For an American aircraft to have an X-designation means it’s an experimental aircraft. It’s an aircraft specifically intended to answer questions about aerodynamics or design, not something that will ever be mass-produced. Some X-planes, like the X-1 and the X-15, are pretty well known. Others, like the X-10, aren't. This X-plane was a drone built with a missile in mind. It was the proof of concept aircraft built by North American Aviation in the 1950s to explore the in-flight intricacies of the company’s SM-64 Navaho supersonic intercontinental cruise missile.
The North American X-15 rocket plane in flight.
The Advent of Guided Missiles
Guided missiles appeared as an offensive weapon during the Second World War in the hands of the Germans. The V-2 missile, which launched vertically like a rocket and flew faster than the speed of sound towards its target, used a simple autopilot system to keep it on course. Two gyroscopes each controlled a pair of fins near the rocket's base, canceling out oscillations and steering the rocket along a preset flight path.
The V-2 wasn't the game-changing weapon Hitler had hoped it would be. The missiles started flying too late in the war for Germany to recover lost ground. But it did anticipate a new kind of warfare where self-guided missiles were poised to take over from bombs dropped by piloted aircraft. It was much safer, and a big enough missile could carry more destructive firepower to its target than an airplane could handle.
One V-2 in the foreground with another just launched in the background.
Immediately after WWII, missile development became a major goal for the United States military. In October of 1945, the U.S. Army Technical Service Command asked aeronautic corporations across the country to submit proposals for guided missiles; the winning design would be funded through its development. Two replies to this initial call for missiles stand out in hindsight. One response came from Convair. The company’s MX-774 missile developed over a number of years into the Atlas Intercontinental Ballistic Missile (ICBM). The other historically interesting reply came from North American Aviation. Best known as the manufacturer behind the X-15 rocket plane and the Apollo Command and Service Modules, NAA pitched the MX-770 missile that was eventually renamed Navaho.
The Navaho Program
The Navaho was designed as an interim strategic weapon, something the U.S. armed forces could have on hand while the first generation of long-range ICBMs came of age. In its final form, the Navaho would mimic the V-2’s flight path, but surpass it in terms of power, range, and accuracy.
The Navaho missile would launch vertically from a launch pad, riding piggy-back on a booster rocket. Gaining speed and altitude, the Navaho’s own engines would only kick in and take over once the missile had reached an altitude of 50,000 feet. Achieving this speed and altitude was necessary because the Navaho was designed to use ramjet engines, engines that compress incoming air without the help of a rotary compressor. They provide efficient thrust at supersonic speeds but can’t get a vehicle moving from a standstill.
Getting the Navaho surpass its German predecessor meant North American would have to develop new booster rocket technology and the largest ramjet engines then in existence. It also demanded a substantial auxiliary power unit to power and the missile's electronics and hydraulics, and a state of the art navigation system, the N-6 or NAVAN (short for North American Vehicle Auto Navigation) built by North American’s Autonetics Division.
It was, simply, a massive undertaking. By the early 1950s, the task had been broken into three stages, the first of which was to build and fly a proof of concept vehicle.
The G-26 two-thirds scale Navaho missile.
The X-10 was this vehicle, a drone designed to test the airframe, general aerodynamics, guidance, and control systems that would go into the Navaho missile. The only things missing were the ramjet engines, which were still in development.
This unmanned high performance jet looked like a sleek airplane missing its cockpit. Where a bump in the fuselage would indicate the presence of a pilot, the X-10 was flat. 71 feet long with a wingspan of 28 feet and two inches, it was powered by two afterburning XJ-40 turbojet engines that delivered 10,000 pounds of thrust apiece. The combination of a triangular delta wing body with an all-moving frontward wing (called a canard) meant the X-10 was aerodynamically sound in the transsonic and supersonic environments (passing through and flying faster than sound) but unstable without active flight control. To keep it steady, the X-10 was radio controlled by a ground operator during take-off, landing, and other flight phases.
Preliminary design of the X-10 was finished in February 1951, and the first vehicle was delivered to Edwards Air Force Base just over two years later in May 1953. It made its maiden flight on October 14, 1953. It flew just 20 flights, all of which returned a wealth of information on how the Navaho would fly, and how all of its part worked.
With the proof of concept flights complete, the second stage of the Navaho program saw development of the G-26. This was a two-thirds scale test version of the missile that brought all the pieces together.
Launched vertically by a twin-thrust-chamber liquid-fuel rocket booster just like the full-sized Navaho was designed to do, the G-26 would rocket upward, reaching Mach 3 and an altitude of 50,000 ft. At this point, the missile would be traveling at ramjet startup speed. The expended booster would fall away and the ramjets would ignite, powering the missile on its final path to its target. Between the booster and its own engines, the G-26 had a range of 2,500 miles and a top speed of Mach 2.75.
The first small-scale test missile launched on November 6, 1956. It failed. As did the second attempt. The third launch ended abruptly in a spectacular launchpad explosion. The fourth launch was a success, but the test failed when the missile’s ramjet didn't ignite.
A Navaho demonstrator on the launch pad.
The final operational version of the Navaho was the G-38. It followed the model of the G-26 but was larger and incorporated a number of new technologies including titanium gimbaled rocket engines to provide pitch, yaw, and roll control in flight, and a kerosene/lox (liquid oxygen) fuel combination. But it never flew. When the third G-26 exploded on the launch pad the future was looking grim for Navaho. The program was canceled in 1957 before a full-scale version was built.
But the Navaho did leave a legacy. Its advanced rocket booster technology was applied to other missiles, including the Atlas, that would go on to launch the orbital Mercury missions in the early 1960s. Its inertial guidance system was later used as the guidance system on the first U.S. nuclear-powered submarines.
The X-10, meanwhile, was itself briefly optioned for a second life as a cruise missile. Able to take off and fly to a target under its own power, arming it with a nuclear warhead would turn it into a formidable weapon. But the success of the Atlas and Titan ICBMs eliminated the need for a weaponized X-10.
On July 13, 1998, exactly 41 years after the Navaho program was canceled, the Air Force Space and Missile Museum rolled out the only Navaho missile still in existence and put it out on display. The X-10 remains the only missile to be classified as an X-plane. The one remaining X-10 can be seen at the National Museum of the United States Air Force in Dayton, Ohio.