Previous designed for a "rotating tether" space elevator set the aircraft rendezvous above 100 km altitude, and used a shorter 500-900 km tether. Problems I've had with this approach is the need for a hypersonic aircraft to reach the rendezvous altitude, like NASA's Hyper-X, and the high rotation rate of the tether, both of which made the scenario rather dubious. However one would expect that a lower rendezvous altitude would increase aerodynamic drag on the tether and the whole system would eventually lose orbit velocity, re-enter the atmosphere, and crash into the Earth.

But playing around with my computer program I have discovered something very interesting.... the aerodynamic drag is decreased significantly if a very long tether is used. In fact, I can lower the aircraft rendezvous altitude to just 10 km high (within range of many aircraft) if I put the orbiting hub at least 1500 km high. At this altitude the tether rotation speed is gradual enough that the maximum drag never exceeds 1 million Newtons. This was the original estimate I made using a spreadsheet. If I further increase the hub orbit altitude to 4000 km, I can lower the aircraft rendezvous right down to sea-level and the peak aerodynamic drag will still never exceed 400,000 Newtons !!!!

Well I must say I am rather proud of myself tonight....