Ablation Cooling

Another potential method for disbursing heat during high-speed flight was the application of an "ablation” material to the outer surface of the structure. An ablator is a material that is applied to the outside of a vehi­cle that burns or chars when exposed to high temperature, thus carry­ing away much of the associated heat and hot gases. Ablators are quite efficient for short duration, one-time entries such as an intercontinen­tal ballistic missile (ICBM) nose cone. Ablators were also used on the early crewed orbiting capsules (Mercury, Gemini, and Apollo), which used ballistic or semiballistic entry trajectories with relatively short peak heating exposure times. They seemed to offer special promise for lifting bodies, with developers hoping to build classes of aluminum-structured spacecraft that could have a cheap, refurbishable ablative coating re­applied after each flight. Indeed, on April 19, 1967, the Air Force did fly and recover one such subscale experimental vehicle, the Mach 27 Martin SV-5D (X-23) Precision Recovery Including Maneuvering Entry (PRIME) lofted over the Pacific Test Range by a modified Atlas ballistic missile.[751]

But for all their merits, ablators are hardly a panacea. Subsonic and transonic testing of several rocket-powered aluminum lifting bodies at NASA’s Flight Research Center showed that this class of vehicle could be landed; however, later analysis indicated that the rough surface of an exposed ablator would probably have reduced the lift and increased the drag so that successful landings would have been questionable.[752]

Flight-test experience with the X-15 confirmed such conclusions. When the decision was made to rebuild the second X-15 after a crash landing, it seemed a perfect opportunity to demonstrate the potential of ablative coatings as a means of furnishing refurbishable thermal pro­tection to hypersonic aircraft and spacecraft. The X-15A-2 was designed to reach Mach 7, absorbing the additional heat load it would experience via MA-25S, a thin Martin-developed silica ablative coating. Coating the aircraft with the MA-25S proved surprisingly time-consuming, as did the refurbishment between flights.

During a flight to Mach 6.7 by Maj. William J. "Pete” Knight, unantic­ipated heating actions severely damaged the aircraft, melting a scramjet boilerplate test module off the airplane and burning holes in the exter­nal skin. Though Knight landed safely—a great tribute to his piloting skills—the X-15A-2 was in no condition to fly without major repairs. Although the ablator did provide the added protection needed for most of the airplane, the tedious process of applying it and the operational problems associated with repairing and protecting the soft coating were quite time-consuming and impracticable for an operational military or civilian system.[753] The postentry ablated surface also increased the drag of the airplane by about the same percentage that was observed on the PRIME vehicle. Clearly the X-15A-2’s record flight emphasized, as NASA engineer John V. Becker subsequently wrote, "the need for maximum attention to aerothermodynamic detail in design and preflight testing.”[754] The "lifting body” concept evolved as a means of using ablative protec­tion for entries of wingless, but landable, vehicles. As a result of the X-15 and lifting body testing by NASA, an ablative coating has not been seriously considered for any subsequent reusable lifting entry vehicle.