After Math - Foamology and Flight Rationale

The Space Shuttle was developed by NASA to be a largely reusable launch system which could provide frequent access to low earth orbit. Like all previous launch systems, safe reentry for the crew and payload required the use of a thermal protection system (TPS). Unlike previous spacecraft though, the Shuttle's TPS was exposed from launch, making it sensitive to debris which could be generated by the vehicle on ascent. The most likely and potentially destructive source of debris was considered to be ice, which could build-up anywhere on the External Tank (ET) where there was exposed metal. Ice could form during ground operations after the cryogenic propellants had been loaded and then be knocked loose on ascent. In order to prevent both ice build-up and boil-off of the propellants, the entire ET and all protuberances (orbiter attach points, pressurization lines, propellant feed lines, etc.) were covered with a spray on foam insulation (SOFI) type TPS. Unfortunately the foam was also susceptible to liberation during ascent, and posed a debris risk of its own. During the early years of the Shuttle Program engineers spent a good deal of effort characterizing the amount of foam that was shed.