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Aerodynamic Testing 217

to decrease after the greatest curvature until the normal shock wave is
reached.

ROCKET MOTORS

It should be noted that this description of a supersonic
venturi is the same as the description of a rocket motor. The
flow of the compressed gas from the combustion chamber is
restricted by the region of Mach 1 that has moved to the
throat of the motor. The gas then expands into a region of
lower pressure so it accelerates. Thus the exhaust of a rocket
motor is supersonic. Since by Newton’s second law the thrust
of the rocket is proportional to the velocity of the gas, this is
a very desirable situation.
The velocity of gas is Mach 1 at the throat of all rocket
motors. This limits the amount of gas that can be expelled. In
designing a motor the engineer must make the throat small
enough so that the gas reaches Mach 1 and thus the exhaust
becomes supersonic. But the throat must also be large
enough to let enough gas out to produce the desired thrust.

Supersonic Wind Tunnels
Supersonic wind tunnels operate differently than subsonic and tran-
sonic wind tunnels. First, because fans are inefficient at supersonic
speeds, they must run subsonic and the air must make a transition
from subsonic to supersonic speeds. Second, supersonic wind tunnels
require an enormous amount of power. Supersonic wind tunnels can
require so much power that if run during periods of peak electricity
demands they can cause a regional brown-out. Very few facilities have
continuous supersonic wind tunnels for this reason.
The key to making a supersonic wind tunnel is to employ a
supersonic venturi. Figure 8.18 shows a schematic of a closed-circuit
supersonic wind tunnel. The fan moves the air in a subsonic
channel. During startup the subsonic section has been pressurized
while the test section remains at a static pressure of 1 atmosphere.
The air accelerates in the first venturi until the speed at the throat
becomes Mach 1. As the channel opens up, since the air is flowing