Re: Orbital Towers.

From: Spike Jones (
Date: Thu Mar 02 2000 - 21:26:23 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.
> Doug Jones wrote: 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.

Ja, Doug is right again, damn him. {8^D The Isp of the shuttle goes
from 3500-ish on the deck to about 4500 m/sec in vacuum if memory

> 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

Now youve given me something to think about, Doug. The classic
rocket equation rule of thumb is that by the time you reach a delta
vee equal to your exhaust velocity the remaining mass is 1/e times
the initial mass, so in that sense the rocket is at best 37% efficient.

My notion was not exactly Carnot efficiency, at which the typical
rocket does really well, since its peak temperature is so high. Your
cold gas thruster example shows why I say this: the carnot efficiency
of cold gas is great, but the Isp is anemic. The cold gas thruster does
a great job of converting what little bit of energy is there, which isnt
much (what is a typical vac Isp of cold N2? 800 m/sec? Cold
hydrogen would be better, if you ignore the weight of the tank).

The skyhook idea allows a theoretical cheap lift, so long as you
ignore the practical difficulties. There would be less structure
required, since you need no aerodynamic shroud, etc. The biggest
real world difficulty of a skyhook is one I seldom see mentioned:
that planes and satellites would hit it. Furthermore, it isnt a free
ride at all. When a payload is lifted, the center of mass of the
cable is lowered, so unless you figure out a way to lift the CM
back to geo the entire structure starts to drift east. spike

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