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From: (Henry Spencer)
Subject: Re: Rotary Rocket style engine design questions
Date: Tue, 6 Feb 2001 23:17:06 GMT

In article <>,
Ian Stirling  <> wrote:
>Radiation cooling can work in some cases, though it requires very
>expensive nozzles/chambers.
>Os/Ir engines are more expensive than gold.
>I don't know what the practical limits are, but do know they have been
>used on small engines, giving good ISP. (maybe only on the test bench)

I'm not sure if the latest weird alloys have seen actual space use, but
radiation cooling has been moderately common for small engines in the
past, especially for their nozzles -- the Apollo SM and LM RCS systems
used radiation-cooled nozzles, and had partial radiation cooling of their
chambers as well.  (The CM RCS had to be ablatively cooled, since it was
necessarily recessed below the CM surface.)
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

From: (Henry Spencer)
Subject: engine cooling (was Re: Rotary Rocket style engine design questions)
Date: Sun, 11 Feb 2001 23:29:10 GMT

In article <3a870e5a$0$23095$>,
Ian Woollard <> wrote:
>About the only way to shed heat is in the exhaust?

It's not the *only* possibility, just (usually) the most attractive one.
The only other major option is radiative cooling, with the engine running
hot (very hot) and just radiating away the excess.  It's not uncommon to
use that for the last part of the nozzle, which is quite exposed and sees
only limited heating, but doing it for the whole engine is harder.  That's
done a fair bit in small engines like RCS systems, but generally not for
big ones.

The closest any big engine currently comes to full radiative cooling is
the Viking engine, used in Arianes 1-4, which uses radiation cooling for
its whole nozzle, but film cooling for the chamber and throat.  It can get
away with this partly because N2O4/UDMH doesn't burn all that hot (by the
standards of high-pressure flames).

One limitation in radiative cooling is that it does mean running very hot
(by structural standards), and lightweight high-temperature materials are
scarce.  High-temperature metals are not hard to find, but they tend to be
very dense, yielding very heavy engines, and they often pose other
difficulties like being brittle at room temperature.  Some of the newer
radiation-cooled nozzle extensions use things like silicon carbide, but
those often have awkward mechanical properties and I don't think you could
build a whole engine that way.

Incidentally, you do also pay some performance penalty for radiative
cooling.  Heat lost by radiation is heat that's not being used to push
exhaust out faster.
When failure is not an option, success  |  Henry Spencer
can get expensive.   -- Peter Stibrany  |      (aka

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