The First Reusable Spacecraft: The X-15 Flights Above the Karman Line

According to the FAI (Fédération Aéronautique Internationale) which maintains records for aeronautical achievements, the “official” threshold of space is at an altitude of 100 kilometers which corresponds to the Karman Line – the altitude where the air becomes so thin that an aircraft would need to exceed orbital velocity in order to support itself by aerodynamic lift alone. According to Chapter 2.2 of the FAI Sporting Code Section 8 – Aeronautics, a “spacecraft” is “a vehicle capable of flight in space” (with no requirement to necessarily reach orbit) while a “reusable spaceship” is a spacecraft “capable of making two manned consecutive flights in such manner that a minimum of 90% (in mass) of the elements constituting the take-off empty mass of the first flight will be present in the take-off empty mass of the second flight.”

The Gemini 2 reentry module, shown here being recovered after its second suborbital flight on November 3, 1966, is erroneously cited by some as being the first reusable spacecraft. (USAF)

Inevitably during any discussion among space enthusiasts about what was the first reusable spacecraft, younger folks immediately cite NASA’s Space Shuttle which readily meets the FAI definition. Older enthusiasts may cite the Gemini 2 reentry module which flew on an unmanned suborbital test flight on January 19, 1965 (see “The Mission of Gemini 2”) and was then refurbished to fly a second suborbital mission on November 3, 1966 in support of the USAF Manned Orbiting Laboratory program (see “The USAF Manned Orbiting Laboratory Test Flight”). While there are potential issues meeting the FAI definition (e.g. while it was a manned spacecraft design, it was flown unmanned and it did not use 90% of its original hardware mass on its reflight), this was not the first reusable spacecraft either. That distinction goes to the X-15-3 rocket plane (tail number 66672) which flew above the Karman line twice during the summer of 1963.

The X-15

The X-15 was part of an experimental hypersonic rocket plane program run by the USAF and NASA to fly as fast as Mach 6 and at peak altitudes of at least 76 kilometers (see “The First Reusable Spacecraft: The Origins & First Test Flights of the X-15“). Started in 1954, this program would expand the performance envelope explored by the first generation of rocket planes like the X-1 series and penetrate the threshold of space in preparation of future programs like the X-20 Dyna Soar which would push performance limits to even higher speed and altitudes (see “The Future That Never Came: The X-20 Dyna Soar Aerospace Plane”).

Three views of the North American X-15. Click on image to enlarge. (NASA)

Built by North American Aviation (which after decades of corporate mergers is now part of Boeing), the X-15 was 15.2 meters long and had a mass of about 15,100 kilograms at launch. Midway down its fuselage were a pair of low aspect ratio, trapezoidal shaped wings with a span of 6.7 meters. Based on research performed by NACA (NASA’s predecessor in aeronautics), the X-15 used a pair of thick, wedge shaped vertical stabilizers and thin, down sloping horizontal stabilizers to provide directional control during flight. These also gave the aircraft its classic arrow-like profile. It was powered by a throttable 254-kilonewton XLR-99 engine built by Reaction Motors which burned 8,540 kilograms of liquid anhydrous ammonia and liquid oxygen (LOX) held in the X-15’s propellant tanks. A set of a dozen small hydrogen peroxide-fueled jets located in the nose and wingtips with thrusts of 180 and 450 newtons provided attitude control when the X-15 was too high and the air too rarefied for its aerodynamic control surfaces to work.

A cutaway diagram showing the major components of the X-15. Click on image to enlarge. (NASA)

The first X-15, X-15-1 fitted with a pair of 27-kilonewton XLR-11 engines as an interim power plant, was rolled out on October 15, 1958 and made its first unpowered glide flight after being dropped from its NB-52 carrier aircraft on June 8, 1959 eventually followed by the first powered flight using the second X-15 on September 17. X-15-2 made the first powered flight using the XLR-99 engine on November 15, 1960 with the third aircraft, X-15-3, making its inaugural flight on December 20, 1961 with NASA civilian pilot, Neil Armstrong, at the controls.

