History of Hypersonics — The North American X-15
By: Rodney Rodríguez
Tags: Aviation, Technology, Hypersonic, Experimental Aircraft

It was a sunny morning in the Rogers Dry Lake in October 7, 1967. Only the sound of wind and dust dared timidly to break the silence. Suddenly, a big “bang” came out of the blue.

It was the “sonic-boom” of the North American X-15, soaring high at an incredible speed of Mach 6.7 (6.7 times the speed of sound!) at 102,100ft.

Exactly fifty-three years ago, on that morning, USAF test pilot William J. “Pete” Knight flipped the switch to turn on the two XLR-11 rocket engines of the North American X-15 he was piloting, to shatter the world record of the highest speed ever reached by a manned airplane. That day, one of the golden pages of the history of manned hypersonic flights was written. However, it was not an official record. Apparently, to set a world record, the aircraft must make two passes (round trip) of 15 km at the same altitude, something that was never done in the X-15 tests.

The honor of being the fastest manned aircraft still belongs to the Lockheed SR-71, which in 1976 reached a speed of 3,529 km/h. Since then, the US super-spy plane has held the record for the world’s fastest mass-produced aircraft

X-15 in flight shortly after being released from the B-52 mothership. Source NASA

The Early Days of the Space Program

Eight years earlier, the first test with an X-15 had already been performed. It was a test without propulsion carried out by Scott Crossfield himself on June 8, 1959. It was the culmination of a project that had been born years before as a program that aimed to investigate all aspects of manned hypersonic flight, using an aircraft based in a concept that the former general of Nazi Germany, Walter Dornberger, who had worked with Von Braun on the V2 rocket project, had developed for the NACA (the old name of today’s NASA).

Requests for proposals were published in December 1954 for the fuselage, and in February 1955 for the engine. Finally the X-15 would be built by two manufacturers: North American Aviation would be in charge of the structure of the plane, and Reaction Motors of the engines. Both companies put the best of their engineering knowledge with the intention of making the maiden flight before 1960. With the aim of surpassing the Soviets in any field, and therefore, having a high budget, the X-15 program was a dream to work on for any scientist interested in aerospace research.

Cutaway drawing of the North American X-15 showing the various components of the aircraft. Credits: NASA Illustrations

Many were the engineering challenges to tackle during the early design phase of the air vehicle, and particularly the problems of heating in high-speed flight. During the preliminary design phase, it was of great concern whether the aircraft skin might overheat when re-entering the atmosphere in a nose high attitude.

Solving the High-Speed Overheating Problem with Minimum Overweight

Like in any other aircraft of the late 50's, stressed-skin constructions were selected to provide structural resistance while minimizing the aircraft weight. With this solution, the outer skin of the aircraft acted with a double functionality: first to distribute the stress and share the loads with the internal structure via internal bracing, and second and in this case more important, to provide a heat dissipation mean at hypersonic speed.

When engineers started to do the calculations, they benefited from a stroke of serendipity.

It turned out that the skin thickness required to withstand the aerodynamic loads was actually the same as that required to provide enough heat dissipation at the design speed of Mach 7. Thus, the proposed configuration was in a “sweet-spot” with the optimal balance between structural weight, loads distribution and heat dissipation characteristics. Just incredible!

The Flight Test Campaigns

The X-15 flights were short, each one lasting about ten minutes, so the plane used practically all of its fuel on acceleration. It wasn’t even supposed to take off by itself. For the tests, it was carried in a pylon under the wing of a NASA B-52 that took off with it and ascended until it reached a height of about 14,000 meters. Once there, the X-15 was released from the pylon and seconds after turning on the rocket engine, it began its flight.

Moments after being launched from a B-52 bomber. Source NASA

NASA test pilot Milt Thompson recalled his experience in his book To the Edge of Space:

“[The launch] was always a surprise, no matter how many times you did it. You had the feeling that the X-15 would explode once off the hooks … We pilots didn’t have much time to lose after launch. You had to get the engine to start or abort the flight. The problem was, you had to do it in a matter of seconds because you were falling fast — about 12,000ft/min. “

Joe Walker reached 354,200ft, an altitude not exceeded until 2004, when Brian Binnie reached 367,400ft on SpaceShipOne. The pilots could aim at a given altitude but were rarely able to predict exactly how high they would reach. The X-15 climbed at a speed of 3,900ft/s, so if the pilot took a second or so to shut down the engine, he could or could not reach a height never achieved by anyone.

Although the flight lasted about ten minutes from launch to landing, the time with the engines at full power lasted considerably less, approximately 85 seconds. Each mission ended in a non-powered landing after a very rapid descent. As the lift to drag ratio of the X-15 ranged from 2 to 4, depending on the Mach number, during the descents the glide path was very steep (with glide paths ranging from -26deg to -14deg) — similar to how the space shuttles would do later.

X-15 landing on dry lakebed. Observe the skids of the main landing gear. Source NASA

Because the front landing gear wheel lacked of steering control and the main landing gear used skids, the X-15 had to land on a dry lake bed. The Rogers Dry Lake near the Californian bases of Edwards and Dryden — where the flight tests were carried out — was the proposed landing location for all flights, although other dry beds were also selected in case of requiring an emergency landing.

During the almost ten years of the program, there were three accidents. Two of them occurred due to causes related to the engine system and in both cases the pilots were able to make an emergency landing and come out alive, although injured.

Only one X-15 pilot failed to land. USAF Major Michael Adams passed away in 1967, when the plane disintegrated in a hypersonic spin shortly after reentry, due to pitch instabilities generated by the experimental Adaptive Flight Control System (AFCS) that was being tested. The X-15 program ended the following year.

The rocket-powered experimental aircraft flew a total of 199 flights with 12 different pilots at the controls, from 1959 to 1968. During this time, essential data on the effects of hypersonic flight on humans and machines were obtained. This data proved highly value for NASA aerospace researchers, who would use them in their future programs, including the development of a reusable space vehicle, the first, which was also capable of putting satellites into orbit and bringing them back to the surface, the famous Space Shuttle.