Photo by Frederick
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
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
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
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
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
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
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
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
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
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!”