The Space Runway FAQ
Q: What is the Space Runway?
A: The space runway is a non-rocket space-launch concept, dating back to 1979.
Since it is a lot harder to get a spacecraft into orbit than into space, we choose to make a long tube in low-earth orbit that can catch a suborbital spacecraft and rapidly accelerate it to orbital speed (about 7300m/s relative to earth at the equator).
Such a tube could decelerate a spacecraft relative to itself, akin to an aircraft landing on a runway. Assuming the runway is significantly more massive than the spacecraft, the end result will be a small velocity loss for the runway and a large gain for the spacecraft.
Q: Who thought of it?
A: The idea goes back to Arnold and Kingsbury¹ in the late 1970s under the name “spaceport”, but has been reinvented several times as a “hypervelocity landing track”² , or “LEO Catcher”³ or “Capture tube”⁴.
Q: Why bother with this?
A: The expense of space travel today comes from the rocket equation — getting into a minimum stable orbit around earth demands a rocket that is almost entirely made out of fuel, and that creates an extraordinarily expensive vehicle that has a pathetically small payload. A “catcher” in low earth orbit would cut the delta-v required to get to orbit from about 9km/sec to just 2km/sec — the speed required to go to about 200km above earth.
A rocket and runway system to get to low earth orbit could increase the payload per launch by up to a factor of 20, meaning a reusable Falcon 9 lower stage could send hundreds of tons to orbit per launch, and the expendable upper stage wouldn’t be needed. The cost per kilogram to orbit could fall to 20 times lower than whatever the cost for a rocket alone is (which itself is falling due to re-usability)
Q: How would the runway help the spacecraft achieve orbit?
A: Since the runway is already in orbit it just has to slow the spacecraft down relative to itself. Once the spacecraft has “landed” on the runway, it will be in orbit. This deceleration process can be achieved very simply with a magnetic spacecraft and a conductive metal runway through eddy-current braking.
Q: Wouldn’t you need lots of complex magnets and electronic equipment in the runway?
A: No! The runway would just be a metal tube made out of nonmagnetic material like aluminium. The magnets on the ship would induce currents in the runway which create a co-moving magnetic field
Q: How big and heavy would the space runway have to be?
A: An initial space runway would need to be something like 20 kilometers long and 500 times heavier than the spaceship that lands on it.
Q: Wow, that’s a lot of mass! You’d need an infeasibly large number of launches.
A: The runway can bootstrap itself. A shorter, lighter starting runway of just a few kilometers becomes useful to partially help rockets. Once the process gets going, the exponential nature of the rocket equation works in our favor, giving increasing gains.
Q: How will the runway bootstrap itself — it’s hard to build things in orbit!
A: This is something of an open problem with the runway and probably its biggest risk. Possible solutions include welding sections together or even spraying material on to build the runway up.
Q: What keeps the runway in orbit?
A: Over a period of time the runway would be able to regain the lost velocity using efficient ion engines. Ion engines are much more efficient than rocket engines, and they can regain the lost momentum for a relatively small amount of fuel which can be brought up from earth (or even harvested locally from the atmosphere).
Q: How exactly will the spacecraft meet the runway without crashing into it? They will need to coincide correctly to within 20 meters or so, but they are traveling at 7.3 km/sec relative to each other!
A: Objects move more predictably in space than on earth. In the last 30 seconds or so before coincidence, the spacecraft would be moving in an almost perfect parabola due to gravity. Small thrusters on the spacecraft could make some final corrections to ensure it was in the right place at the right time.
Q: How would the runway support the spacecraft against gravity between capture and landing? How would small deviations in position be corrected?
A: The spacecraft could support itself with magnetic levitation. This is also a passive technology — the runway wouldn’t need any magnets or electronics.
Q: How much power would the runway need and how would you power it?
A: The runway would be solar powered. The amount of power it needs would depend on the rate of mass going to space and the runway engines’ power efficiency, but we are not talking about impossible amounts of power.
Q: How long would the runway be?
A: It is likely that the initial versions of the runway would be about 20km long, as this is the shortest runway that spacecraft can land on without rocket assistance for the orbital velocity. Any shorter would cause unacceptably high g-forces for the spacecraft. Over time it might make sense to make it longer.
Q: Would humans or animals be able to land using a relatively short runway?
A: No, the g-forces would be fatal. However most mass that people need in space is food, water, structural materials, fuel, etc. Most objects and substances are much more robust than humans are, and we will need a lot of them in space (to the point where they will dominate the costs of humans existing in space or on other worlds).
Q: Would humans be able to land on a relatively long runway?
