The X-15, a hypersonic rocket plane designed and manufactured by North America for NASA tests, broke unbeatable speed records for over 60 years. The first rocket-powered aircraft was the German Heinkel He-176, better known as the Messerschmitt Me-163 Komet. Even in 1944, Komet managed to reach speeds of 997.8 km per hour in horizontal flight, during the tests an unofficial flight speed record of 1126.54 km per hour was achieved, which was not broken until 1953!

After World War II, as with many other projects, anything that could give the Allies an advantage received great attention, including rocket-powered aircraft. When North American (NA) won the X-15 airframe tender in 1955, Reaction Motors would build the engines, and the first flight was made four years later, in June 1959.

Unlike conventional aircraft design, the X-15 was never intended to take off under its own power. Like other experimental aircraft such as the M2-F3 Lifting Body, the X-15 was designed to be carried on an “aircraft base”.

NASA used the B-52A and B-52B, which were retired in 1969. Both have been modified with a pylon on the right wing to carry test vehicles. The NB-52A has been named “Big and Powerful” and the NB-52B “Kuli 8”.

Balls 8 first flew in June 1955 and was not retired until December 2004, making it the oldest B-52 in flight and the only B model in service at the time.

At the design stage, it should be noted that the X-15 had to fit under the wing of the B-52, which meant the designers had to make it quite compact. It was designed for only one crew member and had an overall length of 15.24 meters and a wingspan of only 6.7 meters.

The X-15 had a very sleek profile designed to reach hypersonic speeds. That means it has to be a serious machine. Three X-15s were built. Improved Kh-15-1, -2 and -3. Each was used on a different number of flights. The X-15-1 made a total of 81 flights, while the X-15-2 made just 53 flights.

The engine was also a key point in early designs: the aircraft was to be built around two XLR11 rocket engines and fuel tanks to take it to the edge of space. Every aspect of the design was aimed at making the X-15 the fastest aircraft ever built.

The new XLR99 engine was almost ten times more powerful than previous engines and was used until the end of the X-15’s service. Incredible performance was achieved with anhydrous ammonia and liquid oxygen.

The speed and altitude achieved by the X-15 hypersonic aircraft broke many records. The highest known flight was in 1963, when test pilot Joseph Walker ascended to an altitude of 107 km, 960 m at a speed of 6,106 km/h. Any flight over 100 km actually gave pilots the right to become astronauts. And in 1967, the X-15 managed to reach a maximum speed of 6206 km per hour. Source