17 September 1959: After previously making one glide flight, North American Aviation Chief Engineering Test Pilot Albert Scott Crossfield made the first powered flight of an X-15 hypersonic research rocket plane.
Carried aloft under the right wing of an eight-engine Boeing NB-52A Stratofortress bomber, USAF serial number 52-003, the first of three North American Aviation X-15s, 56-6670, was airdropped from 35,000 feet (10,668 meters) over Rosamond Dry Lake, 40 miles (64 kilometers) north of Edwards Air Force Base. Launch time was 08:08:48.0 a.m., Pacific Daylight Savings Time (15:08.48.0 UTC).
The X-15 was designed to use the Reaction Motors XLR-99 rocket engine, but early in the test program that engine was not yet available so two smaller XLR-11 engines were used. This was engine the same type used in the earlier Bell X-1 rocket plane that first broke the sound barrier in 1948. Though producing just one-fourth the thrust of the XLR-99, it allowed the functional testing of the X-15 to proceed.
Scott Crossfield wrote:
Two minutes after launch I reached 50,000 feet and pushed over in level flight. Then I dropped the nose slightly for a speed run, meanwhile maneuvering the ship through a series of turns and rolls, conscious of a deep rumbling noise of the rocket and a great rush of wind on the fuselage. It was obvious the black bird was in her element at supersonic speeds. She responded beautifully. I stared in fascination at the Mach meter which climbed from 1.5 Mach to 1.8 Mach and then effortlessly to my top speed for this flight of 2.3 Mach or about 1,500 miles and hour. Then, because I was under orders not to take the X-15 wide open, I shut off three of the rocket barrels. As I slowed down, I recalled the agony at Edwards many years before when we had worked for months pushing, calculating, polishing and who knows what else to achieve Mach 2 in the Skyrocket. Now with the X-15 we had reached that speed in three minutes on our first powered flight and I had to throttle back.
—Always Another Dawn, The Story Of A Rocket Test Pilot, by A. Scott Crossfield with Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960. Chapter 39 at Pages 362.
The X-15 dropped 2,000 feet (610 meters) while Scott Crossfield ignited the two XLR-11 engines and then started “going uphill.” During the 224.3 seconds burn duration, the X-15 reached Mach 2.11 (1,393 miles per hour, 2,242 kilometers per hour) and climbed to 52,300 feet (15,941 meters), both slightly higher than planned.
Problems developed when the rocket engine’s turbo pump case failed, and fire broke out in the hydrogen peroxide compartment, engine compartment and in the ventral fin. Crossfield safely landed on Rogers Dry Lake at Edwards Air Force Base. The duration of the flight was 9 minutes, 11.1 seconds. Damage to the rocket plane was extensive but was quickly repaired. 56-6670 flew again 17 October 1959.
Over the next nine years the three X-15s would make 199 flights, setting speed and altitude records nearly every time they flew, and expanding NASA’s understanding of flight in the hypersonic range. The first two X-15s, 56-6670 and 56-6671, survived the program. 670 is at the Smithsonian Institution National Air and Space museum and 671 is at the National Museum of the United States Air Force.
8 September 1954: Scott Crossfield, a NACA Aeronautical Research Pilot at the High Speed Flight Station, Edwards Air Force Base, California, took the North American Aviation F-100A-5-NA Super Sabre, 52-5778, on its first NACA test flight—and his first flight in an F-100.
Tests of the prototype and early production Super Sabres revealed directional stability problems, a very dangerous inertia coupling characteristic that could cause the aircraft to go violently out of control (and which would result in the death of North American’s chief test pilot, George Welch, in just another three weeks). The highly swept wings could stall at high angles of attack, causing the airplane to pitch up in the deadly “Sabre dance”. NACA wanted to explore the causes of these aerodynamic problems and design solutions.
