From: Doug Jones (djonesxcor@qnet.com)
Date: Thu Mar 15 2001 - 10:47:21 MST
Spike Jones wrote:
>
> Michael Lorrey wrote:
>
> > (btw: what is the
> > equation to use to calculate mass fractions?
>
> mass fraction ~ exp(9100/Ve)
>
> You need a delta vee of around 9100 meters per second to
> acheive orbit, if memory serves. Doug, is this about right?
> This estimates the loss from atmospheric drag and gravity
> losses. I can sharpen this up if you want to get more exact.
Close enough, but of course dense fuels (kerosene vs hydrogen) need less
total delta-V to orbit, since they have lower gravity and drag losses-
lower drag because of lower frontal area, lower gee losses because they
burn off fuel faster. Actual numbers are closer to 8900 m/s for
reasonable designs- and that 200 m/s is a significant difference due to
the exponential. Spaceflight would be a hell of a lot easier if we were
Martians.
Launching from a balloon is not practical for reasonably large vehicles,
though it is a great performance improvement for small sounding rockets
with lower mission delta-V. You can get most of the benefit of a
balloon launch from the back of a 747 or an Antonov, and the development
risk would be far less.
-- Doug Jones
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