Ask the typical space enthusiast to name the first reusable piloted spacecraft and the most likely answer would be NASA’s Space Shuttle. While the Space Shuttle’s external tank was discarded on each mission, its pair of solid rocket boosters as well as the highly complex and expensive orbiter (the actual “spacecraft”) were certainly reusable. But the Space Shuttle was not the first piloted spacecraft that could be flown over and over. Nor was some little known piece of Soviet engineering genius from decades ago. The honor belongs to the grandfather of all modern aerospace planes, the X-15 built by North American Aviation (which, after decades of corporate mergers, is now part of Boeing).

While the X-15 is certainly the most famous of all the X-series aircraft, the fact that it flew high enough on no less than 13 occasions to qualify its pilots for USAF astronaut wings while the Mercury and Gemini programs came and went is frequently overlooked. Even in many “official” counts of American manned space missions, the suborbital X-15 flights are notably absent. This despite the fact that eight NASA and USAF pilots qualified for USAF astronaut wings during the program. This included Joe Engle who went on to command NASA’s Space Shuttle during three approach and landing test flights in 1977 as well as in orbit for the STS-2 mission in 1981 and STS-51-I in 1985.

There are several possible reasons the X-15 spaceflight accomplishments are often forgotten: First the majority of the X-15’s 199 flights were never meant to fly high enough to qualify as a spaceflight by any definition. Extremely high altitude flights were only one of this long-running program’s many objectives. Combined with the almost routine nature of what was really a test program, X-15 flights did not generate the media coverage afforded to the far less “routine” space missions of the Mercury and Gemini programs. Finally, when the X-15 passed the threshold into space, it barely did so and only briefly – hardly newsworthy to some when astronauts were spending days or weeks in orbit in preparation for a manned lunar mission. But exactly where is this threshold where one “officially” passes into space?

The Edge of Space

There really is not a clearly defined altitude where one passes out of the sensible atmosphere and into the vacuum of space. In the late 1950’s the USAF decided to award astronaut wings to pilots who flew higher than 50 statute miles (80.45 kilometers) above sea level. Besides being a nice round number (at least in Imperial measurement units), this altitude is higher than any balloon or conventional aircraft has ever flown (about 50 kilometers) yet lower than the lowest perigee of a marginally stable satellite orbit (about 90 kilometers). Eventually flying as high as about 108 kilometers where it passed even the 100-kilometer Karman line (which is widely accepted today as the threshold of space), the X-15 was the first craft capable of flying in the transition region between the sensible atmosphere and space (see “A History of Suborbital Crewed Spaceflights”).

The need to explore this region as well as the effects of hypersonic flight (i.e. at speeds exceeding five times the speed of sound or Mach 5), had been recognized in the early 1950s. In the years after the World War II, rocket-powered aircraft such as the USAF’s X-1 series and the X-2 as well as the US Navy-sponsored D-558 series of test aircraft first broke the sound barrier and proceeded to set a string of speed records up to Mach 3. In addition, these aircraft also flew at increasingly greater altitudes eventually reaching as high as 38 kilometers above sea level. But military planners anticipated the need for future aircraft to fly faster and higher still. In addition, since the prevailing view of crewed spaceflight at the time called for a pilot to fly his rocket powered aircraft into orbit and back, there was an obvious need to explore the issues associated with high altitude hypersonic flight.

After meeting held by the Executive Committee of NACA (NASA’s pre-Space Age predecessor, the National Advisory Committee for Aeronautics) it was recommended that NACA start research into the problems of flight at speeds of Mach 4 to 10 and at altitudes from 19 to 80 kilometers. What would become the X-15 was designed to meet this goal. A further resolution on July 14, 1952 extended NACA goals to speeds from Mach 10 to escape velocity and altitudes from 80 kilometers to infinity. Attempting to meet this latter goal eventually led to the USAF X-20 Dyna Soar (see “The Future That Never Came: The X-20 Dyna Soar Aerospace Plane”).

During the coming months NACA engineers performed numerous studies on hypersonic aircraft designs and soon the USAF took an interest. They had been performing similar studies and, along with NACA officials, knew that such a research program would be best carried out by pooling the resources of several agencies. By July 9, 1954 a joint NACA/USAF/US Navy committee started meeting to discuss the need for such a vehicle and its basic design. By late 1954 the base design criteria were determined and on January 17, 1955 the USAF officially assigned the new aircraft the “X-15” designation.

The Design

Four aircraft manufacturers responded to the joint NACA/USAF/US Navy call for proposals. Bell (the builder of the X-1 series) submitted its D-171 design, Douglas its Model 684 D-558-3, and Republic its Model AP-76. While every agency involved had its favorite design, ultimately they agreed on the North American NA-240 proposal and a contract for three aircraft was signed on September 30, 1955. This design was chosen because of its simplicity and ease to modify to meet the agencies’ various specifications. The final design that emerged from this long process became an aerospace classic.