During the flight there was an engine fire warning and Crossfield shut down the Pratt & Whitney J57-P-7 turbojet engine. The F-100A had no flaps and North American’s own test pilots did not think a “dead stick” landing was possible due the very high landing speed required.
Scott Crossfield tells the story in his autobiography:
. . . As a matter of fact, North American tests pilots were then flipping coins to see who would bring an F-100 in dead-stick to fulfill a requirement of the Air Force acceptance tests. I was not concerned. Dead-stick landings in low L-over-D[Lift-over-Drag]airplanes were my specialty. Every test pilot develops a strong point. I was certain that my talent lay in dead-stick landings.
With the engine idling and generating no energy to the plane’s systems, I was running out of hydraulic pressure to operate the controls. Following the handbook instructions, I pulled a lever which extended a miniature “windmill” into the slipstream. This “windmill” churned, building up pressure in the hydraulic lines. Unknown to me, there was a major leak in the line. The windmill was not helping, but hurting me. It was pumping hydraulic fluid overboard as fast as it could turn.
I called Edwards tower and declared an emergency. All airborne planes in the vicinity of the base were warned away from the lake area. I held the ailing F-100 on course, dropping swiftly, following the glide path that I used for the dead-stick Skyrocket.[Douglas D-558-II Skyrocket] I flared out and touched down smoothly. It was one of the best landings I have ever made, in fact. Seconds later, while the F-100 was rolling out, the remaining bit of hydraulic pressure in the control lines drained out and the controls froze.
I then proceeded to violate a cardinal rule of aviation: never try tricks with a compromised airplane. The F-100 was still rolling at a fast clip, coming up fast on the NACA ramp, when I made my poor decision. I had already achieved the exceptional, now I would end it with a flourish, a spectacular wind-up. I would snake the stricken F-100 right up the ramp and bring it to a stop immediately in front of the NACA hangar. This trick, which I had performed so often in the Skyrocket, was a fine touch. After the first successful dead-stick landing in an F-100, it would be fitting.
According to the F-100 handbook, the hydraulic brake system—a separate hydraulic system from the controls—was good for three “cycles,” engine out. This means three pumps on the brake, and that proved exactly right. The F-100 was moving at about fifteen miles an hour when I turned up the ramp. I hit the brakes once, twice, three times. The plane slowed, but not quite enough. I was still inching ahead ponderously, like a diesel locomotive. I hit the brakes a fourth time—and my foot went clear to the floorboards. The hydraulic fluid was exhausted. The F-100 rolled on, straight between the yawning hangar doors!
The good Lord was watching over me—partially anyhow. The NACA hangar was then crowded with expensive research tools—the Skyrocket, all the X-1 series, the X-3, X-4 and X-5. Yet somehow, my plane, refusing to halt, squeezed by them all and bored steadily on toward the side wall of the hangar.
The nose of the F-100 crunched through the corrugated aluminum, punching out an eight-inch steel I-beam. I was lucky. Had the nose bopped three feet to the left or right, the results could have been catastrophic. Hitting to the right, I would have set off the hangar fire-deluge system, flooding the hangar with 50,000 barrels of water and ruining all the expensive airplanes. Hitting to the left, I would have dislodged a 25-ton hangar-door counterweight, bringing it down on the F-100 cockpit, and doubtless ruining Crossfield.
Chuck Yeager never let me forget the incident. He drew many laughs at congregations of pilots by opening his talk: “Well, the sonic wall was mine. The hangar wall was Crossfield’s.” That’s the way it was at Edwards. Hero one minute, bum the next. That I was the first pilot to land an F-100 dead-stick successfully, and memorized elaborate and complete instrument data on the engine failure besides, was soon forgotten.
The F-100 is a tough bird. Within a month NACA’s plane was flying again, with Crossfield back at the helm. In the next few weeks I flew forty-five grueling flights in the airplane, pushing it to the limits, precisely defining the roll coupling. (On one flight the coupling was so severe that it cracked a vertebra in my neck.) These data confirmed, in actual flight, the need for a new F-100 tail, which North American was planning to install on later models of the airplane.
