From Chapter 1 of Elememts of Propulsion: Gas Turbines and Rockets
, Jack D. Mattingly, 2006, AIAA
Introduction to Air Breathing Engines
Each engine type will operate only within a certain range of
altitudes and Mach numbers (velocities). Similar limitations in
velocity and altitude exist for airframes. It is necessary, therefore,
to match airframe and propulsion system capabilities. Figure 1
shows the approximate velocity and altitude limits, or corridor
of flight, within which airlift vehicles can operate. The corridor
is bounded by a lift limit, a temperature limit, and an aerodynamic
force limit. The lift limit is determined by the maximum level-flight
altitude at a given velocity. The temperature limit is set by
the structural thermal limits of the material used in construction
of the aircraft. At any given altitude, the maximum velocity attained
is temperature-limited by aerodynamic heating effects. At lower
altitudes, velocity is limited by aerodynamic force loads rather
than by temperature.
The operating regions of all aircraft lie within the flight
corridor. The operating region of a particular aircraft within
the corridor is determined by aircraft design, but it is a very
small portion of the overall corridor. Superimposed on the flight
corridor in Fig. 1 are the operational envelopes of various powered
aircraft. The operational limits of each propulsion system are
determined by limitations of the components of the propulsion
system and are shown in Fig. 2.
Figure 1 - Flight limits
Figure 2 - Engine operational limits
The turbojet, turbofan, turboprop, turboshaft, and ramjet engine
systems will be discussed in this section. The discussion of these
engines will be in the context of providing thrust for aircraft.
The listed engines are not all the engine types (reciprocating,
rockets, combination types, etc.,) that are used in providing
propulsive thrust to aircraft, nor are they used exclusively on
aircraft. The thrust of the turbojet and ramjet results from the
action of a fluid jet leaving the engine, hence, the name "jet
engine" is often applied to these engines. The turbofan,
turboprop, and turboshaft engines are adaptations of the turbojet
to supply thrust or power through the use of fans, propellers,
The "heart" of a gas turbine-type engine is the gas
generator. A schematic diagram of a gas generator is shown in
Fig. 3. The compressor, combustor, and turbine are the major components
of the gas generator which is common to the turbojet, turbofan,
turboprop, and turboshaft engines. The purpose of a gas generator
is to supply high temperature and pressure gas.
Figure 3 - Gas Generator
By adding an inlet and a nozzle to the gas generator, a turbojet
engine can be constructed. A schematic diagram of a simple turbojet
is shown in Fig. 4a and a turbojet with afterburner in Fig. 4b.
In the analysis of a turbojet engine, the major components are
treated as sections. Also shown in Figs. 4a and 4b are the station
numbers for each section.
The turbojet was first used as a means of aircraft propulsion
by von Ohain (first flight August 27, 1939) and Whittle (first
flight May 15, 1941). As development proceeded, the turbojet engine
became more efficient and replaced some of the piston engines.
A cross-section of the J79 turbojet with afterburner used in the
F-4 Phantom II and B-58 Hustler is shown in Fig. 5. The adaptions
of the turbojet in the form of turbofan, turboprop, and turboshaft
engines came with the need for more thrust at relatively low speeds.
Some characteristics of different turbojet, turbofan, turboprop,
and turboshaft engines are included in Tables
I, II, III, and IV.
Figure 4a - Schematic diagram of a turbojet
Figure 4b - Schematic diagram of a turbojet with afterburner
Figure 5 - General Electric J79 turbojet with afterburner
The thrust of a turbojet is developed by compressing air in
the inlet and compressor, mixing the air with fuel and burning
in the combustor, and expanding the gas stream through the turbine
and nozzle. The expansion of gas through the turbine supplies
the power to turn the compressor. The net thrust delivered by
the engine is the result of converting internal energy to kinetic
The pressure, temperature, and velocity variations through
a J-79 engine are shown in Fig. 6. In the compressor section,
the pressure and temperature increase as a result of work being
done on the air. The temperature of the gas is further increased
by burning in the combustor. In the turbine section, energy is
being removed from the gas stream and converted to shaft power
to turn the compressor. The energy is removed by an expansion
process which results in a decrease of temperature and pressure.
