It was powered by both a Westinghouse J-34 turbojet engine and a Reaction Motors, Inc. rocket motor.
The D-558-II "Skyrockets" were among the early transonic research airplanes like the X-1, X-4, X-5, and X-92A. 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 Calif., redesignated in 1949 the High-Speed Flight Research Station (HSFRS); the Navy-Marine Corps; and the Douglas Aircraft Co. The HSFRS was now known as the NASA Dryden Flight Research Center. The Skyrocket made aviation history when it became the first airplane to fly twice the speed of sound The II in the aircraft's designation referred to the fact that 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.
Douglas pilot John F. Martin made the first flight at Muroc Army Airfield (later renamed Edwards Air Force Base) in Calif. on February 4, 1948. 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.
The need for transonic research airplanes grew out of two conditions that existed in the early 1940s. One was the absence of accurate wind tunnel data for the speed range from roughly Mach 0.8 to 1.2. The other was the fact that fighter aircraft like the P-38 "Lightning" were approaching these speeds in dives and breaking apart from the effects of compressibility--increased density and disturbed airflow as the speed approached that of sound, creating shock waves. People in the aeronautics community--especially the NACA, the Army Air Forces (AAF), and the Navy--agreed on the need for a research airplane with enough structural strength to withstand compressibility effects in the transonic region. The AAF preferred a rocket-powered aircraft and funded the X-1, while the NACA and Navy preferred a more conservative design and pursued the D-558, with the NACA also supporting the X-1 research.
The Navy contracted with Douglas to design the airplane, and in the course of the design process, the D-558 came to be divided into two separate phases, with phase one being a straight-wing turbojet aircraft and phase two consisting of a swept-wing design with turbojet and rocket propulsion. At the NACA's suggestion, based on the research of Robert Jones at Langley and captured German documents, Douglas and the Navy had agreed to the swept-wing design, and to provide sufficient power to propel the swept-wing airplane past Mach 1, they also agreed to add rocket propulsion.
Then, to fit both a turbojet and rocket engine in the phase two aircraft required a new fuselage. Like the D-558-I, the Skyrocket featured a horizontal stabilizer high on the vertical tail to avoid the wake from the wing. As with the X-1 and the D-558-1, the Skyrocket also featured, at NACA suggestion, a horizontal stabilizer that was thinner than the wing and movable in flight so as to avoid simultaneous shock wave effects for the wing and horizontal tail and to provide pitch (nose up or down) control when shock waves made the elevators ineffective. While Douglas was constructing the D-558-IIs, the NACA continued to furnish the contractor data it needed on aircraft performance based on tests in Langley wind tunnels and with rocket-propelled models from the Wallops Island Pilotless Aircraft Research Station.