Every night after landing, I taxied the F-100 slowly to the NACA ramp. At the bottom, placed there on orders of Walt Williams, there was a large new sign, symbolic of the new atmosphere at Edwards. It said:
PLEASE COME TO A COMPLETE STOP BEFORE TAXIING UP RAMP
—Always Another Dawn, The Story Of A Rocket Test Pilot, by A. Scott Crossfield with Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960. Chapter 20 at Pages 196–199.
8 June 1959: At Edwards Air Force Base, California, North American Aviation’s Chief Engineering Test Pilot, A. Scott Crossfield, made the first flight of the X-15A hypersonic research rocketplane.
56-6670 was the first of three X-15s built for the U.S. Air Force and NASA. It was airdropped from a Boeing B-52 Stratofortress, NB-52A-1-BO 52-003, at 37,550 feet (11,445 meters) over Rosamond Dry Lake at 08:38:40 a.m, Pacific Time.
This was an unpowered glide flight to check the flying characteristics and aircraft systems, so there were no propellants or oxidizers aboard other than hydrogen peroxide which powered the pumps and generators.
The aircraft reached 0.79 Mach (522 miles per hour, 840 kilometers per hour) during the 4 minute, 56.6 second flight.
In his autobiography, Scott Crossfield described the first flight:
“Three” . . . “Two” . . . “One” . . .
Inside the streamlined pylon, a hydraulic ram disengaged the three heavy shackles from the upper fuselage of the X-15. They were so arranged that all released simultaneously, and if one failed they all failed. The impact of the release was clearly audible in the X-15 cockpit. I heard a loud “kerchunk.”
The X-15 hung in its familiar place beneath the pylon for a split second. Then the nose dipped sharply down and to the right more rapidly than I had anticipated. The B-52, so long my constant companion, was gone. The X-15 and I were alone in the air and flying 500 miles an hour. In less than five minutes I would be on the ground. . . .
There was much to do in the first hundred seconds of flight. First I had to get the “feel” of the airplane, to make certain it was trimmed out for landing just as any pilot trims an airplane after take-off or . . . when dwindling fuel shifts the center of gravity. Then I had to pull the nose up, with and without flaps, to feel out the stall characteristics, so that I would know how she might behave at touchdown speeds . . . My altimeter unwound dizzily: from 24,000 to 13,000 feet in less than forty seconds. . . .
The desert was coming up fast. At 600 feet altitude I flared out. . . .
In the next second without warning the nose of the X-15 pitched up sharply. It was a maneuver that had not been predicted by the computers, an uncharted area which the X-15 was designed to explore. I was frankly caught off guard. Quickly I applied corrective elevator control.
The nose went down sharply. But instead of leveling out, it tucked down. I applied reverse control. The nose came up but much too far. Now the nose was rising and falling like a skiff in a heavy sea. Although I was putting in maximum control I could not subdue the motions. The X-15 was porpoising wildly, sinking toward the desert at 200 miles an hour. I would have to land at the bottom of an oscillation, timed perfectly; otherwise, I knew, I would break the bird. I lowered the flaps and the gear. . . .
. . . With the next dip I had one last chance and flared again to ease the descent. At that moment the rear skids caught on the desert floor and the nose slammed over, cushioned by the nose wheel. The X-15 skidded 5,000 feet across the lake, throwing up an enormous rooster tail of dust. . . .
—Always Another Dawn: The Story of a Rocket Test Pilot, by A. Scott Crossfield and Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960, Chapter 37 at Pages 338–342.
Before the drop, it was discovered that the aircraft’s Stability Augmentation System was inoperative in pitch mode. During the flight it was found that the hydraulic-assisted flight control system was responding too slowly to Crossfield’s inputs. Engineers analyzed the problem and increased the hydraulic system pressure. The problem never recurred.
