The second built X-15 (55-6671) was getting ready for its first flight after extensive modifications, piloted by USAF Major William J. Knight
55-6671 in its white ablative paint with external fuel tanks
It would be the 53rd flight for the airframe, but the first after a landing incident. The fuselage was extended 2 inches (0,71 meters) to create space for a hydrogen tank that would supply a scramjet installed in the ventral fin. Additionally, external propellant tanks were installed to enable the aircraft to reach higher altitudes and speeds. An ablative paint (for better heat resistance) was applied to make the aircraft better capable of the heat generated at the extremely high speeds planned for the flight).
After some minor delay the mothership, NB-52B Stratofortress 52-008 "Balls 8", lifted off with the X-15 under its right-wing
The X-15 with the scramjet attached to the ventral fin, under the wing of the B52
After climbing to an altitude of 45.000 ft (13.716 meters) the X-15 was released and the rocket engine (Reaction Motors XLR99-RM-1) was fired and the aircraft started o climb and accelerate. 60 seconds after ignition of the engine the external tanks are emptied to reduce aerodynamic drag.
The X-15 just after release from the B52. The steam trail is hydrogen peroxide used to power the rocket engine turbopump.
The X-15 continued to climb and at 102.100 feet the aircraft levelled off, still accelerating. just 104 seconds after the start of the engine it was shut down. With the thrust slowly decreasing the aircraft continued to accelerate reaching its top speed for the flight of 6630 feet per second. or Mach 6.72 (about 7275 kph) Surface temperatures at the tail section of the aircraft reached temperatures of 2700 F (1482 C) as a result of the shockwaves caused by the scramjet. During the subsequent approach and increased air density the scramjet broke off the aircraft. After an 8 minute and 17-second flight USAF Major William J. Knight set the aircraft down on runway 18 at Rogers Dry Lake.
The speed reached this flight still stands as a world record, "The greatest speed ever reached by a manned aircraft that is not a spacecraft"
Firefighters cooling the ventral fin down after landing
Some specifications of the X-15:
Length: 50 ft 9 in (15.47 m)
Wingspan: 22 ft 4 in (6.81 m)
Height: 13 ft 3 in (4.04 m)
Wing area: 200 sq ft (19 m2)
Empty weight: 14,600 lb (6,622 kg)
Gross weight: 34,000 lb (15,422 kg)
Powerplant: 1 × Reaction Motors XLR99-RM-2 liquid-fuelled rocket engine, 70,400 lbf (313 kN) thrust
Maximum speed: 4,520 mph (7,270 km/h, 3,930 kn)
Range: 280 mi (450 km, 240 nmi)
Service ceiling: 354,330 ft (108,000 m)
Rate of climb: 60,000 ft/min (300 m/s)