The SR-71 was developed in the 1960's by Boeing. This was a revolutionary aircraft in that it could travel at Mach 3 speeds, or 3 times the speed of sound. This plane is quite strange because when it is sitting on the ground the plates of the aircraft don't meet up properly and the plane actually leaks fuel. The reason behind this design is since the plane flies so fast and so high up. At an altitude 80,000 ft the pressure on the metal on the outside of the aircraft is so little that the outside begins to expand, the engineers at Boeing had to account for this so they made the panels fit together loosely. Another factor for why the metal would expand in flight is that it would get very hot flying at mach 3 speeds. At such high speed the external body would reach upwards of 500 degrees, and the inside of the windshield would reach temperatures of 250 degrees, this is all caused with how much friction the air has on the plane causing this massive amount of heat. To deal with this massive amount of heat they had to develop a special cooling system that would take the hot air from inside the cockpit and put it in the fuel right before being used.