Inthe annotated screenplay for First Man, author Josh Singer was asked “why start with the X-15?” for the gripping opening scene in the movie. His answer was simple: “we fell in love with the aircraft. The fastest and highest flying … ever built … [it] flew well over Mach 6 (4,520 miles per hour) and more than 50 miles high, well outside the sensible atmosphere.” Singer’s collaborator and Neil Armstrong’s official biographer, James R. Hansen, adds a fascinating historical footnote: the eponymous first man “really didn’t enjoy talking about the Moon landing, probably because that was all anyone ever asked him about. But ask him about the … X-15 and he’d talk a blue streak.”

It’s not surprising the famously taciturn pilot-first-astronaut-later Neil Armstrong was a chatterbox when it came to this remarkable aircraft. The X-15 had wings and a tail and a cockpit with controls and a pilot sitting at them who was an integral part of the system. As an aircraft, it had to be actively flown — the X-15 pilots had to continuously use of the controls to make the vehicle do what they wanted it to do.

As author Tom Wolfe wrote in The Right Stuff in 1979, there were two competing space programs in the United States in the early 1960s. The first was an ‘East Coast’ program which was attempting to harness ballistic missile technology. There was also a second, parallel ‘West Coast’ program consisting of the X-15 and other super-exotic, one-off test aircraft. Back east, they were blasting space vehicles into orbit using what was basically a controlled and focused explosion. In the west, the program was based on the more elegant notion of flying into space as a seamless extension of what was started by the Wright Brothers at Kitty Hawk a scant sixty years earlier. The unbroken arc of the West Coast story was one of Americans at their ingenious and tenacious best.

Based in Muroc, California — later and more famously known as Edwards Air Force Base — the program out west started to take shape in 1947 with the Bell X–1. Under the quiet and steady hands of Chuck Yeager, the Glamorous Glennis uneventfully flew past the previously unsurpassed and apparently misnamed sound barrier. The X-1 was carried to launch altitude beneath a re-purposed strategic bomber just like the X-15 would be in the future. Also like the X-15, the X-1 was dropped unpowered and bomb-like into the thin, cold atmosphere at high altitude. When clear of the mother ship, Yeager fired a rocket engine — one more similarity shared with the X-15 — which pushed the X-1 easily through the speed of sound which, at the 40,000 feet Yeager was flying on that October day, is about 700 miles per hour. The X-1 flew into history and permanently cemented Yeager’s reputation as the archetypical no-nonsense, cool-under-pressure, air-force-salary test pilot. All other test pilots, including those at the controls of the X-15, drew at least some of their inspiration from Yeager.

The concept behind the X-15 was simple: build an airframe capable of flying in the comparatively syrupy air in the lower atmosphere using conventional wings, tail and control surfaces. At the same time, incorporate features which would enable it to behave like a spacecraft when it began to fly — as Josh Singer would eventually say — “well outside the sensible atmosphere.” By that, Singer means the atmosphere as we know it — the one which wraps our home planet in a breathable, protective blanket and when it flows over the right kind of shape, can generate an upward force capable of lifting an aircraft into the sky.

Nearly ninety percent of this atmosphere is below 50,000 feet. It predictably continues to thin out as you go still higher until it all but disappears entirely at about 330,000 feet. This is known as the Kármán Line and where the formal concept of space begins. With effectively no air at this altitude, a lot of pretty basic ideas about how aircraft fly do not apply at all. Hence the nearly magical stuff which was required to make the X-15 do what it was designed to do — fly up through the atmosphere, behave like a spacecraft for a time and then descend through that same but now surprisingly sandpapery lower atmosphere and then finally glide, once again as an aircraft, for a relatively soft landing back on Earth.

Tohandle flight outside the “sensible atmosphere”, the X-15 incorporated reaction controls. For anybody who has seen 2001: A Space Odyssey or Apollo 13, these are the ‘thrusters’ used to control the the cinematic spacecraft. The moviemakers had it exactly right, though: apply the thruster in one direction, and Newtonian physics guarantees an equal reaction in the opposite direction. Thrust left, move right. Thrust right, move left. The X-15 had these thrusters built into the forward fuselage to control pitch — nose up and down — and yaw which is nose left and right. Similarly, thrusters were also located on the wings to roll the aircraft around its fore-and-aft axis. A limited, onboard supply of hydrogen peroxide was squirted out these thrusters under the direct control of the pilot. As the X-15 flew out of the atmosphere, the traditional control surfaces had less and less air on which to get a grip. They eventually became completely ineffective, in which case the reaction controls — which do not require air to work — were employed instead. The X-15 pilots had to truly master both systems and in particular the transition between the two. Remember, this was the early 1960s, which means controlling the X-15 was without the aid of computer systems to reduce the pilot’s workload which are present in many modern aircraft.

