Chapter 6 - Wind Tunnels in the Space Age

How ICBMs Are Spared Thermal Destruction


[75] The air temperatures around the nose of a reentering ICBM may reach tens of thousands of degrees- hotter than the surface of the Sun. Part of the heat is generated outside the boundary layer surface by shockwave compression. This part is dissipated harmlessly into the surrounding air. The rest of the heat arises within the boundary layer, which is in contact with the missile structure and has the opportunity to melt or damage the vehicle and its contents. Structural heating can be reduced if more of the heat can be shifted outside the boundary layer.

Intuitively, one would think that sleek sharppointed missiles would be best for the atmospheric penetration. In 1952 H. Julian Allen, of NACA's Ames Aeronautical Laboratory, showed analytically that this was not true. The nose cone should be blunt instead. When a blunt-nosed missile enters the atmosphere, a powerful bow shock wave builds up that generates much more heat outside the boundary layer than is the case with a sharp-pointed nose. This revolutionary and unanticipated development was not announced by NACA until 1957 because of its military implications. The blunt nose cone was an important conceptual breakthrough not only for missiles but the future Mercury, Gemini, and Apollo blunt reentry capsules.

But how could the blunt nose cone idea be tested realistically? The conventional hypersonic wind tunnel could not duplicate the sudden increase in air density as the missile plunged into the stationary, ever denser atmosphere at speeds in the range of 15 000 mph.

The earlier Ames counterflow tunnel provided a clue. If a model nose cone is fired from a gun upstream through the air rushing out of a supersonic nozzle, reentry conditions are closely simulated. The gun provides hypervelocities in the neighborhood of 15 000 mph, while a trumpet- shaped supersonic nozzle discharging at Mach 5 into a vacuum creates a volume of ever-denser air with decreasing Mach number in the upstream direction. Analysis showed that the flight history of the nose-cone-shaped bullet would indeed be similar to that of a full-scale reentering nose cone. The aerodynamic heating and thermal stresses would be closely duplicated. Reassured by the computation, Ames built its Small-Scale Atmospheric Entry Simulator, which effectively bridged the gap between wind tunnel and flight testing of missiles. Initial success with the small-scale simulator led to the construction of a larger version in 1958.

But first let's allow politics to catch up to technology.


image of shockwaves

[76] Bow shock waves produced by blunt and sharp-pointed reentry bodies. The strong shock wave from the blunt body dissipates energy far out into the flow field and thereby reduces local body heating.

drawing of atmospheric entry simulator

The small-scale atmospheric entry simulator located at Ames Research Center.