[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Fri, 31 Jan 2020 00:48:18 -0500 (EST)
On Thu, 30 Jan 2020, Norman Yarvin wrote:
You're completely missing the point. For a given amount of heat
addition the constant volume process has a higher pressure rise, a
higher temperature rise, and (most importantly) a lower entropy
increase. This means that more of the energy is available to
accelerate the fluid and less has to be rejected as heat.
I thought you'd agreed that (in a rocket) you can get all those
benefits equally well just from upping the pressure...
Remember that upping the pressure costs energy -- it doesn't happen free,
indeed the cost can be quite significant. (It's not an accident that the
high-pressure engines use topping cycles so that the propellant burned to
power the pumps isn't then thrown away at low velocity -- you can't afford
that, there's too much of it.)
Besides, even setting that aside, such an equivalence would be rather
strange. Disregarding the (important) issue of nozzle efficiency, more
energy to accelerate fluid gives higher Isp. And to an ideal first
approximation, raising chamber pressure does *not* raise Isp -- higher
pressure gives higher thrust by pushing more kg/s of gas out, not by
making it move faster. The velocity at the throat of a choked nozzle is
Mach 1, which is determined by gas properties and temperature only, and
what happens downstream is likewise.
(A second approximation gets to consider some non-ideal behavior: higher
pressure tends to suppress dissociation and give a hotter flame, and it
also permits a higher expansion ratio in the presence of non-zero ambient
pressure.)
Henry
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