On October 3, 1967, the North American X-15A-2, piloted by USAF Major William J. "Pete" Knight, was released from its NB-52 mother plane at 35,000 feet above the Mojave Desert to begin its dash for an absolute speed record of 4,520 miles per hour, a record which stood unbroken until exceeded by the Space Shuttle.
Because of the lack of oxygen at the altitudes at which the X-15 flew, the XLR99 rocket engine used anhydrous ammonia as fuel and liquid oxygen as the oxidizer. Fitted with external fuel tanks to prolong engine burn up to 70%, the A-2 climbed to 102,000 feet and reached a speed of Mach 6.7.
A-2 differed from previous Marks in that it had an ablative coating to protect it from the high temperatures encountered at the high Mach velocities. It also had a dummy scram-jet engine fitted to the stub ventral fin to test the feasibility of the engines for sustained high-Mach flight.
The X-15 expanded the envelope of manned, winged flight to the very limits of the atmosphere and opened aviation to horizons only dreamed of previously. In all its variants, X-15 will be remembered as one of the most successful X-planes.
As a direct result of test data obtained by early X-plane flights, and advances in stealth technology, super fighters have been developed that will carry America and her Allies well into the 21st Century. Using exotic fuels and oxidizers, these aircraft will operate at extreme altitudes and be capable of speeds approaching Mach 30, or 17,500 miles per hour, which speed will allow these aircraft to be placed into orbit after unassisted lift-off from their desert bases.
As aircraft technology advances, airplanes are beginning to look more and more like ovate ellipsoids, or 'Flying Saucers." The newest stealth fighters will most certainly be capable of vertical takeoff and landing and may, until the orbital goal is met, operate at speeds approaching Mach 8 or even Mach 10 with ease.
Because of the constraints inherent in protecting the pilot from death during severe maneuvers, many of these aircraft will be capable of only straight and level flight or shallow dives and climbs when operating at their Mach limits. As new data is evaluated, this problem may be solved by designing the aircraft to stop in mid-air, hover, or change directions quickly (but not instantly), or they may be flown remotely from simulators on the ground thousands of miles away.
But while we occasionally see glimpses of these new aircraft or hear rumors of their development, even more remarkable aircraft and devices are being tested in complete secrecy in underground facilities in the American Southwest deserts. If the rumors are correct (and there is no reason at this time to doubt them), galactic travel and even time-travel may be only weeks away!