When NASA pilot Bill Dana brought the X-15 to a halt on Rogers dry lake on October 24th 1968, it marked the end of the research plane’s flying career. At the time there were hopes that a final 200th flight could be made before the end of the year, but following a number of cancellations and aborts it wasn’t to be.

The X-15 programme drew to a close just as the world’s attention turned to the Moon with Apollo 8’s successful lunar orbital flight and the push towards a landing during 1969 to meet Kennedy’s goal.

Dana’s final flight was made in the first X-15, the only remaining airworthy vehicle of the three originally constructed by NAA. The second airframe, converted to the X-15A-2 configuration had been badly damaged during Pete Knight’s record breaking Mach 6.7 flight and although repaired, never returned to flight status. The third aircraft was tragically lost along with USAF pilot Mike Adams following a loss of control and hypersonic spin on November 15th 1967. The effective loss of these two aircraft put an end to plans for follow-on configurations which had aimed to expand the X-15’s flight envelope.

Although the lifting body programme ran concurrently with the latter part of the its career and carried on until the mid 1970s, there was no direct successor to the X-15’s mantle as the fastest and highest flying of the research planes. What had looked at the outset of the programme as just another step in aviation’s steady evolutionary path into space turned actually marked the end of the line for many years. As the NACA transformed into NASA and competition with the Soviet Union became the prime motivator behind human spaceflight, so the winged route into space fell from favour when compared to the faster missile and capsule approach. When Project 1226, which became the X-15, was first envisaged during the mid-1950s some within the NACA questioned whether including the so-called ‘Space Leap’ requirement was even necessary given how far in the future the practical requirement for human spaceflight seemed.

The X-15 Legacy
It can be tempting to look at the X-15 and NASA’s higher profile human spaceflight endeavours, Mercury, Gemini and Apollo as operating in isolation from one and other, but in reality the X-15 did influence the capsule programme in a number of ways. Although not the first cooperative project between the parties, the collaboration between the NACA (later NASA), Air Force and Navy on the X-15 helped pave the way for the close collaboration needed on other major space projects. While the NACA provided the technical direction for the programme, the Air Force was responsible for managing the construction of the vehicle with the Navy providing much of the human-factors and biomedical expertise. All three bodies contributed funding to the project, although the bulk of the responsibility fell on the military. Each body also contributed pilots to the programme. This collaborative arrangement held throughout the development and flight test phases of the programme and was deemed highly successful by those involved. As NASA’s capsule based programmes also required significant cooperation and contributions from the military, the X-15 experience provided a useful reference.

One of the most notable contributions made by the X-15 programme towards the fledgling Project Mercury was the experience in tracking and control gained via the establishment and use of the High Range. The Mission Control model that became so closely associated with the orbital and lunar programmes was a direct descendant of the network established to support the X-15’s flights over the desert, with multiple radar stations reporting to a central control room where a single point of contact – always a fellow pilot – would communicate with the X-15’s pilot. Walt Williams, so influential as an architect of this system as manager of the Flight Research Center, was transferred to NASA’s Space Task Group at their Langley centre, later becoming Director of Operations for Project Mercury.

NARA RG-342-B1957, Wright Field, centrifugre, XMC-2 pressure suit
Huge amounts of human factors work carried out for the X-15 programme contributed to later NASA projects [IMG: NASA]
The X-15 also made many important contributions to the understanding of human factors for spaceflight. When the prospective Mercury astronauts first climbed into the centrifuge at Johnsonville, they were following in the footsteps of the X-15 pilots who had used this facility extensively during the early phases of the project. An interesting observation gleaned during the early X-15 flights was that the pilots tended to exhibit extremely high heart rates during periods of intense workload and dynamic stress, but this appeared to create a state in which they could work rapidly and effectively – some commented that it was as if time had slowed for them. The X-15 also pioneered the development of side stick cockpit controls for use under high G conditions, although as it happened Mercury astronauts would get to use these in space before an X-15 pilot got the chance. The X-15 did however provide a great deal of useful data on the pilot’s ability to accomplish flying tasks while under high positive and negative G conditions which became more relevant to later programmes where lifting reentry profiles were flown in capsules.

