On Sat, 6 Nov 2004, Bill Clawson wrote:
> My information is a bit rusty, but doesn't the engine
> efficiency also increase with size? I think this is
> related to gas flow & resistance in the nozzle & bell,
> in addition to loss of heat from the working fluid
> into the engine walls...
You have to go a long way down in size for such things to matter much, but
yes, scaled down far enough, various problems ensue as the thickness of
the boundary layer becomes a sizable fraction of the throat diameter,
surface areas become very large in proportion to the gas contained, etc.
The most obvious manifestation of this is that it's difficult to build
small regeneratively-cooled engines -- there is too much surface area to
cool and not enough coolant to cool it with.
> I assume there is such a thing as too big of an engine...
Only in the sense that larger engines are increasingly prone to problems
with combustion stability: to a crude first approximation, various
details of injector design determine what oscillation frequencies might be
prone to amplification, and the question is whether the chamber is big
enough for any of those frequencies to be resonant within it.
Otherwise, most everything gets better as the size grows.
> but there is also such a thing as too small of an engine, no?
Only at the extreme low end, and again, it's not so much a matter of
little engines being completely unworkable as of them posing increasingly
difficult engineering problems which get harder and harder to solve.
Henry Spencer
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