"Zoltan Szakaly" wrote in message
om...
When I looked at the video of how they simulated the foam block
colliding with the reinforced carbon carbon leading edge of the
shuttle, I noticed that behind the leading edge (which is a thin sheet
of RCC) there was nothing.

Isn't this highly irresponsible, in other words stupid? Any
homebuilder of kit airplanes knows that filling the cavity with foam
would greatly enhance the strength of the leading edge without
increasing the weight of the structure. The carbon fibers comprising
the composite sheet are strong in tension/compression but can be bent.
This lack of support from the inside was the direct cause of the hole
that the foam block punched in it.

Zoltan

I realize this thread is old by now but what the heck...

Just a little backgrounder on impact damage. There are two main failure
modes going on that take place in different time scales.

If the impactor is moving fast enough, it can cause a shock wave in the
material that travels laterally through the thickness, originating from the
point where the impact occurs. This is obviously a compression wave . What
"fast enough" means is that the speed of the impactor is fast compared to
the wave speed in the material. I don't know what the wave speed in RCC
might be but in metals it's on the order of 11,000 mph, so I doubt this was
part of the Columbia failure mode. At any rate, what typically happens is
the compression shock reflects off the back face of the material and turns
into a tension wave which rips the material apart.

The other mode is usually called global structural response. That is, what
happens to the structure as a whole when it gets loaded by the impact. In a
"slow" impact the response is quasi-static, whereby the structure responds
(for example, local plate bending) as it would if you simply pushed on it
with a force equal to the peak force seen in the impact. At higher speeds
(smaller time scales) you can see dynamic structural effects, like plate
bending waves, rippling outwards from the impact location. This is usually
the killer.

Focusing on the plate-bending mode, depending on the shape of the bending
waves the induced bending stresses can be very high. Obviously the long lazy
waves will induce small stresses but a "fast" impact might give you a sharp
wave shape with astronomical stresses that purée your material.

So, what would a foam backup do for you? If you have a really fast impact
that causes a compression wave, obviously nothing since the modulus of foam
is always orders of magnitude less than primary structural materials, and
you are still going to get the reflected tension wave.

What about the dynamic global response mode? Hypothetically it's possible
foam could help by absorbing some of the impact energy thereby reducing the
wave amplitudes. However, this never happens, again because the modulus of
foam is so small (measured in the thousands of psi vs. millions of psi for
"real" materials.) It just isn't stiff enough to attract much load. The best
you could hope for is that the foam will survive the impact well enough to
hold some of the broken bits of the primary material in place. I wouldn't
want to be responsible for a structure that was supposed to work like that.
Not to mention that cracks in the RCC would let plasma in which would
instantly vapourize the foam, at best. At worst the gas created by the
vapourizing foam would explode and blow the whole leading edge off. No
thanks.

Patrick