An X-15 contrail after launch. (NASA)
The X-15 was not the first rocket-powered aircraft, but it is probably the best one ever built and flown. Before the first X-15 took flight in the late 1950s, the fastest speed airplanes had reached was Mach 3. The X-15 doubled that. And, remarkably, it also went on to fly into space more than a dozen times.
The US Air Force and NASA developed the X-15 to better understand flight under extreme conditions, including reentry through the Earth’s atmosphere. Yet more than half a century later, the exceptional plane still holds the world record for speed by a piloted, powered aircraft after William Knight flew the vehicle at Mach 6.70 in 1967.
The X-15 program also boasts an exclusive club of pilots—only a dozen aviators can claim to have flown the aircraft, which made 199 flights in total. (They were all men, given the era.) Before he landed on the Moon, Neil Armstrong flew seven X-15 missions between 1960 and 1962. The movie First Manvividly depicts one of these flights.
Another X-15 pilot, Joe Engle, also went on to a remarkable career at NASA. Slated to land on the Moon during Apollo 17 alongside Eugene Cernan, Engle got booted from that mission for largely political reasons so that a scientist, Harrison Schmitt, could be added to the final Apollo mission. Later, Engle would command the second space shuttle mission.
Engle is also the last of the X-15 pilots still alive. So to mark the 60th anniversary of the X-15’s first powered flight, during which Scott Crossfield took the plane up to 52,000 feet on September 17, 1959, Ars spoke with Engle about flying the baddest airplane the world has ever known. Spoiler alert: It was thrilling.
NASA nixed
Engle graduated from the US Air Force Test Pilot School in 1961, and he served in the fighter test group at Edwards Air Force Base in California. Two years later, as NASA’s efforts to send humans to the Moon began to gain traction, Engle and many of the other Air Force test pilots, such as Mike Collins, applied to become an astronaut as part of the agency’s third class in 1963. That summer, Engle recalls being summoned to the office of the commander of the Air Force test center at Edwards, Major General Irving Branch, a legendary figure who had led Pacific bombing raids during World War II.
As he walked to Branch’s office, Engle tried to figure out what he might have done wrong. Branch ordered Engle to sit when he entered. “I see you’ve applied for the NASA astronaut program,” Branch told Engle. “Don’t you like it here?”
The test pilot allowed that yes, he had applied to NASA for its third class of astronauts. It seemed like the next logical step in his career, and he liked the idea of going to the Moon. This did not mollify Branch. As Engle tells the story, he said Branch held up his NASA application, and tore it in half.
“Well, I think I’m just going to disapprove of it,” Branch said.
An X-15 is seen shortly after igniting its rocket engine. (NASA)
The X-15 is seen moving away from its B-52 drop ship.(U.S. Air Force)
In 1960, an X-15 rocket plane is seen on a captive-carry test with its B-52 drop ship. (NASA)
X-15 Pilots in 1965. Left to Right: Air Force pilot William J. "Pete" Knight, Air Force Major Robert A. Rushworth, Air Force Captain Joseph H. Engle, NASA pilot Milton O. Thompson, NASA pilot Bill Dana, and NASA pilot John B. "Jack" McKay. (NASA)
NASA research pilot Jack McKay was injured in a crash landing of the X-15 #2 on November 9, 1962. Following the launch from the B-52, he started the X-15's rocket engine, only to discover that it produced just 30% of its maximum thrust. He had to make a high-speed emergency landing on Mud Lake, NV, without flaps but with a significant amount of fuel still in the aircraft. As the X-15 slid across the lake bed, the left skid collapsed; the aircraft turned sideways and flipped onto its back. McKay suffered back injuries but was eventually able to resume X-15 pilot duties, making 22 more flights.
Neil Armstrong is photographed with the X-15 rocket plane in 1960. (NASA)
An X-15 is shown in flight, attached under the wing of the B-52 aircraft. (NASA)
An X-15 lands alongside its chase plane in the early 1960s. (NASA)
A close-up view of the X-15 cockpit. (NASA)
Close-up view of the XLR-99-RM-2 rocket engine nozzle installed on the North American X-15. (NASA)
After this inauspicious meeting, Engle said he spent a lot of time trying to figure out what he could possibly have done to get crosswise with the boss. Weeks later, Engle learned that one of the X-15 pilots on the base, Robert White, was rotating out of the program. Engle was to be his replacement.
A year earlier, White had flown the X-15 to an altitude of 95km, crossing the Air Force threshold for spaceflight for the first time in the experimental airplane. This qualified White as an astronaut. Now, Engle stood a very good chance to follow in his footsteps.