Scott Crossfield was the most experienced rocketplane pilot with 82 rocketplane flights before the X-15 program. “. . . he was intimately involved in the design of the aircraft and contributed immensely to the success of the design.”
—At The Edge Of Space, by Milton O. Thompson, Smithsonian Institution Press, 1992, Introduction, at Page 3.
North American Aviation X-15A 56-6670 made the first glide flight and the first and last powered flights of the X-15 Program. It made a total of 82 of the 199 X-15 flights. It is in the collection of National Air and Space Museum at Washington, D.C.
19 April 2006: Former experimental test pilot Albert Scott Crossfield, Jr., was enroute from Prattville, Alabama, to Manassas, Virginia. Scott Crossfield¹ was flying his personal Cessna 210A, N6579X. The Cessna was cruising at 11,000 feet (3,353 meters) under Instrument Flight Rules (IFR), under the control of the Atlanta Air Route Traffic Control Center (ARTCC).
During the flight, he encountered a Level 6 thunderstorm.
Scott Crossfield requested to deviate from his planned course to avoid the severe turbulence. Atlanta Center authorized his request and he began to turn. Approximately 30 seconds later, at 11:10 a.m., radar contact was lost near Ludville, Georgia. The last indication was that the Cessna was descending through 5,500 feet (1,676 meters).
The wreckage of N6579X was located the following day by a Civil Air Patrol search team, 3.3 nautical miles (6.1 kilometers) northwest of Ludville at an elevation of 1,269 feet (386.8 meters) above Sea Level. [N. 34° 30.767′, W. 84° 39.492′] The airplane had descended through the forest canopy nearly vertically and created a crater approximately 4½ feet (1.4 meters) deep and 6 feet (1.8 meters) across. Albert Scott Crossfield’s body was inside.
N6579X was a Cessna Model 210A, serial number 21057579, built in 1960 by the Cessna Aircraft Company, Inc., of Wichita Kansas. It was a six-place, single-engine, high-wing monoplane with external struts to brace the wings, and retractable, tricycle landing gear. The airplane was certified for instrument flight by a single pilot. At the time of the crash, N6579X had been flown 4,987.4 hours, total time since new (TTSN).
The Cessna 210A was 28 feet, 2 inches (8.585 meters) long with a wingspan of 36 feet, 6 inches (11.125 meters) and overall height of 9 feet, 7 inches (2.921 meters). The airplane had an empty weight of 1,839 pounds (834.2 kilograms) and maximum gross weight of 2,900 pounds (1,315.4 kilograms). It had a fuel capacity of 65 gallons (246 liters), with 10 gallons (37.9 liters) unusable, and 12 quarts of engine oil (11.4 liters).
N6579X was powered by an air-cooled, fuel-injected, 471.239-cubic-inch-displacement (7.722 liters) Teledyne Continental IO-470-E horizontally-opposed six-cylinder direct-drive engine with a compression ratio of 8.6:1. The engine was rated at 260 horsepower at 2,625 r.p.m. for takeoff, using 100LL aviation gasoline. It weighed 429 pounds (195 kilograms). This engine, serial number 77583-0-E, was original to the airplane and accumulated 4,987.4 hours, total time since new (TTSN). It had been overhauled by Victor Aviation, Palo Alto, California, 1,259.8 hours prior to the accident (TSO). A three-bladed McCauley constant-speed propeller with a diameter of 6 feet, 10 inches (2.083 meters) was installed in 2005.
The Cessna Model 210A has a maximum structural cruise speed of 175 miles per hour (282 kilometers), and maximum speed (Vne) of 200 miles per hour (322 kilometers per hour). Maneuvering speed, which should be used in turbulent conditions, is 130 miles per hour (209 kilometers per hour). The 210A has a maximum rate of climb of 1,300 feet per minutes (6.6 meters per second) and service ceiling of 20,700 feet (6,309 meters). Its maximum range is 1,284 miles (2,066 kilometers).
