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D-558 II Skyrocket

The X-1's were joined in the quest to expand the high-speed and stratospheric frontiers by the Navy-National Advisory Committee for Aeronautics (NACA)- Douglas D-558-II Skyrocket. The Douglas D-558-2 "Skyrockets" were among the early transonic research airplanes like the X-1, X-4, X-5, and X-92A. The mission of the D-558 II program was to investigate the flight characteristics of a swept-wing aircraft at high supersonic speeds. Particular attention was given to the problem of "pitch-up," a phenomena often encountered with swept-wing configured aircraft. The Skyrocket made aviation history when it became the first airplane to fly twice the speed of sound.

Three of the single-seat, swept-wing aircraft flew from 1948 to 1956 in a joint program involving the National Advisory Committee for Aeronautics (NACA), with its flight research done at the NACA's Muroc Flight Test Unit in CA, redesignated in 1949 the High-Speed Flight Research Station (HSFRS). Also partners in the flight research were the Navy-Marine Corps and the Douglas Aircraft Co. The HSFRS became the High-Speed Flight Station in 1954 and is now known as the NASA Dryden Flight Research Center.

Shortly after the D-558 I Skystreak research project was started, the contract was modified to include investigation of sweptback wings in addition to the straight wings of the Skystreak. In August of 1945, the decision was reached to split the program into a Phase 1 that was a straight-wing aircraft and a Phase 2 that was a swept-wing airplane. The redesigned aircraft was named D-558 II Skyrocket. The Skyrocket was the phase-two version of what had originally been conceived as a three-phase program, with the phase-one aircraft having straight wings. The third phase, which never came to fruition, would have involved constructing a mock-up of a combat-type aircraft embodying the results from the testing of the phase one and two aircraft.

Three D-558 II "Skyrockets" were built by Douglas Aircraft. The three airplanes flew a total of 313 times--123 by the number one aircraft (Bureau No. 37973--NACA 143), 103 by the second Skyrocket (Bureau No. 37974--NACA 144), and 87 by airplane number three (Bureau No. 37975--NACA 145). Skyrocket 143 flew all but one of its missions as part of the Douglas contractor program to test the airplane's performance.

The D-558 II was a single-place, 35 degree swept-wing aircraft measuring 45 ft in length. It was 11.5 ft high and had a wingspan of 25 ft. Fully fueled it weighed about 16,000 lb. Until configured for air launch, NACA 143 featured a Westinghouse J-34-40 turbojet engine rated at 3,000 pounds of static thrust. It carried 260 gallons of aviation gasoline and weighed 10,572 pounds at take-off. NACA 144 (and NACA 143 after modification in 1955) was powered by an LR-8-RM-6 rocket engine rated at 6,000 pounds of static thrust. Its propellants were 345 gallons of liquid oxygen and 378 gallons of diluted ethyl alcohol. In its launch configuration, it weighed 15,787 pounds. NACA 145 had both an LR-8-RM-5 rocket engine rated at 6,000 pounds of static thrust and featured a Westinghouse J-34-40 turbojet engine rated at 3,000 pounds of static thrust. It carried 170 gallons of liquid oxygen, 192 gallons of diluted ethyl alcohol, and 260 gallons of aviation gasoline for a launch weight of 15,266 pounds.

The Douglas D-558-II Skyrocket was originally designed to take off under jet power and then use onboard rockets for high speed runs. When the first planes were delivered, the rockets had not yet been installed and the first test flights were made on jet power alone. Unfortunately, the engine originally installed could not even provide takeoff power and so JATO bottles had to be used. Although capable of using four JATO bottles, in the cash-strapped period following World War 11, the NACA crews only made use of two bottles, per flight to save money.

Two of the aircraft were later modified to be air launched from a Boeing B-29 (actually a Navy P2B-1 variant) and powered by one Reaction Motors XLR-8 rocket motor. The first of the three D-558 IIs had a Westinghouse J-34-40 jet engine and took off under its own power. The second was equipped with a J-34-40 jet engine and a Reaction Motors Inc., LR8-RM-6 rocket engine. The jet engine was for takeoff and climbing to altitude and the four-chambered rocket engine was for reaching supersonic speeds. This aircraft was modified so it could be air-launched from a B-29 carrier aircraft and the jet engine was removed to make room for extra propellant tanks. The third Skyrocket had the jet engine and the rocket engine but was also modified so it could be air-launched. The rocket engine was rated at 6,000 lb of thrust.

