Pooling under the SR-71 Blackbird it was assumed that it had a mechanical flaw. No way, this was planned!
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.
The high temperatures generated in flight required special design and operating techniques. Major sections of the skin of the inboard wings were corrugated, not smooth. Aerodynamicists initially opposed the concept, disparagingly referring to the aircraft as a Mach 3 variant of the 1920s-era Ford Trimotor, which was known for its corrugated aluminum skin. The heat would have caused a smooth skin to split or curl, whereas the corrugated skin could expand vertically and horizontally and had increased longitudinal strength.
Fuselage panels were manufactured to fit only loosely with the aircraft on the ground. Proper alignment was achieved as the airframe heated up and expanded several inches. Because of this, and the lack of a fuel-sealing system that could handle the airframe’s expansion at extreme temperatures, the aircraft leaked fuel on the ground prior to takeoff.
The SR-71 used a vintage military specification called MIL-T 38219, or Jet Propellant 7 (JP7 to its friends). Pooling under the Blackbird it was assumed that it had a mechanical flaw. No way, this was planned! The aircraft did not have a traditional fuel bladder. JP7 would dissolve the tank liners that were available and at that time. The six main fuel tanks formed its exterior skin with room for expansion.
When the Blackbird took to the sky, its airframe started leaking, leaving a trail of jet fuel on the tarmac. While many were concerned that this would render the plane useless, the Blackbird was designed to eject its specialized fuel. When developing the aircraft, engineers accounted for the temperature fluctuations that it would experience. More specifically, they realized that the plane’s components would heat up as the aircraft gained more speed.
This friction would inevitably prompt the plane’s body to expand, so they had to fit some pieces together loosely as mentioned above. The fuselage panels, in particular, were purposely positioned farther apart. These loose-fitting parts are what produced the leakage, but it was a necessary design quality to prevent overexpansion. As the SR-71 reached 2,200 miles per hour, the panels enlarged as expected. Fortunately, there was enough room between them to allow for this inflation. Though the SR-71 heated up while it took flight, it cooled down while landing. As a result, the panels contracted, causing them to return to their original state.
The SR-71 was functioning as planned.. It was not malfunctioning when you would see the leaking.
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