Falling chunks of insulating foam brought the space shuttle Challenger to a tragic end. Discovery’s latest voyage looked for a while to be similarly doomed. What is this foam that is causing so many problems?
The shuttle fuel - liquid oxygen and hydrogen - is stored in an external tank and must stay cool. This is done by two systems: an ablator - a material that dissipates heat by eroding - made of silicone resins and cork; and that infamous spray-on foam that keeps falling off in chunks. Polyurethane-based, the low-density, closed-cell foam comprises: polymeric isocyanate, flame retardants, surfactants, blowing agents and a catalyst.
The foam keeps the fuel cool and the tank ice-free. Various causes for it breaking off the tank have been hypothesised. These include the tank shrinking by as much as six inches in the extreme cold temperatures, although Nasa claims this amount is insignificant.
Nasa is now looking at ultra high temperature ceramics as thermal protectors. Made of hafnium and zirconium diboride, they will be used on the shuttle’s leading edges - its nose. As for foam, latest research is directed at titanium or silicon fillers, which react with preceramic polymers and make ceramic foam.