As I recall, NASA was concerned about propellant pooling in horizontal
tests of the LOX-methane engine we did for them, so we ran purge tests
after first filling the chamber with RO water up to the throat edge.
(Filling it with liquid methane wasn't deemed worth the trouble.)
I don't recall drying things out afterwards being an issue at all; the
engine was easy to disassemble and inspect and we did so regularly. I'd
guess it just got taken apart and blown out with dry clean air after
that test. (I expect it got a more thorough cleaning before we actually
went to hot-firings.)
The purge cleared out all but a few stray droplets, after which the
customer never mentioned it as an issue again.
I'd be inclined not to do such tests with actual fuel - it'd be messy
and add hazard; that's a fair amount of atomized fuel-air mix your purge
would be kicking out the back. Messy even if it doesn't encounter an
ignition source, and if it does, WHOOM! Potential FAE time.
If for some reason I did run such a test with actual fuel, I'd plan on a
lot more safety precautions and a far more thorough engine cleaning
afterwards. As you say, engines with injector geometry such that pooled
fuel might back-flow into oxidizer openings would bear careful watching.
Fuel working its way back through injector oxidizer openings ALWAYS
bears careful watching. Set up purges so the oxidizer side purge
doesn't lag the fuel side, nor end before it. Less obviously, strange
transient things, pops bangs & backfires, can happen during engine
shutdowns - there any advice would be a lot more dependent on engine and
injector geometry, so I'll just say, keep an eye on the possibility.
Henry
On 4/25/2016 5:21 PM, Robert Watzlavick wrote:
Randall,
For a horizontally mounted engine, it is likely that if you pour fuel in
the end of the engine that it will also back up into the oxidizer
injector holes, depending on their position. Is a gaseous purge really
enough to clean out all traces of hydrocarbon in those lines? It must
work since you're suggesting it (and using it) but it seems like there
could still be some fuel left in the lines afterwards. How long of a
purge do you use?
-Bob
On 04/25/2016 05:42 PM, Randall Clague wrote:
Easy to test, too. Load fuel. Do not load LOX. Disconnect igniter
power. Disconnect LOX Pre and LOX Main, because Murphy. Cold flow
fuel. Safe the stand. Pour fuel into chamber until it runs out the
throat. Return to bunker. Purge. Safe the stand and inspect the chamber.
If you want to be extra conservative, test with a liquid of higher
viscosity than your fuel.
Check your LOX system for fuel contamination afterward. Not because
there's any reason to believe there is any, but because if there is,
this is a good opportunity (fewer variables than usual) to find it and
isolate it.
-R