Getting into the cockpit
After joining the X-15 research program, Engle flew his first free flight in October 1963. He would go on to fly a total of 16 missions over the course of two years, reaching a maximum of 85.5km, above the US Air Force’s threshold to be considered an “astronaut.” He also achieved a top velocity of Mach 5.71.
Once selected for a particular mission, Engle said a pilot would spend most of his waking hours in a simulator, or with test engineers, to understand what data they hoped to obtain from a particular flight. They also did a lot of practicing in Lockheed F-104 Starfighter aircraft, particularly landing them in the dry lake beds near Edwards Air Force Base in case of an engine failure during an X-15 mission.
The engineers had two basic flight profiles they wanted to gather flight data from. During one designed for higher altitudes, the X-15 would drop from its B-52 aircraft, fire its rocket engine, and fly upward at a pitch as steep as 45 degrees. This would provide enough energy and lift to send the aircraft on a ballistic flight as high as 107km. During such a flight, the X-15 would collect data about atmospheric re-entry. The other profile, in which the X-15 launched at a pitch of about 30 degrees, leveled out around 30km. During these flights, the engineers sought data about flight at high velocity or heating due to shock impingement as air flowed around the X-15 surfaces.
On the night before a flight, technicians would mate the X-15 to the B-52 aircraft, pressurizing the plane’s fuel tanks and preparing it for flight. As the pilot, Engle would arrive shortly after sunrise to don a pressure suit and ensure the integrity of its seals. About 45 minutes before the carrier aircraft took off, he would climb into the X-15 cockpit and connect to life support.
Then, the wheels would roll, and the B-52 would take off for its bumpy ride up to the launch location and an altitude of 13.7km. This would take a little more than an hour, during which time Engle would continue to review his cue cards. In particular, he would memorize the various dry lake beds he could make for, were there an engine failure aboard the X-15, based on how many seconds the engine had burned before failing. During the rare moments of reflection at this time, he doesn’t recall being nervous. Rather, he said, “You’re thrilled to death to have the opportunity to be there.”
“Covered in sweat”
At the drop-off point, Engle would hit the launch button to release the X-15 from its carrier aircraft. As soon as the rocket plane was clear of the B-52, the pilot would grab the throttle as quickly as possible to open the propellant lines.
The liquid-fueled XLR99-RM-2 rocket engine, built by Reaction Motors, had 70,400 pounds of thrust. This was a powerful engine six decades ago, and even today it has about two-thirds of the thrust that Blue Origin’s New Shepard rocket uses to send a six-person capsule into space. At the time of launch, the engine was in “igniter idle” status, with enough propellant mixture in the engine chamber to immediately ignite when propellant began flowing from the main fuel tanks. Because the X-15 weighed about 30,000 pounds fully loaded with fuel, the initial acceleration was about 2Gs.
“You were pushed back rather abruptly,” Engle said. “You didn’t have any doubt that the engine had lit OK. But it wasn’t debilitating in any way. It didn’t keep you from reaching up and turning off switches, or maneuvering the airplane with the stick.”
The engine would burn for about 90 seconds, and the force pushing against Engle would build to about 4Gs, making it more difficult to reach upward, toward the engine panel. This initial part of the flight required absolute attention, because any course corrections had to be made early, before the X-15 climbed out of the lower atmosphere. Once on a ballistic trajectory, there wasn’t much that could be done to alter the flight.
Cut-away of the X-15 rocket plane. (NASA)
For high-altitude flights, the X-15 would have about two to four minutes above the bulk of the Earth’s atmosphere in space. “The first time, I naively thought there would be a lot of time,” Engle said. “I thought I would be enjoying the view out the window.” But reaching space brought a new series of tests. Guidance and navigation engineers wanted ultraviolet and infrared sensors on the plane to gather data about the atmosphere, so the X-15 needed to be rolled 90 degrees one way, back to zero pitch, and then rolled 90 degrees in the opposite direction to sweep the instruments over the horizon at different Sun angles. Some of this data was used to better control the flight of intercontinental ballistic missiles.
Upon returning through the upper atmosphere, the mission would generally begin with a 20-degree angle of attack, increasing to about 26 degrees lower in the atmosphere to minimize heating on the X-15 while also maximizing slowing the vehicle.
During such a flight, Engle said he did not have any real concerns for his safety. Mostly, he just wanted to make sure he nailed the flight profile as accurately as possible and held the right pitch during ascent and re-entry. “I honestly can say I was so terribly focused on flying those profiles as closely as possible that I didn’t really have time to look out the window and see how pretty it was,” he said.
