The development of the Pratt &
Whitney F100 turbofan began in August of 1968 when the USAF awarded contracts to both P
& W and General Electric for the development of engines to be used in the projected
F-X fighter, which was later to emerge as the F-15 Eagle.
In 1970, Pratt and Whitney was declared
the winner of the competition and was awarded the contract for the engine for the F-15.
The engine was to be designated F100. Two versions of the engine were planned, the F100
for the USAF and the F401 for the Navy. The latter engine was intended for later models of
the F-14 Tomcat, but was cancelled when the size of the planned Tomcat fleet was cut back
in an economy move.
The F100 is an axial-flow turbofan with a
bypass ratio of 0.7:1. There are two shafts, one shaft carrying a three-stage fan driven
by a two-stage turbine, the other shaft carrying the 10-stage main compressor and its
two-stage turbine. For the F100-PW-200 version, normal dry thrust is 12,420 pounds, rising
to a maximum thrust of 14,670 pounds at full military power. Maximum afterburning thrust
is 23,830 pounds.
The F100 engine was first tried in
service with the F-15 Eagle. The Air Force had hoped that the F100 engine would be a
mature and reliable powerplant by the time that the F-16 was ready to enter service.
However, there were a protracted series of teething troubles with the F100 powerplants of
the F-15, compounded by labor problems at two of the major subcontractors. Initially, the
Air Force had grossly underestimated the number of engine powercycles per sortie, since
they had not realized how much the F-15 Eagle's maneuvering capabilities would result in
abrupt changes in throttle setting. This caused unexpectedly high wear and tear on the
engine, resulting in frequent failures of key engine components such as first-stage
turbine blades. Most of these problems could be corrected by more careful maintenance and
closer attention to quality control during manufacturing of engine components.
Nevertheless, by the end of 1979, the Air Force was being forced to accept engineless F-15
airframes until the problems could be cleared up.
However, the most serious problem with
the F100 in the F-15 was with stagnation stalling. Since the compressor blades of a jet
engine are airfoil sections, they can stall if the angle at which the airflow strikes them
exceeds a critical value, cutting off airflow into the combustion chamber which results in
a sudden loss of thrust. Such an event is called a stagnation stall. Stagnation stalls
most often occurred during high angle-of-attack maneuvers, and they usually resulted in
abrupt interruptions of the flow of air through the compressor. This caused the engine
core to lose speed, and the turbine to overheat. If this condition was not quickly
corrected, damage to the turbine could take place or a fire could occur.
Some stagnation stalls were caused by
"hard" afterburner starts, which were mini-explosions that took place inside the
afterburner when it was lit up. These could be caused either by the afterburner failing to
light up when commanded to do so by the pilot or by the afterburner actually going out. In
either case, large amounts of unburnt fuel got sprayed into the aft end of the jetpipe,
which were explosively ignited by the hot gases coming from the engine core. The pressure
wave from the explosion then propagated forward through the duct to the fan, causing the
fan to stall and sometimes even causing the forward compressor stage to stall as well.
These types of stagnation stalls usually occurred at high altitudes and at high Mach
numbers.
Normal recovery technique from stagnation
stalls was for the pilot to shut the engine down and allow it to spool down. A restart
attempt could be made as soon as the turbine temperature dropped to an acceptable level.
When it first flew, the YF-16 seemed to
be almost free of the stagnation stall problems which had bedeviled the F-15. However,
while flying with an early model of the F100 engine, one of the YF-16s did experience a
stagnation stall, although it occurred outside the normal performance envelope of the
aircraft. Three other incidents later occurred, all of them at high angles of attack
during low speed flights at high altitude. The first such incident in a production F-16
occurred with a Belgian aircraft flying near the limits of its performance envelope.
Fortunately, the pilot was able to get his engine restarted and land safely. The F-16 was
fitted with a jet-fuel starter, and from a height of 35,000 feet the pilot would have
enought time to attempt at least three unassisted starts using ram air.
When the F100 engine control system was
originally designed, Pratt & Whitney engineers had allowed for the possibility that
the ingestion of missile exhaust might stall the engine. A "rocket-fire"
facility was designed into the controls to prevent this from happening. When missiles were
fired, an electronic signal was sent to the unified fuel control system which supplied
fuel to the engine core and to the afterburner. This signal commanded the angle of the
variable stator blades in the engine to be altered to avoid a stall, while the fuel flow
to the engine was momentarily reduced and the afterburner exhaust was increased in area to
reduce the magnitude of any pressure pulse in the afterburner. Tests had shown that this
"rocket-fire" facility was not needed for its primary purpose of preventing
missile exhaust stalls, but it turned out to be handy in preventing stagnation stalls.
Engine shaft speed, turbine temperature, and the angle of the compressor stator blades are
continuously monitored by a digital electronic engine control unit which fine-tunes the
engine throughout flight to ensure optimal performance. By monitoring and comparing spool
speeds and fan exhaust temperature, the unit is able to sense that a stagnation stall is
about to occur and send a dummy "rocket-fire" signal to the fuel control system
to initiate the anti-stall measures described above. At the same time, the fuel control
system reduces the afterburner setting to help reduce the pressure within the jetpipe.
