A Sample Feature From Aviation News

First double Mach-Buster

Now that supersonic flight is routine, the huge scale of effort involved in first breaking the ‘sound barrier’ is easily forgotten. Here Peter Holthusen focuses on one such programme and describes how the Douglas Skyrocket become the first aircraft to exceed Mach 2.

Above: Developed in parallel with the jet-powered straight-wing transonic D-558-I Skystreak, the swept-wing supersonic D-558-II Skyrocket represented one of the first-generation, post-WW2, rocket-powered flight test aircraft, seen here with a North American F-86A Sabre flying ‘chase’ during the mid 1950s. (All photos NASA unless otherwise stated).

Below: In a bid to extract higher speeds and altitude from the Skyrocket, the first and second of the three aircraft were modified for air-launch from NACA’s Boeing P2B-1S carrier, a converted B-50. Carried under the aircraft centre-line in a position normally occupied by the bomb bay, the rear of the P2B-1S is raised by jacking up the main wheels allowing the Skyrocket to be secured.

When the first D-558-II Skyrocket (Bu No 37973) arrived at Muroc Dry Lake in early December 1947 from the EI Segundo plant of the Douglas Aircraft Company, the Bell XS-1 had already shattered the myth of the ‘sound barrier’. However, much still remained for aerodynamicists to learn about transonic and supersonic flight conditions and it fell to the D-558-II to explore another aerodynamics problem, this one a potentially dangerous instability predicted by wind-tunnel tests – the pitch-up phenomenon.

In August 1945 the US Navy and the National Advisory Committee for Aeronautics (NACA) asked the Douglas Aircraft Company for initial design studies for a more advanced research aircraft capable of supersonic speed. As a result of research into captured German aerodynamic reports by Douglas engineers who had joined the US Naval Technical Mission to Europe in May 1945, the new aircraft – D-558-II – was designed around swept wings. To supplement the insufficient thrust of available turbojets, a rocket engine with sufficient fuel for two minutes operating at 4,000lb thrust was eventually to be installed.

On January 27, 1947, an order for three aircraft was finalised under Contract No NOa(s) 6850. The D-558-II Skyrockets flew from 1948 to 1956 in a joint programme involving NACA at the Muroc Flight Test Unit in the Mojave Desert, Calif, (re-designated in 1949 the High-Speed Flight Research Station (HSFRS) now known as the NASA Dryden Flight Research Centre) the US Navy, Marine Corps and Douglas.

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 programme, with the phase-one aircraft having straight wings. The third phase, which never came to fruition, would have involved constructing an engineering mock-up of a combat-type aircraft embodying the results from the testing of the phase one and two designs.

The swept-wing D-558-II was, in many ways, the workhorse of NACA high-speed flight-testing, bridging the era from the subsonic aeroplane almost to the era of the hypersonic North American X-15.

It is now 58 years since Douglas pilot John F Martin made the first flight with the D-558-II on February 4, 1948, at Muroc (later renamed Edwards Air Force Base) in Calif. Douglas selected Martin, its chief pilot, because of his experience and service with the company. He had joined Douglas in 1940 after flying for United Air Lines and went on to test a variety of types, including the A-20 Havoc, A-26 Invader and the C-54. The story, allegedly true, of how Martin became project pilot illustrates how the company’s own test pilots viewed the D-558-II Skyrocket project. At the time the Skyrocket neared completion, the ‘sound barrier’ still loomed as a dangerous unknown in the minds of most pilots: Yeager’s epochal flight lay in the future. When it came time to select a project pilot for the aeroplane, Douglas El Segundo sent notice to the flight test office at Santa Monica for the company test pilots to submit bids for the flight programme. The pilots did not desire to fly the somewhat imposing aircraft and, after talking among themselves, decided to submit exceptionally high bids that almost certainly would not be accepted. Martin was the only pilot not in on the plot, for he was away delivering an aeroplane for Douglas. Unaware of the conspiracy in Santa Monica, he submitted a reasonable bid, so the company designated him as project pilot.

The glistening white Skyrocket waited at Muroc in January 1948 while Douglas technicians completed installing its Westinghouse 24C (better known as the J34-WE-40) turbojet engine and research instrumentation. By the beginning of February, the aircraft was ready for flight and, on the morning of February 4, Martin lifted the first Skyrocket off the vast lakebed at Muroc. The D-558-II had a sluggish performance on just its turbojet engine (it only produced 3,000lb thrust) and Douglas decided to improve take-off performance by installing JATO solid-fuel booster bottles. These early flights also revealed a ‘Dutch-roll’-type oscillation – a combined yawing and rolling motion as if the aircraft was hunting back and forth to find a comfortable position for itself – and the flush cockpit canopy guaranteed poor visibility for the pilot. Douglas remedied the two problems by increasing the height of the vertical fin by roughly 18in, raising it to an even 13ft (coincidentally giving it an even more graceful line) and installing a raised cockpit similar to the D-558-I Skystreak. After making 15 flights in the Skyrocket, Martin turned it over to Gene May.

