Roystonea Regia

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.

Crew

Position	Astronaut
Pilot	Joseph A. Walker Second (FAI-recognized) / Third (U.S.-recognized) and last spaceflight

Mission parameters

• 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

Mission highlights

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.

Notes

References

• 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.