The afterburner-induced stalls were
addressed by a different mechanism. In an attempt to prevent pulses from coming forward
through the fan duct, a "proximate splitter" was developed. This is a forward
extension of the internal casing which splits the incoming air from the compressor fan and
passes some of this air into the core and diverts the rest down the fan duct and into the
afterburner. By closing the gap between the front end of this casing and the rear of the
fan to just under half an inch, the designers reduced the size of the path by which
high-pressure pulses from the burner had been reaching the core. Engines fitted with the
proximate splitter were tested in the F-15, but this feature was not introduced on the
F-15 production line, since the loss of a single engine was less hazardous in a
twin-engined aircraft like the Eagle. However, this feature was adopted for the
single-engined F-16.
These engine fixes produced a dramatic
improvement in reliability. Engines fitted to the F-16 fleet (and incorporating the
proximate splitter) had only 0.15 stagnation stalls per 1000 hours of flying time, much
better than the F-15 fleet.
In recent years, the USAF became
interested in acquiring an alternative engine for the F-16, partly in a desire to set up a
competitive process between rival manufacturers in an attempt to keep costs down, as well
as to develop a second source of engines in case one of the suppliers ran into problems.
In search of a source for an alternate engine for the F-16 and for the Navy's F-14 Tomcat,
in 1984 the Department of Defense awarded General Electric a contract to build a small
number of F101 Derivative Fighter Engines (DFE) for flight test. The DFE was based on the
F101 used in the B-1 but incorporated components derived from the F404 engine used in the
F/A-18. The Navy decided to adopt the DFE as a replacement for the Tomcat's TF30 turbofan,
but the USAF announced that they were going to split future engine purchases between Pratt
& Whitney and General Electric. GE was given a contract for full-scale development of
its new engine, which was to be designated F110.
The General Electric F110 is similar in
size to the Pratt & Whitney F100. The F110 has a three-stage fan leading to a
nine-stage compressor, the first three stages of which are variable. The bypass ratio is
0.87 to 1. The annular combustion chamber is designed for smokeless operation, and has 20
dual-cone fuel injectors and swirling-cup vaporizers. The single-stage HP turbine is
designed to cope with inlet temperatures as high as 2500 degrees F (1370 C). Blades are
individually replaceable without rotor disassembly. An uncooled two-stage LP turbine leads
to a fully-modulated afterburner. When afterburning is demanded, fuel is injected into
both the fan and core flows, which mix prior to combustion.
All F110s ordered by the USAF were for
the F-16 fleet, with the F-15 retaining the F100. The choice of engines for the Fighting
Falcon began with the Fiscal Year 1985 Block 30 F-16C/Ds. About 75 percent of the F-16s
purchased from that time on by the USAF were powered by the GE engine, with the remainder
being powered by the P & W engine. However, it is not intended that individual units
operate with F-16s powered by two different engine types, since that would create a spare
parts and logistics nightmare. The choice of engines for the F-16 is made at the Wing
level.
In an attempt to address some of the
reliability problems of its engine, Pratt & Whitney developed the -220 model of its
F100 turbofan. It has the same thrust as the -200, but is much more reliable, having
improvements which radically lowered the number of. unscheduled engine shutdowns. Many
older -200 engines were rebuilt to the -220E standard, becoming directly interchangeable
with new-build -220 engines.
In an attempt to make the F100 more
competitive with the General Electric F110, Pratt & Whitney introduced the more
powerful F100-PW-229 version in the early 1990s. This engine is rated at 29,100 pounds of
thrust with full afterburner. It has a higher fan airflow and pressure ratio, a
higher-airflow compressor with an extra stage, a new float-wall combustor, higher turbine
temperatures, and a redesigned afterburner. It has about 22 percent more thrust than
previous F100 models. The first F-16s powered by the -229 engines began to be delivered in
1992. However, the degree of mechanical changes introduced in the -229 make it impractical
to rebuild -200 or -220E engines to -229 standards.
On the export market, the higher thrust
of the F110 made it the engine of choice through the mid to late 1980s. The more powerful
F100-PW-229 finally gave P&W the chance of re-entering the export market. In 1991,
South Korea chose the F100-PW-229 for its license-built F-16s, maintaining engine
commonality with F-16Cs and Ds that were purchased earlier from the USA.
The F100-PW-200+ is intended for foreign
air forces which operate significant numbers of F-16s that are powered by -200 and -220E
engines, but which are denied access to the more powerful -229. It combines the core of
the -220 with the fan, nozzle, and digital control system of the -229. It develops around
27,000 pounds of thrust with afterburning.
Sources:
- Combat Aircraft F-16, Doug Richardson,
Crescent, 1992.
- General Dynamics Aircraft and their
Predecessors, John Wegg, Naval Institute Press, 1990.
- The American Fighter, Enzo Angelucci and
Peter Bowers, Orion, 1987.
- United States Military Aircraft Since
1909, Gordon Swanborough and Peter M. Bowers, Smithsonian, 1989.
- F-16 Fighting Falcon--A Major Review of
the West's Universal Warplane, Robert F. Dorr, World Airpower Journal, Spring 1991.
- The World's Great Interceptor Aircraft,
Gallery, 1989.
- Modern Military Aircraft--F-16 Viper, Lou
Drendel, Squadron/Signal Publications, 1992.
- Lockheed F-16 Variants, Part 1, World
Airpower Journal, Volume 21, Summer 1995.
- E-mail from Ben Marselis
