In article ,
OM om@our_blessed_lady_mary_of_the_holy_NASA_researc h_facility.org wrote:
Gemini bipropellant thrusters had considerably better performance, and
originally looked like they would cause fewer hassles. (It's not clear
that they really did...)

...Well, at least for *one* flight it didn't :-)

More true than you probably realized. :-)

One of the little headaches of N2O4/varioushydrazines thrusters, not
appreciated early on, is that they are prone to small valve leaks in the
N2O4 side after the first firing. The reason is that any trace of water
in the N2O4 plumbing gives nitric acid, which tends to dissolve bits of
the metal plumbing, forming metal nitrates... and those tend to
crystallize out in random places downstream. Essentially, there's now a
bit of grit in the liquid, and that can easily prevent a valve from
closing completely. If the valves are closed (and leak-checked) with
the plumbing empty, and it's then filled, you're okay as long as it's
just sitting there inactive... but the first time you open those valves,
they just might fail to seal cleanly on closing again.

And *that* is why Gemini mission rules called for immediate descent, to
the first available splashdown area, after any firing of the reentry RCS:
because the reentry-RCS valves couldn't be guaranteed leakproof after
first use, you had to descend while you were still reasonably assured of
a functioning RCS.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |