“It gave you the greatest speed, the greatest excitement, the greatest terror. We haven’t built anything since that flies within the atmosphere like the X-15.”
— X-15 pilot Bill Dana
Bold, black and blazing fast, the North American X-15 was flown by an elite team of just 12 pilots, including Neil Armstrong, who would be the first man on the moon in 1969.
A plane unlike any other, it first flew over 60 years ago and is still the quickest manned aircraft ever to fly.
Shaped more like a bullet than a conventional plane, the rocket-powered X-15 completed 199 test flights over nine years, starting in 1959.
It could reach the edge of space and then glide back down to Earth, capturing data that informed the design and engineering of later American spacecraft, including NASA’s space shuttles.
To some it was the last research aircraft before men entered space. To others, it was the first true space plane. To its pilots, it was a unique aircraft that combined three modes of control into one: It was a jet, a spaceplane, and a glider.
They don’t make ’em like they used to
The X-series consisted of more than 60 experiment aircraft produced by US government agencies, including the Air Force and NASA, since the end of World War II. They were often extreme machines, designed to push the envelope, and the X-15 had a particularly ambitious goal.
In 1952, when the development of the X-15 started as the joint brainchild of the National Advisory Committee for Aeronautics (NACA, the precursor to NASA) and the US Air Force, the official speed record for an aircraft was just under 700 mph. The plane’s mission was to reach Mach 5 — five times the speed of sound, or nearly 4,000 mph. It would reach that, and more.
Almost more of a pilotable rocket tube than an aircraft, the X-15 was tiny. Just 50 feet long with a 23-foot wingspan, it held one pilot in a cramped cockpit. The rest of its body was filled with fuel tanks to fire its massive engine. All told, the aircraft weighed just 34,000 pounds fully fueled — not too heavy for a space plane.
To simulate orbital reentry, the X-15 flew high enough that it actually re-entered the atmosphere from near-orbital heights. This was as close to the real thing as it could get. To reach these heights and achieve speeds equivalent to those associated with orbital flight, the X-15 flew a unique flight path.
Launched from a B-52 bomber
The X-15 began its flight from a dry lakebed around Edwards, California strapped underneath the wing of a converted B-52 bomber. The X-15’s pilot, meanwhile, waited.
When the B-52 reached about 30,000 feet, roughly the altitude of a cross-country commercial flight, it released the X-15. Its pilot ignited his main engine and shot up and away from the B-52.
Reaching speeds close to Mach 7 (seven times the speed of sound), it flew like a traditional jet aircraft until it exhausted its fuel store.
Momentum carried the X-15 further until its acceleration was matched by gravity. It was weightless, floating on top of its arcing flight path for a few minutes. But the pilot couldn’t enjoy the view, which featured the curvature of the Earth from a peak altitude close to 270,000 feet.
His window was pretty tiny but, more importantly, he had to make sure his aircraft was in the right alignment for reentry.
Without any air, traditional controls were useless. And so a second set of controls were inside the X-15’s nose and wings — ballistic controls. These fired hydrogen peroxide through tiny thrust rockets to adjust the aircraft’s orientation. For these few weightless moments, the X-15 was a true space plane.
From rocket plane to glider
As gravity took over and the aircraft began to fall from its peak altitude, the pilot had the difficult task of landing the tiny airplane without any fuel — he’d spent it all during his ascent. This was a challenge.
“It flew aerodynamically like a normal airplane, but it climbed like nobody’s business,” said Christian Gelzer, chief historian at NASA’s Armstrong Flight Research Center. “Milt Thompson, who was one of the pilots, said that it was the only airplane he ever flew in which he was glad when the engine quit.”
With no fuel he had no chance to try landing a second time if he missed the runway on a first attempt. To make it harder, the X-15 wasn’t exactly a good glider. It generated some lift, but it was really designed with atmospheric entry in mind, not gliding.
“From the moment it ran out of fuel, or the pilot turned off the engine, it was a glider. A very heavy, very fast glider with very small wings — so not even a great glider. At that point, the pilot only had speed and altitude to exchange for reaching his destination,” said Gelzer.
Gliders usually have a wingspan much broader than the fuselage is long. The X-15 had the opposite arrangement.
Landing on skids at 200 mph
To slow his descent to a relatively manageable landing speed of 200 mph, the pilot traced circles over the runway. He landed on skids. They didn’t provide much directional control, but they did slow the speeding aircraft faster than any wheels and traditional brakes could.
Flying at thousands of miles per hour, the outer skin of the X-15 also became very hot due to aerodynamic friction and was therefore made of a special nickel-chromium alloy called Inconel X. “The aircraft heated up to 1,200 degrees Fahrenheit,” Gelzer said. “And the pilots could hear it expand behind them.”
This flight path wasn’t an easy one, even for the skilled X-15 pilots. On its first flight, pilot Scott Crossfield smashed the aircraft’s nose gear. Subsequent repairs set the program back a few months. Crossfield later landed so hard in the aircraft that it broke halfway between nose gear and rear landing skids.
Crossfield, however, was lucky. His crash landings were at least on the runway. Neil Armstrong, before he became an astronaut, flew one of the most dangerous flights in the X-15 program.
He took the aircraft up to his peak altitude in the upper atmosphere as planned, but couldn’t get the X-15’s nose down enough to start its fall to Earth. He was stuck, skipping along the atmosphere and moving further and further from the safety of the dry lakebed.
A near miss for Armstrong
Eventually, the friction between the X-15’s underside and the atmosphere caused the aircraft to begin its descent, but Armstrong was 50 miles south of his landing point. He could try and make it back, or try and land at a small commercial airport.
He chose the lakebed — without power, trying to get in a queue for landing would be impossible and his landing skids wouldn’t grip the asphalt. When he got back to the runway at Edwards, he was flying level with nearby trees. It was a near miss but he made it.
“By the time they got the aircraft back on the ground it was not the same airplane that it had been when it left the base. There were holes burned in from the heat,” said Gelzer.
The X-15 was retired in 1970 after setting records that remained unbroken into the 21st century. The highest flight of the program was made by US Air Force Pilot Joe Engle. At 353,760 feet or 67 miles, he was the only pilot whose flight qualified as a space flight by the FAI (International Aeronautical Association).
Out of the entire test flight program, only two resulted in crash landings, one of which killed pilot Michael Adams. On November 15, 1967, Adams went into a spin during re-entry and could not straighten the aircraft, which broke up in the air.
The inherent risks of flying this type of aircraft, half-plane and half spaceship, is among the reasons why the X-15’s records have never been beaten.
Nevertheless, the X-15 is consigned to history as one of the most successful flight research programs ever conducted, garnering a wealth of data about high-speed flight, returning from space and human physiology.
And in 1967, pilot Pete Knight reached the record speed of 4,520 mph, or Mach 6.7.
(Compilation of sources: Amy Shira Teitel, Vice.com; Jacopo Prisco, CNN.com; and NASA.gov)