The sweptwing D-558-II completed its first flight at Muroc (later Edwards AFB) on February 4, 1948, flown by John Martin, a Douglas test pilot. It was originally powered by a 3,000-lb. thrust J34 turbojet and then by a combination of the J34 and a 6,000-lb. thrust XLR-8 rocket engine for ground takeoffs. The goals of the program were to investigate the characteristics of swept-wing aircraft at transonic and supersonic speeds with particular attention to pitch-up (uncommanded rotation of the nose of the airplane upwards)--a problem prevalent in high-speed service aircraft of that era, particularly at low speeds during take-off and landing and in tight turns.

The three aircraft gathered a great deal of data about pitch-up and the coupling of lateral (yaw) and longitudinal (pitch) motions; wing and tail loads, lift, drag, and buffeting characteristics of swept-wing aircraft at transonic and supersonic speeds; and the effects of the rocket exhaust plume on lateral dynamic stability throughout the speed range. (Plume effects were a new experience for aircraft.) The number three aircraft also gathered information about the effects of external stores (bomb shapes, drop tanks) upon the aircraft's behavior in the transonic region (roughly 0.7 to 1.3 times the speed of sound). In correlation with data from other early transonic research aircraft such as the XF-92A, this information contributed to solutions to the pitch-up problem in swept-wing aircraft.

NACA 144 began its flight program with a turbojet powerplant. NACA pilots Robert A. Champine and John H. Griffith flew 21 times in this configuration to test airspeed calibrations and to research longitudinal and lateral stability and control. In the process, during August of 1949 they encountered pitch-up problems, which NACA engineers recognized as serious because they could produce a limiting and dangerous restriction on flight performance. Hence, they determined to make a complete investigation of the problem.

In 1950, Douglas replaced the turbojet with an LR-8 rocket engine, and its pilot, William B. Bridgeman, flew the aircraft seven times up to a speed of Mach 1.88 (1.88 times the speed of sound) and an altitude of 79,494 feet (the latter an unofficial world's altitude record at the time, achieved on August 15, 1951). In the rocket configuration, a Navy P2B (Navy version of the B-29) launched the airplane at approximately 30,000 feet after taking off from the ground with the Skyrocket attached beneath its bomb bay. During Bridgeman's supersonic flights, he encountered a violent rolling motion known as lateral instability that was less pronounced on the Mach 1.88 flight on August 7, 1951, than on a Mach 1.85 flight in June 1951 when he pushed over to a low angle of attack (angle of the fuselage or wing to the prevailing air-flow direction).

The NACA engineers studied the behavior of the aircraft before beginning their own flight research in the airplane in September 1951. Over the next couple of years, NACA pilot A. Scott Crossfield flew the airplane 20 times to gather data on longitudinal and lateral stability and control, wing and tail loads, and lift, drag, and buffeting characteristics.

The X-1 had achieved a top speed of Mach 1.45 (957 mph) in 1948 and, in August of 1949, Maj. Pete Everest had flown it to a peak altitude of 71,902 feet. These marks were soon surpassed by the Skyrocket in its pure rocket configuration, as Douglas test pilot Bill Bridgeman accelerated to a speed of Mach 1.88 (l,180 mph) and climbed to an altitude of 74,494 feet in August of 1951. Then, in August of 1953, Marine Lt. Col. Marion Carl flew the airplane to a new (unofficial) altitude record of 83,235 feet on August 21, 1953, and to a maximum speed of Mach 1.728. Its performance, thus far, was not bad for an airplane which had not been designed to exceed Mach 1.6.

Following Carl's completion of these flights for the Navy, NACA technicians at the High-Speed Flight Research Station (HSFRS) near Mojave, Calif., outfitted the LR-8 engine's cylinders with nozzle extensions to prevent the exhaust gas from affecting the rudders at supersonic speeds. This addition also increased the engine's thrust by 6.5 percent at Mach 1.7 and 70,000 feet.