The X-15 was designed to attain speeds of Mach 6 and peak altitudes well in excess of 76 kilometers. It was 15.2 meters long and had a mass of 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 NACA research, 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. 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 similar system was later used by NASA’s Mercury spacecraft for attitude control.

The bulk of the X-15 airframe was made from titanium while most of the outer skin was composed of the heat resistant and then exotic nickel-based alloy, Inconel X. Such materials were needed to withstand the anticipated 650° C temperatures generated during hypersonic flight. The air conditioned, climate controlled cockpit provided enough room for a single pressure suit-clad pilot. It was equipped with an advanced ejection seat that would work safely at speeds up to Mach 4 and an altitude 37 kilometers – the regime where an ejection was deemed most likely. It provided an extra safety margin for what was recognized as a risky test program.

Most of the X-15 fuselage housed a set of tanks holding 8,540 kilograms of propellant for the X-15’s single rocket engine. The XLR-99 engine built by Reaction Motors, Inc. (which became part of Thiokol in 1958) was ultimately chosen based on a bid the company submitted in December of 1955. Based on the earlier XLR-30 engine which itself was an enlarged version of the XLR-10 engine employed by the US Navy’s Viking sounding rocket (see “America’s First Space Rocket: The Origin & First Flights of the Viking Rocket“), the XLR-99 produced 254 kilonewtons of thrust at altitude and was intended to be restartable and throttleable in flight. Initially the engine was to be throttled from 30% to 100% of its maximum thrust. Early versions of the engine would only throttle between 50% and 100% but even later versions were limited to a minimum 42% rating to avoid running problems encountered during test flights at low thrust settings.

The turbopump-fed XLR-99 ran on an unusual combination of liquid anhydrous ammonia and liquid oxygen (LOX). While there are certainly rocket fuels more powerful than liquid ammonia available, Reaction Motors had much experience with this propellant combination as a result of an earlier R&D effort and knew that engines burning this volatile fuel were very forgiving during restarts – a very important safety consideration. This powerful engine would easily allow the X-15 to exceed its speed and altitude design goals. Ultimately the performance of the X-15 would be limited by its structure’s ability to withstand the heat generated during high speed flight or reentry and not by its engine.

But as development of the X-15 and its XLR-99 engine proceeded it became clear that the first X-15 airframes would be available long before their innovative powerplants. By February 1958 it was decided that the first two X-15 aircraft would initially be equipped with a pair of smaller Reaction Motors XLR-11 engines similar to the ones that powered the Bell X-1 series and the Douglas D-558-II aircraft. As configured for the X-15 program, each XLR-11 engine consisted of four thrust chambers that could be fired independently allowing for an eight-step throttle capability. With all eight chambers running, the pair of ethanol/LOX fueled XLR-11 rocket engines produced a total of 71 kilonewtons of thrust. While this was less than a third of the maximum thrust generated by the XLR-99, it did allow the X-15 test program to proceed with the previously planned low speed trials with minimal modifications made to the aircraft.

But even with the powerful XLR-99, the X-15 would waste far too much propellant taking off directly from the ground. Like many other rocket powered test aircraft of the era, the X-15 would be carried by a large carrier aircraft to altitude before being dropped for the beginning of a test flight. But the B-29 and its sibling, the B-50, bombers used by earlier X-series aircraft were too small to handle the X-15. After much debate a modified B-52 bomber, which was just entering service, was selected to be the X-15 mother craft. The X-15 would be mounted under the B-52’s starboard wing on a special pylon that provided a variety of support functions before the X-15 was launched. B-52A tail number 52-003 and B-52B tail number 52-008 were sent to North American for their transformation into the NB-52A and NB-52B carrier aircraft.

By the time the first X-15 (designated X-15-1 with tail number 66670) was rolled out on October 15, 1958 the Space Age was already a year old and there was a new sense of urgency in the program. The X-15 was the first craft ever built that was capable of sending a person into space and it had a chance of not only beating the Soviet Union, but NASA’s just announced ballistic man-in-space initiative eventually known as “Mercury”. Because of the advanced state of development, North American even proposed using an X-15 variant called the “X-15B” to be launched into orbit using Titan boosters. While NASA continued participation in the X-15 program in place of the NACA as part of their aeronautics research program, even by this early date NASA was committed to using a simple ballistic capsule for its first manned spaceflights and the North American plan lost out to the proposal submitted by the McDonnell Aircraft Company (see “The Origins of NASA’s Mercury Program”).

