On July 4th, 2014, I photographed A-12 #06938, on display at the USS Alabama Museum in Mobile, Alabama. Even though I’ve photographed #06938 numerous times, I always attempt to create fresh, interesting photos. This time, I photographed the two the liquid nitrogen tanks in the nose gear bay, shown in the final photo. The liquid nitrogen was stored in these tanks, converted into gaseous nitrogen, and used to inert the atmosphere in the aircraft’s fuel tanks. This inert nitrogen atmosphere was required, because the fuel heated 350° Fahrenheit inside the tank during flight. At that temperature, an ambient air environment could have caused combustion inside the fuel tank. If the nitrogen environment could not be achieved during flight, there was a danger of combustion inside the fuel tank.
The Blackbird aircraft has what we call “wet wings”, which means that the skin panels of the wings and fuselage double as a fuel tank. There is no bladder inside the aircraft to hold the fuel, and every joint and screw has to be sealed from the inside, to prevent fuel leakage. When the aircraft flew at full speed, Mach 3.2, the compression of the air against the surface of the aircraft would cause serious heating, up to 620° Fahrenheit in some places. This heating would cause the entire length of the aircraft to grow about five inches in flight.
When the aircraft would constantly contract and expand, it would cause the sealant in the fuel tanks to wear out, and fuel leaks would take place. These leaks were monitored by maintenance crews, measuring them in drips per minute (DPM). If the DPM reached its tolerance in a certain area, maintenance crews would go inside the fuel tanks, and reseal the area, which was a nightmarish process.
Nearly every time I photograph a Blackbird in a museum, I hear a museum guest mistakenly saying, “The Blackbird had to refuel mid-air immediately after takeoff, because it leaked so badly.” This is not true. The real reason they refueled after takeoff was, when the Blackbird was fueled on the ground, the atmosphere inside the tank was ambient air. This had to be replaced with gaseous nitrogen before they reached full speed. When the tanker aircraft topped off the Blackbird’s tanks, all of the ambient air would be expelled from the tanks through relief valves. Then, as the aircraft consumed fuel, the space created in the fuel tanks would be replaced with gaseous nitrogen. This created a safe, inert atmosphere in the fuel tanks. If the aircraft, for some reason, could not create this 100% nitrogen atmosphere, the flight could not exceed 2.6 Mach.
It was possible to fully fuel, then defuel the aircraft to a partial load on the ground, before flight, to create this inert nitrogen tank environment, but this was a maintenance nightmare. This procedure was called a “maintenance yo-yo.” When you put the gaseous nitrogen head pressure in the fuel tanks on the ground, it caused excessive leaking, so maintenance always preferred to perform this procedure in the air, after takeoff.