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Flight Plans Newsletter

07. Redefining the Limits: the X-15 Program

By Bret Grady – a senior at Mt. Rainier High School and a volunteer with The Museum of Flight Museum Apprentice Program

From left to right, pilots are: Joe Engle, Robert Rushworth. Phot courtesy of Dryden Research Center (NASA)

October 4, 1957: The USSR launches Sputnik, the first artificial satellite to orbit Earth. Radio and television broadcasts, as well as newspaper headlines, report on the basketball-sized object for days on end.

April 12, 1961: Yuri Gagarin becomes the first man to travel into space, receiving media coverage comparable to that of the Sputnik launch four years earlier.

August 4, 1960: Joseph A. Walker sets a new flight speed record of Mach 3.31 (2,196 miles per hour, or three times the speed of sound) in the X-15. Walker receives little media attention for his feat.

August 12, 1960: Robert M. White sets a new flight altitude record of 136,500 feet in the X-15. Like Walker, White’s accomplishments go almost unnoticed by the general public.

Where did the little black rocket-plane go wrong? Why was it relegated to the catacombs of history instead of taking front seat? More than likely, it was due to the public’s interest in orbital (space) flight rather than suborbital flight. Yet despite the plane’s lack of popularity, its mission was vital: conducting research on high-speed, high-altitude flight, which the United States would apply to spaceflight, giving them a lead over the Soviets in the Space Race.

The Plane

The X-15 was a research aircraft built in response to a Request for Proposal put out in 1954, asking for an aircraft capable of hypersonic, high-altitude flights. North American Aviation, Bell, Douglas, and Republic all submitted designs, with North American Aviation’s design being deemed most suitable for the mission. The company manufactured all three X-15s that were produced. Essentially, the plane was a fuel-filled rocket engine loaded with instrumentation. It was a mere 50 feet long and had a stubby pair of wings with a span of only 22 feet. The instrumentation alone weighed 1,200 pounds. The fuselage was made of a high-tech, heat resistant alloy called Inconel X-750 to keep the plane from burning up at high speed. In 1964, X-15 #2 was augmented with external fuel tanks and an ablative coating (a spray-on coating to protect against heat), enabling it to achieve speeds of nearly Mach seven. The plane was renamed the X-15A-2.

The Pilots

The program drew pilots from North American Aviation, NASA, the U.S. Air Force and the U.S. Navy, many of whom would receive astronaut wings during or after the program. Pilots Neil Armstrong and Joseph Engle would go on to achieve fame in the U.S. space program: Neil Armstrong for the Gemini and Apollo missions; Joseph Engle for Shuttle missions.

The U.S. Air Force considered any flight reaching an altitude over 50 kilometers a space flight, so it awarded astronaut wings to pilots Michael Adams (USAF), Bill Dana (NASA), Joseph Engle (USAF), Pete Knight (USAF), John McKay (NASA), Robert Rushforth (USAF), Joe Walker (USAF) and Robert White (USAF). The FAI (Fédération Aéronautique Internationale) standard for space flight was any flight reaching an altitude over 100 kilometers, so only Joe Walker qualified while in the program (Neil Armstrong and Joseph Engle would qualify later in their careers).

A Typical Mission

Two of the primary reasons for the development of the X-15 were to test how fast, and how high, a human pilot could go.

A typical X-15 flight would start with the aircraft being fueled and mounted under the wing of a B-52 Stratofortress. The bomber would take off, climb to 45,000 feet, and reach a speed of Mach 0.8. At that point, the X-15 would be released from the bomber and the X-15 pilot would switch to his secondary stick (capable of altitude adjustment only) and engage his engine. If the flight was a speed test, the pilot would remain straight and level. If the flight was an altitude test, the pilot would climb to a peak altitude of 250,000 feet (more or less, depending on the mission). After the plane ran out of fuel and the pilot had lost enough speed, he would switch to a typical joystick for landing. He would deploy his gear, touching down with tail skids first to slow the craft and keep it from bouncing off the runway.

The X-15A-2 continues to hold the record for fastest human-operated, rocket-powered aircraft. Peter Knight set the bar for the speed record on October 3, 1967, when he took the plane up to a speed of 7,274 kilometers per hour on Flight 188.

Joe Walker set an altitude record on Flight 91 on August 22, 1963. He flew to a height of 107,960 meters, which was the unofficial world record for highest, manned rocket-plane flight until 2004, when it was broken by SpaceShipOne.

The Aftermath

In total, 199 missions were performed by the three X-15s. There was only one fatal crash in the program. The X-15 ended its career on October 24, 1968. X-15 #1 was sent to the National Air and Space Museum in Washington D.C., where it is now on display; X-15 #2 was reconfigured to its original form and sent to the National United States Air Force Museum in Dayton, Ohio, where it is currently on display