19 May 1976: A Strategic Air Command Boeing B-52D Stratofortress eight-engine bomber took off from Carswell Air Force Base, Fort Worth, Texas on a training flight. As the airplane’s landing gear was retracting, the hydraulic system failed leaving the right front gear with its 2-wheel bogie partially retracted and unlocked. The hydraulic system failure also disabled the B-52’s steering, brakes and rudder. Captain James A. Yule, an Instructor Pilot, took command of the aircraft. SAC headquarters at Omaha, Nebraska, diverted the airplane to Edwards Air Force Base in California so that the bomber could land on the large dry lake bed there.
After a five-hour flight and making several practice approaches, Captain Yule landed the aircraft. With no brakes, it coasted for two-and-a-half miles before coming to a stop. During the roll out, the right front bogie bounced up and down, providing no support. However, with the limited control available, Captain Yule successfully landed the Stratofortress with no damage and no injuries to the crew. He and another pilot received the Air Medal, and the rest of the air crew were awarded the Air Force Commendation Medal.
Captain Yule was the recipient of the Mackay Trophy for 1976. Established in 1911 and administered by the National Aeronautic Association, the Mackay Trophy is awarded to the “most meritorious flight of the year” by an Air Force person, persons, or organization. His citation reads:
“For gallantry and unusual presence of mind while participating in a flight as an instructor pilot of a B-52D Stratofortress.
“Captain James A Yule, distinguished himself by gallantry and unusual presence of mind while participating in an aerial flight as an instructor pilot of a B-52D aircraft on 19 May 1976. Captain Yule’s flight developed a unique multiple emergency and he assumed command of the aircraft, and at great personal risk, checked out the hydraulic open wheel well area to detect the problem. Using initiative, he coordinated with ground agencies and crew members and determined that a safe landing could be made after loss of braking and complete failure of steering. Captain Yule’s professional competence and outstanding airmanship under extreme stress resulted in successful recovery of the crew and a valuable aircraft. His courageous acts in landing a malfunctioning aircraft reflect great credit upon himself and the United States Air Force.”
24 March 1960: After North American Aviation’s Chief Engineering Test Pilot, Albert Scott Crossfield, had made the first flights in the new X-15 hypersonic research rocketplane (one gliding, eight powered), NASA Chief Test Pilot Joseph Albert Walker made his first familiarization flight.
The X-15, 56-6670, the first of three built by North American Aviation, Inc., was carried aloft under the right wing of a Boeing NB-52A Stratofortress, 52-003, flown by John E. Allavie and Fitzhugh L. Fulton.
The rocketplane was dropped from the mothership over Rosamond Dry Lake at 15:43:23.0 local time, and Joe Walker ignited the Reaction Motors XLR-11 rocket engine. The engine burned for 272.0 seconds, accelerating Walker and the X-15 to Mach 2.0 (1,320 miles per hour/2,124.3 kilometers per hour) and a peak altitude of 48,630 feet (14,822.4 meters). Walker landed on Rogers Dry Lake at Edwards Air Force Base after a flight of 9 minutes, 8.0 seconds.
Joe Walker made 25 flights in the three X-15 rocket planes from 24 March 1960 to 22 August 1963. He achieved a maximum Mach number of 5.92, maximum speed of 4,104 miles per hour (6,605 kilometers per hour) and maximum altitude of 354,200 feet (107,960 meters).
Joe Walker was killed in a mid-air collision between his Lockheed F-104N Starfighter and a North American Aviation XB-70A Valkyrie near Barstow, California, 1 June 1966.
The number one ship, 56-6670, made 81 of the 199 flights of the X-15 Program. It was the first to fly, and also the last, 24 October 1968. Today, it is in the collection of the Smithsonian Institution National Air and Space Museum.
7 March 1961: Launched over Silver Lake, a dry lake bed near the California/Nevada border, at 10:28:33.0 a.m., Pacific Standard Time, test pilot Major Robert M. White, U.S. Air Force, flew the number two North American Aviation X-15 hypersonic research rocketplane, 56-6671, to Mach 4.43 (2,905 miles per hour/4,675 kilometers per hour) and 77,450 feet (23,607 meters), becoming the first pilot to exceed Mach 4.
This was the first flight for the number two X-15 with the Reaction Motors XLR99-RM-1 engine, which was rated at 57,000 pounds of thrust (253.55 kilonewtons).
The flight plan called for a burn time of 116 seconds, an altitude of 84,000 feet (25,603 meters) and a predicted maximum speed of Mach 4.00. The actual duration of the engine burn was 127.0 seconds. Peak altitude was lower than planned, at 77,450 feet (23,607 meters). The longer burn and lower altitude translated into the higher speed.
