|
|
<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
<< Lancair Builders' Mail List >>
<<<<<<<<<<<<<<<<--->>>>>>>>>>>>>>>>
In a message dated 4/6/00 0:23:18 AM EST, lancair.list@olsusa.com writes:
<< He then went into a quite detailed explanation about
composites and "surface excitement and resonance" which can cause
fasteners, nuts etc. to vibrate loose. He also warned of holes
penetrated by bolts becoming enlarged by the constant resonating. >>
There are many reasons why NASA's space projects all cost a bazillion dollars. This is one of them. Armies of intelligent people paid to think of anything that could possibly go wrong, no matter how unlikely. (He says, at the risk of igniting a firestorm...)
The Space Shuttle and the expendable launch vehicles undergo a horrendous amount of vibration unlike anything we see in general aviation. It can be a challenge to get mechanisms and electronics to survive these environments. Just becuase they armor their vehicles against this onslaught, it doesn't mean we have to. Ours aren't subjected to anything near what their vehicles are subjected to. (Satellites and space stations, too. Their worst loads are often the launch loads.)
If any screws get loose, there's no opportunity to inspect and correct the situation. That's one reason why every single fastener I used on the Space Shuttle payload I flew in 1985 (I was Chief Engineer) had to use a cotter pin, safety wire, or other positive locking device. No self-locking nuts for these guys, unless you did a vibration test to prove that it would hold. There's another NASA tradition that doesn't exist in general aviation. Screws and nuts may loosen due to vibration. That's why we use self-locking nuts, self-locking nutplates, cotter pins and safety wire for safety-critical fasteners. We attach non-critical items like wingtip fairings, access panels and such with sheet metal screws and inspect them during preflight and annual inspections.
As an engineering manager at Orbital Sciences, I'd have to say that 25-35% of all engineering discussions involved vibration and shock qualification of electronic components. We'd never use an FAA-PMA part without an extensive vibration qualification program. Another space flight tradition that isn't followed in genral aviation.
(For what it's worth, we did an extensive experimental study of various vibration isolation grommets for shock-mounting stuff. The E*A*R grommets were by far the best.)
So in conclusion, keep listening to your NASA engineer patients. But don't blindly apply what they say, since they may be suggesting a solution to a problem that doesn't exist in our world.
But then, that's why you sent us this posting, isn't it?
- Rob Wolf
(FWIW, VP/Engineering at Pioneer Rocketplane, www.rocketplane.com)
LML website: http://www.olsusa.com/Users/Mkaye/maillist.html
Builders' Bookstore: http://www.buildersbooks.com/lancair
Please send your photos and drawings to marvkaye@olsusa.com.
|
|