thrust. Because of the high fuel consumption 
during afterburner operation and the adverse 
effect on endurance, the use of the afterburner 
should be limited to short periods of time. 
In addition, there may be limited time for the 
use of the afterburner due to critical heating 
of supporting or adjacent structure in the vicin- 
ity of the afterburner. 
The specific fuel consumption of the basic 
engine will increase with the addition of the 
afterburner apparatus. The losses incurred by 
the greater fluid friction, nozzle and flame- 
holder pressure drop, etc. increase the specific 
fuel consumption of the basic engine approxi- 
mately 5 to 10 percent. 
The principal advantage of afterburner is the 
ability to add large amounts of thrust with 
relatively small weight penalty. The applica- 
tion of the afterburner is most common to the 
interceptor, fighter, and high speed type 
aircraft. 
The use of wafer injection in the turbojet en- 
gine is another means of thrust augmentation 
which allows the combustion of additional fuel 
within engine speed and temperature limits. 
The most usual addition of water injection de- 
vices is to supplement takeoff and climbout 
performance, especially at high ambient tem- 
peratures and high altitudes. The typical 
water injection device can produce a 25 to 35 
percent increase in thrust. 
The most usual means of water injection is 
direct flow of the fluid into the combustion 
chamber. This is illustrated in figure 2.14. 
The addition of the fluid directly into the com- 
bustion chamber increases the mass flow and 
reduces the turbine inlet temperature. The 
drop in temperature reduces the turbine power 
and a greater fuel flow is required to maintain 
engine speed. Thus, the mass flow is increased, 
more fuel flow is allowed within turbine limits, 
and greater, energy is imparted to the exhaust 
gases. 
The fluid injected into the combustion cham- 
bers is generally a mixture of water and alco- 
hol. The water-alcohol solution has one 
NAVWEPS 00-30T-30 
AIRPLANE PERFORMAPJCE 
immediate advantage in that it prevents fouling 
of the plumbing from the freezing of residual 
fluid at low temperatures. In addition, a large 
concentration of alcohol in the mixture can 
provide part of the additional chemical energy 
required to maintain engine speed. In fact, 
the large concentration of alcohol in the in- 
jection mixture is a preferred means of adding 
additional fuel energy. If the added chemical 
energy is included with the water flow, no 
abrupt changes in governed fuel flow are 
necessary and there is less chance of underspeed 
with fluid injection and overspeed or over- 
temperature when fluid flow is exhausted. Of 
course, strict proportions of the mixture are 
necessary. Since most water injection devices 
are essentially an unmodulated flow, the use 
of this device is limited to high engine speed 
and low altitude to prevent the water flow 
from quenching combustion. 
THE GAS TURBINE-PROPELLER COM- 
BINATION. The turbojet engine utilizes the 
turbine to extract suflicient power to operate 
the compressor. The remaining exhaust gas 
energy is utilized to provide the high exhaust 
gas velocity and jet thrust. The propulsive 
efficiency of the turbojet engine is relatively 
low because thrust is produced by creating a 
large velocity change with a relatively small 
mass flow. The gas turbine-propeller combin- 
ation is capable of producing higher propulsive 
efficiency in subsonic flight by having the pro- 
peller operate on a much greater mass flow. 
The turboprop or propjet powerplant re- 
quires additional turbine stages to continue 
expansion in the turbine section and extract 
a very large percent of the exhaust gas energy 
as shaft power. In this sense, the turboprop 
is primarily a power producing machine and 
the jet thrust is a small amount of the output 
propulsive power. Ordinarily, the jet thrust 
of the turboprop accounts for 15 to 25 percent