[AR] pressure and thrust (was Re: relativity abandons...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Mon, 17 Apr 2023 18:37:16 -0400 (EDT)
On Mon, 17 Apr 2023, kevin ward wrote:
Can anyone help with this equation...
More psi in combustion chamber, no matter what, results in more thrust ?
BurnSim says so...
But dad insists NASA knows best and may 1 at the throat is penultimate.
Both are right. :-)
Yes, under anything resembling normal rocket conditions, gas velocity at
the throat is Mach 1 (note that this is relative to the speed of sound in
the exhaust gas there, which is a function of its composition and its
temperature -- often faster than the normal sea-level speed of sound).
However, higher pressure makes the gas *denser*. It's still coming out at
the same velocity, but there's more mass per cubic centimeter and hence
more mass per second. And so more thrust, at the cost of more fuel
consumption.
Higher pressure also has some secondary effects, like encouraging more
complete combustion, but those are smaller.
Beware of analogies to liquids -- the gas is compressible, and that makes
a BIG difference in flow behavior.
I could see a different analogy that proves me wrong as in with a boat
and it's propeller. There is a point before velocity increases, that if
you apply too much torque, you're going to cavitate. Does that apply in
rockets ?
Not in the chamber. (In pumps and in the feed system in general, yes.)
Again, gases and liquids are quite different.
I insist that NASA is limited in acceleration due to what they're
sending up; people, equipment, O-rings that go bang, etc. And the whole
point is to "attain escape velocity as quickly as possible".
"Other things being equal." But other things aren't equal. Notably,
higher thrust generally requires heavier engines, which reduces the
payload, the amount of fuel that can be carried, or both.
Optimizing for maximum payload, or for maximum final velocity with a given
payload, usually pushes upper stages toward small, light engines with long
burns and big tanks.
First stages are a bit different, because they've got to haul the upper
stages up off the ground and clear of the atmosphere for long enough to do
those long burns (I oversimplify slightly). But even they often optimize
to thrust less than twice takeoff weight, depending somewhat on details.
That's the general picture for sizable rockets. For small ones, air drag
during ascent is more of an issue, and that can push toward lower thrust,
to keep velocity down until the rocket has cleared most of the atmosphere.
There are a bunch of other issues that can also affect the numbers.
Henry
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