[AR] Re: static tests (was Re: SpaceX F9 Launch/Update -- Live Link)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 27 Dec 2015 21:58:42 -0500 (EST)
On Thu, 24 Dec 2015, William Claybaugh wrote:
I think the key question is whether a static test is a
sufficiently traumatic :-) event to need significant inspection
and refurbishing afterward...
It seems to me more likely that SpaceX static tests--in part--simply
because they can: the vehicle was designed for reuse and accordingly
scared for the "extra" weight of the hardware needed...
Yes, could be.
I've not been able to make the statistics work w/ regard to SpaceX's
testing--the risk still appears higher than the benefit--but you are the
statistician, perhaps you can explain?
While I may know a bit more about statistics than most, I don't claim to
be a real statistician. From what I do know, I'd suggest that the source
of the discrepancy is more likely to be found in the assumptions than the
calculations. It's hard to assess the question properly because so much
relevant information is not public. A few speculations:
+ Maybe the risk really is minimal, because the hardware is designed for
reuse and putting an extra cycle on the engines just isn't significant.
+ Maybe that's the *long-term intent*, and the static tests are a standard
part of the process today because Elon wants to drive the hardware and the
procedures in that direction, and is willing to tolerate some risk and
inconvenience now to help ensure that things move in the right direction.
(I think a frequent failing of traditional space-industry evaluations of
decisions is short-term thinking: because there is no long-range plan,
everything must be justified by benefits today. SpaceX definitely has
long-range plans -- whether they are good ones is a separate question --
and might make short-term sacrifices for long-term benefit.)
+ Maybe the benefits are larger than they seem, say because SpaceX's
testing revealed a rare not-yet-understood problem that shows up in a
static test and is serious enough that catching it is deemed mandatory.
+ Maybe such a problem did show up, and is now believed solved, but
continued static testing is deemed useful insurance against the
possibility that there are more such problems. (Another frequent failing
of the traditional process, highly visible in such episodes as Apollo 13,
is to think that because a determined and expensive effort was made to
anticipate all possible problems, there can be no further surprises, and
hence there is no need to take generic precautions against them.)
+ Maybe such a problem did show up, and is now believed solved, and so the
test that revealed it is now sacrosanct and must be continued, whether or
not it provides any meaningful further risk reduction. (The flip side of
the previous speculation. Traditional space does a *lot* of this --
snapping your fingers to keep the elephants away, the absence of elephants
being proof that it's effective and thus necessary. I'd be a bit
surprised to see SpaceX yielding to this temptation, but they have been
dealing with NASA and the USAF for quite a while now...)
Henry
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