X-15 flights over 50 miles

The X-15, whose programme operated between June 1959 and October 1968, was a rocket-powered aircraft built by North American Aviation. The programme was operated as a joint NASA/USAF/USN venture, for aeronautical research at speeds in excess of Mach 6 and altitudes up to and beyond 50 miles. The fastest recorded speed was eventually 4,520 mph (Mach 6.7) and the highest altitude achieved was 354,200 feet (66.8 miles).

Date Free-flight Pilot Aircraft Altitude (miles)

1962 Jul 17 62 White 3 59.16 (95.18km)

First FAI-certified world altitude record; 3 aborts preceded the attempt. The rocket engine fired for one second longer than planned resulting in a speed 248mph (399kph) faster than planned. At peak altitude White could see a panorama that stretched from San Francisco in California down to Mexico.

1963 Jan 17 77 Walker 3 51 (82.05km)

Walker’s flight was to study the handling of the X-15 without its ventral fin at extreme altitudes and to conduct an infra-red experiment.

1963 Jun 27 87 Rushworth 3 55 (88.49km)

This flight was aimed at providing the pilot with experience of high-altitude handling and phenomena.

1963 Jul 19 90 Walker 3 65.3 (105.06 km)

On this flight, Walker was to study the expansion of the airframe during re-entry with the ventral fin removed. He also deployed and towed a nitrogen-filled balloon and conducted horizon-scanning, photo­metric, infra-red and ultraviolet observations, all in ten minutes.

1963 Aug 22 91 Walker 3 66.75 (107.40km)

Walker’s third “astro-flight’’ reached the highest altitude attained by an X-15 in 199 free-flights. He also attained a speed of Mach 5.58 (3,794mph or 6,104.5kph).

1965 Jun 29 138 Engle 3 53.14 (85.50 km)

Engle’s first “astro-flight’’ included a horizon-scanning experiment.

1965 Aug 10 143 Engle 3 51.7 (83.18km)

Engle’s second ‘‘astro-flight’’ occurred just 8 months prior to his selection as a NASA astronaut. He was the first astronaut selected who already held ‘‘astronaut-pilot wings’’.

1965 Sep 29 150 McKay 3 56 (90.10km)

After surviving a crash of the X-15 #2 aircraft that almost killed him in 1962, McKay finally completed an ‘‘astro-flight’’ which investigated boundary-layer noise and structural loads on the horizontal tail, as well as horizon-scanning experiments.

1965 Oct 14 153 Engle 1 50.17 (80.72km)

In his third ‘‘astro-flight’’, Engle completed a programme that included taking measurements of atmo­spheric pressure and further experiments in the scanning of Earth’s horizon.

1966 Nov 1 174 Dana 3 58 (93.32km)

This flight included the objectives of collecting micrometeorites, and tests of a dual-channel radiometer and a tip-pod accelerometer. Precise measurements of the attitude and density of the atmosphere were also taken.

1967 Oct 17 190 Knight 3 53.4 (85.92km)

Further collection of micrometeorites, the recording of wing-tip pod deflection during re-entry, observations of the ultraviolet plume of the XLR99 rocket exhaust, and studies of the solar spectrum above 200,000ft (60,960 metres) were all objectives assigned to this mission.

1967 Nov 15 191 Adams 3 50.4 (81.09 km)

The scientific objectives of this 12th ‘‘astro-flight’’ of the programme, included a UV study of the rocket exhaust plume, observations of the solar spectrum and the bow shockwave of the wing-tip pod. Nose-gear loads were to be observed, micrometeoroids collected and an ablative material tested for use on the Saturn 5 booster. Adams was killed on this flight and was awarded his USAF Astronaut Wings posthumously.

1968 Aug 21 197 Knight 1 50.7 (81.57km)

The final X-15 ‘‘astro-flight’’ was just two missions prior to the end of the programme. The planned 200th flight was cancelled. In 199 free-flight missions, the three X-15s had logged 30 hours 13 minutes 49.4 sec­onds in flight and had flown 41,763.8 miles (67,197.95 km). Pilot experience at Mach 4 was almost 6 hours, with a further 90 minutes at Mach 5 and 78 seconds at Mach 6.

Fifteen pilots were selected to fly the X-15, although there was no formal selection process. They were all qualified test pilots prior to assignment to the programme. Eventually, only twelve flew X-15 missions, of which there were 199 completed by the three X-15 vehicles. In addition, several captive flights were executed, where the X-15 was not released from under the wing of the B-52 launch aircraft.

Although not considered as a spacecraft, the X-15 did operate in a region of the upper atmosphere whose conditions were only fractionally different from those encountered by a vehicle in Earth orbit. In the early 1960s, the USAF had declared that flights above 50 miles (80.45 km) would be classified as a space flight. They would award USAF Astronaut Wings to honour those USAF pilots that attained this altitude. In contrast, the Federation Aeronautique International (FAI), the inter­national aeronautical record-keeping body, decided that flights over 100 kilometres (or 62 miles) would be classified as space flights.

Of the 199 X-15 flights, thirteen surpassed the 50-mile altitude barrier, and these have been designated astro-flights, rather than space flights. Eight of the X-15 pilots (Walker, White, Rushworth, Engle, McKay, Dana, Knight and Adams) flew these thirteen missions. Of these, only five (White, Rushworth, Engle, Knight and Adams) were USAF pilots who received the USAF wings. The remaining three (Walker, McKay and Dana) were civilians and did not qualify for the USAF title. However, Walker completed two X-15 flights in excess of the FAI qualification altitude.