NAVWEPS 00-30T-30  AIRPLANE PERFORMAPJCE  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.  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 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 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 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 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  of the total thrust output.  Since the turbo-  prop is primarily a power producing machine,  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  131 