[AR] Re: Orions and PDEs (was Re: More MAX delays.)
- From: Norman Yarvin <yarvin@xxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Fri, 31 Jan 2020 13:00:36 -0500
On Fri, Jan 31, 2020 at 02:02:42AM -0500, Henry Spencer wrote:
On Fri, 31 Jan 2020, Norman Yarvin wrote:
thread (particularly with the mention of rotary PDEs and some of the
terminology) had me second guessing :)
The puzzle for me with rotary PDEs is how you'd get the pressure low
enough that you could inject propellant at a lower pressure. With the
speed of a detonation wave being thousands of meters/second, injecting
propellants and having them mix thoroughly before the detonation wave
returns is a challenge already, even if you're injecting them from a
higher-pressure source.
Using lower injection pressure just means you need larger orifices to get
the desired flow rate.
Ah, but this is not just about low injection pressure, but about
*negative* injection pressure: the pressure in the propellant
manifolds needs to be lower than the average chamber pressure, if
you're chasing after the thermodynamic advantages. Considerably
lower, too: combustion can boost the pressure by a factor of five, and
that's the sort of pressure difference that makes for the advantage
you're chasing after. (I've been arguing that you shouldn't be
chasing after it, but if that's not what you're chasing after, you're
just playing with loud noises.) Instead of the commonly-recommended
20% surplus of pressure behind the injector, this'd be an 80% deficit
of pressure.
With the truly pulsed sort of pulse detonation engine, it's easy to
get the chamber pressure lower before introducing the next batch of
propellants: just wait. That kills your average thrust and messes
with your nozzle efficiency, but it works. But when you have a
detonation wave repeatedly and unrelentingly making a complete circle,
taking less than a millisecond for each cycle, you can't just wait;
you'd need to somehow use rarefaction waves in the chamber to pull
propellant in.
Of course it's natural that they didn't try to solve all the problems
at once in those experiments; as you mention, they had more than
enough trouble as it was:
Then they
discovered that while *most* of the combustion happens in the wave, there
is enough still-reacting garbage in the gas behind it that the next batch
of fuel vapor tends to ignite immediately rather than waiting for the wave
to come around. By the sounds of it, the funding ran out before anybody
solved that. :-) )
I wonder if they'd have had more luck with ammonia. According to
Clark, in _Ignition!_, LOX-ammonia is horrible for combustion
instability, producing lots of noisy engine runs when they used it in
the X-15 engine, but in this case that's actually the goal.
("At night, I could hear the motor being fired, ten miles away over
two ranges of hills, and could tell how far the injector design had
progressed, just by the way the thing sounded.")
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