> Mike Lorrey writes:
> > A new NASA program is the Ultra Long Duration Balloon, which can carry
> > up to 1600 kg at an altitude minimum of 33.5 km for periods of up to 100
> > days. What is the mass fraction required to launch from the 33.5 km
> > altitude at the equator and gain a 200 km orbit? (btw: what is the
> > equation to use to calculate mass fractions? Spike, can you post up your
> > spreadsheet?
> I don't know the answer, but I do know that getting to altitude is a small
> part of the problem of reaching earth orbit. LEO velocity is 7.5E3 m/s.
> Putting 1 kg into this orbit takes 1/2 mv^2 or 2.8E7 Joules. Raising 1 kg
> by 33.5 km adds 9.8 * 33.5E3 Joules or 3.3E5 Joules, just over 1% of the
> needed energy to reach orbit. So you haven't gained much.
Ok, well, how about as a suborbital launch platform for the x-prize? All
that matters for that is altitude and turnaround time, it seems, no
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