In the nozzle, the gas stream is further expanded to produce a
high exit kinetic energy. All the sections of the engine must
operate in such a way to efficiently produce the greatest amount
of thrust for a minimum of weight.
Figure 6 - Property variations through the General Electric
J79 afterburning turbojet engine
The turbofan engine consists of an inlet, fan, gas generator,
and nozzle. A schematic diagram of a turbofan is shown in Fig.
7. In the turbofan, a portion of the turbine work is used to supply
power to the fan. Generally the turbofan engine is more economical
and efficient than the turbojet engine in a limited realm of flight.
The thrust specific fuel consumption (TSFC, fuel mass flow rate
per unit thrust) is lower for turbofans and indicates a more economical
operation. The turbofan also accelerates a large mass of air to
a lower velocity than a turbojet for a higher propulsive efficiency.
The frontal area of a turbofan is quite large compared to a turbojet
and for this reason more drag and weight results. The fan diameter
is also limited aerodynamically when compressibility effects occur.
Several of the current high bypass turbofan engines used in subsonic
aircraft are shown in Figs. 8a through 8f.
Figure 7 - Schematic diagram of a high-bypass-ratio turbofan
Figure 8a - Pratt & Whitney JT9D turbofan
Figure 8b - Pratt & Whitney PW4000 turbofan
Figure 8c - General Electric CF6 turbofan
Figure 8d - Rolls-Royce RB-211-524G/H turbofan
Figure 8e - General Electric GE90 turbofan
Figure 8f - SNECMA CFM56 turbofan
The Afterburning Turbofan Engine
The afterburning turbofan engine is shown in Figure 9a. In
this engine, the bypass stream is mixed with the core stream before
passing through a common afterburner and exhaust nozzle. Figures
9b and 9c show the Pratt & Whitney F100 turbofan and the General
Electric F110 turbofan, respectively. These high performance afterburning
turbofan engines are used in both the F15 Eagle and F16 Falcon
supersonic fighter aircraft.
Figure 9a - Station numbering for mixed-flow, afterburning
Figure 9b - Pratt & Whitney F100-PW-229 afterburning turbofan
Figure 9c - General Electric F110-GE-129 afterburning turbofan
The Turboprop and Turboshaft
A gas generator that drives a propeller is a turboprop engine.
The expansion of gas through the turbine supplies the energy required
to turn the propeller. A schematic diagram of the turboprop is
shown in Fig. 10a. The turboshaft engine is similar to the turboprop
except that the power is supplied to a shaft rather than a propeller.
The turboshaft engine is used quite extensively for supplying
power for helicopters. The turboprop engine may find application
in VTOL (vertical takeoff and landing) transporters. The limitations
and advantages of the turboprop are those of the propeller. For
low speed flight and short field takeoff, the propeller has a
performance advantage. At speeds approaching the speed of sound,
compressibility effects set in and the propeller loses its aerodynamic
efficiency. Due to the rotation of the propeller, the propeller
tip will approach the speed of sound before the vehicle approaches
the speed of sound. This compressibility effect when approaching
the speed of sound limits the design of helicopter rotors and
propellers. At high subsonic speeds, the turbofan engine will
have a better aerodynamic performance than the turboprop since
the turbofan is essentially a "ducted turboprop." Putting
a duct or shroud around a propeller increases its aerodynamic
performance. Examples of a turboshaft engine are the Pratt &
Whitney of Canada PT6 (Fig. 10c) used in many small commuter aircraft,
and the Allison T56 (Fig.10b) used to power the C-130 Hercules
and the P-3 Orion.
Figure 10a - Schematic diagram of a turboprop
Figure 10b - Allison T56 turboshaft
Figure 10c - Pratt & Whitney of Canada PT6 turboshaft
The ramjet engine consists of an inlet, a combustion zone,
and a nozzle. A schematic diagram of a ramjet is shown in Fig.
11. The ramjet does not have the compressor and turbine as the
turbojet does. Air enters the inlet where it is compressed and
then enters the combustion zone where it is mixed with the fuel
and burned. The hot gases are then expelled through the nozzle
developing thrust. The operation of the ramjet depends upon the
inlet to decelerate the incoming air to raise the pressure in
the combustion zone. The pressure rise makes it possible for the
ramjet to operate. The higher the velocity of the incoming air,
the more the pressure rise. It is for this reason that the ramjet
operates best at high supersonic velocities. At subsonic velocities,
the ramjet is inefficient and, in order to start the ramjet, air
at a relatively high velocity must enter the inlet.
