"The drop from the B-52 carrier aircraft was pretty abrupt, and then when you lit that rocket a second or two later you definitely felt it," said Joe Engle, another X-15 test pilot and member of the same exclusive fraternity of flyboys that included Crossfield and the eventual first man on the moon, Neil Armstrong. All took the X-15 to speeds and altitudes that extended the frontiers of flight.

The X-15 was a research scientist's dream. The experimental, rocket-boosted aircraft flew 199 flights with 12 different pilots at the controls from 1959 through 1968. It captured vital data on the effects of hypersonic flight on man and machine that proved invaluable to the nation's aeronautics researchers, including NASA and developers of the space shuttle.

"That first powered flight was a real milestone in a program that we still benefit from today," said Engle.

Engle knows what he's talking about.

The Kansas native flew the X-15 for the U.S. Air Force 16 times from 1963 to 1965 and went on to command two missions of NASA's space shuttle.

Still an active pilot, the retired major general fondly recalled what it was like to fly the X-15 and how lessons learned then made possible the space shuttle program years later.

"It was a very busy airplane to fly, but it also was a beautiful airplane to fly; a very, very good solid flying vehicle. Particularly when you were subsonic, in the landing pattern- even at the lower supersonic speeds," Engle said. Three times Engle flew an X-15 higher than 50 miles, officially qualifying him for Air Force astronaut wings and providing him a brief moment for sightseeing at the edge of space.

"I didn't really have time to soak up the view in the X-15 like I did later when I flew the space shuttle," Engle said.

"You could glance out and see the blackness of space above and the extremely bright Earth below. The horizon had the same bands of color you see from the shuttle, with black on top, then purple to deep indigo, then blues and whites.

"You were just so terribly busy flying the airplane, keeping everything under control and watching for any deviations. And in many cases, during re-entry flights for example, making sure the airplane was perfectly lined up as you started to enter the atmosphere."

Engle credits the X-15 for laying the foundation for many of the operational techniques of the space shuttle, and for providing designers with confidence that certain design and control concepts for the winged orbiter would work: With similar flying characteristics, the X-15 proved the shuttle could re-enter the atmosphere and glide to a precision landing, in part relying on a maneuver known as Terminal Area Energy Management where speed and altitude are carefully controlled so the vehicle can reach the runway instead of falling short or overflying it.

Using technology developed and tested on the X-15, pilots learned how to transition control smoothly from reaction control jets at high altitudes or in space to wing- and tail-mounted control surfaces in the atmosphere closer to the ground.

While not a benefit to the space shuttle alone, the X-15 was the first flight test program to make extensive use of simulators to work out certain problems and train pilots before going up-a practice since employed for nearly every flight test program.

The X-15 flights proved the usefulness of having chase aircraft follow a test vehicle during its approach to the runway to make sure, as Engle put it: "Everything that is supposed to be up is up, and everything that is supposed to be down is down."

The X-15 was suggested in the early 1950s by Bell Aircraft's Walter Dornberger as a vehicle for exploring the realm of hypersonic flight, which was defined as a speed in excess of Mach 5, or five times the speed of sound. The earliest days of the X-15 program were shaped by the National Advisory Committee for Aeronautics, the federal agency which NASA replaced in 1958.

The NACA, Air Force and Navy all had an interest in the program and all provided resources, including pilots. Eventually the Navy stopped supporting the X-15 in order to concentrate on aircraft carrier operations, Engle said.

By the time contracts for the airframe and engine were signed with North American Aviation in 1955 and Reaction Motors in 1956, the program had goals of flying the X-15 to a speed of Mach 6 and an altitude of 225,000 feet.

"It was a pretty aggressive move, a pretty gutsy step. We had reached Mach 1, 2 and even 2.5 in other airplanes. But then we lost a pilot when he crashed in one of those airplanes after reaching Mach 3," Engle said. "So the next step was Mach 6?"

As the prime contractor for the X-15 airframe, North American Aviation was responsible for checking out the vehicle before turning it over to the NACA, Air Force and Navy team so research flights could begin. It was up to the company's chief test pilot, Scott Crossfield, to take the controls for the initial flights.

Crossfield flew a handful of captive flights with the X-15 slung beneath the wing of a B-52 mother ship. Some were intentional and some were not, as initial attempts for a drop test were aborted. Crossfield and his rocket plane finally were released from the B-52 on June 8, 1959, to make an unpowered glide to the lakebed below at Edwards Air Force Base in California.

With the X-15's primary rocket engine, the XLR-99, still a few months away from being ready to fly, two of the smaller XLR-11 rockets were installed into the X-15 for Crossfield to use in making the first powered flight on Sept. 17, 1959.

The X-15 worked as anticipated that day, reaching a modest altitude of 52,341 feet, but easily breaking the sound barrier and recording a top speed of Mach 2.11 during the nine-minute flight.

"It was a big step, you bet," Engle said. "It showed that the propulsion unit was compatible with the airframe and that it would work. Crossfield was able to demonstrate the airplane would launch, fly free from the B-52, and that it could go supersonic without picking up any handling problems going through the transonic region."

The X-15 set world records for altitude and speed, but more importantly the research conducted during those test missions provided data that would benefit future operations and investigations related to aeronautics and spaceflight.

"I think they far exceeded what they thought was going to be the design parameters for the X-15 program. They wanted to achieve Mach 6 and they got to Mach 6.7. The design altitude was 225,000 feet and (NASA pilot) Joe Walker got it to 354,200 feet," Engle said.

But reaching those numbers didn't automatically allow the X-15's designers and pilots to declare success, Engle said. The whole process they went through to get to that point is where the lessons were taught and learned, sometimes harshly. In 1967, Air Force pilot Michael Adams was killed in the crash of an X-15.

"You learn so very much during the envelope expansion. Yes, there are some potholes that live within the envelope that you have to learn how to solve or avoid. But it's just as valuable to learn those as it is the lessons that wait for you at the edge of the envelope," Engle said.

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