The SR-71 Blackbird came home from every mission 3 to 4 inches longer than it took off, and Lockheed built it that way on purpose.
Story Highlights
- At about Mach 3.2, more than three times the speed of sound, air friction heated the jet’s titanium skin to about 620 degrees Fahrenheit.
- That heat stretched the 107-foot jet 3 to 4 inches, and it shrank back as it cooled after landing.
- Lockheed fitted the panels loose on purpose, so the heat would press them tight in flight.
- That is why a cold Blackbird leaked fuel on the ramp until the joints sealed after takeoff.
The Blackbird first flew on Tuesday, Dec. 22, 1964. Lockheed’s secret Skunk Works shop, led by engineer Kelly Johnson, built it to fly higher and faster than any enemy missile could reach. Holding that speed for hours at a time is exactly what made the plane grow.
Built to Fly Hot
At cruise, the Blackbird flew near 80,000 feet, where the outside air sat around 70 degrees below zero. Even so, air rushed across the skin so fast that friction heated the metal to about 620 degrees on average. The nose edges and engine inlets ran hotter still, near 1,000 degrees at the hottest spots.
Aluminum, the usual metal for airplanes, would have gone soft at that heat. So Lockheed built the jet mostly from titanium, which made up between 85 and more than 90 percent of its structure. Titanium keeps its strength when it is hot, but like any metal, it grows as it heats up.
Loose on the Ground, Tight in the Sky
A normal airframe would have warped, buckled or torn as it grew. Johnson’s team planned for the growth instead of fighting it. They left small gaps between panels and used slip joints, which let one piece of metal slide against the next as it expands. Parked on the ramp, a cold Blackbird looked rough, with gaps you could see.
The fuel tanks were built into the body of the plane and shared those same joints. So on the ground the jet dripped JP-7, a special fuel made for it, onto the concrete, and crews set out drip pans as a matter of routine. The leaks were not a defect. Once the jet was flying fast and hot, the metal swelled and pressed the joints shut.
The wings had shallow ridges running front to back, a little like a washboard. They let the skin grow and shrink like an accordion without bending out of shape.
Fuel That Doubled as a Coolant
The fuel had a second job. About 12,000 gallons of JP-7 flowed past the hottest parts of the plane, soaking up heat from the frame, the engine oil, the hydraulics and the cockpit before it reached the engines. The black paint helped too. It was chosen to shed heat into the sky, and it lowered the skin temperature by dozens of degrees.
Most missions began with a partial fuel load, and the jet topped off from a tanker plane soon after takeoff. After landing, the Blackbird was too hot for ground crews to handle until it cooled and shrank back to size.
Cold War Ingenuity
The Air Force retired the Blackbird in 1990, brought a few back in 1995 and retired them for good in 1998. Over more than three decades of flying, thousands of missiles were fired at it, and none ever hit. Its secret was simple to say and hard to build: fly so high and so fast that nothing could catch it, and design the jet to grow into the job.
Sources:
migflug.com, grc.nasa.gov, 19fortyfive.com
