After watching SpaceShipOne on TV this morning, I was thinking again about my electromagnetic double-barreled space gun which captures the energy of a returning craft to launch the next one. Obviously, since a hotel on the moon is my ultimate goal, the gun needs to be able to fire its projectile at what is for all intents and purposes escape velocity, plus extra for the loss due to drag the first few miles of flight. Since the acceleration would have to be moderate in deference to my squishy guests, I knew that to get up to that kind of speed would take a launcher several kilometers long.
But how long? Well, this morning I did the math. Escape velocity is about 11100 m/s. We’ll shoot for 12000 m/s to give us a little cushion. Plus, it makes the math easier. OK, the first warning bells started to go off when I realized that in the last second of the launch the capsule would need almost 12 Km of launcher. Uh, oh.
Get out your shovels, boys and girls, because to get the capsule up to target velocity at the only moderately-stressful acceleration of 40m/s/s would take 300 seconds, or five minutes. In that time the capsule would travel 1800 Km, almost 1/4 of the diameter of the Earth.
The electrogun could still be used for an initial boost for a ship which also carried its own rockets to allow it to claw up out of the gravity well, and indeed several people have already thought of that. (I have not seen any design that recaptures the energy on return, however.) A gun with much greater acceleration and a shorter barrel could also be used to launch non-squishy payloads.
It’s also worth noting that it would take only a tiny fraction of the energy to needed achieve escape velocity to match the feat of SpaceShipOne, so I’ll still happily accept any large donations to make my dream a reality. In the meantime, I’m back to rooting for the Space Elevator boys to deliver my hotel guests.