RCS Threat

wrote in news:1146147688.536719.219130
@j33g2000cwa.googlegroups.com:

Out of all the things that could go wrong, at least from knowing what I
know, this would worry me the most as an astronaut, since it has
happened five times already.

This is what happens when the press takes things out of context - I know
that Florida Today has published the above factoid several times.

The five past occurrences were caused by a design flaw in the orbiter's
Display Driver Units (DDUs), which also provide power to the orbiter's hand
controllers. The flaw (caused by capacitor discharge) caused the DDUs to
briefly report an erroneous Translational Hand Controller (THC) deflection
when the crew flipped the power switch to the controllers. If the flight
software happened to be polling the hand controllers during that brief
period (it polls once every 320 ms), an uncommanded RCS thruster firing
would result.

The old DDUs were used only with the pre-"glass cockpit" (MCDS) orbiters.
As part of the glass cockpit (MEDS) upgrade, the dedicated displays were
removed and the DDUs were replaced with Device Driver Units (DDUs, same
acronym, yes I know it's confusing...) that only power the hand
controllers. The new DDUs do not share the design flaw. STS-113 was the
last flight with the old DDUs.

Therefore, the root cause of the five *actual* uncommanded firings has been
eliminated and cannot recur.

I don't know where they come up with the
odds of 1 in 10,000 to 1 in 1,000,000 when it has happened five times
through 114 missions. I'd be quite uncomfortable.

The two remaining failure modes for an uncommanded RCS firing are a
Darlington pair transistor failure in the Reaction Jet Drivers (RJDs), or a
"smart" wire-to-wire short along the lines leading from the RJDs to the
thrusters. The odds you quote are for those two causes. They have never
occurred in any of the previous 114 missions. The space shuttle program is
considering an RJD modification to mitigate the former, and increased
wiring inspections for the latter.
--
JRF

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