29 November 1957: The third production Boeing B-52A-1-BO Stratofortress strategic bomber, 52-003, was flown from Boeing’s Seattle plant to the North American Aviation facility at Air Force Plant 42, Palmdale, California, to be modified to carry the new X-15 hypersonic research rocketplane.
Modifications began on 4 February 1958. A pylon was mounted under the bomber’s right wing. A large notch was cut into the trailing edge of the inboard flap for the X-15’s vertical fin. A 1,500 gallon (5,678 liter) liquid oxygen tank was installed in the bomb bay.
A station for a launch operator was installed on the upper deck of the B-52 at the former electronic countermeasures position. A series of control panels allowed the panel operator to monitor the X-15’s systems, provide electrical power, and to keep the rocketplane’s liquid oxygen tank full as the LOX boiled off during the climb to launch altitude. The operator could see the X-15 through a plexiglas dome, and there were two television monitors.
After modifications were completed at Palmdale, 52-003 was flown to Edwards Air Force Base, 14 November 1958.
NB-52A 52-003 is on display at the Pima Air and Space Museum, Tucson, Arizona.
3 November 1965: Major Robert A. Rushworth made the first flight of the modified X-15A-2 rocketplane, Air Force serial number 56-6671. After a landing accident which caused significant damage to the Number 2 X-15, it was rebuilt by North American Aviation. A 28-inch (0.71 meter) “plug” was installed in the fuselage forward of the wings to create space for a liquid hydrogen fuel tank which would be used for an experimental “scramjet” engine that would be mounted on the the ventral fin. The modified aircraft was also able to carry two external fuel tanks. It was hoped that additional propellant would allow the X-15A-2 to reach much higher speeds.
The first flight with the new configuration was an “envelope expansion” flight, intended to test the handling characteristics of the X-15A-2, and to jettison the tanks (which were empty on this flight) to evaluate the separation and trajectory as they fell away from the rocketplane in supersonic flight.
The X-15A-2 was dropped from the Boeing NB-52A Stratofortress 52-003, over Cuddeback Lake, 37 miles (60 kilometers) northeast of Edwards Air Force Base in the Mojave Desert of southern California. This was the only time during the 199-flight X-15 Program that this lake was used as a launch point.
The X-15 was released at 09:09:10.7 a.m., PST. Bob Rushworth ignited the Reaction Motors XLR99-RM-1 rocket engine and it ran for 84.1 seconds before its fuel supply was exhausted. This engine was rated at 57,000 pounds of thrust (253.549 kilonewtons).
The X-15 climbed to 70,600 feet (21,519 meters) and reached Mach 2.31 (1,514 miles per hour/2,437 kilometers per hour.)
The test flight went well. The external tanks jettisoned cleanly and fell away. The recovery parachute for the liquid oxygen tank did not deploy, however, and the tank was damaged beyond repair.
Rushworth and the X-15A-2 touched down on Rogers Dry Lake after a flight of 5 minutes, 1.6 seconds.
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.
22 August 1963: On his twenty-fifth and last flight with the X-15 program, NASA Chief Research Test Pilot Joseph Albert Walker would attempt a flight to Maximum Altitude. Engineers had predicted that the X-15 was capable of reaching 400,000 feet (121,920 meters) but simulations had shown that a safe reentry from that altitude was risky. For this flight, Flight 91, the flight plan called for 360,000 feet (109,728 meters) to give Walker a safety margin. Experience had shown that slight variations in engine thrust and climb angle could cause large overshoots in peak altitude, so this was not considered an excessive safety margin.
For this flight, Joe Walker flew the Number 3 X-15, 56-6672. It was the only one of the three North American Aviation X-15s equipped with the Honeywell MH-96 flight control system, which had been developed to improve control of the rocketplane outside Earth’s atmosphere. This flight was the twenty-second for Number 3.
Walker and the X-15 were airdropped from the Boeing NB-52A Stratofortress 52-003, The High and Mighty One, at 45,000 feet (13,716 meters) above Smith Ranch Dry Lake, Nevada, about half-way between the city of Reno and the NASA High Range Tracking Station at Ely. Launch time was 10:05:57.0 a.m., PDT. Walker ignited the Reaction Motors XLR99-RM-1 rocket engine. This engine was rated at 57,000 pounds of thrust. Experience had shown that different engines varied from flight to flight and that atmospheric conditions were a factor. Thrust beyond 60,000 pounds was often seen, but this could not be predicted in advance. The flight plan called for the duration of burn to be 84.5 seconds on this flight. The X-15 climbed at a 45° angle.
As Walker was about to shut down the engine according to plan, it ran out of fuel. The total burn time was 85.8 seconds, just slightly longer than planned.
“At burnout, Joe was passing 176,000 feet [53,645 meters] and traveling at 5,600 feet per second [1,707 meters per second]. He then began the long coast to peak altitude. It would take almost 2 minutes to reach peak altitude after burn out. Two minutes does not seem like a lot of time, but try timing it. Just sit back in your easy chair and count off the seconds. It is almost impossible to believe that you can continue to coast up in altitude for that length of time after the engine burns out. It gives you some feel for how much energy is involved at those speeds. For comparison, when you throw a ball up in the air as hard as you can, it only coasts upward a maximum of 4 or 5 seconds. The X-15 coasted up for 120 seconds.
“The airplane would coast up another 178,000 feet [54,254 meters] during that time to peak out at 354,200 feet. . . .” [107,960 meters]
—At The Edge of Space: The X-15 Flight Program, by Milton O. Thompson, Smithsonian Institution Press, Washington, 1992, Chapter 5 at Page 125.
Joe Walker and the X-15 reached the peak of their ballistic trajectory at 354,200 feet (67.083 miles, 107,960 meters). Walker pitched the nose down to be in the proper attitude for atmospheric reentry. The X-15 decelerated as it hit the atmosphere and Walker experienced as much as 7 Gs. The rocketplane’s aerodynamic control surfaces again became operational as it descended through 95,000 feet (28,956 meters) and Walker leveled at 70,000 feet (21,336 meters). He then glided to a landing on Rogers Dry Lake at Edwards Air Force Base, California, after 11 minutes, 8.6 seconds of flight.
Flight 91 was the highest flight achieved by any of the X-15s. It was Joe Walker’s second flight into space. His record would stand for the next 41 years.