Hi Guy and Michael, Yes, you would think that by now the insulation would not be falling off the Shuttle's external fuel tank. However, the problem is not that the adhesive is inadequate. In the late 1980's our company was contacted by the NASA engineer in charge of quality control at the Stennis Space Center for the shuttle fuel tank insulation. He was looking for a better method for testing the adhesive bond between the tank and the insulation (we couldn't help). He stated that the process currently used in installing the foam insulation to the tank sometimes resulted in unbonded regions and that these regions were difficult to detect. The detection method used was entirely manual, and consisted of a person using a small vacuum pump to try to suck the foam away from the tank. It's my recollection that since it wasn't practical to test the entire surface this way, only suspect regions, and those known to have been difficult to bond in the past were tested. You would think that this simple process could have been at least partially automated so that most of the tank could be inspected and those remaining bits tested by hand. Perhaps this is currently done. Incidentally, a few years later another NASA engineer contacted us about methods for nondestructively testing the bonding of the ceramic (foam?) tiles on the Shuttle. Noninvasive or nondestructive testing of bond integrity, especially over large areas can be a very difficult problem. Allen Grahn At 11:32 PM 7/27/2005 -0600, you wrote:
Just heard the news that the shuttle fleet is grounded once again. There's a clear photo of a big chunk of foam just missing Discovery as it falls off of the external tank. Given the fact that NASA has spent the last two years playing with foam formulations and adhesives, you've got to wonder how the shuttle program goes forward after this. Hindsight being what it is (20/20), it seems that the decision to place the main engines on the orbiter wasn't such a good idea. Economically, it appears to make sense, but it means that any external tank must be placed ahead of the engines. This in turn means that physical problems on the tank (stuff falling off) pose an immediate hazard to the orbiter. Placing only maneuvering engines and retros on the orbiter would allow a more traditional booster BEHIND the orbiter. Then you wouldn't care a lot if chunks of foam or ice fell off (as long as you weren't the guy in the boat underneath).
This may also spell doom for the ISS. I've never been a fan of ISS (we got more done on Skylab), and an untrustworthy shuttle makes it unlikely that it can ever carry a staff of more than 2. That's how many can fit into the Soyuz escape vehicle. And it takes two people just to keep that bucket together. They'll keep a stiff upper lip for a while, but the handwriting is on the wall. More importantly, it may mean the real end of Hubble. The new administrator at NASA seemed to be inclined to reverse Sean O'Keefe's decision. Can't do it without a shuttle though.
I wasn't too worried about the sensor issue. There are small gremlins for any launch. They always get fixed, clearing the way for a different set of gremlins on the next launch. But the foam problem is fundamental. It's the reason that seven astronauts died, and two years of focused effort don't appear to have resolved it.
Michael
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