On Tue, 25 Feb 2003, Ian Woollard wrote:
> "This was a result of flow separation on the highly over expanded nozzle..."
>
> Hmm interesting. I'd idly wondered about this before. If you throttle
> back an engine presumably the chamber pressure scales down, but the
> nozzle expansion ratio is fixed, and as the ISP is nearly the same, so
> presumably the chamber is now under expanded?
No, the engine is overexpanded -- it's expanding the gas too far, to below
ambient pressure. Underexpansion is what happens when you take a sea-level
nozzle into vacuum.
Throttling tends to hurt Isp somewhat, by the way, but you're correct that
its main effect is lower chamber pressure. And yes, the result, when
fired in atmosphere, can be overexpansion. For the same reason, most
engines are momentarily very overexpanded during startup and shutdown in
atmosphere, and this can cause trouble if the designer hasn't made
allowances for things like nozzle side loads.
> ...doesn't this
> mean that you can deliberately use this to altitude compensate?
Not well. For one thing, as noted above, there often is an Isp penalty
for throttling, which is just what you don't want later in ascent. Worse,
you need a lot of throttling to altitude-compensate effectively, and most
engines become unstable if throttled deeply. (The injector pressure drop
and the chamber pressure scale differently with changes in flow: the
former falls off faster as flow decreases, which makes it easier for
chamber disturbances to propagate back into the injector, and tends to
cause injector-coupled forms of combustion instability.)
Henry Spencer
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