>Why not use a main rocket engine at the base during liftoff, and just use the rotors during landing?

>Why are the rotors used during takoff?

The final roton design did use a rocket at the base, but that removed some of the niftiness of the original roton design. As I understand it, the change was due to some development and scaling issues, but the original concept still seems viable for small vehicles.

Turning a rotor with tip rockets gets you 10x the lift efficiency of just pointing rockets down. The advantage goes down with altitude and speed, eventually becoming negative when supersonic, but it can be a very good way to get out of the thick air.

Look up the original roton paper on www.spacefuture.com, it is worth reading.

John Carmack
www.armadilloaerospace.com