As the X-15 moved away from it’s original mission having achieved its initial design and research goals by 1963, it increasingly became a flying laboratory carrying out an extensive follow-on programme whereby it carried a range of experiments beyond the atmosphere or into required dynamic flight regimes. Many missions were undertaken in support of other NASA programmes including Apollo with the X-15 providing a convenient means to test the properties of insulation and ablative materials.

Famous alumni

Armstrong suiting up for Flt 1-18-31 11/30/60 NASA DFRC
Neil Armstrong preparing for an X-15 flight in 1960 [IMG: NASA]
There was another significant contribution that the X-15 programme made to the moon-race. Two of the X-15’s pilots went on to be selected as NASA astronauts. Most famously, Neil Armstrong was selected in NASA’s second astronaut intake, the so-called ‘next nine’, following his time as a research pilot in the Mojave which included 7 X-15 flights and a substantial amount of work in support of the Air Force’s X-20 Dyna-Soar project. Interestingly Armstrong was named as an astronaut for both the Air Force’s Man In Space Soonest (MISS) and Dyna-Soar programmes in spite of being a civilian research pilot at the time and was NASA’s first civilian astronaut. Cleary he was destined to fly in space sooner or later!

Engle with X-15A-212/2/65 NASA DFRC
Joe Engle became the only pilot to fly both the X-15 and Space Shuttle [IMG: NASA]
Joe Engle joined the X-15 programme as an Air Force research pilot in 1963 and went on to make sixteen flights, two of which exceeded the 50-mile altitude mark earning him his Air Force astronaut wings. Engle joined NASA in 1966 and trained as Lunar Module pilot for Apollo 17 only to lose his seat to Harrison Schmitt when it became clear this would be the final lunar expedition. Engle transferred to the Shuttle programme flying many of the Approach and Landing Test (ALT) flights at Dryden before commanding STS-2, the second orbital test flight and becoming the only person to pilot the X-15 and Shuttle.

Follow-ons that weren’t to be

NASA LaRCTerry Panopalis Collection
Large amounts of work were done on the delta wing follow-up configuration, but the loss of X-15-3 put an end to these plans [IMG: NASA]
Many follow-on developments were proposed for the X-15 with some, such as an orbital version of the vehicle proposed by NAA post-Sputnik, being more far fetched than others. Probably the most seriously researched option was fitting an X-15 with a delta wing to research the suitability of this configuration to high heats and dynamic pressures. This research would have had obvious application to a number of proposed Air Force spaceplane projects as well as proposed hypersonic aircraft and would likely have contributed data of use to the later Shuttle programme. Unfortunately this project relied the availability of sufficient funding and one of the existing airframes. X-15 3 was slated for the conversion but when this was lost in 1967 the chance for a delta winged X-15 went with it.

Even during the X-15’s design and construction, both the Air Force and NACA were looking to what the next logical step would be. The Air Force had developed an interest in the idea of winged space vehicles following evaluation of research carried out by Eugene Sänger and Irene Bredt during the 1930s and 40s into a spaceplane capable of skipping on the atmosphere to prolong its range. Under the Nazi regime Sänger’s Silbervogel became an ‘Antipodal’ intercontinental bomber and following the fall of Nazi Germany, both the USA and USSR became fascinated by the concept as a means to deliver atomic weapons.

Early in the X-15’s flight programme it was still expected that it would lead to a line of Lifting Reentry vehicles. Here we see the X-15 shown leading to the Dyna-Soar and beyond [IMG: NASA]
While further analysis by the NACA proved that the atmospheric ‘skip-glide’ technique would be impractical due to the repeated extreme temperatures and dynamic loads, this did lead to the idea of a ‘boost-glide’ vehicle which would initially be boosted into space vertically by a rocket, then follow a shallow gliding path to a controlled reentry and landing. Bell Aircraft were instrumental in pursuing this idea with the help of Werner von Braun’s former Commander at Peenemünde, Walter Dornberger. This led to a variety of Air Force programme including BoMi (Bomber Missile), RoBo (Rocket Bomber), Brass Bell and HYWARDS all aiming to produce an exoatmospheric weapons system.