Figure 11 - Schematic diagram of a ramjet
The combustion process in an ordinary ramjet takes place at
low subsonic velocities. At high supersonic flight velocities,
a very large pressure rise is developed that is more than sufficient
to support operation of the ramjet. Also, if the inlet has to
decelerate a high supersonic velocity air stream to a subsonic
velocity, large pressure losses can result. The deceleration process
also produces a temperature rise and, at some limiting flight
speed, the temperature will approach the limit set by the wall
materials and cooling methods. Thus when the temperature increase
due to deceleration reaches the limit, it may not be possible
to burn fuel in the air stream.
In the past few years, research and
development have been done on a ramjet that has the combustion
process taking place at supersonic velocities. By using a supersonic
combustion process, the temperature rise and pressure loss due
to deceleration in the inlet can be reduced. This ramjet with
supersonic combustion is known as the SCRAMJET (supersonic
combustion ramjet). Figure 12a shows the schematic of a scramjet
engine similar to that proposed for the National AeroSpace Plane
(NASP) research vehicle, the X-30 shown in Fig. 12b. Further development
of the scramjet for other applications (e.g., the Orient Express)
will continue if this research and development produces a scramjet
engine with sufficient performance gains. It must be remembered
that since it takes a relative velocity to start the ramjet or
scramjet, another engine system is required to accelerate aircraft
like the X-30 to ramjet velocities.
Figure 12a - Schematic diagram of a scramjet
Figure 12b - Conceptual drawing of the X-30
Turbojet/Ramjet Combined Cycle
Two of the Pratt & Whitney J58 Turbojet engines (see Fig.
13a) are used to power the Lockheed SR71 Blackbird (see Fig. 13b).
This was the fastest aircraft (Mach 3+) when retired in 1989.
The J58 operates as an afterburning turbojet engine until it reaches
high Mach when the six large tubes (Fig. 13a) bypass flow to the
afterburner. When these tubes are in use, the compressor, burner,
and turbine of the turbojet are essentially bypassed and the engine
operates as a ramjet with the afterburner acting as the ramjet's
Figure 13a - Pratt & Whitney J58 turbojet
Figure 13b - Lockheed SR-71 Blackbird
Aircraft Engine Performance Parameters
The two most useful performance parameters for airbreathing engines in aircraft propulsion are the thrust of the engine (F) and the trust specific fuel consumption (S). Values of thrust
(F) and fuel consumption (S) for various jet engines
at sea level static conditions are listed in Tables
I, II, III, and IV. The predicted variations of uninstalled
engine thrust (F) and uninstalled thrust specific fuel
consumption (S) with Mach number and altitude for an advanced
fighter engine are plotted in Figs. 14a through 14d. Note that
the thrust (F) decreases with altitude and the fuel consumption
(S) also decreases with altitude until 36k ft (the start
of the isothermal layer of the atmosphere). Also note that the
fuel consumption increases with Mach number and the thrust varies
considerably with Mach number. The predicted partial throttle
performance of the advanced fighter engine is shown at three flight
conditions in Fig. 14e.
Figure 14a Uninstalled Thrust at Maximum Power
Figure 14b Uninstalled Thrust Specific Fuel Consumption at
Figure 14c Uninstalled Thrust at Military Power
Figure 14d Uninstalled Thrust Specific Fuel Consumption at
Figure 14e Uninstalled Partial Power Performance
The takeoff thrust of the JT9D high bypass turbofan engine
is given in Figure 15a versus Mach number and ambient air temperature
for two versions. Note the rapid fall-off of thrust with rising
Mach number that is characteristic of this engine cycle and the
constant thrust at a Mach number for temperatures of 86F and below
(this is often referred to as a flat rating). The partial throttle
performance of both engine versions is given in Figure 15b for
two combinations of altitude and Mach number.
Figure 15a JT9D-70/-70A Takeoff Thrust at Sea Level
Figure 15b JT9D-70/-70A Cruise Performance