Both Flight 90 and Flight 91 of the North American X-15 crossed the Kármán line, reaching altitudes of 106.01 and 107.96 km respectively. Both flights were piloted by Joseph A. Walker, who became in 1963 the "United States' seventh man in space" and "qualified him as an astronaut under the rules of the U.S. Air Force and the Fédération Aéronautique Internationale (FAI)"
By November 1960, Reaction Motors was able to deliver the XLR99 rocket engine, generating 57,000 pounds-force (250 kN) of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single engine configuration. The XLR99 used anhydrous ammonia and liquid oxygen as propellant, and hydrogen peroxide to drive the high-speed turbopump that delivered propellants to the engine. It could burn 15,000 pounds (6,804 kg) of propellant in 80 seconds.
I hadn't heard of anhydrous ammonia as a fuel before reading this. It needs to be either pressurized or cryogenic (about -33C) to remain as a liquid, unlike organic fuels like alcohols or the heavier hydrocarbons. What were the various reasonings behind developing an ammonia burning engine in this case? Has ammonia been used again after the X-15? Or before for that matter!
above: Test pilot and astronaut Joseph A. Walker
above X-15 from here.