The X-15 didn’t have brakes or the landing gear of a traditional airplane, rather it had a front wheel and skids at the back. For nominal flights, the landing took place at a large, dry lake bed, 21km long. Pilots flying the X-15 had practiced landing here and other similar, flat lake beds, which had the consistency of a clay tennis court, hundreds of times in an F-104 airplane.
“Once you touched down you had one more negative surprise,” Engle said. As the X-15 came in for landing, most of its weight was aft, where the rocket engine was located. After the skids came down, the nose landing gear would slap down really hard onto the ground. “That was the most disconcerting part of the whole flight, knowing no matter how nice of a landing you would make, how slow you came in, the nose was going to slam down and it would look like you botched the landing.”
But once the plane had skidded to a stop, a pilot could finally relax and enjoy the moment.
“The X-15 was the most demanding airplane I’ve ever flown, and also the most rewarding airplane I’ve ever flown,” Engle said. “You’d work your tail off during that 10 minute flight. You’d be covered in sweat. But once it stopped, you really felt like you’d accomplished something, like you’d scored a goal with a USA uniform on at the Olympics.”
Missing the Moon
In the fall of 1965, knowing he would eventually rotate out of the X-15 program, Engle again applied to NASA. Along with 18 others, he joined the space agency’s fifth astronaut class, the last one whose members had a chance to go to the Moon.
After Engle served as a back-up crew member for the Apollo 14 mission, the agency’s chief astronaut, Deke Slayton, selected him to fly as the Lunar Module pilot on Apollo 17. Slayton rated Engle as a better qualified astronaut than Harrison Schmitt, a geologist who also had been up for the assignment.
Joe Engle with X-15 aircraft in 1965. (NASA)
Thermal Vacuum Test with Astronauts Joe Engle, Dr. Joseph Kerwin and Vance Brand in 1968.
Astronaut portrait of Joe Engle in 1971. (NASA)
Engle, commander of the second crew for the Space Shuttle Approach and Landing Tests, is photographed at the Rockwell International Space Division's Orbiter assembly facility at Palmdale, California in 1976. (NASA)
In 1981, NASA's space shuttle Columbia climbs toward space for a return visit from Launch Pad 39A. Aboard the space shuttle, astronauts Joe H. Engle and Richard H. Truly man the flight deck. (NASA)
Engle, rear, and Richard H. Truly egress the space shuttle Columbia after spending two days, six hours and 13 minutes in space. (NASA)
Former NASA Astronaut and Enterprise Commander Joe Engle looks at an exhibit in the Intrepid Sea, Air & Space Museum's Space Shuttle Pavilion where the space shuttle Enterprise is on Thursday, July 19, 2012 in New York. (NASA)
In 2018, NASA astronaut Joe Engle attends the premiere of Universal's feature film "First Man” Thursday, Oct. 4, 2018 at the Smithsonian National Air and Space Museum in Washington. (NASA)
“He was a terrific stick and rudder guy,” Slayton wrote about Engle in his autobiography, Deke!. “I thought it would be nice to send a geologist to the Moon, but it wasn’t my business to be nice. I had at least one other guy I thought was more qualified.”
But the Moon was not to be for Engle after the final three Apollo missions were canceled in 1970 due to budgetary reasons. NASA Headquarters wanted Schmitt to fly on the final Apollo mission instead of Engle so that it would send at least one scientist to the Moon during the program's six flights to the lunar surface. In his book, Slayton writes that Engle took the decision better than he would have.
Nearly half a century later, Engle remains philosophical about the experience.
“It was very disappointing,” he said. “I was crushed to learn that. However, going to the Moon was really all about geology. Learning where the Moon came from, and about the craters there. Jack Schmitt had a degree in field geology. To me, it just made sense that he should be on the Moon picking up rocks. So I understood the decision, even if it wasn’t my idea.”
For taking the decision in stride, Engle got his pick of future missions. Would he prefer an Apollo mission in low-Earth orbit, or did he want to wait for the space shuttle to come online? He opted for the shuttle because it was more like an airplane, with a reentry profile not entirely dissimilar to the X-15. So Engle ended up commanding the second spaceflight of the shuttle, in 1981, as well as another mission in 1985.
He loved it—sitting in the cockpit again, returning from space. The shuttle was a trip. He remembers holding the stick in his hand, trying to remain entirely focused on that, while descending through the ionized gas of the atmosphere. It felt like flying into the middle of an acetylene torch, he recalled.
More than three decades on, Engle still misses it like hell.
Listing image by U.S. Air Force
Eric Berger is the senior space editor at Ars Technica, covering everything from astronomy to private space to wonky NASA policy, and author of the book
Liftoff, about the rise of SpaceX. A certified meteorologist, Eric lives in Houston.