Albert Scott Crossfield, Jr., was born 2 October 1921 at Berkeley, California. He was the second of three children of Albert Scott Crossfield, a chemist who was employed as the superintendant of the Union Oil Company refinery in Wilmington, California, and Lucia M. Dwyer Crossfield.
When he was five years old, young “Scotty” contracted pneumonia. He was comatose for a while and was not expected to survive, but after several weeks he began to recover. A year later, he again became seriously ill, this time with rheumatic fever. He was confined to total bed rest for four months, and continued to require extensive bed rest until he was about ten years old. It was during this time that he became interested in aviation.
Scott Crossfield attended Boistfort Consolidated School, southwest of Chehalis, Washington, graduating in 1939, and then studied engineering at the University of Washington until taking a job at Boeing in late 1941. During this time, Scotty learned to fly in the Civilian Aviation Training Program.
The week following the attack on Pearl Harbor and the United States’ entry into World War II, Scott Crossfield enlisted as an aviation cadet in the United States Army Air Corps. After numerous delays, he joined the United States Navy on 21 February 1942, and resigned from the Air Corps. He began aviation cadet training at NAS Sand Point, near Seattle, and then was sent to NAS Corpus Christi, Texas. In December 1942, he graduated, received his gold Naval Aviator wings and was commissioned as an Ensign, United States Naval Reserve.
Ensign Crossfield was assigned to NAS Kingsville as an advanced bombing and gunnery instructor. He was promoted to Lieutenant (junior grade), 1 March 1944. He continued as a gunnery instructor for two years before being transferred to Air Group 51 in the Hawaiian Islands, which was preparing for the invasion of Japan. He was promoted to Lieutenant, 1 August 1945, while serving aboard the Independence-class light aircraft carrier USS Langley (CVL-27). With the end of World War II, though, the Navy was cutting back. Lieutenant Crossfield was released from active duty 31 December 1945.
In April 1943 at Corpus Christi, Texas, Ensign A. Scott Crossfield married Miss Alice Virginia Knoph of Seattle. They would have five children.
Following the War, Scotty returned to the University of Washington to complete his degree. He took a part time job operating the University’s wind tunnel. At the same time, he remained in the Naval Reserve, assigned to VF-74, a fighter squadron which flew both the Grumman F6F Hellcat and Chance Vought F4U Corsair out of NAS Sand Point, back where his naval career began.
Crossfield graduated from the University of Washington with a bachelor’s degree in aeronautical engineering in June 1949, and a master’s degree in 1950.
In 1950 Crossfield joined the National Advisory Committee for Aeronautics (NACA) as a research test pilot at the High-Speed Flight Station, Edwards Air Force Base, California. He flew the Republic YF-84, F-84F Thunderstreak, and North American Aviation F-86 Sabre. Crossfield made 25 flights in the delta-winged Convair XF-92A, which he described as “the worst flying airplane built in modern times.” He also flew the Northrop X-4 and Bell X-5. He made 17 flights conducting stability tests in the Douglas D-558-1 Skystreak. Scotty made 65 flights in the North American Aviation F-100A Super Sabre, including a test series which discovered a fatal flaw which led to the death of North American’s chief test pilot, George S. Welch.
Crossfield is known as a rocketplane pilot. He made 10 flights in the Bell X-1, and 89 in the Douglas D-558-II Skyrocket. He became the first pilot to exceed Mach 2 when he flew the Skyrocket to Mach 2.005, 20 November 1953.
Crossfield flew for NACA for approximately five years. During that time, approximately 500 flights were made at Edwards by NACA test pilots. Scott Crossfield flew 181 of them.
