Space elevator for surface transport

Two weeks ago I was at Space Access '13 and ran into Michael Laine, since when I've been thinking about novel uses for a Lunar Space Elevator.  I've come up with one for which a few minutes of searching could turn up no prior art, so I'm publishing it here lest some troll try to patent it.  I'm still trying to think of a snappy name for it, and you can help.

How it works

The easiest way to describe it is as a tetherball with the main elevator cable in place of the central post and a vehicle in place of the ball.  For the tether we'd use some more of the material we used for the main cable, and we'd need some kind of swivel at the place where the tether and post met.  The tether would be longer than for a playground tetherball, long enough to reach ground level with a bit to spare; the vehicle would contain a winch that could reel in enough tether so it (the vehicle) would hang at or slightly above ground level.  By some means of propulsion that might range from a push with a stick to a high-velocity rocket thruster, the vehicle would start itself moving away from the main cable, and would reel the tether in to lift itself up to clear any obstacles, then out again to descend to ground level and land at some distance from the main cable, if a landing were desired.  If the mission were to survey the surface near the main cable without landing, then when the vehicle had almost come to a stop, thrust would be applied at right angles to the direction of travel to send it on a circular or elliptical path around the main cable.  An elliptical path will precess, so that the vehicle could survey the interior of a circle centred at the main cable, rather than just the circumference of such a circle.

Ready for some snappy names?  The ones I've thought of aren't very snappy: lunar pendulum, Tarzanator, non-o-rail.  Suggestions welcome!

Energy and time

The equation for a pendulum with a rigid, massless rod under 0.16 of an Earth gravity gives, for a length of 10km, a period of oscillation about 490 seconds, so we could travel from the main cable to the end of the swing in 123 seconds, just over two minutes (journey time would be independent of distance traveled, up to a point -- if you don't believe me, ask Galileo).  I think the speed at the bottom of the swing is pi/2 times the average speed, so to travel 1km would require a starting speed of nearly 13 m/s; a ballistic launch at the same speed 45 degrees above the horizontal would give a parabolic flight that would last some 11 seconds and travel some 100 metres, so the pendulum travels further for the same amount of energy, and can have a softer landing with no extra fuel, but it is still subject to a square law: for a given length of tether,  twice the distance means four times the energy.  If we want to travel further without using more energy, we have to go slower. Making the tether 1,000km long gives a journey time of  roughly 1,225 seconds, so for the same starting speed we could travel 10 km, or for ten times the speed (a hundred times the energy) we could travel 100km.

I should not have used that equation, because the tether is neither rigid nor massless, though if it is made of a fibre such as Kevlar and is no thicker than it needs to be, well, OK, let's give a safety factor of 2, then a 1,000km tether will have about as much mass as the vehicle (including its payload).  But giving a sudden push to a heavy weight at the bottom of the tether would lead to a lot of energy being wasted in lateral oscillations of the tether: Twang!  Plausibly we could fix this by accelerating the vehicle over a time comparable to the time it takes the lateral wave to reach the top of the tether.   I estimate the speed of a lateral wave along the tether at 1km/s, so a cautious acceleration time for a 10km tether would be about 10s, which is small compared to the journey time; for a 1,000km tether I get 1,000s, and for longer tethers the acceleration time would exceed the journey time, so it seems that long tethers don't offer much advantage.

The tether would tug on the swivel and cause lateral oscillations in the main cable.  These are already a problem for space elevator designs, and I don't propose a solution here; I hope somebody develops one.

Economics

The pendulum could be constructed together with the main elevator cable and would need little additional technology development, so the cost should be modest.  For a young Lunar settlement it could provide occasional point-to-point transport and obviate the need to design and construct some ground vehicles and roads, which would require more tech.  As the settlement grew, a road, rail, or tunnel system would be added to provide more capacity than a pendulum.

Further applications

On Mars, where there is enough wind to provide thrust, a vehicle with a sail could travel modest distances with no input power; how it could achieve a trip upwind is left as a exercise for the reader. There are obvious applications to deploying instruments in the upper atmosphere of Mars, at altitudes below the reach of satellites but above that of balloons.

If a terrestrial space elevator were constructed, a pendulum carrying a current would be magnetically pulled eastwards or westwards from the main cable, and could mount a defence against space debris.

The ability to suspend a moving mass from a space elevator implies that launching from the elevator into a circular orbit may become possible, for a wide range of heights and inclinations, using a low-thrust engine.  A vehicle circling about the elevator cable would impose a hefty oscillating lateral load at the swivel; this could be overcome by adding a counterweight, but there may be other problems.