KC-135Q
A team of Lockheed personnel, led by Clarence “Kelly” Johnson, then vice president of Lockheed’s Advanced Development Company Projects (commonly known as the “Skunk Works” and now part of Lockheed Martin Corp.), designed the SR-71. The SR-71 was the most sophisticated aircraft in the Blackbird family, which also included the A-12 and YF-12.
What made the SR-71 remarkable was its ability to cruise at Mach 3.2, exceeding three times the speed of sound or more than 2,200 miles per hour, while reaching altitudes up to 85,000 feet.
Without air refueling, the SR-71 would have been limited to approximately 2,000 NM, but it was through multiple refueling operations that the aircraft’s range could be extended to the limits of crew endurance, with many missions surpassing 12,000 NM, and it was the forward basing of both the SR-71 and KC-135Q tankers that allowed for enhanced efficiency through faster response times, shorter range missions, reduced duration, and fewer air refuelings.
What set KC-135Q crews and their aircraft apart from the rest of the Air Force were several unique characteristics: their aircrews were exclusively certified in Blackbird’s specific radio-silent rendezvous procedures, and it was only their boom operators who were qualified to refuel the SR-71.
Why did the Blackbird have to refuel right after takeoff?
The Q-model tankers had special plumbing between their fuel tanks, allowing them to transfer JP-4 and JP-7 fuel between various tanks. Their engine could burn transfer JP-4 or JP-7 fuel. If the SR-71 landed somewhere JP-7 fuel was not available, the Q-model tankers flew in with the fuel and, through the use of transfer hoses on the ground, were able to refuel the SR-71. One of the best advantages of flying the Q-model tankers is that their crews did not have to be on twenty-four-hour alert status like the rest of the SAC’s tankers’ crew members.
No story on the SR-71 would be complete without an understanding and appreciation of just how valuable the KC-135Q model tankers and their crews were to the successful and safe completion of every mission.
Most SR-71’s operational sorties started with a refueling after takeoff.
“Many people believe we refueled after takeoff because the aircraft leaked fuel so profusely that we needed to fuel up quickly,” says Col. Richard H. Graham, a former Blackbird pilot, in his book SR-71 The Complete Illustrated History of THE BLACKBIRD The World’s Highest, Fastest Plane. “We had to refuel right after takeoff for only one reason, and it wasn’t because we leaked JP-7 fuel on the ground. Yes, the plane does leak fuel, but not enough to require refueling after takeoff.
SR-71 Pilot on why the Blackbird had to refuel after takeoff
“The JP-7 fuel reaches temperatures well over 300 degrees F. during Mach 3 cruise, making the fumes in each of the six fuel tanks very volatile and potentially explosive. The metal skin of the aircraft approaches 400 degrees F., adding to the volatility of the fuel inside the tanks. One of our aircraft limitations was a maximum speed of Mach 2.6 without an inert atmosphere inside the fuel tanks.
“The aircraft had three liquid nitrogen Dewar flasks containing 260 liters of liquid nitrogen, located in the nose wheel well. The only way to ensure 100 percent inert atmosphere in each fuel tank was to refuel the plane inflight completely full of JP-7, allowing ambient air in each fuel tank to vent overboard. Once full of fuel, gaseous nitrogen would now dominate each fuel tank’s empty space above as it burned off JP-7. The nitrogen gas pressurized each fuel tank to 1.5 psi above ambient pressure and inerts the space above the heated fuel to prevent autogenous ignition. This is why we refueled after takeoff. Then we could safely accelerate beyond Mach 2.6.”
“Yo-Yo”
Graham concludes:
“There was one other way of achieving tank inerting, called a “Yo-Yo,” but this was a maintenance nightmare. A few of our missions required the SR-71 to accelerate to Mach 3+ right after takeoff with a 65,000-pound fuel load. The Yo-Yo procedure had the crew chief completely refuel the plane to full tanks of 80,000 pounds of fuel. Then, with the nitrogen pressurization system working, they de-fueled 15,000 pounds of JP-7, ending up with a 65,000-pound fuel load and a plane that was capable of going immediately to Mach 3+.”
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Photo by U.S. Air Force
