Flying at altitudes and speeds unheard of for contemporary jet aircraft, the Blackbird family required a fuel with unique properties to perform in the extremes the aircraft would be subject to.
The Mach 3+ speeds of the Lockheed A-12, M-21, YF-12A, and SR-71 would heat fuel up to 350 degrees, and, lacking insulated fuel tanks, the aircraft required a fuel with a high-flash point that would not vaporize or explode under tremendous heat and pressure. Available jet fuels at the time would not work in this environment, someone would have to create such a fuel, and, in addition to fueling the aircraft, the new fuel also was used as an internal coolant, cooling important components of the aircraft.
A Flying Fuel Tank
With a total of six fuel tanks, the thirsty Lockheed SR-71 had a fuel capacity of over 80,000 pounds of fuel. The tanks occupied areas within the fuselage and wings holding 12,219 gallons total. Each of the SR-71’s two cockpits had large fuel-quantity gauges.
In addition to feeding the twin Pratt & Whitney JP-58 engines, the fuel was used to cool the aircraft as well as components of the aircraft. A temperature control valve insured the hottest fuel was sent to the engines while the coolest was used to maintain a proper temperature for the retracted landing gear, hydraulics, avionics, and other systems, including keeping the cabin at comfortable temperature.
An automated fuel transfer system ensured the fuel was consumed in a manner maintaining the aircraft’s center of gravity (CG) during supersonic flight. Forward fuel transfer was controllable manually by the pilot when returning to subsonic flight. A system of 16 centrifugal, fuel-cooled, AC-powered electrical fuel boost pumps moved the fuel to the engines and through heat exchangers through left and right manifolds.
An engine-driven pump provided 1,800 psi (pounds per square inch) of recirculated fuel to actuate several engine components before being returned to the aircraft’s fuel system and burned. Some of the components were used to control the afterburner nozzles and to control position of the two-position inlet guide vanes.
Liquid nitrogen was carried in three Dewar flasks located in the nose wheel well of the aircraft. All fuel tanks were pressurized with the nitrogen to 1.5 psi above ambient pressure, forcing ambient air to vent. The empty space above the fuel in the tanks would be dominated by the gaseous nitrogen preventing autogenous ignition. The nitrogen also prevented pressure from crushing the empty tanks when the SR-71 descended down to higher air pressure altitudes.
A New Fuel
Developed by chemist Clarence Brown Eichman for use in the Pratt & Whitney J58 engine in 1955 and produced by Shell Oil in accordance with Pratt & Whitney specification PWA-535 and the government’s MIL-DTL-38219 specs, Jet Propellant 7, commonly known as JP-7, was what the J58 consumed. Originally developed for a Navy aircraft, the J58 found a new home in the CIA (Central Intelligence Agency) Lockheed A-12 spy plane and later aircraft of the Lockheed family. Some sources also link the J58 to the XB-70 Valkyrie project.
MIL-DTL-38219 specified a fuel that was non-nauseating and “water-white, clean and bright” at room temperature. A high flash point and high thermal stability would also be required. The fuel would also be used in the engine hydraulic systems and as a heat sink for various components of the aircraft. Additional requirements stated fuel could not break down and deposit coke and varnishes in the fuel system. A high luminometer number (brightness of flame index) was required to minimize heat transfer to burner parts.
JP-7 is unique in it is not a distillate fuel, but a blending of very low concentration of highly volatile components such as benzene or toluene with very small amounts of oxygen, sulfur, or nitrogen impurities. I can be used in a wide range of temperatures, from extreme cold of high altitudes to the high temperatures created in the uninsulated fuel tanks of the Blackbirds during supersonic flight. Composed mainly of hydrocarbons with the addition of fluorocarbons to increase lubricating properties, it also contains a compound known as A-50 which aided in disguising the radar signature of the exhaust plume.
The melting point of the fuel is -22 degrees Fahrenheit with a boiling point of around 550 degrees Fahrenheit. The flashpoint is 140 degrees Fahrenheit, and it is said a lighted match dropped on a spill would not ignite the fuel. Fuel specialists would use a special kit with a Bunsen burner to ensure the proper flash point of JP-7.
In order to for the fuel to be ignited, the Blackbirds carried triethylborane (TEB) in small cylinders mounted atop the J58 engines. TEB, which ignites when coming into contact with air of even very low oxygen content, would be used to ignite both the main engine and afterburner sections. As the TEB exploded, it would in turn ignite the JP-7. Holding a pint and a quarter, the tank contained enough TEB for 16 starts/restarts of the engine or afterburners. TEB produced a telltale green flash when it exploded to start the engines.
In addition to being challenging to ignite, JP-7 would also dissolve linings in hoses and the fuel tanks used on fuel trucks. Special techniques required treatment of these items by flushing multiple times with JP-7 to dissolve the linings and ensure the fuel placed in the Blackbirds was free of contamination.
A Flying Fuel Station
With JP-7 being unique and the Blackbirds requiring in-flight refueling support, a tanker would be required to handle the fuel and refueling operations. Boeing KC-135 Stratotankers were modified with special plumbing between fuel tanks, allowing JP-7 and JP-4 to be moved around among various tanks. This isolated the KC-135’s JP-4 from the JP-7. Almost all aircraft in the United States Air Force inventory flew on JP-4 as well. A total of 56 KC-135’s were modified to the KC-135Q version to handle the JP-7, with some later becoming KC-135Ts.
Crews of the KC-135Qs had special training for radio-silent procedures. A special radio known as the ARC-50 installed on both the tankers and the Blackbirds allowed coded communications with variable power levels for the transmitter. Once set to the same frequency and the correct codes set, the range and bearing of each aircraft would be displayed in the other aircraft. Boom operators had to be certified to refuel the SR-71. Later some McDonnell Douglas KC-10s would also be used for refueling.
Without the support of the tankers, the range of the SR-71 would have been very limited; most missions required at least one refueling, but often multiple instances of refueling. The tankers could even refuel the Blackbirds on the ground if need be using transfer hoses between the two aircraft.
JP-7’s Last Gasp
JP-7 was only used in one other vehicle besides the A-12, M-12, YF-12A, and SR-71 families. That was the experimental Boeing X-51A Waverider equipped with a Pratt & Whitney SJY61 scramjet engine. The X-51A was an unmanned test demonstrator for the United States Air Force, with a total of four being built.
Carried aloft by a Boeing B-52, the Waverider was launched with the solid rocket booster accelerating it to Mach 4.5 before being jettisoned. The scramjet then takes over, burning JP-7 after being ignited by ethylene. The vehicle accelerated to Mach 5.1 during the final flight occurring on May 1, 2013, and traveled over 230 nautical miles.
