[AR] hybrids (was Re: Re: List of universities ...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 21 Mar 2017 17:50:23 -0400 (EDT)
On Tue, 21 Mar 2017, Team Icarus SDSU wrote:
I am interested to learn more about staged combustion hybrids; it seems to
me that under the right circumstances this technology could use burst disks
and pyrotechnic igniters instead of cryo valves for motor startup, which
could reduce mechanical complexity at the sounding rocket scale.
Remember that you can do the same things with liquids too... if you aren't
concerned about being able to turn the thing off, and don't mind replacing
parts after each run. Much depends on the exact application, and also on
how thorough a test program you want to run.
(A really thorough test program for a liquid-engine design -- the sort
you'd run if you wanted to use the engine for more than just one or two
experimental launches -- involves many many starts, and replacing burst
disks etc. gets tiresome very quickly. Arguably, a similar number of
starts would actually be appropriate for hybrids and solids too, except
that everybody runs out of money or patience first.)
...could the gas from a solid propellant gas generator feasibly be used
to run a flometrics style pistonless pump instead of helium and a liquid
fuel gas generator?
Depends -- do you mean the same s-p-g-g that's supplying fuel gas in a
Cesaroni-style hybrid? The answer to that is *NO*, because you *really*
don't want to be pumping oxidizer with fuel-rich gas! That's asking for
explosions.
(Goddard originally used engine-heated LOX to supply warm GOX for tank
pressurization. He was plagued by in-flight fuel-tank explosions, which
stopped abruptly when he switched to LN2/GN2 for fuel pressurization.)
There's also an issue with using *hot* drive gas in a pistonless pump, but
that actually shows up even for warm helium pumping a cryo liquid. The
fix is a float, to limit gas-liquid contact. (No, that's not enough to
let you safely pressurize with fuel-rich gas -- Goddard had floats.)
Mind you, problems like this obviously get worse as the gas gets hotter.
There's no question that you could drive a pistonless pump from *a*
solid-fuel gas generator. There are commercial off-the-shelf gas
generators that produce not-too-hot nitrogen, although you might not like
how much they weigh and how much they cost. Their main limitation is that
you can't shut them off once you start them.
Henry
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