Hi Frank

JP are being cagey so they don't give away any proprietary information, or so some speculate. John Powell's not giving much away details wise.

Robert Shawyer's thruster could be a good way to go, if it can get a high enough 'Q' factor. Basically it consumes minimal energy to counteract gravity, and starts eating power once it starts building up speed. If sufficient thin-film PVs could coat a JP airship then the ship could 'levitate' on thruster power, expending minimal power in darkness (thus needing only a light 'battery pack') and accelerate when in sunlight. Drag would be a pain, but then it might lift to a much higher acceleration level to minimise it.

Thus a slow ride to orbit all the way. Once there the thrusters can be used for station-keeping. Or big inflatable PV arrays could be deployed to really revv-up the Shawyer thrusters.

Adam