The SR-71 Blackbird was one of the most remarkable machines ever built. Designed during the Cold War for long-range reconnaissance, the SR-71 served with both the CIA and the U.S. Air Force and operated at the bounds of technical possibilities.

With a top speed of Mach 3, a service ceiling of 85,000 feet, and an 11,820-foot-per-minute rate of climb, the SR-71 could fly so fast, and so high, that the supersonic jet was capable of simply outrunning surface-to-air missiles.

But operating the SR-71 at such speeds and altitudes required a lot of specialized equipment—which in turn required a lot of investment in research and development, and in some respects, the creation of specialized components that were used only with the SR-71. The fuel source in particular required special attention, on account of how hot the SR-71’s engines and surfaces became while flying at Mach 3. Let’s take a closer look at the demands of fueling the SR-71.

Starting from Scratch

As 19FortyFive reported, “Every last component of the Blackbird was specialized and distinct; the jet needed to do things no other jet could do. As a result, the SR-71 shared roughly zero commonality with any other airframe.” Every single piece of the SR-71 had to be funded, designed, and built from scratch. As you might imagine, the project became very expensive.

The SR-71’s fuel supply is indicative of the jet’s unique nature. Most jets in the U.S. military’s inventory were fine to function on JP-4, or later, JP-8—standardized fuels across the U.S. and NATO platforms. But neither fuel was suitable for the SR-71. Why not? The SR-71 generated temperatures beyond the fuel’s flashpoint, meaning the engine would have reached temperatures that would have caused the fuel to ignite, surely a fatal proposition for all aboard. Instead, the SR-71 would require a unique fuel source, derived from scratch. The answer would be Pratt & Whitney’s JP-7, which was designed with a super-low flashpoint and high thermal stability. Rumor maintains that JP-7’s flashpoint was low enough to permit contact with an open flame. But while JP-7’s low flashpoint was capable of serving the SR-71’s voracious fuel consumption without igniting, the new fuel had a different problem: igniting at all for the sake of engine ignition. Again, specialized equipment would need to be developed to allow the JP-7-fueled SR-71 to achieve engine ignition. The solution was to inject triethyl borane (TEB) into the SR-71’s engines—a complex and expensive procedure that boosted the complexity and the expenses of the overall project. 

Refueling with JP-7

None of the U.S. Air Force’s existing tanker aircraft were capable of holding JP-7. Instead, the KC-135 would need to be modified to accommodate the novel fuel source. The result was the KC-135Q, an entirely new variant of aircraft.

While designing a new aircraft just to permit the SR-71 to refuel in-air may seem indulgent, it was absolutely necessary for SR-71 operations. The Blackbird consumed fuel so steadily (between 36,000 and 44,000 pounds per hour) that without in-air refueling, the SR-71 would have burned through it’s stock of fuel after just 3,000 miles, which would have hardly been sufficient for a long-range reconnaissance aircraft. Rather, in-air refueling, every 90 minutes, was vital to the SR-71’s function.

Harrison Kass is a defense and national security writer with over 1,000 total pieces on issues involving global affairs. An attorney, pilot, guitarist, and minor pro hockey player, Harrison joined the US Air Force as a Pilot Trainee but was medically discharged. Harrison holds a BA from Lake Forest College, a JD from the University of Oregon, and an MA from New York University. Harrison listens to Dokken.

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