32
$\begingroup$

Would it be technologically and economically practical to build and operate a space elevator that only provided transport between lower and higher orbital paths?

Such a mechanism would only avoid some of the issues with a fixed surface-orbit elevator and would introduce other issues such as needing to dock with a platform moving at a velocity different than the local orbital velocity.

Somewhat related

What technological barriers do we need to overcome to build a space elevator?

Methods for propulsion other than rockets for leaving earth atmosphere? (S.F.'s answer mentions a "partial space elevator" linking a jet-accessible platform to LEO.)

$\endgroup$
  • 1
    $\begingroup$ Another proposed model is to use a spinning tether in orbit, to latch on to a payload at a lower orbit, and rotate it up to a higher orbit, transferring momentum on release as well. $\endgroup$ – geoffc Jul 17 '13 at 14:54
19
$\begingroup$

In short, no. The reason is, the hardest part by far of getting to space is getting to Low Earth Orbit. As the saying goes, once you've done that, you're half way to anywhere.

It would be difficult to make the elevator stay put to non-anchored locations. Furthermore, it would mess with the speed required to get to each of these locations, and in the end, wouldn't be very practical.

Given this, the most practical thing that could be done along these lines would be to build a very large platform on top of a mountain, and raise the object to the top of the platform to launch. That gives very little theoretical benefit, although it does cut down on wind resistance and such.

There is, however, a system that could help once you have obtained orbit, using magnetic tethers. I won't bother to explain all of the physics, but I will point you to Space Tethers, which discusses it further, and also provide a graphic from their website which demonstrates it further.

enter image description here

$\endgroup$
  • 1
    $\begingroup$ Very interesting - I didn't know about that "use Earth as stator, tether as rotor" approach. Of course TANSTAAFL, one would need to produce electricity for the tether current somehow, but that's still far neater than ion jets or the likes. $\endgroup$ – SF. Jul 17 '13 at 15:03
  • $\begingroup$ They tried producing power by using the earth as a stator once in a space shuttle mission, but the cable broke. science1.nasa.gov/missions/tss $\endgroup$ – user39 Jul 19 '13 at 5:30
  • $\begingroup$ @SF. Solar sail to raise orbit >> magnetic/conducting tether in 'generator mode' to produce electricity, stored on-board (frictionless flywheels or supercapacitors) >> feed current back into the tether in 'motor mode' to maneuver against geomagnetic field. 'Easy as Pi', right? $\endgroup$ – hunter2 Jul 31 '13 at 11:15
  • 1
    $\begingroup$ @hunter2: Theoretically. In practice solar sail is very, very weak - to raise something this massive you'd need many km^2 of it. Plus it wouldn't work in shadow of Earth, and would push instead of pulling when on Sun side - it would work only during mornings and evenings. Just powering the cable through solar cells just to make it move would be more efficient. $\endgroup$ – SF. Jul 31 '13 at 11:26
  • $\begingroup$ @SF. Yeah, I suppose. Up to a point, at least - it's conceivable that at some point it could make sense to build/deploy a huge sail instead of a very large array of panels (esp. if you're making the panels on Earth with roughly current methods). Going the other way, maybe it would be useful for micro-sats - but then you might get by with some kind of battery/fuel-cell (and dock with a mothership for recovery or resupply)(or PV, but the point was that for a tiny sat, you wouldn't need too crazy-huge a sail). $\endgroup$ – hunter2 Aug 1 '13 at 3:29
10
$\begingroup$

Practical and useful - yes. Economically viable or technologically possible? Not sure.

Surely it wouldn't require nearly as much materials so incredibly durable as the "full elevator". Jet engine powered airplanes are vastly cheaper and easier to construct and operate than rocket-powered vehicles. Once in LEO any vehicle can engage low-thrust, low-energy propulsion like a solar sail, and depart Earth's gravity pit at fraction of the cost of corresponding jet propulsion. Essentially, that would provide the benefits of space elevator at the part of the travel where its benefits matter most - between the atmosphere (where jet engines work) and the LEO (where orbital engines work). Currently that gap is jumped with rocket thrusters that are simply extremely costly.

That much for benefits. Now for the problems. The problems of delivering several hundreds kilometers of a space rope to the orbit aside (we've been there with classic space elevator) we have actual air drag, and no anchor that would pull it and provide energy, plus each vehicle riding up would pull it down. That thing wouldn't support itself like a classic space elevator, it would require its own propulsion to keep it afloat. (OTOH, fuel for said propulsion could be delivered by the same airplanes, and it could be any of the number of the neat "orbital engines", no need for the troublesome rockets). There would be the whole problem of docking the payload in the stratosphere at supersonic speeds. I'm not sure about meteorology of the stratosphere, but I think it could be troublesome (not worse than against classic space elevator though). And obviously the usage cost would be considerably higher than of classic elevator which could use efficient electric motors to bring payload from ground far beyond geostationary orbit - while jets are an order of magnitude cheaper than rockets, electric motors leave jets far behind in terms of energy efficiency = cost of operation.

One more problem: the active propulsion can't fail for prolonged periods of time. In case of a classic elevator, it would sit there completely inert with no problems. In case of the partial, it would fall. If lengthy repairs are needed, it could be propelled to a higher orbit, as high as needed, and repaired there over time, but unexpected faults would simply destroy it.

$\endgroup$
  • $\begingroup$ Some things to consider: 1) if every pound required by the skyhook (see my response) offsets several pounds required by other vehicles, you need less material net. 2) There is no need to place the skyhook where it would experience drag or need reboosting. 3) The energy the skyhook transfers can be replenished by solar electric propulsion -- which is very efficient (high ISP) and requires very little reaction mass. $\endgroup$ – Erik Jul 18 '13 at 22:09
  • $\begingroup$ @Erik re: #2, the ISS needs boosting, so I think it's fair to assume that an (LEO) orbital/orbiting tether would, too $\endgroup$ – hunter2 Jul 31 '13 at 11:19
  • $\begingroup$ The ISS is in a very low orbit so that the Shuttle was able to get to it. You can be in a higher LEO and not need a reboost nearly as often. $\endgroup$ – Erik Jul 31 '13 at 17:21
  • 1
    $\begingroup$ @Erik: the lower the orbit the more useful the rope. There was another question somewhere around that listed energy costs of various parts of the journey, Earth-LEO was about as costly as LEO-Moon. The lower you put it the more useful it is. $\endgroup$ – SF. Jul 31 '13 at 18:57
  • $\begingroup$ That's true. A higher tether demands more launch vehicle performance. $\endgroup$ – Erik Jul 31 '13 at 18:58
5
$\begingroup$

This is a very practical and useful idea. It is often called a skyhook and the Wikipedia article has lots of useful information and links about it.

Probably the most useful trait of a skyhook is it potential to store orbital momentum over time via electrical propulsion and transfer it to a "hooked" spacecraft quickly -- reducing the need for propellant and the it's associated mass.

Getting to LEO is the hardest part to getting into space and that applies to the fuel you need to go beyond LEO. So something that might reduce this is very valuable.

The biggest challenge here is probably connecting a vehicle on a suborbital trajectory to the end of the skyhook. The rendezvous wouldn't be hard, but you'd only get one chance I suspect.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.