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From: Doug Jones <>
Subject: Re: space-based propulsion (was Re: Precious metals in space)
Date: 12 Jun 1998

Henry Spencer wrote:
> In article <>,
> John Creighton  <> wrote:
> >In a thermal rocket wouldn't you have a compressor? Wouldn't these
> >take away the need for super high temperatures?
> No, because that compressor/pump has to be powered somehow -- it
> makes no net contribution when you look at the whole system.  (In
> fact, often it exacts a penalty.)

Indeed, lower pressure improves the Isp of a solar thermal rocket rather
markedly, because the equilibrium between H2 and atomic H shifts toward
atomic at lower pressures.

Hydrogen, 2300 K chamber, 200:1 area ratio, frozen at throat:

Pc,   Isp,
PSI   sec
1     865
5     853
25    846
1000  840

The dissociation reduces the molecular weight and improves Isp.  As Bob
Truax points out, space-based rocket engines do *not* need pumps, and
the solar engine proposed by Boeing in AvLeak recent had a design
chamber pressure of 25 psi, pressure fed by the hydrogen vapor pressure.
The impulse would be even better at 5 psi, but the engine gets a bit too

Ammonia is also a good candidate propellant, because it dissociates to
H2, N2, and H to give Isp=413 for Pc=25 PSI at 2300K, 200:1 expansion. A
soft cryogen that boils at around 240K, it would be ideal for long-
duration missions.

Doug Jones

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