[AR] Re: Why does fuel not get pushed back into the engine?
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Fri, 1 Jan 2021 20:13:16 -0500 (EST)
On Sat, 2 Jan 2021, Charlie Jackson wrote:
I’m currently designing my own liquid-fuel rocket engine and, when
drawing up the injector, I can’t get my head around what’s stopping hot
combustion gas from travelling through the pipe lines because of the
pressure difference. And could this be prevented with simple non-return
valves?
Do you mean during operation, or during shutdown?
During operation, pressure in the feed lines must exceed chamber pressure
at all times, so flow goes the right way. There is a significant pressure
drop across the injector.
(Orthodox wisdom is that injector pressure drop needs to be 20% or so of
the chamber pressure, to avoid "feed-coupled" low-frequency combustion
instability, in which variations in chamber pressure turn into variations
in flow and thus variations in burn rate and thus variations in chamber
pressure. Some injector designs seem to be fine with less than 20%, but
this isn't a well-explored topic.)
During shutdown, it's a really good idea to provide a purge flow of inert
gas (e.g. nitrogen), in both fuel and oxidizer lines, at sufficiently high
pressure that it sweeps remaining propellant out of the plumbing and then
out of the chamber. This prevents various kinds of unpleasant surprises.
(If the purge gas supply pressure is regulated at a level between feed
pressure and chamber pressure, then you can put check valves [non-return
valves] in the purge lines, and just leave the purge supply on during the
engine run -- when propellant valves close and the feed pressures start to
drop, the check valves open and the purges start automatically.)
Henry
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