[AR] Re: Circa 1968 video about NERVA nuclear rocket program
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 16 Apr 2018 11:46:02 -0400 (EDT)
On Mon, 16 Apr 2018, Troy Prideaux wrote:
In fact, conveniently, rocket efficiency is often somewhere in the
neighborhood of 50%, so Ve*T is a plausible first guess...
Where is that 50% efficiency number coming from? A mid point between
typical sea level and vacuum ops? If you look at the SSME for example,
its efficiency would be over 70% in vacuum ops.
First, note the phrase "somewhere in the neighborhood of". That doesn't
mean 50 +/- 1, and it isn't the same for all engines.
Second, that covers a multitude of sins, including things like divergence,
recombination "freezing", etc. Usually the biggest contributor is energy
that's stashed away in rotational and vibrational modes of the more
complex molecules and can't get out quickly enough -- *in principle* a
sufficiently long nozzle will extract that energy, but in practice
"sufficiently long" is too long and heavy, even with no requirement for
low-altitude operation.
Henry
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