A: Yes, as the runway gets longer the g-forces would fall, eventually to the point that humans could use it. With moderate rocket-assistance (~3300m/s), humans could use a 150km long runway, though it would be “the ride of your life” — 10g for 1 minute.
Q: We have reusable rockets, so why do we need this?
A: Reusable rockets are actually a necessary ingredient of the runway because they facilitate getting to the runway cheaply and with a good payload. Once you have a runway, reusable rockets get a huge payload increase.
Q: How strong would the magnets on the spacecraft have to be?
A: Strong, but not to a point of being unrealistic. The more mass in the runway, the weaker the spacecraft’s magnets can be. They can probably be permanent rare-earth magnets.
Q: Would the runway be stable in its orbit?
A: A runway along the orbital vector is in equilibrium (all forces are in balance), however it is an unstable equilibrium. With some extra parts it can be stabilized, or alternatively it can be kept under control by a series of moving counterweights or thrusters. This is an area of active research.
Q: How can the empty spacecraft get back down to earth? Reentry heating will doom it just like it has doomed many other reusable launchers (the Space Shuttle in particular)
A: A second smaller space runway can be constructed in a retrograde orbit. The empty spacecraft will thrust up or down ~10 kilometers and coincide with the retrograde runway at 16 km/sec, decelerate to 0 or thereabouts relative to earth, and reenter gently. The requirements for the retrograde runway will be lower because the empty spacecraft with no payload will be lighter and exert less force on it. The higher relative speed could cause problems, though. There are other options, such as a using the runway as a hydrogen light gas gun to return the spacecraft with a sabot. Whatever method is used must ensure a rapidly reusable spacecraft, and that probably rules out reentry heating.
Q: Once the spacecraft is back in the atmosphere, how will it get back down to the ground?
A: Final landing could be with folding wings, allowing rapid and full re-usability of the system. Rocket landing like the Falcon 9 is also possible
Q: This sounds too good to be true. Nothing is this good, you must have made a mistake!
A: A number of people have pointed out purported reasons for why the space runway wouldn’t work or wouldn’t be practical. So far no clear showstopper has emerged. Further comments are encouraged at the stackexchange question related to the runway.
Q: Nobody would want to get to space cheaply! And even if they did, nobody would bother to invest the money to do so!
A: Unfounded negative predictions have a poor track record, it’s hard to know what people will not want to do in the future, especially when it becomes much cheaper; it’s hard to know what financiers will or will not fund if the payoff is big enough.
Q: If this idea is so good, why haven’t the professionals already thought of it?
A: There are several factors. Firstly, the space runway is very counter-intuitive. Secondly, the original exposition of the idea wasn’t published in a scientific publication. It was also somewhat overcomplicated in our opinion. Third, the runway isn’t very economical without reusable first-stage rockets (or some other cheap way of getting well past the Kármán Line). It was dismissed early on and forgotten about, but its time may have come.
Q: Wouldn’t some of the other non-rocket launch methods like a Lofstrom loop or a space elevator be better?
A: No, probably not.
Each other non rocket space launch system has serious problems. Space elevators require materials that are almost magical — certainly not things we have access to today. The Lofstrom loop has a long list of problems: it’s 2000km long, covered from end to end in sophisticated electronics, has unsolved stability issues, and is always a microsecond away from releasing a megaton-scale explosion if its rotor collides with the sheath. Rotating and nonrotating tethers can’t actually get a spacecraft to orbital speed unless they have the same magical materials that the space elevator needs, so they rely very heavily on rocket assistance, reducing the benefit to the point that it’s not worth it.
Q: Why is the runway so much better than all of the other systems?
A: Because it doesn’t need to support its own weight, and takes advantage of the fact that large objects are possible in micro-gravity environments, and certain other advantages of space like a lack of atmospheric drag. The tether-based systems all fail because they try to support their own weight, and the earth-based systems have to deal with earth’s gravity and atmosphere, both of which are troublesome. The runway also has a relatively good rate of material to orbit — it can potentially lift its own mass to orbit within a week (though it is not clear at present what the binding constraint on this will turn out to be)
[1]: “The Spaceport, Part I,” Analog Science Fiction/Science Fact, Vol. 99, №11, November 1979, pp. 48–67 and “The Spaceport, Part II,” Analog Science Fiction/Science Fact, Vol. 99, №12, December 1979, pp. 60–77.
[2]: https://en.wikiversity.org/wiki/Hypervelocity_Landing_Track
[3]: https://medium.com/@AlanSE/leo-catchers-as-a-launch-assist-system-7f4d4183109b
[4]:http://www.islandone.org/LEOBiblio/SPBI1CT.HTM