The total duration of the flight, from the air drop from the Boeing NB-52B Stratofortress carrier, 52-008, to touchdown at Edwards Air Force Base, was 8 minutes, 34.1 seconds.
20 December 1968: After 199 flights, the National Aeronautics and Space Administration cancelled the X-15 Hypersonic Research Program. A 200th X-15 flight had been scheduled, but after several delays, the decision was made to end the program. (The last actual flight attempt was 12 December 1968, but snow at several of the dry lakes used as emergency landing areas resulted in the flight being cancelled.)
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 with former NACA test pilot Albert 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.
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
At The Edge Of Space, by Milton O. Thompson, Smithsonian Institution Press, 1992
X-15 Diary: The Story of America’s First Spaceship, by Richard Tregaskis, E.F. Dutton & Company, New York, 1961; University of Nebraska Press, 2004
X-15: Exploring the Frontiers of Flight, by David R. Jenkins, National Aeronautics and Space Administration http://www.nasa.gov/pdf/470842main_X_15_Frontier_of_Flight.pdf
The X-15 Rocket Plane: Flying the First Wings into Space, by Michelle Evans, University of Nebraska Press, Lincoln and London, 2013
20 December 1962: Milton Orville Thompson, a NASA test pilot assigned to the X-15 hypersonic research program, was conducting a weather check along the X-15’s planned flight path from Mud Lake, Nevada to Edwards Air Force Base in California, scheduled for later in the day. Thompson was flying a Lockheed F-104A-10-LO Starfighter, Air Force serial number 56-749, call sign NASA 749.
In his autobiography, At the Edge of Space, Thompson described the day:
“The morning of my weather flight was a classic desert winter morning. It was cold, freezing in fact, but the sky was crystal clear and there was not a hint of a breeze—a beautiful morning for a flight.”
Completing the weather reconnaissance mission, and with fuel remaining in the Starfighter’s tanks, Milt Thompson began practicing simulated X-15 approaches to the dry lake bed.
X-15 pilots used the F-104 to practice landing approaches. The two aircraft were almost the same size, and with speed brakes extended and the flaps lowered, an F-104 had almost the same lift-over-drag ratio as the X-15 in subsonic flight. Thompson’s first approach went fine and he climbed back to altitude for another practice landing.
When Milt Thompson extended the F-104’s flaps for the second simulated X-15 approach, he was at the “high key”— over Rogers Dry Lake at 35,000 feet (10,668 meters) — and supersonic. As he extended the speed brakes and lowered the flaps, NASA 749 began to roll to the left. With full aileron and rudder input, he was unable to stop the roll. Adding throttle to increase the airplane’s airspeed, he was just able to stop the roll with full opposite aileron.
Thompson found that he could maintain control as long as he stayed above 350 knots (402 miles per hour/648 kilometers per hour) but that was far too high a speed to land the airplane. He experimented with different control positions and throttle settings. He recycled the brake and flaps switches to see if he could get a response, but there was no change. He could see that the leading edge flaps were up and locked, but was unable to determine the position of the trailing edge flaps and came to the conclusion that the trailing edge flaps were lowered to different angles.
Thompson called Joe Walker, NASA’s chief test pilot, on the radio and explained the situation:
“I told him the symptoms of my problem and he decided that I had a split trailing edge flap situation with one down and one up.
“He suggested I recycle the flap lever to the up position to attempt to get both flaps up and locked. I had already tried that, but I gave it another try. Joe asked if I had cycled the flap lever from the up to the takeoff position and then back again. I said no. I had only cycled the flap lever from the up position to a position just below it and then back to the up position. Joe suggested we try it his way. I moved the flap lever from the up position all the way to the takeoff position and then back to the up position. As soon as I moved the lever to the takeoff position, I knew I had done the wrong thing.
“The airplane started rolling again, but this time I could not stop it. The roll rate quickly built up to the point that I was almost doing snap rolls. Simultaneously, the nose of the airplane started down. I was soon doing vertical rolls as the airspeed began rapidly increasing. I knew I had to get out quick because I did not want to eject supersonic and I was already passing through 0.9 Mach. I let go of the stick and reached for the ejection handle. I bent my head forward to see the handle and then I pulled it. Things were a blur from that point on.”
—At the Edge of Space: The X-15 Flight Program, by Milton O. Thompson, Smithsonian Institution Press, Washington and London, 1992. Chapter 5 at Pages 119–120.
As Thompson descended by parachute he watched the F-104 hit the ground and explode in the bombing range on the east side of Rogers Dry Lake. He wrote, “It was only 7:30 a.m. and still a beautiful morning.”