[AR] fatigue life (was Re: Re: SpaceX F9 Launch/Update...)
- From: Henry Spencer <hspencer@xxxxxxxxxxxxx>
- To: Arocket List <arocket@xxxxxxxxxxxxx>
- Date: Sun, 27 Dec 2015 23:26:34 -0500 (EST)
On Sun, 27 Dec 2015, Henry Vanderbilt wrote:
Engines can to a considerable extent be life-tested on the ground. A
much larger unknown (as Bill has pointed out) is the fatigue lifetime of
the structure and tankage... The tricky thing is, whether (and where,
and how well hidden) F9 first stage might (or might not) have its
equivalent of the old Comet airliner's square-cornered windows - a place
where fatal fatigue builds up under flight cycles much faster than
expected.
That can also be tested on the ground, to some extent, if you understand
the flight loads halfway well: dedicate a fatigue-test airframe to
sitting in a hangar being flexed by hydraulic jacks, so that it builds up
cycles much faster than the flight vehicles. When that was done for the
Comet -- it wasn't a routine part of aircraft development then -- the
window-corner fatigue failure was successfully duplicated.
Replicating some aspects of the rocket flight environment (notably the
vibration) admittedly might be difficult.
(If you're only building a few flight vehicles, it's really tempting to
think that dedicating the vehicle and the resources to fatigue test is a
waste, that you'll just analyze the problem to death instead. As I've
noted in the past, the rocket world envies the aircraft world's
reliability, but often refuses to pay attention to how it was achieved.)
Henry
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