NAVWEPS OO-EOT-80  AIRPLANE PERFORMANCE  certain type of operation. Of course, the  effect on service life of any particular load  spectrum must be anticipated.  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 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.  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.  .,  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 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:  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  The low efficiency of the afterburner is illus-  trated by the additional 21,CCO  lbs./hr. of fuel  flow to create the additional 5,ooO lbs. of  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.  129 