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The KC-135Q Stratotanker
The KC-135Q was a variant of the iconic KC-135 Stratotanker modified to carry JP-7 fuel necessary for the Lockheed SR-71 Blackbird by separating the JP-7 from the KC-135’s own fuel supply (the body tanks carrying JP-7, and the wing tanks carrying JP-4 or JP-8). The tanker also had special fuel systems for moving the different fuels between different tanks. When the KC-135Q model received the CFM56 engines, it was redesignated the KC-135T model, which was capable of separating the main body tanks from the wing tanks where the KC-135 draws its engine fuel.
KC-135Q crews and their aircraft were unique from the rest of the Air Force in several ways: the KC-135Q’s crews were in fact the only one certified in SR-71’s specific radio-silent rendezvous procedures, and their boom operators were the only ones qualified to refuel the Blackbird.
SR-71 air refueling
The SR-71 would be a very short-ranged aircraft were it not for air refueling, limited to around 2,000 NM. Multiple air refuelings extended the range of the aircraft to the limits of crew endurance. Many missions have exceeded 12,000 NM. Forward basing of the SR-71 and KC-135Q tankers permitted faster response, shorter range, shorter duration missions, fewer air refuelings, and greater overall efficiency.
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.
The SR-71 had to refuel after takeoff
Col. Richard H. Graham, a former SR-71 pilot, explains why the Blackbird had to refuel right after takeoff in his book SR-71 The Complete Illustrated History of THE BLACKBIRD The World’s Highest, Fastest Plane.
“Many people believe we refueled after takeoff because the aircraft leaked fuel so profusely that we needed to fuel up quickly. 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.
The Yo-Yo procedure
“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+.”
Refueling the SR-71 took about 15 minutes or more but sometimes it seems like an eternity. Being assigned to the KC-135Q you did not have to be on alert all the time like other SAC tankers. The crews of the SR-71 fondly remember the crew members of the KC-135Q for being vigilant about keeping them safe.
Be sure to check out Linda Sheffield Miller (Col Richard (Butch) Sheffield’s daughter, Col. Sheffield was an SR-71 Reconnaissance Systems Officer) Facebook Page Habubrats for awesome Blackbird’s photos and stories.
Photo credit: U.S. Air Force