Why the Air Force Had to Invent a New Type of Fuel for the SR-71 Blackbird – Jalopnik

The Lockheed SR-71 Blackbird is unlike virtually any aircraft that came before it, and virtually no aircraft have been made since. Although development of this remarkable machine began in the 1950s and was eventually retired in 1998, it has yet to be officially replaced by anything approaching Mach’s claimed maximum speed of 3.23 or 3,293.2 mph. That’s more than three times the speed of sound, and at those mind-boggling speeds, physics doesn’t work in the plane’s favor. In fact, the SR-71 had no dedicated fuel tanks, as separate metal tanks would add too much weight and lighter plastic tanks would melt due to the heat generated by aerodynamic friction. Therefore, the aircraft used a so-called ‘total wet wing’ fuel tank system, which meant that the fuel was contained within the skin of the aircraft itself.

Some parts of the SR-71’s exterior reach temperatures of more than 1,100 degrees Fahrenheit, partly due to that aerodynamic friction, which would boil other standardized fuels on the U.S. and NATO platforms. The SR-71 therefore required an entirely new, specially designed fuel mix to fly safely. Enter JP-7.

Every jet before the SR-71 was powered by mixtures called JP-4 or later JP-8, but both fuels would ignite and explode when heated to the extreme temperatures the SR-71 reached during supersonic flight. According to nationalinterest.orgShell developed JP-7 fuel specifically for the SR-71, with a boiling point at standard atmospheric pressure between approximately 540 and 550 degrees Fahrenheit and a flash point of 140 degrees Fahrenheit according to thesr71blackbird.com. Even those exceptionally high limits would have been exceeded on the SR-71, so the fuel had to be pressurized with nitrogen gas to keep the fuel stable. The fuel was so stable that it was actually also pumped through the aircraft as a liquid coolant to keep vital components working as intended.

Naturally, the Blackbird’s Pratt & Whitney J58 jet engines used absurd amounts of fuel to reach its absurd speeds—about 59% of the total weight of a fully loaded SR-71 was fuel. Even then it had a short maximum range that dropped even further when flying in high temperatures, but on average the aircraft had a range of about 5,200 miles, or enough to fly a one-way trip from New York to London. It relied heavily on aerial refueling, which posed more challenges due to the JP-7’s unique characteristics. Specialized KC-135Q tankers were actually adapted to handle pressurized refueling of the JP-7 and SR-71 air tankers, so they had to be present whenever a Blackbird would fly.

Greg Mathieson/mai/Getty

The SR-71 remains a marvel of American aviation engineering, and to be honest, I still have a poster of it on the wall in my childhood room. Lockheed Martin is reportedly developing a supersonic successor to the SR-71, called the SR-72, which is said to be an autonomous craft capable of traveling at speeds in excess of Mach 6, or twice the speed of the Blackbird, although details are obviously limited.