Interesting to note that all of SpaceX's "unexpected" failures involved
the helium system (and in the most recent one, the hypergolics).
Starship/Super-Heavy will have neither.
On 2019-07-16 05:46, Henry Spencer wrote:
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