Scott Crossfield left NACA in 1956 to join North American Aviation, Inc., as chief engineering test pilot for the X-15 project. Between 8 June 1959 and 6 December 1960, he made fourteen flights in the X-15. He reached a maximum speed of Mach 2.97 and altitude of 88,116 feet (26,858 meters). Once the contractor’s flight tests were completed and the rocketplane turned over to the U.S. Air Force and NACA, the customers’ test pilots, Joe Walker and Major Robert M. White, took over.
Albert Scott Crossfield made 114 flights in rocket-powered aircraft, more than any other pilot.
After completing his work on the X-15, Crossfield followed Harrison (“Stormy”) Storms, who had been the Chief Engineer of North American’s Los Angeles Division (where the X-15 was built) to the Space and Information Systems Division in Downey, California, where he worked in quality assurance, reliability engineering and systems testing for the Apollo Command and Service Modules and the Saturn S-II second stage.
Crossfield left North American at the end of 1966, becoming Vice President for Technological Development for Eastern Air Lines. In this position, he flew acceptance tests for new Boeing 720 and 727 airliners at Boeing in Seattle.
In The X-15 Rocket Plane, author Michelle Evans quoted Crossfield as to why he had not entered NASA’s space program as an astronaut:
One question that pressed was, with his love of flight and the early responsibility of going into space with the X-15, why would Scott not apply to the NASA astronaut office? He explained, “[Dr.] Randy Lovelace and General [Donald] Flickinger were on the selection board. They took me to supper one night and asked me not to put in for astronaut. I asked them, ‘Why not?’ and they said, ‘Well, we’re friends of yours. We don’t want to have to turn you down.’ I asked, ‘Why would you have to turn me down?’ and they said, ‘You’re too independent.’ “
—The X-15 Rocket Plane: Flying the First Wings into Space, by Michelle, Evans, University of Nebraska Press, 2013, Chapter 1 at Page 33.
The remains of Albert Scott Crossfield are interred at the Arlington National Cemetery.
¹ “Scott Crossfield” is the family name, going back for several generations.
10 March 1959: With North American Aviation’s Chief Engineering Test Pilot Albert Scott Crossfield in its cockpit, the X-15 high speed research rocket plane was airborne for the first time. X-15A 56-6670 was carried aloft under the wing of the Boeing NB-52A Stratofortress mother ship, 52-003, for a series of captive flights. The purpose was to verify that all the systems on both the X-15 and the mothership were properly functioning up to the point that the drop would occur.
Fully settled in my tiny flight office, I could speak by radio to the B-52 pilot, Charlie Bock, who was about thirty feet away in the nose of the mother plane, out of sight. . . .
As we sat, waiting at the end of the long runway while chase planes took off and circled, the clock on the instrument panel of the X-15 showed 0955. . . On signal, B-52 pilot Charlie Bock cobbed the eight engines, standing hard on the brake pedal. As the engines wound up to full military power, the X-15 trembled and the noise was tremendous. Through my radio earphones I heard Bock call a countdown for the benefit of the official movie cameramen who would record every inch of the takeoff:
“Five . . . four . . . three . . . two . . . one. BRAKE RELEASE.”
One hundred thirty tons of aluminum, fuel, Inconel X, five men and the hope of a nation began rolling down the long runway. . .
As we rolled, the huge runway distance markers flashed by, clocking our path: 14,000 . . . 13,000 . . . 12,000 . . . 8,000. When the X-15 air-speed indicator reached 170 knots, I noted only a minor vibration. We would continue the takeoff. 6,000 . . . 5,000 . . . 4,000, and we broke ground. It was smooth and gentle, like the take-off of an airliner. The air-speed indicator crept up to 260 knots. The parched brown desert fell away. . . .
—Always Another Dawn: The Story of a Rocket Test Pilot, by A. Scott Crossfield and Clay Blair, Jr., The World Publishing Company, Cleveland and New York, 1960, Chapters 34 and 35 at Pages 316–321.
