Photos: Looking back at NASA's X-15 aircraft

Back in the "Right Stuff" era, even before there were astronauts, there were test pilots. It was the test pilots who flew the early generations of high-powered aircraft such as the X-1 that opened the way to space programs from Mercury to the space shuttle.

In this slideshow, we'll take a look at the X-15 rocket plane, which first flew 50 years ago this week and which NASA calls "the most successful research aircraft ever built." This image shows the X-15-1 aircraft in 1958, at which point the XLR-99 rocket engine intended for it was not ready, so it was fitted with a pair of older XLR-11 engines, like the one used in the earlier X-1.

The X-15 was launched in midair from a B-52, so that it could conserve the fuel it would need to reach super high speeds and altitudes. NASA says that the hypersonic aircraft--there were three in total, built by North American Aviation--would eventually set unofficial world records for speed (4,520 mph, or Mach 6.7) and altitude (354,200 feet). The X-15s made a total of 199 flights over a decade, from June 8, 1959, to October 24, 1968.

An X-15 takes off from its B-52 mothership, in this photo from 1959. These airborne launches took place at about 45,000 feet with the planes moving at 500 miles per hour or better. The X-15 typically was propelled by its rocket engine for about 1 to 2 minutes, then flew for 8 to 12 minutes without power before--even with its stubby wings--coming to Earth in a 200-mph glide landing.

The inaugural flight in June 1959 was a completely unpowered glide flight intended to check out the handling of the X-15. The second flight, which took place in September of that year, was the first in which the X-15 was powered.

Ignition! The rocket engine kicks in for X-15 number two. The white patches in the middle of the aircraft, according to NASA, are frost from the liquid oxygen used in the propulsion system. (Liquid nitrogen, meanwhile, served to cool areas including the nose and cockpit.) The XLR-99 engine could reach nearly 60,000 pounds of thrust.

The white sealant on the X-15 in this 1967 photo is part of a treatment to help the X-15 deal with the high temperatures generated in hypersonic flights--that is, at speeds above Mach 5. Underneath the white coating was an eraser-like "ablative" layer that also protected against the high temperatures. The tubes below the wing are external fuel tanks, while the boxy item beneath the tail unit is part of a dummy ramjet engine, a mockup in preparation for tests that never occurred.

Correction, 11:34 a.m. PDT: This caption initially gave an incorrect description of the underside of the X-15.

Here's a closer look at the X-15 with its ablative coating applied, in August 1967. The aircraft measured 50 feet long, with a 22-foot wingspan. It weighed 14,000 pounds empty and 34,000 pounds when ready for flight.

This cutaway view of the X-15 shows a number of the plane's components and features. When flying within the atmosphere, the X-15 used conventional aerodynamic controls including rudder and flap surfaces. Outside the atmosphere, it used a reaction control system involving small hydrogen peroxide thrusters in the nose and on the wingtips.

An X-15 gets a checkup after a flight, while its B-52 mothership passes by overhead.

Test pilot Scott Crossfield gets ready for an early X-15 test flight. One of 12 men who flew the X-15, Crossfield was the first, and he eventually made a total of 14 flights.

Another member of the X-15 corps was Neil Armstrong (in the cockpit), who went on to great fame as an Apollo astronaut and as the first person to step on the moon. Armstrong made seven X-15 flights.

This is what Crossfield, Armstrong, and the other pilots saw in the cockpit.

An occupational hazard for test pilots is crash landings. This one took place in November 1962, when the landing gear collapsed. The aircraft was repaired and eventually returned to duty. The pilot, Jack McKay, also recovered and was able to fly X-15s again, according to NASA. Another pilot, Michael Adams, died in an X-15 crash in 1967.

This is a view from an X-15 of the Las Vegas area in June 1965, taken by a Hycon HR-236 camera. The planes operated out of NASA's Dryden Flight Research Center at Edwards Air Force Based in California.