Out of curiosity, did you factor air resistance into that calculation? I ask because I was ball-parking (with estimates because I'm too lazy to actually do the math) around 75-80%. Which, when you factor in the cost of helium, would still make it not really worthwhile. We could use hydrogen as our lifting gas, which would be much cheaper, but a hydrogen balloon that stands a good chance of being in the exhaust of a rocket engine just seems like a bad idea.
Biting the hand that feeds IT
