Why The World’s Fastest Plane SR-71 Runs Out Of Fuel As Soon As It Takes OffWorld's Fastest Plane SR-71 Runs Out Of Fuel worlds-fastest-plane-sr-71-runs-out-of-fuel
The SR-71 Blackbird spy plane flies so high and so fast that during flight its surface stays at over 2000 degrees F. That means that any soft seals in the titanium body would burn up. There are no seals between body sections, and the plane leaks JP-7 jet fuel constantly until it reaches cruising speed and altitude. Therefore the SR-71 has to be refueled immediately after takeoff.
From the video notes, which are very similar to (and probably cut and pasted from) the Wikipedia entry for the SR-71:
On most aircraft, use of titanium was limited by the costs involved in procurement and manufacture. It was generally used only in components exposed to the highest temperatures, such as exhaust fairings and the leading edges of wings. On the SR-71, titanium was used for 85% of the structure, with much of the rest polymer composite materials. To control costs, Lockheed used a more easily worked alloy of titanium which softened at a lower temperature.[N 3]
The challenges posed by the SR-71 led Lockheed to develop entirely new fabrication methods to enable its manufacture, and have since been used in the manufacture of many other aircraft. Welding the titanium requires distilled water, as the chlorine present in tap water is corrosive; commonplace cadmium-plated tools could not be used as they also caused corrosion. Metallurgical contamination was another problem; at one point 80% of the delivered titanium for manufacture was rejected on these grounds.
The high temperatures generated during flight required special design and operating techniques. For example, major portions of the skin of the inboard wings were corrugated, not smooth. (Aerodynamicists initially opposed the concept and accused the design engineers of trying to make a Mach 3 variant of the 1920s-era Ford Trimotor, known for its corrugated aluminum skin.) The heat of flight would have caused a smooth skin to split or curl, but the corrugated skin could expand vertically and horizontally. The corrugation also increased longitudinal strength. Similarly, the fuselage panels were manufactured to fit only loosely on the ground. Proper alignment was achieved only when the airframe heated up and expanded several inches. Because of this, and the lack of a fuel sealing system that could handle the thermal expansion of the airframe at extreme temperatures, the aircraft would leak JP-7 jet fuel on the runway. At the beginning of each mission, the aircraft would make a short sprint after takeoff to warm up the airframe, then refuel before heading off to its destination.
Cooling was carried out by cycling fuel behind the titanium surfaces in the chines. On landing, the canopy temperature was over 300 °C (572 °F).
Why The World’s Fastest Plane SR-71 Runs Out Of Fuel As Soon As It Takes Off