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Airplane Propulsion 119

gas continues to accelerate, producing an exhaust of hyper- The Apollo IV was the noisiest of
sonic gas with a great deal of thrust. A large rocket expels a all rockets. It was so loud it
great deal of gas at a high velocity. The magnitude of the produced seismic readings in
amount of gas expelled can be seen in Figure 5.2, which New Jersey, a distance of 850 mi
shows the launch of Apollo 8. (1400 km).
The thrust from a rocket motor is analogous to the recoil of
a rifle when a bullet is fired. In order to increase the thrust of a rocket,
one can increase the amount of gas expelled per second, the velocity
of that gas, or both. An aircraft propulsion system works much the
same way as the rocket motor. The force on the airplane is a reaction

Fig. 5.2. Apollo 8 launch. (Photo courtesy of NASA.)