Even before Marion Carl had flown the Skyrocket, HSFRS Chief Walter C. Williams had petitioned NACA headquarters unsuccessfully to fly the aircraft to Mach 2 to garner the research data at that speed. NACA test pilot Scott Crossfield believed that, if pushed to its ultimate extreme, the Skyrocket might be able to get to the next magic number: Mach 2. Finally, after Crossfield had secured the agreement of the Navy's Bureau of Aeronautics, NACA director Hugh L. Dryden relaxed the organization's usual practice of leaving record setting to others and consented to attempting a flight to Mach 2.

NACA technicians did everything they could to coax a little more performance out of the airplane, including minor modifications that would provide modest increases in engine thrust and fuel efficiency, and even taping over panel cracks and covering the airframe with a coat of wax in an effort to reduce drag to the absolute minimum. With these preparations and employing a flight plan devised by project engineer Herman O. Ankenbruck to fly to approximately 72,000 feet and push over into a slight dive, Crossfield made aviation history on November 20, 1953. Crossfield he dropped away from the belly of the P2B-1S at 32,000 feet, lit his engine and began to climb. At 72,000 feet he pushed over into a shallow dive and, as he passed through 62,000 feet, he became the first man to reach Mach 2--but just barely, at Mach 2.005 (1,291 mph). He became the first pilot to reach Mach 2 in this, the only flight in which the Skyrocket flew that fast.

Following this flight, Crossfield and NACA pilots Joseph A. Walker and John B. McKay flew the airplane for such purposes as to gather data on pressure distribution, structural loads, and structural heating, with the last flight in the program occurring on December 20, 1956, when McKay obtained dynamic stability data and sound-pressure levels at transonic speeds and above.

Meanwhile, NACA 145 had completed 21 contractor flights by Douglas pilots Eugene F. May and Bill Bridgeman in November 1950. In this jet-and-rocket-propelled craft, Scott Crossfield and Walter Jones began the NACA's investigation of pitch-up lasting from September 1951 well into the summer of 1953. They flew the Skyrocket with a variety of wing-fence, wing-slat, and leading-edge chord extension configurations, performing various maneuvers as well as straight-and-level flying at transonic speeds. While fences significantly aided recovery from pitch-up conditions, leading edge chord extensions did not, disproving wind-tunnel tests to the contrary. Slats (long, narrow auxiliary airfoils) in the fully open position eliminated pitch-up except in the speed range around Mach 0.8 to 0.85.

NACA aircraft 143 was initially powered by a Westinghouse J-34-40 turbojet engine configured only for ground take-offs, but in 1954-55 the contractor modified it to an all-rocket air-launch capability featuring an LR8-RM-6, 4-chamber Reaction Motors engine rated at 6,000 pounds of thrust at sea level (the Navy designation for the Air Force's LR-11 used in the X-1). In this configuration, NACA research pilot John McKay flew the airplane only once for familiarization on September 17, 1956. The 123 flights of NACA 143 served to validate wind-tunnel predictions of the airplane's performance, except for the fact that the airplane experienced less drag above Mach 0.85 than the wind tunnels had indicated.

In June 1954, Crossfield began an investigation of the effects of external stores (bomb shapes and fuel tanks) upon the aircraft's transonic behavior. McKay and Stanley Butchart completed the NACA's investigation of this issue, with McKay flying the final mission on August 28, 1956.

Besides setting several records, the Skyrocket pilots had gathered important data and understanding about what would and would not work to provide stable, controlled flight of a swept-wing aircraft in the transonic and supersonic flight regimes. The data they gathered also helped to enable a better correlation of wind-tunnel test results with actual flight values, enhancing the abilities of designers to produce more capable aircraft for the armed services, especially those with swept wings. Moreover, data on such matters as stability and control from this and other early research airplanes aided in the design of the century series of fighter airplanes, all of which featured the movable horizontal stabilizers first employed on the X-1 and D-558 series.

The Skyrockets were flown until Dec. 1956 and completed a total of 161 flights with the NACA before being retired. D-558-2 #1 Skyrocket is on display at the "Planes of Fame Museum," Chino, California. The number two Skyrocket, the first aircraft to fly Mach two, is on display at the National Air and Space Museum in Washington D.C. The number three is displayed on a pedestal at Antelope Valley College, Lancaster California.