For the first few decades of flight,
aircraft engines simply used the same kind of gasoline that powered
automobiles. But simple gasoline was not necessarily the best fuel for the
large, powerful engines used by piston-driven airplanes that were
developed in the 1930s and 1940s.
Before World War II, Major Jimmie
Doolittle realized that if the United States got involved in the war in
Europe, it would require large amounts of aviation fuel with high octane.
Doolittle was already famous in the aviation community as a racing pilot
and for his support of advanced research and development (and would later
earn even wider fame as head of the 1942 B-25 bombing raid on Tokyo). In
the 1930s, he headed the aviation fuels section of the Shell Oil Company.
Fuel is rated according to its level of
octane. High amounts of octane allow a powerful piston engine to burn its
fuel efficiently, a quality called "anti-knock" because the engine does
not misfire, or "knock." At that time, high-octane aviation gas was only a
small percentage of the overall petroleum refined in the United States.
Most gas had no more than an 87 octane rating. Doolittle pushed hard for
the development of 100-octane fuel (commonly called Aviation Gasoline or
AvGas) and convinced Shell to begin manufacturing it, to stockpile the
chemicals necessary to make more, and to modify its refineries to make
mass production of high-octane fuel possible. As a result, when the United
States entered the war in late 1941, it had plenty of high-quality fuel
for its engines, and its aircraft engines performed better than similarly
sized engines in the German Luftwaffe's airplanes. Engine designers were
also encouraged by the existence of high-performance fuels to develop even
higher-performance engines for aircraft.
A major problem with gasoline is that it
has what is known as a low "flashpoint." This is the temperature at which
it produces fumes that can be ignited by an open flame. Gasoline has a
flashpoint of around 30 degrees Fahrenheit (-1 degree Celsius). This makes
fires much more likely in the event of an accident. So engine designers
sought to develop engines that used fuels with higher flashpoints.
The invention of jet engines created
another challenge for engine designers. They did not require a fuel that
vaporized (turned to a gaseous state) as easily as AvGas, but they did
have other requirements. Instead of using gasoline, they chose kerosene or
a kerosene-gasoline mix. The first jet fuel was known as JP-1 (for "Jet
Propellant"), but the U.S. military soon sought fuels with better
qualities. They wanted fuels that did not produce visible smoke and which
were also less likely to produce contrails (the visible trail of condensed
water vapor or ice crystals caused when water condenses in aircraft
exhaust at certain altitudes). But a major requirement was for fuels that
did not ignite at low temperatures in order to reduce the chance of fire.
Certain types of aircraft operations
also demanded that specific types of fuel be available. For instance, the
U.S. Navy had to carry large amounts of fuel for the planes and
helicopters on its aircraft carriers. When most of the aircraft were
piston-driven, they carried AvGas, which had a low flashpoint and was
therefore dangerous to have on board because it could easily catch fire.
The advent of jets led the Navy to seek jet propellant that had a higher
flashpoint than JP-1. Whereas most Air Force aircraft soon used a
kerosene-gasoline mix called JP-4, which already had a higher flashpoint
than standard AvGas, the Navy developed a fuel known as JP-5 with an even
higher flashpoint than JP-4. It also sought to retire aircraft that used
AvGas. Fortunately, the introduction of turbine engines on helicopters and
for propeller-driven airplanes also reduced the Navy's need for AvGas.
Navy leaders are extremely safety-conscious about fuels. When a Navy jet
is refuelled in flight by an Air Force tanker with Air Force fuel, safety
rules prohibit the plane from being stored below deck on the ship when it
Aircraft operators are constantly
refining their fuels to deal with specific performance concerns. The U.S.
Air Force during the 1990s switched from JP-4 to JP-8 because it had a
higher flashpoint and was less carcinogenic, among other things. By the
mid 1990s, the Air Force further modified JP-8 to include a chemical that
reduced the build-up of contaminants in the engines that affected
performance. JP-8 has a strong odour and is oily to the touch, which makes
it more unpleasant to handle and less safe in some ways (military
personnel who work with it complain that it is difficult to wash off and
causes headaches and other physical problems). About 60 billion gallons
(227 billion litres) were used worldwide by the late 1990s, with the U.S.
Air Force, Army, and NATO using about 4.5 billion gallons (17 billion
litres). It is also used to fuel heaters, stoves, tanks, and other
Commercial jet fuel, known as Jet-A, is
pure kerosene and has a flashpoint of 120 degrees Fahrenheit (49 degrees
Celsius). It is a high-quality fuel, however, and if it fails the purity
and other quality tests for use on jet aircraft, it is sold to other
ground-based users with less demanding requirements, like railroad
engines. Commercial jet fuel as well as military jet fuel often includes
anti-freeze to prevent ice build-up inside the fuel tanks.
The development of the A-12 OXCART
spy plane in the late 1950s created another problem for aircraft and engine
designers. The high speeds reached by the A-12 would cause the skin of the
aircraft to get hot. Temperatures on the OXCART ranged from 462 to 1,050
degrees Fahrenheit (239 to 566 degrees C). The wings, where the fuel was
stored, had external temperatures of more than 500 degrees Fahrenheit (260
degrees C). Even with the lower flashpoint, fuel stored in the wings could
explode. As a result, the engine designers at Pratt & Whitney sought a
fuel with an extremely high flashpoint. Working with the Ashland Shell and
Monsanto companies, the engine designers added fluorocarbons to increase
lubricity (or slipperiness), and other chemicals to raise the flashpoint.
The resulting fuel was originally known as PF-1 but later renamed JP-7. It
was used only by the A-12 OXCART (and its sister YF-12 interceptor) and
later the SR-71 Blackbird. JP-7 has such a high flashpoint that a burning
match dropped into a bucket of it will not cause it to ignite.
Engine designers and fuel chemists
created JP-7 with a high flashpoint that would not explode in the
aircraft's tanks, but this also made the fuel hard to ignite within the
engines themselves. Because JP-7 is so hard to ignite, particularly at the
low pressures encountered at high altitudes, these planes used a special
chemical called tri-ethyl borane (TEB), which burns at a high temperature
when it is oxidized (combined with air). Another problem that the A-12
encountered was that the engine exhaust (particularly shock waves created
in the exhaust when the engines were at full afterburner) was easily seen
by radar. The engine designers added an expensive chemical known as A-50,
which contained caesium, to the fuel for operational flights that reduced
its ability to be detected by radar.