Trouble With The Concept

Kevin Connors is arguing with Sir Arthur C. Clarke (the guy who invented geostationary orbit, and popularized the concept of space elevators) about the technical viability of the latter. All I can say is that he’s a braver man than I. He’s also a little confused about orbital mechanics:

…orbit in the Clarke Belt is achieved because the centrifugal force of the orbiting satellite exactly matches the force imparted upon it by gravity.

Well, this is sort of correct, but oversimplifed. In reality, there’s no such thing as a centrifugal force, but one can pretend there is in the rotating (non-inertial) reference frame. It’s more correct to say that the centripetal acceleration exactly matches that of gravity at that altitude.

Propelling a payload up a tether attached to that satellite would upset that equilibrium. Further, their is the distributed mass of the tether itself to consider. It is therefore necessary that the satellite be in a far lower orbit, in order to maintain tension on the tether.

This is where he goes off the tracks. I don’t know why he thinks a lower orbit would be required (or what he means when he says “satellite”).

A space elevator is designed to have its center of mass at a point beyond geostationary orbit. The idea is to have a balance between the forces that would provide sufficient tension in the cable. During construction, the anchor would initially be in GEO, but as the cable is dropped from it, it will move upward to keep the CM at GEO altitude, to maintain a geostationary period. Once the cable has reached down to earth, the other end is anchored. At that point, you’d continue to reel it out, but moving the anchor up to increase tension in the cable to whatever was desired, at which point the geostationary orbital period is maintained by being attached to the planet. The old conventional wisdom (if such a phrase makes sense in the context of a concept like this) was that one might use a small asteroid for the anchor. Newer concepts don’t require as much mass, but in either case, there will be sufficient mass, at a sufficient supergeostationary altitude to allow motion up and down without major issues.

Indeed, the path the transport vehicle takes to reach the satellite will not be a straight path, as is popularly envisioned, but a great parabolic arc.

Again, I don’t know what he means by this, but (also again) the path will depend on the reference frame. From the reference frame of a rotating earth, the path will follow the cable, which is to say straight up to GEO (where the weightless docking station would be, though the elevator structure would continue on to higher altitude, as described above). From an inertial frame, the path would appear to be a spiral, as the car orbits the earth once per day with increasing altitude. There will be some coriolis force on the moving car as a function of its velocity and altitude (as there is in an earthly elevator car), but the tension of the cable will be designed to be sufficient to prevent it from bending it much.

From a basic physics standpoint, the concept is fine, and can be easily simulated, honest.