|Answers to E-mail Questions About Foam Loss|
NASA has received many e-mails at firstname.lastname@example.org with suggestions or questions about foam loss from the Shuttle's External Tank. Here are answers to some of the most common questions: |
Is there a problem with the foam adhering to the aluminum surface of the External Tank?
Our primary failure mode is cohesive failure of the foam; we have experienced little, if any, adhesive failure of the foam not sticking, per se, to the tank. Cohesive failure of the foam is different from the adhesive failure mode in that the foam itself fails to stay together and portions of the foam breaks free. Cohesive failure can be caused on a small scale by build up in pressure of the closed-cell foam that we use as the foam is heated during ascent. We refer to this phenomenon as "pop-corning." Voids that form within the foam during application to the tank can also lead to cohesive failure. Air entrapped within the voids can expand with the heating experienced during ascent, increasing the pressure, and ultimately cohesively failing the foam between the void and the foam surface. Note that the delta pressure (change in pressure) across the foam between the void and the surface is not only influenced by ascent heating, but also by the ever-decreasing ambient pressure until the vacuum of space is realized. Where voids form near or at the interface between the foam and the tank structure, entrapped air will be liquefied in the presence of the Liquid Hydrogen or Liquid Oxygen temperatures at the tank's aluminum surface, the potential for cohesive failure is exacerbated.
Why doesn't NASA apply paint, a cover, or net over the tank?
One might remember that we painted the first couple of External Tanks with white paint in the early 1980's. In both cases, we had a significant amount of foam loss during ascent. Although at face value applying a net or some other foam entrapping method to the External Tank sounds easy, it is not without concern. After careful examination of this approach, NASA's conclusion is that portions of the net could become in itself an undesirable debris source. Depending on the material used (Kevlar, aluminum, etc.), the density of the netting material would present a more critical debris source than foam to the Orbiter Thermal Protection System. Through a rigid certification process, we would also have to understand if and when the netting material could come off and in what quantities or mass that the netting material could present. Our assessment is that the process of certifying a netting material for flight would take several years and would not be available until late in the Space Shuttle Program life. NASA's goal remains to eliminate the potential for critical debris from all sources, including the External Tank foam.