For its high altitude flights, the pilot let the X-15’s rocket accelerate the aircraft while pulling the nose sharply upwards towards the blue and eventually black sky. Once the fuel was exhausted — 60 to 90 seconds was typically all it took — the X-15 coasted up to apogee like a stone hurled skyward. As they went ‘over the top’ at this high point of the flight, the X-15 pilots would experience weightlessness — precisely the same weightlessness astronauts feel while in orbit. The sudden absence of gravity created its own problems particularly with respect to the orientation of the pilot and vehicle. Quite literally, it was difficult to know which way was up. The X-15 solved this problem with what looked like a supersized ballpoint pen in its nose. It was the flow direction sensor and provided the pilot with the essential orientation information used to control the flight. Not only was it a novel solution to a brand new problem, the engineering of this device was also one of the earliest exercises in managing the harsh extremes of space flight where airframe skin temperatures can soar as high as 1,900° F. The flow direction sensor not only had to survive these temperatures, but it had to keep working properly. Without the precious, detailed information it provided, the X-15 was almost guaranteed to be fatally uncontrollable no matter how proficient the pilot.

The latter flights of the X-15 had a ghostly quality to them, perhaps metaphorically foreshadowing the end of the program. In these flights, the X-15 airframe was covered in a material consisting of resin and glass bead powder and then slathered with white sealer. The resulting coating was intended to be scraped off by the suddenly abrasive air when the X-15 smoked back into the Earth’s atmosphere. Paradoxically, sacrificing the outer coating meant the underlying vehicle structure would survive the scorching re-entry. Once back on the ground, a new coating was simply applied. The design goal, even in these early days, was re-use of the vehicle rather than throwing it away after a single flight. This is still the holy grail of space flight. Present day efforts, such as SpaceX and Blue Origin for example, have reusability and its attendant cost reduction as one of the prime objectives of their designs. Space is made more accessible for more applications simply by lowering the cost of admission.

No matter how inconsequential or irrelevant it may seem, the sacrificial coating transformed the X-15 from its characteristic ultra-cool matte black colour to a milky white. At the same time, larger fuel tanks were added under the wings in order to extend the time the rocket engine would burn and shove the X-15 to even higher speeds. However, I have always felt the combination of these incremental developments spoiled the pure, Vaderesque-before-that-was-a-thing aesthetic of the X-15. I think it’s also possible that even an untrained observer back then might have concluded the program was close to running its course. That turned out to be true. However, it was not before the concept of sacrificial coatings to save the underlying, reusable structure was substantially proven. It was eventually incorporated into more modern spacecraft like the Space Shuttle. The ceramic tiles which coated the outside of the Shuttle are a direct descendent of the material and the approach first proved out on the X-15 flights.

The X-15 program reached its apex in August of 1963 with Flight 91 of the planned 200 flight program. World War II veteran aviator and NASA test pilot Joseph A. Walker accelerated the X-15 for about 85 seconds to a final velocity of 3,794 miles per hour while pulling up to an apogee of 354,200 feet. This was the highest altitude the X-15 ever reached and well clear of the space-defining Kármán Line. As he went over the top, Walker was weightless for about five minutes. The entire flight took just 12 minutes from the moment the X-15 was dropped from the mother ship until it touched down on the dry lake bed at Edwards.

Walker had actually earned his astronaut wings on Flight 90, the previous X-15 flight about a month earlier, when he flew to 347,800 feet using a similar flight profile. He was one of eight pilots to earn their astronaut wings in the X-15 and the first person to travel into space twice.

Joe Walker survived all of his 25 flights in the X-15, and in doing so became one of the most experienced pilots of the program. Ironically, in June of 1966, Walker was flying in a close formation photo shoot with the XB-70 Valkyrie when his F-104 Starfighter collided with the prototype supersonic bomber, tragically killing the pilots of both aircraft.

