An SR-71 Blackbird in flight over California in June 1988. The Blackbird’s astonishingly high speed led to high temperatures that warped its design during flight, leading to a variety of unusual design changes. (US Air Force)

The SR-71 Blackbird Got Bigger in the Air—and It Caused Problems on the Ground

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The SR-71’s design team could not build an aircraft that had sealed fuel tanks on both the ground and the air. It chose the more important of the two.

A little-known fact about the SR-71 Blackbird during its three decades of operation from 1966 to 1999 was its tendency to physically grow larger in the air. At tremendous speeds—often exceeding Mach 3.2, or around 2,300 mph (3,700 km/h)—aerodynamic heating caused the Blackbird’s airframe to expand several inches from front to back.

This property led the SR-71’s designers to make a curious design choice. While the aircraft sat on the ground, its fuel tank was never fully sealed, and the sleek black aircraft—by most metrics the world’s most advanced during its time in service—would visibly leak fuel through gapped seams like a battered jalopy. Yet once the aircraft took off, the expanding frame would tighten its seams and stem the leakage.

That sounds like poor engineering, but it was an intentional and necessary design choice to facilitate the aircraft’s extreme performance envelope.........

© The National Interest