An Experimental Investigation of Potential Icing of the Space Shuttle External Tank

摘要

The thermal protection system tiles on the Space Shuttle orbiter are extremely sensitive to impact damage. Such impacts could be caused by ice particles dislodged from the outer surface of the external tank (ET) during the launch. The ET, which contains the cryogenic propellant tanks, is covered with a spray-on foam insulation (SOFI) to minimize ice formation. The objective of this investigation was to experimentally explore a range of environmental conditions for which significant icing potential exists for the ET. A significant finding, which became evident early in the experimental program, was that the computer models based upon the average SOFI thickness predicted panel surface temperatures that were considerably higher than those observed. Ice formation of a size beyond that specified as hazardous to the thermal protective tiles of the orbiter was observed in all tests having liquid water available on the surface. Polyethylene glycol was applied to the SOFI surface and tested as an approach to ice suppression. The three compounds tested were basically successful in this capacity except at the thinnest SOFI spots. Serious questions remain, however, concerning the longevity of the coating during high moisture availability conditions. A forced air flow on the panel was found to be an effective and fast-acting icing control technique.