Home » A smart valve on the SR-71 Blackbird allowed to use JP-7 fuel as internal coolant

A smart valve on the SR-71 Blackbird allowed to use JP-7 fuel as internal coolant

by Till Daisd
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Only the hottest fuel was transferred to the engines by the SR-71 Blackbird smart valve, while cooler fuel was sent to the avionics and to retract landing gear

The Lockheed A-12 and YF-12A aircraft were the basis for the long-range, Mach 3+ strategic reconnaissance aircraft known as the SR-71, or “Blackbird,” as it was informally called. The 4200th (later 9th) Strategic Reconnaissance Wing at Beale Air Force Base, California, received the first SR-71 to begin service in January 1966, while the first SR-71 flight occurred on December 22, 1964.

The SR-71 Blackbird spy plane has been the fastest and highest-flying operational aircraft in the world for about 24 years. At a speed of Mach 3+, it could cover 100,000 square miles of Earth’s surface per hour while hovering above 80,000 feet. And if an enemy tried to shoot it down with a missile, all the Blackbird had to do was speed up and outrun it.


Everything that had come before the Blackbird was not comparable. “Everything had to be invented. Everything,” in an interesting article that appeared on the Lockheed Martin website, Kelly Johnson, the founder and famed aircraft designer of Skunk Works, recalled.

In the book Skunk Works: A Personal Memoir of My Years of Lockheed, Ben Rich, the second Director of Lockheed’s Skunk Works from 1975 to 1991 (who succeeded Johnson), explains that the design team led by him and Johnson looked to Shell Gas to develop a fuel with high thermal stability so that it would not break down and deposit coke and varnish in the fuel system passages.

Rich says, “We needed the fuel to remain stable at enormous temperature ranges. The JP-7 (as it was later known), was at -60° when a KC-135 tanker pumped fuel into the SR-71. The fuel acted as an internal coolant. All the heat built up inside the aircraft was transferred to the fuel by heat exchanges. They designed a smart valve that could sense temperature changes. It supplied only the hottest fuel to the engines. The smart valve sent cooler fuel to the retracted landing gear and avionics.”

The cockpit was cooled by the fuel, as already explained. The air conditioning bled off the engine compressor, dumped it through a fuel cooler, and then through an expansion turbo into the cabin at a temperature of -40°.

Refueling made Butch Sheffield, the SR-71’s eight-year RSO, happy since it allowed the cockpit to chill down. As fuel ran low, it would get more and more uncomfortable in the cockpit.

Check out  Habubrats SR-71 and  Born into the Wilde Blue Yonder Facebook pages for further Blackbird photos and stories.

Photo by U.S. Air Force

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