A problem of flying at Mach 3+ was that at the time when the SR-71 was devised no sealant would withstand the extreme heat that an airplane cruising at that speed would make. Hence the Blackbird leaked fuel.
In the 1960’s, the US Air Force (USAF) developed the SR-71 Blackbird, a plane that could travel more than 3 times as fast as the sound produced by its own engines.
Throughout its nearly 24-year career, the SR-71 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, given that the Blackbird became so hot because it cruised at a speed of Mach 3.2 conventional jet fuel could not be used in it. A jet fuel with a high flash point, and high thermal stability was required. To satisfy this requirement Shell produced a special blend of fuel called JP-7 which has a high flashpoint to prevent it from being ignited by the heat of the airframe.
Another problem of flying at Mach 3+ was that at the time when the SR-71 was devised no sealant would withstand the extreme heat that an airplane cruising at that speed would make.
Hence the Blackbird leaked fuel.
Everyone talks about how the SR-71 leaks fuel. They often snicker about it like they’re insinuating that the Blackbird was flawed in a significant way. But, as we have just explained, this is not true.
I asked Master Sgt. Floyd Jones (ret.) who worked on the Blackbird for nearly 20 years (according to Air Zoo, Jones entered the USAF in 1966. Assigned to Beale AFB in 1967, Jones became an SR-71 Crew Chief/Phase Inspector. After international assignments, he returned to Beale in 1980 where he became an SR-71 Inspection Dock Chief. Later, Jones worked with SR-71 flight testing and scheduled an SR-71 aircraft wash at Beale) if the Blackbird continued to leak fuel, even when it was flying. Here’s his answer.
‘During the PDM [Phase Depot Maintenance], over 10,000 man-hours were expended for fuel tank repair. When the SR was turned over to the Air Force the fuel leaks were mostly fixed. But after flying at speed, the leaks started to return.
‘You need to understand what constitutes a fuel tank on the SR-71.
‘It starts with an aircraft structure that is sealed with a polymer sealant on all edges and seems.
‘After flying at speed and altitude, the sealant starts to crack, thus creating a leak. Since we couldn’t find a sealant that could withstand the extreme temperature changes a plus 500 degrees to a minus 30.’
‘So yes, the SR-71 would continue to leak under all conditions. It never stopped even today 30 years after they were put on public display. They are weeping JP-7 fuel.’
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Twitter Page Habubrats SR-71 and Facebook Page Born into the Wilde Blue Yonder for awesome Blackbird’s photos and stories.
Photo credit: U.S. Air Force