The X-20 Dyna-Soar represented a direct descendant of the X-15 and aimed to probe the next steps in speed and altitude [IMG: Wikipedia via CC licence]
Concurrent to these Air Force projects the NACA Ames laboratory in California put forward plans for a Mach 10 capable research aircraft to cover the flight regimes beyond the X-15’s design limits. Where research aircraft such as the X-1, D-558-11 and X-2 were considered ‘Round One’ and the X-15 ‘Round Two’, the proposed Ames vehicle would have represented ‘Round Three’ of a prolonged programme to produce high-speed winged access to orbit. The planned Air Force boost-glide weapons systems would have benefitted from both the X-15 and Round Three research. As it was, the Ames Mach 10 design never went much further than a paper study and by the time the Air Force’s various plans coalesced into what became Dyna-Soar (after Dynamic Soaring – it later gained the X-20 designation) the design that emerged was an uneasy mix of research aircraft and weapons system – a joint arrangement that led to constant programme re-evaluation and eventually helped lead to its demise.

The cancellation of Dyna-Soar dealt a blow to the Air Force’s plans for an orbital spaceplane for many years [IMG: Wikipedia via CC licence]
It will always be a matter for conjecture whether an effective follow-on to the X-15 could have led to a more practical space access system to follow the politically led Apollo moon landings. When NASA again started to seriously examine a winged route to space leading to the Space Transportation System (STS – better known as the Shuttle), there were many unanswered questions about flight dynamics and materials that may well have benefitted from the experience of a vehicle such as Dyna-Soar. As it was, many of the lessons learned by the X-15 were of use to the shuttle designers, indeed both it and the lifting bodies which arose alongside it had a direct influence on the decision not to include landing engines in the Orbiter. Sadly, budget cuts and design compromises led to a Shuttle that could never deliver the rapid airplane-like flight operations and high flight rates originally envisaged for it and although the idea of a reusable spaceplane is regularly revived, NASA has reverted to the expendable rocket and capsule model for its SLS/Orion system.

Over half a century has now elapsed since the X-15’s first tentative flights into space. While the Shuttle Orbiter and SpaceShipOne may have exceeded its record speed and altitude marks for winged vehicles, the X-15 still remains one of the most successful research vehicles of all time. Prior to its first flight the unofficial altitude record, attained by Iven Kincheloe in the X-2, was 126,200ft. The unofficial record speed was Mach 3.19 – also recorded in the X-2 on Milburn Apt’s fatal first flight. The X-15 was able to stretch these out to 354,200ft and Mach 6.7 respectively. Rather than offering an interim step between aircraft and fully reusable spaceplanes, the X-15 became something of an endpoint. This evolutionary line may be picked up again in the future as the legacy of the lifting bodies lives on in SNC’s Dream Chaser, but for now the X-15 remains an amazing footnote in mankind’s conquest of space.

On August 23, 2005 NASA finally bestowed the title of Astronaut to Joe Walker, Bill Dana and Jack McKay for their flights above 50 miles. For Walker and McKay this award came posthumously. Walker’s final flight to 354,200 feet on August 22 1963 marked the last time a NASA pilot or astronaut flew a solo spaceflight.

Disposition of remaining X-15s
X-15 Number 1 (66670) is currently on display at the Smithsonian National Air & Space Museum, Washington DC – More details here

X-15A-2 (66671) restored to its original black finish is on display at the National Museum of the US Air Force at Wright-Patterson AFB Ohio – More details here

Part 1 of this article, Toward the Unknown can be found here

Part 2 of this article, Riding the Bull can be found here

At the Edge of Space – Milton O Thompson

Illustrated History of Space Shuttle: US Winged Spacecraft X-15 to Orbiter – Melvyn Smith

The X-Planes: X-1 to X-45 – Jay Miller

Hypersonic: The story of the North American X-15 – Dennis R. Jenkins & Tony R. Landis

The X-15 Rocket Plane: Flying the First Wings into Space – Michelle Evans

The Right Stuff – Tom Wolfe

X-15: Extending the Frontiers of Flight – Dennis R. Jenkins

Hypersonic Before the Shuttle: A Concise History of the X-15 Research Airplane – Dennis R. Jenkins

Space Shuttle: The History of the National Space Transportation System – Dennis R. Jenkins