From Eric Kvaalen
Jeff Hecht says we need to look seriously at space elevators and alternative propulsion for getting into space (18 July, p 24). Looking seriously won’t make them any more feasible.
A space elevator needs a cord that can support thousands of kilometres of its own weight. Even carbon nanotubes aren’t good enough. Alternative propulsion systems are great, once you are in space. But to blast off and get into orbit, you need to impart a speed of nearly 8 kilometres per second, in a few minutes.
Rocket engines using chemical fuels need much more fuel than the weight of the payload. The next option is nuclear fuel, which I wouldn’t recommend.
Les Essarts-le-Roi, France
