“We had to refuel right after takeoff for only one reason, and it wasn’t because we leaked JP-7 fuel on the ground…” Col. Richard H. Graham, former SR-71 Blackbird pilot
If it weren’t for the SR-71’s ability to refuel in the air, its range would only be about 2,000 NM. The aircraft’s range was increased through numerous air refuelings to the limit of the crew’s endurance. Several missions have exceeded more than 12,000 NM. The SR-71 and KC-135Q tankers’ forward basing enabled them to respond more quickly, conduct missions with shorter ranges and shorter durations, require fewer air refuelings, and operate more efficiently overall.
There were various ways in which KC-135Q crews and their aircraft were different from the rest of the Air Force. Their boom operators were the only ones qualified to refuel the SR-71, and their aircrews were the only ones certified in Blackbird’s special radio-silent rendezvous procedures. The JP-4 and JP-7 fuel could be moved between different tanks thanks to the Q-model tankers’ unique plumbing between fuel tanks. Their engine could burn transfer JP-4 or JP-7 fuel.
If the SR-71 landed somewhere where JP-7 fuel was not accessible, the Q-model tankers flew in with the fuel and were able to refill the SR-71 using transfer hoses on the ground. 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 as the rest of the SAC’s tankers’ crew members.
The importance of the KC-135Q tankers and their crews to the effective and secure completion of every mission cannot be underestimated in any story of the SR-71. Indeed, after takeoff, the majority of SR-71 operational flights began with refueling.
“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.
“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 a 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.”
Graham says in his conclusion:
“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+.”
Photo by U.S. Air Force
