The SR-71 Blackbird used a special blend of fuel called JP7 which has a high flashpoint to prevent it from being ignited by the heat of the airframe.
Throughout its nearly 24-year career, the SR-71 Blackbird spy plane remained the world’s fastest and highest-flying operational aircraft. Flying at Mach 3+ from 80,000 feet, it could survey 100,000 square miles of Earth’s surface per hour. And in the off chance an enemy tried to shoot it down with a missile, all the Blackbird had to do was speed up and outrun it.
Its engineering was so cutting edge that even the tools to build the SR-71 needed to be designed from scratch.
In fact, the SR-71 fuel tanks were unique in the sense it was the only aircraft that was built to allow fuel to deliberately leak out of them.
The problem with the SR-71 is at its cruise speed of Mach 3.2 the aircraft gets really hot. So hot that conventional jet fuel could not be used in it. They use a special blend of fuel called JP7 which has a high flashpoint to prevent it from being ignited by the heat of the airframe.
The issue of heat of the fuel was a solved problem but the heat of flight on where it was stored was not. Bladders wouldn’t work because the temperature of Mach 3+ flight would melt them.
They originally tried to seal the fuel tanks of the SR-71 with a variety of sealants like they would with any conventional “wet” wing but found the sealants wouldn’t hold under the high temperatures and the expansion/contraction cycles the airframe would go through during a mission. Gaps had to be built into the entire structure of the SR-71 to accommodate this. So the fuel tanks would leak despite their best efforts to prevent it.
Since the high flashpoint of the JP7 fuel didn’t make it dangerous to handle on the ground and pans under the aircraft were cheap and since sealants failed due to the extreme temperatures the skin attained, so the tanks sealed when the skin expanded, after a while it was decided to let the fuel tanks leak. In flight, the SR-71 would heat up, the airframe would stretch and the tanks would self-seal. When the SR-71 came back down to tank or return home, the airframe would cool and the fuel would leak out again.
As explained by Matt P., an aviation expert, on Quora an interesting side note is you can tell how long an SR-71 has been in a museum by the wet streaks on the underside and the number of pans underneath it to catch any drops of fuel. Even with extensive cleaning and preparation, it is impossible to get every drop of fuel out from every crevice. So newly donated SR-71s, like the one at Udvar-Hazy, leak fuel like a sieve but older examples that have been in a collection for 20 years are bone dry.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Facebook Page Habubrats for awesome Blackbird’s photos and stories.
Photo credit: U.S. Air Force