SR-71 Blackbird: unique fuel and a new tanker for a special aircraft

Developers at Lockheed Corporation sought to create a machine that could perform tasks that no other aircraft could. But this required a unique energy source with an ultra-low ignition temperature and high thermal stability.

None of the existing U.S. Air Force tanker aircraft could operate on JP-7 fuel. Therefore, the KC-135 tanker had to be modified to accommodate the new fuel source. The result was the KC-135Q model — a completely new version of the tanker aircraft.

Focus translated the article by journalist Harrison Cass about the SR-71 strategic reconnaissance aircraft.

The SR-71 Blackbird is one of the most iconic aircraft in history. Developed during the Cold War for long-range reconnaissance, the SR-71 served both the CIA and the US Air Force and was pushed to the limits of its capabilities.

The SR-71 had a top speed of Mach 3, a ceiling of 82,000 feet, and a rate of climb of 12,000 feet per minute. The supersonic SR-71 could fly so fast and so high that it could easily outrun a surface-to-air missile.

But operating the SR-71 at such speeds and altitudes required a lot of specialized equipment, which in turn required a large investment in research and development, and in some cases, the creation of specialized components that were used only in the SR-71. In particular, the fuel source required special attention, since the SR-71's engines and surfaces became very hot during flight at Mach 3. Consider the fuel requirements for the SR-71.

Starting from scratch

As 19FortyFive reports, “every component of the Blackbird was specialized and unique; the plane had to do something no other plane could do. As a result, the SR-71 had virtually nothing in common with any other plane.” Every part of the SR-71 had to be financed, designed, and built from scratch. As you can imagine, the project was very expensive.

The SR-71's fuel system also demonstrates the aircraft's uniqueness. Most jets in service with the U.S. military could run on JP-4 fuel, and later JP-8, standardized for U.S. and NATO platforms. But neither of these fuels was suitable for the SR-71. Why? The SR-71 generated temperatures that exceeded the ignition temperature of the fuel. This meant that its engine could heat up to a temperature that would ignite the fuel, which would undoubtedly be fatal to everyone on board. Instead, the SR-71 needed a unique power source, designed from the ground up. The answer was Pratt & Whitney's JP-7 fuel, designed with an ultra-low ignition temperature and high thermal stability. Rumor has it that JP-7's ignition temperature was low enough to allow contact with an open flame. But while the low ignition temperature of JP-7 made it possible to safely power the voracious SR-71, the new fuel had another problem: engine ignition. And in this case, it was necessary to develop special equipment that would allow the SR-71, fueled with JP-7, to achieve engine ignition. The solution was to inject triethylborane (TEB) into the SR-71 engines – a complex and expensive procedure that increased the complexity and cost of the entire project.

Refueling with JP-7

None of the existing tanker aircraft of the US Air Force could operate with JP-7 fuel. Therefore, the KC-135 tanker had to be modified to adapt to the new fuel source. The result was the KC-135Q model – a completely new version of the tanker aircraft.

While it may seem excessive to develop a new aircraft just to allow the SR-71 to refuel in the air, it was absolutely necessary for the SR-71 to operate. The Blackbird consumed fuel so steadily (16.3 to 19.9 tons per hour) that without aerial refueling, the SR-71 would have exhausted its fuel supply in only 4,800 km, which would hardly be enough for a long-range reconnaissance aircraft. On the contrary, aerial refueling every 90 minutes was vital to the SR-71's operation.

About the author

Harrison Cass — a defense and national security journalist with over 1,000 articles to his name on international affairs. A lawyer, pilot, guitarist, and professional hockey player, Harrison served as a trainee pilot in the United States Air Force but was discharged due to ill health. He earned a bachelor's degree from Lake Forest College, a juris doctorate from the University of Oregon, and a master's degree from New York University. Harrison listens to the band Dokken.