Autzoo.1619
net.space
utzoo!henry
Mon May 3 19:08:58 1982
Saturn 5 plume
The underexpanded-exhaust explanation is, I think, the correct one.
There are several reasons why a rocket engine may not expand its exhaust
to the ambient pressure, like constraints on how long and heavy the
nozzle can be, but the big limit is that any fixed nozzle is necessarily
right for only *one* ambient pressure. This means that as the rocket
climbs, underexpansion inevitably occurs as the outside pressure drops.
In fact, I seem to recall that there was an optional nozzle extension
designed for the F-1. I don't think it ever got used, but its existence
suggests that the F-1 may not have been optimally expanded even at
sea level.
There are engine concepts that are optimally expanded over a considerable
range of pressures, but they are very different from orthodox nozzles,
and as far as I know none of them has ever been used "for real".
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