Thursday, July 28, 2005

Sometimes you just have to start over.

NASA is in a bind. It is one of thier own making as well. Years ago, when the shuttle design was still on the draftng table, NASA decided to make the external fuel tank a basic tank with Urethane foam for insulation. This insulation is vital to prevent the very air around the tank from freezing and forming chunks of ice that could dislodge and strike the orbiter. Not having been involved with the design myself, I can only speculate why urethane foam was chosen instead of another means of insulating the tank. I suspect it had a lot to do with cost. The intent was to make the tank disposable, and foaming the outside of it was cheap. This was obviously penny wise and pound foolish in hindsight. Shedding foam has already caused the loss of one orbiter and it's crew of 7. It has again caused a halt to all flights until a fix can be determined. In other industries that use cryogenic liquids, tanks are made like a thermos bottle. They are double walled, and there is a vacuum between the two walls. This is called a Dewar. The vacuum acts as a very good insulator. Much better than urethane foam could ever be (Actually vacuum is a PERFECT insulator, but since there must be structural connections to the inner tank, those structural connections act as a thermal leak path. But the urethane tank has the same problems, which is what the new heaters are supposed to prevent). Dewars are not cheap to make however, ergo, the foam. But dewars do have one saving grace, they can be made lightweight. The design considerations have changed since then but I believe NASA's engineers have tunnel vision, too much time, effort, and money is tied up in the current design for them to be willing to look at the alternatives that were considered but abandoned years ago without some serious pressure. I believe that pressure is now being applied. I believe the time has come for NASA to go all the way back to the drawing board and redesign this tank using the design considerations that exist today, not 30 years ago. And if it were me, a Dewar would be the place to start.


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