naca-tn-3922
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National Advisory Committee for Aeronautics, Technical Notes - Analytical Investigation of the Effect of Water Injection on Supersonic Turbojet Engine Inlet Matching and Thrust Augmentation
An analytical investigation of the effectiveness of water injection
for engine—inlet matching and thrust augmentation was made at Mach num-
bers from 1.5 to 2.0. One-dimensional equations for complete evaporation
in a constant-area channel were applied to a fixed-geometry inlet, and
its flight performance was compared with.bypass and translating-spike
inlets.
No-spillage engine-inlet matching was achieved over the Mach number
range studied with maximum liquid—air ratios of about 0.03. Augmented
thrust due to injection ranged from 17 to 56 percent higher than that of
the best performing variable-geometry inlet. However, the specific im-
pulse (thrust/(lb)(sec of liquid consumption” of the water-injection in-
let configuration was considerably lower than that of the bypass inlet
and about equal to that of a fixed-geometry configuration without water
injection. The total liquid consumed by the water-injection system, at
matching, was one-half to two and one-half times greater than the bypass
inlet configuration.
Because of the cooling effect on the air entering the turbojet en—
gine, it was found that the maximum allowable flight speed of a
temperature—limited turbojet engine (Mach 2.0 in tropopause) could be
increased by 25 percent, while the cruise altitude of the bypass inlet
system could be increased from 55,000 to 60,000 feet and for a fixed in-
let, from 55,000 to 67,000 feet.
High-speed aircraft operating at supersonic speeds with fixed-
geometry inlets spill large amounts of air when the inlet capacity ex-
ceeds that required by the engine. This condition imposes high drags upon
the airplane. In order to provide engine-inlet matching to prevent such
mass flow spillage and the attendant drags, much mechanical complication
has been introduced. Many current designs provide moving compression
surfaces and bypass flow systems which, in general, increase airframe
complexity, add considerable dry weight, and present difficult control
problems. Less complex solutions to thefmatching problem are desirable.
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