Spray-on Foam Insulation Development for the Space Shuttle External Tank Thermal Protection System

The External Tank (ET) has two major roles - to contain and deliver quality cryogenic propellants to the Space Shuttle main engines and to serve as the structural backbone for the attachment of the orbiter and solid rocket boosters. The Thermal Protection System (TPS), composed of cryoprotective foam insulation and ablator (a sacrificial heatshield material), is applied to the outer surfaces of the ET to maintain the cryogenic propellant quality, to protect the structure from ascent heating, and to prevent ice from forming. Super Light Ablator (SLA) -561, originally developed for the Viking project, was and remains the baseline ablator. The original foam insulation, BX-250, was replaced by CPR-421, then CPR-488 as requirements changed. Ablator reduction accompanied the changes in foam with their attendant material-properties improvements. Ultimately, both CPR-488 and the ablator were replaced by NCFI 22-65 on the aft dome of the External Tank. The TPS design requirements, including thickness and configuration, are primarily defined by allowable structural design temperatures, propellant quality considerations, and weight and debris concerns. The TPS design was verified by a series of rigorous tests that simulate the flight environments.

Key facilities were automated to attain the precision and repeatability of material application necessary to insure specification material properties, and to meet high volume production, schedule and budgetary constraints. Developmental emphasis has been to maintain high reliability, reduce launch weight and to reduce recurring costs. Primary foam materials are applied by computer-controlled spray equipment in environmentally controlled application cells. Several secondary foams, used to close-out assemblies, lend themselves to a wide variety of production techniques, such as spray, pour and mold in place. These closeout materials, primarily urethane foams, require little environmental control. Ablator development initially centered on a sprayable version of SLA-551, then extended to mold-in-place, premold-and-bond, and hand-packed-in-place versions; each tailored to specific design and production needs.