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Subdeacon Joe

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  · Why did the SR 71 have to refuel immediately after takeoff? I found the answer in an interview with Rich Graham. It was so the SR-71 could accelerate past Mach 2.6
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 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.”
Posted by Linda Sheffield
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Then there were the "Yo-Yo" missions.


“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+.”


JP-7, The Fuel That Powered the Blackbirds

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