The A-12 Oxcart, which would evolve into the SR-71, Annie Jacobsen explains (in Area 51), was a flying fuel tank:
It held eleven thousand gallons, which made the tanks the largest portion of the airplane. The fuel had requirements the likes of which were previously unknown. During the refueling process, which would happen in the air, at lower altitudes and lower airspeeds, the temperature of the fuel would drop to -90 degrees Fahrenheit. At Mach 3, it would heat up to 285 degrees Fahrenheit, a temperature at which conventional fuels boil and explode. To allow for this kind of fluctuation, JP-7 was designed to maintain such a low vapor pressure that a person could not light it with a match. This made for many practical jokes, with those in the know dropping lit matches into a barrel of JP-7 to make those not in the know duck and run for cover.
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Flying at speeds of 2,200 miles per hour, an Oxcart pilot would need a 186-mile swath just to make a U-turn. This meant an additional 38,400 acres of land around the base were withdrawn from public access, allowing the Federal Aviation Administration to extend the restricted airspace from a 50-square-mile box to 440 square miles. FAA employees were instructed not to ask questions about anything flying above forty thousand feet. The same was true at NORAD, the North American Aerospace Defense Command.