The gross weight of the combined aircraft was 258,000 pounds (117,000 kilograms). After a takeoff roll of 6,200 feet (1,890 meters) the B-52/X-15 lifted of at 168 knots (193 miles per hour/311 kilometers per hour). During the 1 hour, 8 minute flight the the B-52 climbed to 45,000 feet (13,716 meters) and reached a speed of 0.83 Mach (548 miles per hour/881 kilometers per hour).
The X-15A rocketplane was designed and built for the U.S. Air Force and the National Advisory Committee for Aeronautics (NACA, the predecessor of NASA) by North American Aviation, Inc., to investigate the effects of hypersonic flight (Mach 5+). Design work started in 1955 and a mock-up had been completed after just 12 months. The three X-15s were built at North American’s Los Angeles Division, at the southeast corner of Los Angeles International Airport (LAX), on the shoreline of southern California.
The first flight took place 8 June 1959, again, with Scott Crossfield in the cockpit of the Number 1 ship, 56-6670.
While earlier rocketplanes, the Bell X-1 series, the the Douglas D-558-II, and the Bell X-2, were airplanes powered by rocket engines, the X-15 was a quantum leap in technology. It was a spacecraft.
Like the other rocketplanes, the X-15 was designed to be carried aloft by a “mothership,” rather than to takeoff and climb to the test altitude under its own power. The carrier aircraft was originally to be a Convair B-36 intercontinental bomber but this was soon changed to a Boeing B-52 Stratofortress. Two B-52s were modified to carry the X-15: NB-52A 52-003, The High and Mighty One, and NB-52B 52-008, Balls 8.
From 8 June 1959 to 24 October 1968, the three X-15s were flown by twelve test pilots, three of whom would qualify as astronauts in the X-15. Two would go on to the Apollo Program, and one, Neil Alden Armstrong, would be the first human to set foot on the surface of the Moon, 20 July 1969. Joe Engle would fly the space shuttle. Four of the test pilots, Petersen, White, Rushworth, and Knight, flew in combat during the Vietnam War, with Bob White being awarded the Air Force Cross. Petersen, Rushworth and White reached flag rank.
One pilot, John B. (“Jack”) McKay, was seriously injured during an emergency landing at Mud Lake, Nevada, 9 November 1962. Michael James Adams, was killed when the Number 3 ship, 56-6672, went into a hypersonic spin and broke up on the program’s 191st flight, 15 November 1967.
Flown by a single pilot/astronaut, the X-15 is a mid-wing monoplane with dorsal and ventral fin/rudders and stabilators. The wing had no dihdral, while the stabilators had a pronounced -15° anhedral. The short wings have an area of 200 square feet (18.58 square meters) and a maximum thickness of just 5%. The leading edges are swept to 25.64°. There are two small flaps but no ailerons. The entire vertical fin/rudder pivots for yaw control.
Above 100,000 feet (30,840 meters) altitude, conventional aircraft flight control surfaces are ineffective. The X-15 is equipped with a system of reaction control jets for pitch, roll and yaw control. Hydrogen peroxide was passed through a catalyst to produce steam, which supplied the control thrusters.
The forward landing gear consists of a retractable oleo strut with steerable dual wheels and there are two strut/skids at the rear of the fuselage. The gear is retracted after the X-15 is mounted on the NB-52 and is extended for landing by its own weight.
The rocketplane’s cockpit featured both a conventional control stick as well as side-controllers. It was pressurized with nitrogen gas to prevent fires. The pilot wore an MC-2 full-pressure suit manufactured by the David Clark Company of Worcester, Massachusetts, with an MA-3 helmet. The suit was pressurized below the neck seal with nitrogen, while the helmet was supplied with 100% oxygen. This pressure suit was later changed to the Air Force-standardized A/P22S.
The X-15 is 50.75 feet (15.469 meters) long with a wing span of 22.36 feet (6.815 meters). The height—the distance between the tips of the dorsal and ventral fins—is 13.5 feet (4.115 meters). The stabilator span is 18.08 feet (5.511 meters). The fuselage is 4.67 feet (1.423 meters) deep and has a maximum width of 7.33 feet (2.234 meters).
