In earlier counterflow tunnels, the firing of projectiles upstream into supersonic wind tunnels generated hypersonic test conditions, but simulation fell well short of the actual reentry conditions. In the late 1950s, however, model launchers (the guns) had attained muzzle velocities of 25 000 to 30 000 feet per second, and impulse tunnels could create air velocities of 10 000 to 15 000 feet per second. Mach numbers of 80 or more could therefore be generated by shooting solid projectiles into the bulletlike masses of gas shot out of impulse tunnels. Although reentry conditions are closely simulated, it is almost impossible to measure local aerodynamic parameters on the tiny, free-flying models in test periods limited to less than a millisecond. Nevertheless, one can determine model stability, gross aerodynamic characteristics radiative characteristics of the nose shock, and even some heat-transfer parameters.