Before becoming an astronaut, Neil Armstrong (shown here in 1960) was a civilian pilot working for NASA on the X-15 program. (NASA)

During the first few years of operation, the performance envelope of the X-15 was slowly expanded to ever higher speeds and altitudes. During Flight 52 of the X-15 program on April 30, 1962, NASA test pilot Joseph Walker flew X-15-1 to an altitude of 75.1 kilometers achieving the aircraft’s original design goal. During Flight 62 on July 17, USAF test pilot Robert White flew X-15-3 to a then record-setting altitude of 95.9 kilometers. Since the flight exceeded the USAF threshold of 50 miles or 80.5 kilometers for space (but is below the 100-kilometer Karman line widely recognized today), White qualified for his USAF astronaut wings on this flight. Walker was similarly qualified during Flight 77 on January 17, 1963 when he flew X-15-3 to an altitude of 82.8 kilometers. During Flight 87 on June 27, USAF test pilot Robert Rushworth flew X-15-3 to an altitude of 86.7 kilometers qualifying for his astronaut wings as well.

A schematic of a typical high-altitude X-15 flight profile. (NASA)

Flights 90 & 91 Above the Karman Line

While these series of flights were higher than the design goal of the X-15, it was still well below what this aircraft was actually capable of doing as it was built by North American. Theoretically, the X-15 could fly to altitudes in excess of 122 kilometers and safely return if everything went precisely as planned. To test the X-15 to its limits yet leave some margin for safety due to unavoidable deviations in pilot and hardware performance, it was decided to fly X-15-3 in two successive flights to build on current experience up to an altitude of 110 kilometers – above the 100-kilometer Karman line but still safely below the design limits of the X-15.

Civilian NACA test pilot Joseph Walker shown wearing his pressure suit next to the X-1E rocket plane. (NASA/AFRC)

Joseph A. Walker was chosen to pilot the two high altitude X-15 missions designated Flights 90 and 91. Walker joined NACA after leaving the US Army Air Forces (the predecessor of today’s USAF) where he served as a weather reconnaissance pilot during World War II. Initially he worked as an experimental physicist at NACA Aircraft Engine Research Laboratory (today’s NASA Glenn Research Center) and later became a civilian test pilot. In 1951, Walker transferred to the Edwards Flight Research Facility (today’s NASA Armstrong Flight Research Center) and eventually became the Chief Research Pilot. During this time, Walker flew three different variants of the Bell X-1, two versions of the Douglas D-558 Skyrocket, the Douglas X-3 Stiletto, the Northrop X-4 Bantam and the Bell X-5. After NACA was absorbed into NASA when it was founded in October 1958, Walker continued on with the new organization as a civilian test pilot in the X-15 program. Walker made his first flight in the X-15 (which was also NASA’s first flight) on March 25, 1960 for Flight 9 of the program. By the early summer of 1963, the 42-year old Walker had made a total of 23 flights in the X-15.

Joseph Walker was chosen to pilot the X-15 to its maximum safe altitude limit above the Karman line in the summer of 1963. (NASA/AFRC)

For Flight 90, Walker was to burn the XLR-99 engine for 83 seconds to achieve a peak speed of Mach 5.40 and an altitude of 96.0 kilometers approximately equaling Bob White’s record set a year earlier. During the flight, Walker would deploy a balloon on the end of a line to measure air density while other experiments were performed with a ultraviolet photometer used to measure Earth backgrounds and an infrared photometer to measure the craft’s exhaust signature.

The X-15-3 aircraft shown here shortly after starting powered flight. (NASA)

NB-52B number 8, nicknamed “The Challenger”, took off at 9:20 AM PST on July 19, 1963 with X-15-3 under its starboard wing and Walker onboard for Flight 90. At 10:20:05, X-15-3 was dropped while over a location known as Smith Ranch and Walker quickly brought the XLR-99 engine up to full power. The XLR-99 burned for a total of 84.6 seconds and hit a peak speed of 1,658 meters per second or Mach 5.50. The slightly longer burn of the XLR-99 engine in conjunction with slightly higher than expected thrust as well as a 0.5° error in climb angle combined so that X-15-3 overflew its intended peak altitude and reached a record setting 106.0 kilometers – ten kilometers higher than planned and the first flight of a winged craft above the Karman line. Walker successfully piloted the X-15 through reentry and safely landed on Rogers Dry Lake at Edwards Air Force Base after a flight of 11 minutes and 24 seconds. The only notable failure was an instrumentation malfunction in the balloon experiment.

