Icons of Aviation History: The X-15 Rocket Plane | Hidden History

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The rocket-powered X-15 took its pilots to the very edge of space

X-15 on display at the Smithsonian Air and Space Museum

After the success of the X-1, the National Advisory Committee on Aeronautics (NACA) and the military were interested in going further. The Air Force was working on the concept of a “space bomber“, one which could deliver a nuclear weapon to any point on Earth within an hour by flying through space. But before a workable spaceplane could be designed, there were many problems to be worked out. How fast can an aircraft actually go? Could we reach an atmospheric speed exceeding Mach 4? (four times the speed of sound, or “hypersonic“). Could an aircraft actually fly above the atmosphere? How can we control the orientation and flight path of a vessel in the airless void of outer space? How do we deal with the enormous temperatures that a spacecraft would encounter with air friction upon re-entry? And what about the pilot–how would the human body react to the conditions of weightlessness, and how do we protect it from the vacuum? At this time, no one knew the answer to any of those questions. In 1954, then NACA and the Air Force laid out a joint program to build a rocket-powered research aircraft, the X-15, to study all of these issues.

The $5.3 million contract was won by the North American Aviation Company in the summer of 1956. They were operating in unknown territory: while rocket planes like the D-558-I and II had been built before, their primary focus had been on speed, and they had not flown in the near-space conditions that the X-15 was expected to deal with.

The X-15 faced daunting physical limits. There was no way for it to carry enough fuel to power itself from ground level to the upper edges of the atmosphere, so it would need to be carried aloft by a B-52 bomber, which set stringent limits on its size and weight. The final design was just 50 feet long with a wingspan of 22 feet, and it would be made mostly of titanium metal to reduce the weight as much as possible.

During its high-speed flight, friction with the air was calculated to produce temperatures as high as 1200F, and to withstand this, the exterior skin would be made from a new heat-resistant nickel-chromium alloy called Inconel X. The skin would be made from many separate plates which were each individually shaped to produce a smooth aerodynamic surface, and this required that new methods of welding be worked out and tested. Further, since different areas of the plane would encounter different temperatures during flight, these plates were each carefully milled to their own specific thickness, which both reduced extra weight and gave maximum thermal protection.

“Flight control“ also presented design issues. In a normal airplane, changes in pitch, yaw and roll were accomplished by moveable control surfaces (ailerons and elevators) which worked by deflecting the airflow over the wings and tail. In the dense air of lower altitude the X-15 could use a similar system, which would allow it to glide down to an unpowered dead-stick landing once the rocket fuel had run out. The steep angle of descent necessary for this, however, would block off the airflow over the upper tail fin, and this was compensated for by adding another vertical tail fin below the fuselage. This, however, would then interfere with the landing skids (the X-15 used skids instead of wheels in order to save weight), and so the lower tail fin was rigged with explosive bolts and would be blown off during the landing approach.

At the extreme altitudes that the X-15 would be operating in, however, there was not enough air to make the ailerons or tail fins work, and the craft would need an entirely new system for flight control that would work in a complete vacuum. The solution was to install a set of small hydrogen peroxide attitude-control thrusters in the nose, wings and tail which would, when activated, push the aircraft up or down and steer it into the desired flight path. In the cockpit this “reaction control system“ had its own joystick controller separate from the normal one used for atmospheric flight.

Almost the entire fuselage was taken up by the two fuel tanks, one of which contained the anhydrous ammonia fuel and the other the liquid oxygen oxidizer. There was enough fuel for about 85 seconds of powered flight, though the hypersonic test flights used a pair of external fuel tanks mounted on the X-15’s belly to increase the burn time.

The rocket engine presented another design challenge, as only a liquid-fueled engine would be suitable, and these were still very new technology. It had to be small and light, but it also had to produce sufficient thrust to reach space altitudes and hypersonic speeds. The XLR-99 engine produced by the Reaction Motors company initially presented issues with overheating and melting, which were solved by circulating the cryogenic LOX...