[AR] Re: fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
- From: Henry Vanderbilt <hvanderbilt@xxxxxxxxxxxxxx>
- To: arocket@xxxxxxxxxxxxx
- Date: Wed, 30 Dec 2015 14:12:27 -0700
On 12/29/2015 8:32 PM, Henry Spencer wrote:
On Tue, 29 Dec 2015, Henry Vanderbilt wrote:
And for fatigue, even *flight* testing is always only an approximation,
because fatigue behavior can be quite sensitive to details...
You elided what I think is the most interesting part of my post, this
modest proposal:
"It might actually be easier, cheaper, and more effective to come up
with a boilerplate upper-stage mass&aerodynamics simulator, then
instrument the hell out of a first stage and fly it repeatedly till
things start breaking."
Well, as witness the bit of my commentary I've excerpted above, even
that has its problems. :-) One particular snag is that it's kind of
hard to say just how representative that upper-stage simulator has to
be, because that can matter to things like the vibration environment.
(The very large difference in Saturn V vibration behavior between the
Apollo 4 and 6 unmanned tests was mostly due to a more realistic fake LM
on Apollo 6!) So there are still issues with deciding which stresses
matter enough to be worth simulating.
It's not a ridiculous idea, but I'm not sure it's an obvious winner
either. One big advantage of fatigue testing on the ground is that if a
problem shows up suddenly, it's easier to collect the pieces for study!
Agreed, the practicality of testing F9 first stage useful life via
flight rather than ground test is not necessarily slam-dunk obvious.
But as you say, it's not obviously ridiculous either.
Some additional pluses have occurred to me in the meantime:
- It could require much less of the SpaceX R&D team's finite (and
already heavily committed) design and analytical bandwidth, being a
variation on existing flight-ops rather than an entire new ground-test
hardware complex.
- Related, it could begin happening considerably sooner than in-depth
ground flight-cycle fatigue testing. Time is money.
- In terms of gut-convincing customer decision-makers that reuse is
practical, a half-dozen flight-plus-landing videos trumps any number of
gigabytes of ground-test data. "It reflew N times without breaking" is
massively self-explanatory.
It'll be interesting to see what SpaceX actually does next in pursuing
reusability. Given their decision to retire this first recovered stage
for history, the obvious next step is to get a second copy back intact.
After that, we'll see.
Henry V
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