With this success under their belts, the X-15 team prepared for a second flight to test the limits of the aircraft’s design with a flight to 110 kilometers. It was planned that the XLR-99 engine would burn for 84.5 seconds to reach a top speed of Mach 5.38. For this flight, an experiment to observe the exhaust plume in the ultraviolet was carried. Over the course of two weeks, three launch attempts were aborted. On August 3, 1963 the flight was called off due to the weather as “The Challenger” was climbing to its launch point. A week later, problems with the APU #1 (Auxiliary Power Unit) on the X-15 forced a scrub. On August 15, more APU and weather issues along with a radio problem ended the third launch attempt.

Joe Walker shown getting into X-15-3 on August 22, 1963 for his record setting Flight 91. (USAF)

Finally, on August 22, 1963 at 9:09 PST NB-52A number 003 lifted off with X-15-3 for the fourth launch attempt of Flight 91. At 10:05:57 PST, X-15-3 was dropped over Smith Ranch for the start of its record setting flight. Walker used a newly installed altitude predictor in his instrument panel to adjust his climb angle so that he would hit his target altitude. When the XLR-99 shutdown as it exhausted its propellants after a burn of 85.8 seconds, X-15-3 was passing through an altitude of 53.6 kilometers and travelling at 1,695 meters per second or Mach 5.58 – a touch faster than planned. After climbing for almost two more minutes, X-15-3 with Walker at the controls hit a record setting altitude of 107.96 kilometers – just two kilometers shy of the goal but sufficient to meet the mission objectives. The only other issue encountered during the flight was a frozen left roll thruster controlled from the X-15’s side control stick.

X-15-3 shown gliding in for a landing on the Rogers Dry Lake at Edwards AFB, California. (NASA)

Walker successfully piloted the X-15 through reentry and a touchdown on Rogers Dry Lake. He had covered 491 kilometers across the ground from the drop point over Smith Ranch to landing in 11 minutes and 8.6 seconds of flight. With this flight, Joe Walker became the first person to fly into space twice and the X-15-3 became the first craft to do the same. It would be 41 years before another rocket-powered aircraft would beat this record (see “The Spaceflights of SpaceShipOne”)

Postscript

Following his record-setting flight, Joe Walker left the X-15 program to work on other projects including the initial test flights of Lunar Landing Research Vehicle (LLRV) used by NASA astronauts to practice lunar landings. Walker tragically died on June 8, 1966 when his F-104 chase aircraft collided with a XB-70 Valkyrie bomber during a tight formation publicity photo shoot.

Joe Walker shown at the controls of the Lunar Landing Research Vehicle (LLRV) in 1964. (NASA/AFRC)

The X-15 program would continue for another five years after Walker’s record-breaking mission making an additional 108 flights. While the X-15 would never fly above the Karman line again, it did make another eight flights which qualified five more pilots for their astronaut wings. Unfortunately, X-15-3 and its pilot were lost on one of these missions. On Flight 191 flown on November 15, 1967, USAF test pilot Michael Adams died when X-15-3 broke up during a botched reentry because of an instrumentation malfunction while descending from a peak altitude of 81.1 kilometers. The USAF awarded Adams his astronaut wings posthumously after the only fatal accident in the otherwise highly successful, decade-long X-15 flight program.

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General References

Dennis R. Jenkins and Tony R. Landis, Hypersonic: The Story of the North American X-15, Specialty Press, 2003

Milton O. Thompson, At The Edge of Space: The X-15 Flight Program, Smithsonian Institution Press, 1992

FAI Sporting Code Section 8 – Aeronautics, 2009 [Link]