Since the X-15 was built of steel rather than light-weight aluminum, as are most aircraft, it is a heavy machine, weighing approximately 14,600 pounds (6,623 kilograms) empty and 34,000 pounds (15,422 kilograms) when loaded with a pilot and propellants. The X-15s carried as much as 1,300 pounds (590 kilograms) of research instrumentation, and the equipment varied from flight to flight. The minimum flight weight (for high-speed missions): 31,292 pounds (14,194 kilograms) The maximum weight was 52,117 pounds (23,640 kilograms) at drop (modified X-15A-2 with external propellant tanks).
Initial flights were flown with a 5 foot, 11 inch (1.803 meters)-long air data boom at the nose, but this would later be replaced by the “ball nose” air sensor system. The data boom contained a standard pitot-static system along with angle-of-attack and sideslip vanes. The boom and ball nose were interchangeable.
The X-15s were built primarily of a nickel/chromium/iron alloy named Inconel X, along with corrosion-resistant steel, titanium and aluminum. Inconel X is both very hard and also able to maintain its strength at the very high temperatures the X-15s were subjected to by aerodynamic heating. It was extremely difficult to machine and special fabrication techniques had to be developed.
Delays in the production of the planned Reaction Motors XLR99 rocket engine forced engineers to adapt two vertically-stacked Reaction Motors XLR11-RM-5 four-chamber rocket engines to the X-15 for early flights. This was a well-known engine which was used on the previous rocketplanes. The XLR-11 burned a mixture of ethyl alcohol and water with liquid oxygen. Each of the engines’ chambers could be ignited individually. Each engine was rated at 11,800 pounds of thrust (58.49 kilonewtons) at Sea Level.
The Reaction Motors XLR99-RM-1 rocket engine was throttleable by the pilot from 28,500 to 60,000 pounds of thrust. The engine was rated at 50,000 pounds of thrust (222.41 kilonewtons) at Sea Level; 57,000 pounds (253.55 kilonewtons) at 45,000 feet (13,716 meters), the typical drop altitude; and 57,850 pounds (257.33 kilonewtons) of thrust at 100,000 feet (30,480 meters). Individual engines varied slightly. A few produced as much as 61,000 pounds of thrust (271.34 kilonewtons).
The XLR99 burned anhydrous ammonia and liquid oxygen. The flame temperature was approximately 5,000 °F. (2,760 °C.) The engine was cooled with circulating liquid oxygen. To protect the exhaust nozzle, it was flame-sprayed with ceramic coating of zirconium dioxide. The engine is 6 feet, 10 inches (2.083 meters) long and 3 feet, 3.3 inches (0.998 meters) in diameter. It weighs 910 pounds (413 kilograms). The Time Between Overhauls (TBO) is 1 hour of operation, or 100 starts.
The XLR99 proved to be very reliable. 169 X-15 flights were made using the XLR99. 165 of these had successful engine operation. It started on the first attempt 159 times.
The highest speed achieved during the program was with the modified number two ship, X-15A-2 56-6671, flown by Pete Knight to Mach 6.70 (6,620 feet per second/4,520 miles per hour/ kilometers per hour) at 102,700 feet (31,303 meters). On this flight, the rocketplane exceeded its maximum design speed of 6,600 feet per second (2,012 meters per second).
The maximum altitude was reached by Joe Walker, 22 August 1963, when he flew 56-6672 to 354,200 feet (107,960 meters).
The longest flight was flown by Neil Armstrong, 20 April 1962, with a duration of 12 minutes, 28.7 seconds.
North American Aviation X-15A-1 56-6670 is on display at the Smithsonian Institution National Air and Space Museum. X-15A-2 56-6671 is at the National Museum of the United States Air Force.