The Lockheed SR-71 Blackbird is unlike virtually any plane that came before it, and virtually no planes like it have been made since. Even though the remarkable machine’s development began in the 1950s and it was finally retired in 1998, it still has yet to be officially replaced by anything that can approach its claimed Mach 3.23 or 2,193.2 mph maximum velocity. That’s over three times the speed of sound, and at those mind-bending velocities, physics are not working in the plane’s favor. In fact, the SR-71 didn’t have dedicated fuel tanks since separate metal tanks would add too much weight and lighter plastic ones would melt due to the heat generated from aerodynamic friction, so the plane used what’s called a total wet wing fuel tank system which meant the fuel was contained by the skin of the plane itself.
Some parts of the SR-71’s exterior reach temperatures in excess of 1,100-degrees Fahrenheit due in part to that aerodynamic friction, which would boil other standardized fuels across the U.S. and NATO platforms, so the SR-71 required a totally new, specially engineered mix of fuel to fly safely. Enter JP-7.
JP-7 fuel had a super low flashpoint and high thermal stability
Every jet prior to the SR-71 was fueled by mixtures called JP-4 or later JP-8, but both of those fuels would ignite and explode when heated to the extreme temperatures the SR-71 reached in supersonic flight. According to nationalinterest.org, Shell developed JP-7 fuel specifically for the SR-71, with a boiling point at standard atmospheric pressure between about 540 and 550 degrees Fahrenheit and a flashpoint of 140 degrees Fahrenheit according to thesr71blackbird.com. Even those exceptionally high limits would’ve been exceeded in 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 around the plane as a liquid coolant to keep vital components operating as intended.
Naturally, the Blackbird’s Pratt & Whitney J58 jet engines used absurd amounts of fuel to achieve its absurd speeds – about 59% of the total weight of a fully laden SR-71 was fuel. Even then it had a short maximum range that dropped even further when it operated in high temperatures, but on average the plane had a range of about 3,200 miles, or about enough to fly one-way from New York to London. It relied heavily on aerial refueling, which posed more challenges because of the unique characteristics of JP-7. Specialized KC-135Q tankers were actually modified to handle pressurized JP-7 and SR-71 aerial refueling, so they needed to be present whenever a Blackbird was going to fly.
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The SR-71 remains a marvel of American aeronautical engineering, and full disclosure, I still have a poster of one on my childhood bedroom wall. Lockheed Martin is reportedly developing a supersonic successor to the SR-71 called the SR-72 that is rumored to be an autonomous craft capable of traveling at speeds in excess of Mach 6, or twice the speed of the Blackbird, though naturally details are slim.
