[AR] Re: What blew up Crew Dragon...
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Tue, 16 Jul 2019 08:46:18 -0400 (EDT)
On Mon, 15 Jul 2019, Henry Vanderbilt wrote:
But yeah, AIUI some systems do use a common He bottle to pressurize both
fuel and oxidizer, and depend on check valves to keep the two from ever
meeting back in the common parts of the press plumbing...
In spacecraft propulsion systems, the popularity of this declined sharply
after it was tentatively implicated in the loss of Mars Observer. Some
now use completely separate pressurization systems; some have nothing but
the helium bottle in common. (MESSENGER had a helium bottle with two
separate outlets.)
This strikes me as workable for a short-life expendable system that
starts from a clean-parts-assembled known state.
It can also be workable, with one extra touch, for a long-life expendable
system that does most of its firing early, which is common for things like
comsat propulsion systems. Launch it, pressurize it, do the big burns...
and then fire pyro valves that permanently close off the pressurization
lines, with the nearly-empty tanks running blowdown thereafter.
(The classical Atlas also did roughly that -- it jettisoned its
pressurization system with the booster-engine ring! After that, the tanks
were blowdown, with some assistance from hydraulic head as the
acceleration built up. Mind you, that was a pump-fed system that didn't
need very high tank pressures.)
Henry
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