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Answers to E-mail Questions About Foam Loss


Answers to E-mail Questions About Foam Loss

NASA has received many e-mails at rtfsuggestions@nasa.gov 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 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 exasperated.

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.
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