[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 02:02:42 -0500 (EST)
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. Adequate mixing is just a Small Problem Of
Injector Design :-), although one obvious idea would be doing the mixing
as you injected rather than hoping the propellants would mix in the
chamber. (Premixing injectors quickly got a bad name in ordinary rocket
engines, because of explosions, but here the injectors have to be able to
take full detonation pressure anyway...)
According to the Bratkovich paper, the original rotary PDE did *not* do
anything clever about sucking fuel into the chamber; it simply relied on
the pressure well behind the detonation wave being rather lower than that
of the wave itself, to permit modest injection pressure. Yes, it needed
fast-acting valves for its injection orifices (this is not entirely
unexplored territory -- consider a fuel-injected piston engine's injector
valves at high RPM). That actually wasn't the problem that made it not
work.
(There were two problems, in fact. First, it proved rather difficult to
start a detonation wave propagating around the annular chamber in only
*one* direction -- simple ignition techniques invariably sent detonation
waves both ways, and when they met on the other side they fizzled out.
They eventually solved that with a frangible diaphragm. 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. :-) )
Henry
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