certain type of operation. Of course, the 
effect on service life of any particular load 
spectrum must be anticipated. 
One exception to the arbitrary time standard 
for operation at high temperatures or sus- 
tained high powers is the case of the after- 
burner operation. When the cooling flow is 
only that necessary to prevent excessive tem- 
peratures for adjacent structure and equipment, 
sustained operation past a time limit may cause 
damage to these items. 
THRUST AUGMENTATION. Many op- 
erating performance conditions may require 
that additional thrust be provided for short 
periods of time. Any means of augmenting 
the thrust of the turbojet engine must be ac- 
complished without an increase in engine speed 
or maximum turbine section temperature. The 
various forms of afterburning or water injection 
allow the use of additional fuel to provide 
thrust augmentation without increase in engine 
speed or turbine temperature. 
The aftsrbumer is a relatively simple means 
of thrust augmentation and the principal fea- 
tures are light weight and large thrust increase. 
A typical afterburner installation may add only 
10 to 20 percent of the basic engine wei,ght but 
can provide a 40- to 60-percent increase in the 
static sea level thrust. The afterburner con- 
sists of an additional combustion area aft of 
the turbine section with an arrangement of 
fuel nozzles and flameholders. Because the 
local flow velocities in the afterburner are 
quite high, the flameholders are necessary to 
provide the turbulence to maintain combustion 
within the afterburner section. The turbojet 
engine operates with airflows greatly in excess 
of that chemically required to support combus- 
tion of engine fuel. This is necessary because 
of cooling requirements and turbine tempera- 
ture limitations. Since only 15 to 30 percent 
of the engine airflow is used in the combustion 
chamber, the large excess air in the turbine 
discharge can support combustion of large 
amounts of additional fuel. Also, there are 
no highly stressed, rotating members in the 
NAVWEPS OO-EOT-80 
AIRPLANE PERFORMANCE 
afterburner and very high temperatures can be 
tolerated. The combustion of fuel in the after- 
burner brings additional increase in tempera- 
ture and volume and\ adds considerable energy 
to the exhaust. gases producing increased jet 
velocity. The major components of the after- 
burner are illustrated in figure 2.14. 
One necessary feature of the turbojet engine 
equipped with afterburner is a variable nozzle 
area. As the afterburner begins functioning, 
the exit nozzle area must increase to accom- 
modate the increased combustion products. 
If the afterburner were to begin functioning 
without an increase in exit area, the mass flow 
through the engine would drop and the tem- 
peratures would increase rapidly. The nozzle 
area must be controlled to increase as after- 
burner combustion, begins. As a result, the 
engine mass flow is given a large increase in 
jet velocity with the corresponding increase in 
thrust. ., 
The combustion of fuel in the afterburner 
takes place at low pressures and is relatively 
inefficient. This basic inefficiency of the low 
pressure combustion is given evidence by the 
large increase in specific fuel combustion. 
Generally, the use of afterburner at least will 
double the specihtfuel consumption. As an 
example, consider a turbojet engine capable 
of producing 10,000 lbs. of thrust which can 
develop 15,ooO lbs.. of thrust with the use of 
afterburner. Typical values for specific fuel 
consumption would. be c,= 1.05 for the basic 
engine or t,= 2.1 when the afterburner is in 
use. The fuel flow during operation would be 
as follows: 
fuel flow = (thrust) (specific fuel consump- 
tion) 
without afterburner, 
fuel flow=(10,000) (1.05) 
= 10,500 lbs./hr. 
with afterburner, 
fuel flow=(15,COO) (2.1) 
=31,500 lbs./hr. 
The low efficiency of the afterburner is illus- 
trated by the additional 21,CCO lbs./hr. of fuel