There is no better first-hand account of the X-15 program than At The Edge of Space by Milton O. Thompson. The main impression with which Thompson’s book left me was the X-15 was really the last flight test program where they really didn’t have much of a clue about the outcome. The pilots were on the ragged edge of known science and their first hand observations were like gold when they returned from their flights, including the 14 flown by Thompson himself and which he describes in beautiful, fine-grained detail. At the same time, however, he also describes the first computer simulations coming into use at the time. The computer models were compared to the empirical data gathered during the flights and they matched almost perfectly. In fact, the match was so good you can almost imagine non-pilot bureaucrats who would ask “so why do we the risk lives when we can accurately predict flight behaviour without ever leaving the ground?”

Supporting the bureaucrat’s argument was the death of X-15 pilot Major Michael J. Adams in November of 1967 during Flight 191. Earlier in that same flight Adams had achieved a peak altitude of 266,000 feet for which he would posthumously earn his astronaut wings. Likely as a result of some combination of technical problems with the X-15’s control systems and pilot disorientation, Adams’ aircraft ended up sliding sideways back into the atmosphere. A few moments later he entered a spin while still moving at five times the speed of sound. Adams fought to regain control of the aircraft as it continued to descend through the atmosphere at over 160,000 feet per minute. He had mere seconds to get control back and land the aircraft safely. But time quickly ran out. Adams’ X-15 was finally torn apart at around 60,000 feet, scattering the wreckage over a 50 square mile area near Randsburg, California. There would only be eight more flights in the X-15 program before it was permanently shut down in December of 1968.

The end of the X-15 program wasn’t just the end of a project after it had successfully achieved virtually all of its goals. It really represented the end of an era, which also began with the Wrights at Kitty Hawk. This was the brief period in history when brave young souls got into unproven aircraft and went up to see how they flew or to “push out the envelope.” They did that knowing full well the odds of everybody coming back were just not that good. Yet they went ahead and did it anyway.

To provide some perspective as to what was accomplished with the X-15, think of being asked to engineer a meteorite which arrives intact at a precise point on the surface of the Earth without burning up into ash. Once your metorite got there, make sure it is still in sufficiently good shape to go back and do it all over again. Also, do all of this engineering with a pencil, paper and slide rule and without much in the way of computers. Finally, think of doing this without the benefit of GPS navigation systems. They were still well over the horizon, let alone in the palm of our hands as we now expect them to be. The constellation of GPS satellites was still the fever dream of those who would subsequently work on easy and affordable access to space built on the lessons learned from the X-15’s 199 flights.

Watch any video of SpaceShipOne being dropped from the White Knight into the same Mojave sky as the X-15 and compare it with the much grainier — but still magnificent — footage captured 40 years earlier from cameras mounted on the X-15. Look familiar? It should because it’s exactly the same in many ways. The team behind SpaceShipOne, Scaled Composites of Mojave, California are likely to credit the X-15 flights for having done the heavy science and risky experimentation on which their program was built. It helped them win the $10,000,000 Ansari X-Prize for the first private space flight, in 2004. Virgin Galactic, one of the first efforts to commercialize space travel, is a direct spin-off of SpaceShipOne and therefore also owes at least some of its existence to the X-15 as well.

One of the most ambitious private, commercially-oriented space programs today is Stratolaunch. It is at least a second cousin of SpaceShipOne in that it was also a collaboration between Scaled Composites’ founder — and aeronautical engineering legend — Burt Rutan and the late Microsoft co-founder and philanthropist Paul G. Allen. In the case of Stratolaunch, however, it’s not a further effort at building a new space vehicle, but rather the mother ship from which it is dropped. The Stratolaunch is the largest aircraft, by wingspan, ever built and is scheduled to fly for the first time in 2019. It can directly trace its family tree back to the programs which began with Yeager and the X-1 at Muroc and reached their apex with the X-15. Like the Air Force bombers used in those programs, the role of Stratolaunch is to fly as high as possible above the Earth and then drop its payload into the thin, cold atmosphere at high altitude. From there the now untethered space vehicle can fly into orbit and beyond.

When they do, they will be following the still-fastest-and-highest trail blazed by the X-15.

©2018-2025 Terence C. Gannon

Thank you so much for reading. Although I have made every effort to ensure accuracy of all facts, I would love to hear from you if I have somehow missed something or have it wrong. Please leave a comment below and I’ll see it is promptly corrected. Also, this article is available as episode of the Not There Yet podcast. — TCG