Photo by Frederick A. Johnsen Retired Air Force Maj. Gen. Joe Engle enthusiastically described his experiences in the hypersonic X-15 while using a T-38 as a stand-in during Friday afternoon’s Warbirds in Review session.

August 2, 2009 - Oshkosh, Wisconsin  - When a pilot needs an X-15 in a hurry and none is available, a glossy black supersonic T-38 makes a fine stand-in. 

Retired Air Force Maj. Gen. Joe Engle enthusiastically described his experiences in the hypersonic X-15 while using a T-38 as a stand-in during Friday afternoon’s Warbirds in Review session. 

The NASA/Air Force/Navy X-15 program has been called the most successful X-plane program. The three X-15s performed 199 flights between 1959 and 1968, hitting speeds as high as Mach 6.72—or 4,535 miles an hour— and climbing to an altitude of 67 miles. One was lost with its pilot, Air Force Maj. Mike Adams, in a crash in the Mojave Desert. 

General Engle circled the T-38 as he described the span of the X-15 rocket plane at about 22 feet compared with the T-38’s marginally larger 25 feet; the two are similar in size. But that’s where the likeness evaporated; the aluminum of the mildly supersonic T-38 would melt at the 1,200-degree Fahrenheit skin temperatures endured routinely by the Inconel alloy of the X-15. 

The X-15 grew more than 3 inches in length as it heated, General Engle explained to the audience. 

That made for some exciting times when the airframe stretched, but cables holding landing skids in their retracted position did not extend because they were in a cooler interior environment; that caused the skids to extend at an inopportune time, he recalled. 

General Engle walked around the tail of his T-38 stand-in, pointing to where the far aft-mounted skids were located on the X-15. Unlike conventional landing gear that reside somewhere near the aircraft’s center of gravity, the extreme aft location of the X-15’s main skids radically altered the rocket plane’s landing profile. “As soon as the main gear touched down, the nose slammed down,” General Engle explained. 

The X-15 was specifically designed to land on the hard clay bed of Rogers Dry Lake at Edwards Air Force Base. In the event of an in-flight emergency, missions had a ground track that permitted emergency landings on any of a number of other dry lakebeds that provided flat havens in the otherwise rough terrain of Nevada and California.

General Engle animated the X-15 program with his remarks. He said the idea was to build something that would go faster than five times the speed of sound “and go into space.”

With 50 miles defining the edge of space, General Engle and several of his X-15 pilot compatriots earned astronaut ratings for their exploits in the winged rocket. He was the youngest to do so on June 29, 1965, when he nosed the X-15 through an arc that topped out at 280,600 feet.

At such altitudes, the X-15 required reaction rocket motors for attitude control; the aircraft’s aerodynamic controls were useless above the atmosphere. The X-15 first employed a separate set of controls for each system. When this proved less than desirable, Honeywell devised a unified cockpit control system that sensed when rocket augmentation was necessary.

An X-15 ride was not a stunt to set speed and altitude records, although such records were a byproduct of the research program. Engle described two basic mission profiles and purposes in the X-15 program.

One variation was “speed, or heating missions,” he said, to gather skin friction data. Heat was the goal; speed was the means.

The X-15 used a heat-tolerant metal alloy to survive, and its speed missions provided a data base on high-temperature survivability.

The other mission was an altitude profile with piloted flight into and back from the edge of space. “Not just fall back in a can with a parachute,” General Engle said.

Altitude profiles provided about four minutes of zero gravity. General Engle described a precise pitch angle profile that must be followed to keep from skipping back up in altitude as the ever-denser atmosphere pushed on the airframe as it descended.

Rate of climb of the X-15 could reach 50,000 feet a minute with its 57,000- pound thrust XLR-99 engine, he said. This thrust was nearly 10 tons more than the gross weight of the X-15. “The altimeter was spinning so fast you couldn’t read it,” he remembered.

“It was just a blur.”

As rocket fuel burned off, the X-15 became lighter and handled like a fighter, very responsive in roll, pitch, and yaw, General Engle said. “How beautiful it flew after burnout.”

He recalled jousting with the F-104 chase planes that escorted the X-15—at that point the world’s fastest glider—on its return to Earth.

“It was fun to turn into them,” fighter pilot Engle said.

But such hijinks were only momentary; X-15 pilots relied on the F-104s to help with landing cues for the rapid rocket.

On one mission, Engle noted the dry lakebed at Edwards passing beneath him at about 45,000 feet. Concerned that he might overshoot, he rolled the empty X-15 and lost altitude that way. “To fighter pilots a roll is a very natural maneuver,” Engle told his crowd. The data plotters of the era went off the page during the maneuver, making proof of the roll inconclusive until after looking at the motion picture film from an onboard camera several days later.

Engle was called on the carpet for that roll in the experimental rocket plane. He explained his predicament and his rationale, and was told, “I guess that’s the right thing to do, but don’t do it again!”