| Mission type | Test flight |
|---|---|
| Operator | US Air Force/NASA |
| Mission duration | 11 minutes, 8.6 seconds |
| Distance travelled | 543.4 kilometers (337.7 mi) |
| Apogee | 107.96 kilometers (67.08 mi) |
| Spacecraft properties | |
| Spacecraft | X-15 |
| Manufacturer | North American |
| Launch mass | 15,195 kilograms (33,499 lb) |
| Landing mass | 6,260 kilograms (13,800 lb) |
| Dry mass | 6,577 kilograms (14,500 lb) |
| Crew | |
| Crew size | 1 |
| Members | Joseph A. Walker |
| Start of mission | |
| Launch date | August 22, 1963, 18:05:57 (1963-08-22UTC18:05:57Z) UTC |
| Launch site | NB-52A #52-003, Edwards Dropped over Smith Ranch Dry Lake 39°20′N 117°29′W / 39.333°N 117.483°W / 39.333; -117.483 |
| End of mission | |
| Landing date | August 22, 1963, 18:17:05 (1963-08-22UTC18:17:06Z) UTC |
| Landing site | Rogers Dry Lake, Edwards |
 Joe Walker | |
X-15 Flight 91 was an August 22, 1963 American crewed sub-orbital spaceflight, and the second and final flight in the program to fly above the Kármán line, which was previously achieved during Flight 90 a month earlier by the same pilot, Joseph A. Walker. It was the highest flight of the X-15 program.
Flight 91 was the first internationally recognized spaceflight of a reused spacecraft, as Walker had also flown plane number three on the previous sub-orbital spaceflight over the Kármán line on July 19. The flight was air-launched from a modified Boeing B-52 Stratofortress support plane over Smith Ranch Dry Lake, Nevada, United States. Walker piloted the X-15 to an altitude of 107.96 km and remained weightless for approximately five minutes. The altitude was the highest crewed flight by a spaceplane to that time, and remained the record until the 1981 flight of Space Shuttle Columbia.
Walker landed the X-15 about 12 minutes after it was launched, at Rogers Dry Lake, Edwards Airforce Base, in California. This was Walker's final X-15 flight.
| Position | Astronaut | |
|---|---|---|
| Pilot |  Joseph A. WalkerSecond (FAI-recognized) / Third (U.S.-recognized) and last spaceflight | |
On this flight, Joe Walker became the first person to enter space twice. He had a maximum speed of 3,794 mph (6,106 km/h) and a maximum altitude of 354,200 feet. Second and final X-15 flight over 67 miles.[1] Unofficial altitude record set for class. Highest altitude achieved by X-15.[1] Last flight for Walker in X-15 program. Number 1 left RCS nozzle froze up. First flight with altitude predictor instrument (needed calibration).
The mission was flown by X-15 #3, serial 56-6672 on its 22nd flight.[1]
Launched by: NB-52A #003, Pilots Russell Bement & Lewis. Takeoff: 17:09 UTC. Landing: 18:56 UTC.
Chase pilots: Wood, Dana, Gordon and Rogers.
The X-15 engine burned about 85 seconds. Near the end of the burn, acceleration built up to about 4 G (39 m/s²). Weightlessness lasted for 3 to 5 minutes. Re-entry heating warmed the exterior of the X-15 to 650°C in places. During pull-up after re-entry, acceleration built up to 5 G (49 m/s²) for 20 seconds. The entire flight was about 12 minutes from launch to landing.
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