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and ram drag. However, afterburning increases the jet thrust by a fixed par cent, independent of airplane speed. Thus, the net thrust increase which can be obtained with afterburning increases markedly with airplane speed. In the example mentioned above, the static thrust was increased by 33 per cent. Figure 5 shows the per cent boost which could be obtained at various speeds with the same afterburner configuration. It will be noted that at 600 MPH the thrust boost is 66%.
Since the pressure at Station 4 of Figure 2 where the heat is added for afterburning is lower than the pressure in the engine combustion chamber, the thermal efficiency of the afterburner cycle is lower than the thermal efficiency of the basic engine cycle. Furthermore, since the Jet velocity is increased with the afterburner in operation, the propulsive efficiency of the power plant is lowered. These two factors combine to make the specific fuel consumption of the afterburner cycle very high. For the example mentioned previously, the afterburner fuel-air ratio required to raise the gas temperature from 17000R to 30000R would be approximately
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where Cp is the specific heat, h is the heating value of the fuel, and is the combustion efficiency. In the same example, since the jet velocity vas varied from 1820 ft./sec. to 2420 ft./sec., the thrust gain per pound of air is
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The fuel consumption will be the fuel air ratio divided by thrust per lb. of air per hr. or
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This compares with the fuel consumption of 1 to 1.2 pounds of fuel per hour per pound of thrust for a typical jet engine under static condition. Thus, the afterburner burns more than four times as much fuel per pound of thrust as the basic engine,
The above figures apply to the static condition. As the speed through the air increase, the pressure at which the heat is added in the afterburner rises due to ram, and thus the thermal efficiency of the cycle improves. Also the jet velocity increases at a lower rate than the airplane speed and the propulsive efficiency accordingly improves. These two effects cause the specific fuel consumption of the afterburner to improve with speed, Figure 6 shows the approximate variation of specific fuel consumption with speed for the example stated previously.
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