The X-15 Rocket Plane

Introduction

After World War Two, the United States as well as the United Kingdom and Soviet Union all began programs to develop rocket propulsion as follow on to the German V2. While the main focus of these programs was the deployment of ballistic missiles for defense, much thought was given to the potential for the development of rocket powered space planes.

As result of a study sponsored by the National Advisory Committee for Aeronautics, predecessor to NASA, a concept was developed for a new vehicle to be part of the X series of experimental aircraft. An RFP for an airframe was issued in December, 1954. In February of 1955 a request for proposal was sent for rocket propulsion. With these actions the X-15 research and test vehicle program began.

X-15 Background

The X-15 was a hypersonic rocket powered craft operated jointly by the United States Air Force and NASA. The goals of the program where straightforward. Determine how airframe structures, materials and control surfaces would preform at hypersonic speeds and extreme altitudes. The X-15 flew 199 missions and not only exceeded the goals of the program but supplied far more information than anticipated. This program is one of the most successful flight research programs in history and paved the way for human space flight.

Each X-15 flight was short, less than 10 minutes. The rocket plane was carried to about 45,000 feet under the wing of a specially modified B-52. After the aircraft reached planned altitude, the X-15 was dropped, its rocket engine ignited and burned for 85 seconds. Each mission ended in an unpowered landing after a steep descent. The pilots flew a low-lift glide that helped develop the energy management techniques that Shuttle pilots would use many years later. Only one X-15 pilot did not make it back for a landing. Air Force Major Michael Adams was killed in 1967 when the aircraft broke apart in a hypersonic spin shortly after reentry. The program ended the following year.

The Mission

The X-15 was operated under several different scenarios, including attachment to a launch aircraft, drop, main engine start and acceleration, ballistic flight into thin air/space, re-entry into thicker air, unpowered glide to landing, and direct landing without a main-engine start.

Because the X-15 also had to be controlled in an environment where there was too little air for aerodynamic flight control surfaces, it had a reaction control system (RCS) that used rocket thrusters.

The X-15 program set records for altitude and speed in a piloted vehicle. During November, 1966 the X-15 set a speed record of 4,520 miles per hour (7,274 KPH) which as of today, still stands. The following year, October, 1967 mission #91 entered sub-orbital space, the second X-15 mission to do so. The altitude achieved during this flight was 354,200 feet or 108 kilometers. Space as defined by the Karman Line is 100 kilometers. This record was not broken until October 2004 when SpaceShipOne, a civilian space plane flew to an altitude of 112 kilometers meters.

The Vehicle

The airframe was built by North American Aviation, which after a number of mergers and acquisitions is now part of Boeing. The initial 24 flights used two Reaction Motors XLR11 liquid-propellant rocket engines, enhanced to provide a total of 16,000 pounds of thrust. By November 1960, Reaction Motors was able to deliver the XLR99 rocket engine, generating 57,000 pounds of thrust. The remaining 175 flights of the X-15 used XLR99 engines, in a single engine configuration. Reaction Motors eventually became part of Morton-Thiokol and is now part of Northrup Grumman.

Three X-15s were built, flying 199 test flights, the last on 24 October 1968.

The first X-15 flight was an unpowered glide flight by Scott Crossfield, on June 8, 1959. Crossfield also piloted the first powered flight in September, 1959. His first flight with the more powerful XLR-99 rocket engine took place November 15,1960. Twelve test pilots flew the X-15. Among these were Neil Armstrong, later a NASA astronaut and first man to set foot on the Moon, and Joe Engle, later a commander of NASA Space Shuttle missions.

Legacy of the X-15

The X-15 program was unique and its importance cannot be understated. No other space faring nation at the time had a comparable program. Benefits derived from the 199 missions allowed NASA to eventually develop the flight profile used for the Shuttle program. The X-15 demonstrated a pilot’s ability to control a high performance vehicle in a near space environment and new knowledge was gained on the techniques and problems associated with non-powered lifting body reentry.

Eventually, manned single stage to orbit space planes, using a horizontal take-off profile, will be in service. The promise and legacy of the X-15 will finally then be realized.