The X-15 holds a unique place among aircraft

They swept the dirt from its path as the old rocket plane was rolled out of the Smithsonian's National Air and Space Museum on Tuesday night.

Beneath the spotlights, it still looked menacing, with its long, dark silhouette, tiny cockpit windows and stubby wings. Once the legend of the skies, the X-15 flew faster than a rifle bullet and lofted its pilots to the edge of space 60 miles up.

And as workers crowded around, it seemed as if it was being set for another mission. But its wings were wrapped in padding. Its skin was coated in dust. And around midnight it was plucked by a crane, lowered backward onto a truck and driven off into storage.

But for a moment as it hung over Independence Avenue, with the yellow-and-black NASA logo on its tail and "U.S. Air Force" on its fuselage, it was a reminder of the days when pilots called it "the bull," and it took them on harrowing, sometimes deadly, rides.

The Smithsonian's X-15 had hung from the ceiling of the museum since its opening in 1976. It was lowered for the first time in 43 years on Aug. 21, one of 40 airplanes that have been moved out of the popular museum on the Mall as the building undergoes a seven-year renovation.

The museum remains open, with other famous aircraft such as Charles Lindbergh's Spirit of St. Louis and the 1903 Wright brothers' Flyer still on display.

But the X-15 holds a unique place among aircraft.

It's flight was a controlled explosion. It was carried like a huge bomb, shackled to the wing of a B-52 "mother ship." The pilot sat in an ejection seat that suggested the electric chair, in front of a fuel compartment containing 2,400 gallons of liquid oxygen and anhydrous ammonia.

At about 40,000 feet, the X-15 was dropped, the engine was "lit" and the airplane streaked away, marked by a looping white contrail.

The engine burned up its fuel in about 90 seconds. It pushed the plane so fast that the NASA insignia near the cockpit often burned away in flight, and the nose had to be cooled with liquid nitrogen so it didn't melt.

And when the X-15 came back to earth, it landed with a nose wheel and rear skids that left a rooster tail of dust on the dry lakebed strip before it came to a top.

"It was, and still is, the fast highest-flying piloted airplane in history," scientist historians John Anderson and Richard Passman wrote in their 2014 book about the X-15.

The engine was liquid-fueled and had a throttle. But it operated best when the throttle was wide open and being fed 30 gallons of fuel a second, according to test pilot Milton Thompson's 1992 memoir.

"It's the world's first hypersonic aircraft," said Bob van der Linden, the Smithsonian museum's curator of special purpose aircraft. "This is in essence an early version of the space shuttle. . . . It's fast. . . . And it's really sharp-looking."

It was also a bull, Thompson recalled of his hair-raising flight in 1965. "When it decided to do something on its own, it did it. There was nothing you could do to stop it. I had momentarily lost control of the bull."

While Thompson regained control and landed safely that January day in 1965, two years later, on Nov. 15, 1967, pilot Michael Adams did not. His X-15 went into a spin, was torn apart, and he was found dead on the desert floor.

It could also be dangerous on the ground.

On June 8, 1960, the late test pilot and Herndon, Virginia, resident Scott Crossfield was sitting in an X-15 for ground test when the engine blew up. He was lucky and unhurt. (Crossfield, then 84, was killed in 2007 when his Cessna was ripped apart in a storm over northeastern Georgia.)

The X-15 is being stored in a Smithsonian facility at Dulles International Airport, said Zachary Guttendorf, a supervisory museum specialist. It will probably be in storage for the next four years, museum spokeswoman Alison Mitchell said.

Three X-15s were built. The Smithsonian has the first. The National Museum of the U.S. Air Force says it has the second one built. The third was destroyed in Adams's crash. The planes made 199 flights between 1959 and 1968 out of Edwards Air Force Base in California.

The second plane flew at six times the speed of sound - about 4,600 mph - and reached an altitude of 365,000 feet, said van der Linden, of the Smithsonian.

The museum's plane "only did 4,000 mph," he said. "It's plenty fast. It got as high as 250,000, 300,000 feet."

The X-15 was a research aircraft developed in the late 1950s to study the effects of hypersonic speed - roughly five times the speed of sound - and high-altitude flying.

It was built of super heat-resistant nickel alloy called Iconel-X, which gave it its dark color. And it was equipped with small "attitude rockets" in the nose and wings, much like the space shuttle 20 years later.

They were designed to control the aircraft at ultra-high altitudes, where the air is so thin the wings are not effective. Among the test pilots was Neil Armstrong, later to be the first man on the moon, who made seven flights in the Smithsonian's X-15.

"This was all part of NASA and the Air Force's pushing to go higher, faster and farther," van der Linden said, as he stood overlooking the plane before it was moved. "They knew very little about hypersonic flight and so they built this strictly as a test vehicle."

"The challenges . . . were remarkable," he said. "Aerodynamics was a problem. Flight control at very high altitude was a problem. When you're so high in altitude the flight control surfaces don't work. Hypersonic speed. The air does funky things."

In his 1992 book, Thompson, the test pilot, recounts the radio transmissions during the final moments of Adams's fateful X-15 flight. At an altitude of about 260,000 feet and going 3,500 mph, Adams started losing control of the plane.

The nose of the plane drifts slightly to the right, and then more dramatically to the right as the plane streaks ahead.

As the nose continues to drift sideways, Adams radios that the plane feels "squirrelly." The plane turns completely around so that it's facing backward while hurtling forward.

"I'm in a spin," Adams radioed.

"Say again," a ground controller replied.

"I'm in a spin," the pilot repeated.

Thompson was in the control room and recalled thinking: "What the hell do you mean you're in a spin? How can you spin traveling 3,500 mph? There's no such thing as a hypersonic spin."

There was no further voice contact with Adams. The X-15 came apart as it fell.

A few seconds later the pilot of a chase plane radioed: "I got dust on the lake down there."

The X-15 had crashed.

A search party was organized to look for the pieces, and a helicopter quickly found the cockpit with Adams's body still in it.

What had gone wrong?

"My personal conclusion," Thompson wrote, "was that Mike was thrown off the bull, and the bull killed him."