On 12/28/2015 3:18 PM, Henry Spencer wrote:
On Sun, 27 Dec 2015, Henry Vanderbilt wrote:
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...
Replicating some aspects of the rocket flight environment (notably the
vibration) admittedly might be difficult.
There are also pressure-cycles on the tankage and plumbing, as well as
thermal cycles on the variously cryo-cooled and aero-heated portions...
Pressurization cycles are routinely part of airliner fatigue-life
testing, I believe. Even thermal cycling, although more difficult,
should not be impossible to add, at least first approximations. The
tough part would be the very dynamic things, like acoustic and vibration
loads.
How precisely such tests reproduce real loading environments is always a
tradeoff, a question of how much you want to pay for insurance against
surprises. Even a rough approximation boosts confidence quite a bit.
And for fatigue, even *flight* testing is always only an approximation,
because fatigue behavior can be quite sensitive to details -- even
nominally-identical structures can differ a lot. This is why you want
the fatigue-test airframe to build up cycles *much* faster than the
flying vehicles, to provide a generous safety factor. 5000 cycles of
testing doesn't clear the flight vehicles for 4000 cycles -- more like
1000, maybe 2000 if you've been careful.
(This isn't a test you whip off quickly during development; fatigue-test
airframes sit there for *years*. The idea is not to finish testing
before real flights start, but just to stay well ahead of the fleet
leader.)