglev train. But in the case of a maglev train, the track stays still and the train travels around it at up to hundreds of miles an hour. Here the track travels around the world at its orbital velocity of 7.8 km / sec (if you build it in LEO), and your levitated platform travels along the track in the opposite direction nearly as fast relative to the track, racing in order to stay still as it were.
At 16,500 mph relative to the track, it’s traveling more than twenty seven times faster than the fastest maglev train (603 km/h) and also many times faster than a hypervelocity bullet. But there are no earthquakes, no weather, and its in the hard vacuum of space. If you do that, your platform will be stationary relative to the ground.
So now you can lower a cable down from your magnetically levitated platform to the ground. If you do that just anywhere then there is no way the orbital ring will support it, but you can arrange to have a kink like this in your ring to support it from:
Now the orbital ring consists of two elliptical segments (the diagram is hugely exaggerated in scale; they would both be close to circular) and then the kink gives an upward force that let’s suspend your skyhook, or “Jacob’s ladder” as Paul Birch called it when he proposed the idea in a series of three articles for the British Interplanetary Society.
As soon as you have the first one built, however small, you can then bring up materials along the cable and use it to make your orbital ring more massive, or build new ones. You can build an orbital ring like this at any height. You can also build it at any angle too. It doesn’t have to orbit above the equator, so you can build it along a slanting orbit like the one the ISS follows. You can arrange the balance of forces on the whole system, including the weight of the skyhooks, so that it slowly precesses, and if you do that carefully you can get it to precess once every 24 hours, which keeps it stationary relative to Earth.
So in this way you can actually build elevators up into space from anywhere on the Earth to a platform hovering at the same height as LEO, but stationary relative to the Earth. You wouldn’t be at orbital velocity yet when you get there, you’d just be above the atmosphere and stationary relative to the Earth looking down. But you can then have, say, a maglev train that accelerates along a track along the orbital ring, until you reach orbital velocity.
It’s safe also. The ladders into space are stationary relative to the Earth, so would just fall down vertically into an area a few kilometers long and a few hundred meters wide if they broke. As for the orbital rings themselves, if damaged, they would actually “fall” upwards into a higher orbit. You can read the techy details here: Orbital Rings-I, II, III.
You can build orbital rings around the Moon too. And though orbital rings around Earth would be so expensive to build with rockets, perhaps a trillion dollars (in his 1980s calculation), the cost would go down a lot if you could get materials from the Moon.
It would be a huge asset once built, as the cost per kilogram to send materials into orbit would be only cents. It’s a “bootstrapping problem”. If only we had an orbital ring already, even a small one, it would be very easy and low cost to build lots of orbital rings. But how do we get started? See also the wikipedia article Orbital ring
If one of the other techniques mentioned already gives you a very low cost way to get materials to low Earth orbit (such as skylon, orbital airship, or launch loop) or from the Moon (Hoyt’s cislunar tether transport system), then perhaps we can build an orbital ring as well, for a reasonable cost.
Check out my book for more surprising astronomy answers like this:
Simple Questions - Surprising Answers - In Astronomy,
It is a compilation of some of my quora answers . I’ve just added this answer as the last one in the book.
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