Aerothermodynamics: Meeting the Heating Challenge

The prediction of structural heating on airplanes flying at hypersonic speeds preceded the actual capability to attain these speeds in con­trolled flight. There were dire predictions of airplanes burning up when they encountered the "thermal thicket,” similar to the dire predictions that preceded flight through the sound barrier. Aerodynamic heating is created by friction of an object moving at very high speed through the atmosphere. Temperatures on the order of 200 degrees Fahrenheit (°F) are generated at Mach 2 (the speed of an F-104 Starfighter of the mid – 1950s), 600 °F at Mach 3 (that of a 1960s SR-71 Blackbird), and 1,200

°F at Mach 6 (typical of the X-15). Reentry from orbital speeds (Mach 26—the entry velocity of the Space Shuttle orbiter) will generate tem­peratures of around 2,400 °F. Airplanes or spacecraft that fly in, or reenter, the atmosphere above Mach 2 must be designed to withstand not only aerodynamic forces associated with high Mach number but also the high temperatures associated with aerodynamic heating. The advent of blunt body reentry theory radically transformed the mental image of the spacecraft, from a "pointy” rocket to one having a far more bluff and rounded body. Conceived by H. Julian Allen with the assis­tance of Alfred Eggers of the then-NACA Ames Aeronautical Laboratory (now NASA Ames Research Center), blunt-body design postulated using a blunt reentry shape to form a strong "detached” shock wave that could act to relieve up to 90 percent of the thermal load experienced by a body entering Earth’s atmosphere from space.[749] Such a technical approach was first applied to missile warhead design and the first crewed spacecraft, both Soviet and American. But blunt bodies, for all their commendable thermodynamic characteristics, likewise have high drag and poor entry down-range and cross-range predictability. Tailored higher L/D lifting body and blended wing-body shapes (such as those pioneered by the Air Force Flight Dynamics Laboratory), while offering far better aerodynamic and cross-range performance and predictability, pose far greater cooling chal­lenges. So, too, do concepts for hypersonic air-breathing vehicles. These diverse requirements have stimulated the design and development of sev­eral potential solutions for thermal protection of a vehicle. For purposes of discussion, these concepts are addressed as heat sink structures, abla­tion, hot structures, active cooling, and advanced ceramic protection.