First Flights

In the meantime, the X-15 was being prepared for its first test flights. At the controls of the X-15 for these initial flights was North American’s test pilot Scott Crossfield – a former NACA pilot who had flown the X-1, D-558-I as well as the D-558-II and had been the first man to fly faster than Mach 2 on November 20, 1953. The first captive flight with the X-15-1 attached to the NB-52 carrier took place on March 10, 1959. The interfaces of the X-15 and NB-52 were checked as were the aerodynamic and handling qualities of the pair. Deployment of the X-15’s landing gear after being cold soaked at altitude was also successfully tested in anticipation of the first flight of the rocket plane.

For the first flight scheduled for April 1, 1959, X-15-1 was to be carried by the NB-52A to an altitude of 12.2 kilometers and dropped for an unpowered glide towards a landing on Rogers Dry Lake at Edwards Air Force Base. This flight would test the low speed handling characteristics of the X-15 and the landing procedures. The April 1 launch attempt and three more over the next two months were aborted due to a variety of issues including repeated problems with the radio and the APUs (Auxiliary Power Units). Finally, at 8:38 AM PDT on June 8, X-15-1 was dropped from its carrier aircraft at an altitude of 11.45 kilometers and began its first free flight. All was going well until the final approach for landing when the X-15’s pitch began to oscillate. Crossfield was able to safely land X-15-1 after almost five minutes of free flight. The oscillation issue was fixed by an adjustment to the flight control system.

Next was the first flight of the second X-15, X-15-2 with tail number 66671. With Crossfield in the cockpit, X-15-2 was flown fully fueled for a captive test flight under the wing of the NB-52A on July 24, 1959. In addition to checking the flight characteristics of the combined aircraft, the LOX top off system in the NB-52 was checked out. On September 17, Crossfield flew X-15-2 for the first powered flight of the program. Firing all eight chambers of the XLR-11 engines for a total of 224 seconds, X-15-2 hit a peak speed of Mach 2.11 and a maximum altitude of 15.95 kilometers during a flight lasting just over nine minutes. For Flight #3 of the X-15 program one month later, X-15-2 successfully performed a near repeat of this initial powered flight.

Flight #4 on November 5, 1959 was supposed to repeat the plan of the previous pair of powered flights in order to continue characterizing the aircraft. Unfortunately, problems were immediately encountered during engine start up when one of the XLR-11 chambers in the lower engine exploded forcing an abort after only 14 seconds of powered flight. Crossfield attempted to dump the propellants remaining in the X-15’s tanks but was unable to completely empty them because of the steep descent required to reach the abort landing site at Rosamond. Coming in with a landing mass of 6,867 kilograms (almost a ton heavier than for a normal landing), X-15-2 landed hard on the dry lakebed and broke nearly in two just ahead of its propellant tanks. The X-15 skidded to a stop after only 457 meters – less than a third of the normal 1,500-meter rollout. Fortunately, Crossfield was unharmed and X-15-2 escaped any further damage. The aircraft was returned to North American where the structural failure was found to be the result of the sheering of most of the 70 fasteners at the affected frame. In addition to fixing the crash damage, the number of fasteners were doubled and the other two X-15 aircraft were modified to reinforce their structures.

As X-15-2 was being repaired, Crossfield flew X-15-1 under power for the first time on Flight #5 on January 23, 1960 to continue testing of the new aircraft. For Flight #6 launched on February 11, Crossfield piloted the now-repaired X-15-2 on yet another flight to continue characterizing the aerodynamic characteristics of the X-15. Over the coming months the X-15 performance envelope was gradually expanded to speeds as great at Mach 3.50 and a maximum altitude of 41.6 kilometers as new USAF, US Navy and NASA pilots began flying this manned bullet. After 25 flights with the XLR-11 engines, X-15-2 made the program’s first XLR-99 powered flight on November 19, 1960 with Crossfield once again at the controls. The performance envelope would then be expanded further until the X-15 began to brush the threshold of space in 1962. While the delay in the delivery of the XLR-99 powerplant meant that the X-15 would not make the first manned spaceflight, it did mark the beginning of an unprecedented test program that blazed the trail for future aerospace planes.

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Related Video

Here is an excellent documentary produced by North American Aviation in 1959 about the early test flights of the X-15 program.

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Related Reading

“A History of Suborbital Crewed Spaceflights”, Drew Ex Machina, May 5, 2016 [Post]

“The Future That Never Came: The X-20 Dyna Soar Aerospace Plane”, Drew Ex Machina, April 10, 2016 [Post]

“The Origins of NASA’s Mercury Program”, Drew Ex Machina, December 17, 2018 [Post]

General References

Ben Guenther, Jay Miller, and Terry Panopalis, North American X-15/X-15A-2, Aerofax, Inc., 1985

Robert S. Houston, Richard P. Hallion, Ronald G. Boston, “Transiting from Air to Space: The North American X-15”, from The Hypersonic Revolution Case Studies in the History of Hypersonic Technology Air Force History and Museums Program, 1998

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 Institute, 1992