In article ,
Herb Schaltegger wrote:
(also related to thinner air: rocket engines are more efficient
with less back pressure.

My undergraduate propulsion prof would be gagging at your use of the
term "back pressure" Henry...

This is the difference between someone whose idea of an unsophisticated
audience is upper-year engineering students, and someone who's actually
had practice writing for, and talking to, non-captive audiences. :-)

Is "back pressure" strictly correct? Arguably not, although the issue is
more complicated than it looks (for one thing, ambient pressure at the
nozzle exit isn't necessarily the same as ambient pressure elsewhere on
the engine, which in turn isn't necessarily the same as ambient pressure
on the vehicle -- rocket exhausts can be powerful ejector pumps). But it
*is* what you say if you want to give the right general impression to an
audience that doesn't care to hear the rigorous details.
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