X-15 Flight 91

Flight 91

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. Walker Second (FAI-recognized) / Third (U.S.-recognized) and last spaceflight

• Mass: 15,195 kg fueled; 6,577 kg burnout; 6,260 kg landed
• Maximum altitude: 107.96 km
• Range: 543.4 km
• Burn time: 85.8 seconds
• Mach: 5.58
• Launch vehicle: NB-52A Bomber #003

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.

• Goodwin, Robert (2000). X-15: the NASA mission reports, incorporating files from the USAF. Burlington, Ontario: Apogee Books. ISBN 1-896522-65-3.
• Jenkins, Dennis R. (June 2000). Hypersonics Before the Shuttle: A Concise History of the X-15 Research Airplane (PDF). Monographs in Aerospace History. NASA. hdl:2060/20000068530. ISBN 0-16-050363-9. SP-2000-451. Archived (PDF) from the original on April 7, 2023.
• Price, A. B.; Martin Marietta (January 12, 1968). Thermal protection system X-15A-2 Design Report (PDF) (Technical report). NASA. hdl:2060/19680016245. CR-82003. Archived (PDF) from the original on August 30, 2023.
• Stillwell, Wendell H. (1965). X-15 Research Results - With a Selected Bibliography (PDF). NASA. SP-60. Archived (PDF) from the original on August 30, 2023.
• Watts, Joe D. (October 1968). Flight experience with shock impingement and interference heating on the X-15-2 research airplane (PDF) (Technical report). NASA. TM X-1669. Archived (PDF) from the original on 30 August 2023.