From: Spike Jones (spike66@ibm.net)
Date: Wed Mar 01 2000 - 21:16:31 MST
> >[Eugene Leitl knows] nothing about tethers or orbital towers. However, what
> is wrong
> >with hoisting rockets up to a tethered baloon (10..20 km height), and
> >launching them from there?
> Darin Sunley: The vast majority of the energy you spend to attain a stable
> orbit is in
> building up your horizontal velocity relative to the ground, not fighting
> gravity to gain 100+km of height.
Right. Launching from a high altitude allows you to use a lighter
aeroshroud, but buys you surprisingly little other than that. If you
could launch from the top of Mt. Everest for example, the peak
dynamic pressure on the nose is less than a third what it would
be if launched from the deck. Depending on the type of rocket
that savings might be worthwhile in terms of payload capacity.
> You CAN launch from a couple km up. But it only buys you a vanishingly small
> % of the total energy you have to spend.
>
> Even if we HAD a tower right up to LEO, you would still have to spend approx
> 80% of the energy (something in this neighborhood anyways) to get from the
> top of that tower to a stable orbit at that altitude.
This isnt right tho. Remember in the rockets it is momentum that is conserved
not energy. Rockets are enormously wasteful of energy. If a skyhook
existed, one could climb sufficiently high on it, then let go of the cable
and fall into orbit. I just did a quick back of the envelope calc on that
and it looks like if you climb one Earth diameter up the cable and let
go, one would fall into a minimal orbit. And, if you had an elevator
that could climb a stationary cable, the energy expenditure would
be a fraction of that expended by a rocket.
Doug Jones has likely already jumped on this. I havent downloaded
for several hours.
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