From: Doug Jones (random@qnet.com)
Date: Thu Mar 02 2000 - 11:49:18 MST
Spike Jones wrote:
>
> 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.
The real advantage of high altitude launch is at the other end of the
rocket- the expansion area ratio can be far higher, and the thrust goes
up dramtically for the same mass flow rate. This implies an increase in
Isp- which reduces the mass ratio needed.
> > 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.
Not so. The overall thermodynamic efficiency of rockets can be as high
as 50% -the kinetic energy of the total mass at high velocity is half of
the chemical energy of the fuels on the launch pad (the best efficiency
is at MR=5). I have the derivation of this squirreled away somwwhere,
I'll either dig it out or derive it again.
Decent rockets convert the heat energy in the chamber to kinetic energy
of the exhaust at better than 80% Carnot efficiency- simply due to the
*very* high upper temperature of the cycle, over 5500 Kelvin. Typical
vacuum cold gas thrusters are over 90% efficient, since the temperature
drops from say, 300 K to around 20 K in a simple 200:1 nozzle.
> 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.
Perhaps an order of magnitude, not more.
> Doug Jones has likely already jumped on this. I havent downloaded
> for several hours.
Got it...
-- Doug Jones Rocket Plumber, XCOR Aerospace http://www.xcor-aerospace.com
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