Above: The clean aerodynamic lines of the D-558-II and the blended tail and cockpit canopy presage the era of integrated design, essential for optimum performance with supersonic aircraft and characterised by all high performance combat aircraft designs from the late 1960s. Guided by the legendary Ed Heinemann the aircraft was designed by a team led by Leo Devlin with Robert C. Donovan as project engineer. In its aerodynamic form can be found the mould line of the Douglas A-4 Skyhawk.

Below: The cockpit, like the aircraft, was superior in its simplicity, excluding all extraneous and unnecessary features. Essential flying and attitude instruments are located around the centre of the panel with power, motor and fuel switches along the bottom and around the top of the displays. Control is maintained through conventional stick and rudder pedals.

Feeling the air

The need for transonic research aircraft 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 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 – NACA, USAAF and USN – agreed on the need for a research aeroplane with enough structural strength to withstand compressibility effects in the transonic region. The AAF preferred a rocket-powered aircraft and funded the XS-1, while NACA and USN preferred a more conservative design and pursued the D-558-II, with NACA also supporting the XS-1 research.

Over the spring and summer of 1948, Douglas engineers at Muroc readied the second D-558-II (Bu No 37974) for flight. Like the first Skyrocket, this aeroplane lacked its XLR-8-RM-6 rocket engine, but NACA agreed to accept the aircraft and utilise it with the provision that when the rocket engine became available the agency could return the Skyrocket to Douglas for its installation. Gene May completed two demonstration flights for the company on November 2 and 7, 1948, after which the aircraft, still only powered by a J34-WE-40 turbojet, was delivered to NACA for its flight research programme on December 1, 1948. Equipped only with its J34, the Skyrocket was limited to about Mach 0.9, but since the NACA planned to use it for general stability, control and air-loads in research to Mach 0.85, this did not constitute a serious limitation. Engine difficulties kept it grounded until May 24, research pilot Robert A Champine completed the Skyrocket’s first NACA flight, beginning its research programme with the second flight on June 1, 1949.

By early August, NACA had completed six flights and Champine had reached Mach 0.87 in a dive, the fastest flight yet. During the plane’s seventh NACA flight on August 8, 1949, Champine banked into a 4g turn at Mach 0.6. Suddenly, without warning, the nose pitched upwards violently, attaining a positive acceleration of 6g. Shaken, Champine applied full down elevator and the Skyrocket responded rapidly. Not taking any chances, he landed immediately on Rosamond Dry Lake, west of Muroc. Though model tests had indicated that swept-wing aeroplanes might experience longitudinal instability resulting in a ‘pitch-up’ phenomenon, this seventh NACA flight provided aerodynamicists with the first indication of the severity and seriousness of the problem, particularly those which had their horizontal tail located high on the vertical fin. The remainder of the aircraft’s programme, prior to the installation of ten SLR-8-RM-6 rocket motors, concentrated on the pitch-up problem, soon encountered with other high-speed swept-and-delta-wing aircraft.

In September 1949, John Griffith checked out in the Skyrocket and on November 1 he too entered a 4g turn at Mach 0.6 and as on the earlier flight, the aircraft became longitudinally unstable and pitched up. Griffith recovered from the snap-roll and after checking that everything still functioned normally, continued the flight. During a stall approach with the flaps and landing gear extended and the wing slats closed at 14,000ft, the Skyrocket remained stable down to 130mph, but then pitched up. Again, Griffith attempted to fly beyond the point of instability and the bucking aircraft rolled into a spin. During the spin, Griffith found the D-558-II would not recover with flaps and gear extended. After he retracted the gear and flaps and initiated spin recovery, it returned to normal flight at 7,000ft. The NACA pilots made 21 jet-powered flights between May 24, 1949, and January 6, 1951, when the High-Speed Flight Station returned the aircraft to Douglas to be modified for high-speed, air-launched flights and fitted with an XLR-8-RM-6 rocket engine with additional fuel tanks replacing the turbojet.

A decision to modify Skyrocket Bu No 37974 and later Bu No 37973 to the air-launched configuration, like the XS-1, was made by NACA and the US Navy on the basis of the flight-test results with 37975. This aircraft, the third D-558-II, had initially been fitted with both the J34-WE-40 turbojet and XLR-8-RM-5 rocket engine and had made its first flight, on turbojet power alone, on January 8, 1949, with test pilot Gene May, making its first rocket-powered flight over a month later, on the morning of February 25. The rocket engine increased the aircraft’s speed over 100kt compared with the jet-only Skyrockets, raising the maximum level-flight Mach number from approximately 0.82 at 20,000ft to 0.99 at the same altitude. It could attain approximately Mach 1.08 at 40,000ft in level flight on both turbojet and rocket power.

Supersonic sprint

On June 24, 1949, the third Skyrocket exceeded the speed of sound for the first time. There were problems with the conventional take-off system as the combination of heavily-laden aircraft full of high-explosive rocket propellants was a dangerous cocktail. Fuel burnt during the take-off and climb to altitude restricted endurance and limited the speed to Mach 1.08 – 720mph at 40,000ft – before running out of fuel. As the plane went supersonic, May noted, ‘the flight got glassy smooth, placid, quite the smoothest flying I had ever known.’ The US Air Force’s Bell X-1 had demonstrated the practicality of air-launched operations, so during November 1949 the US Navy Bureau of Aeronautics instructed Douglas to modify 37975 to air-launch configuration with the original turbojets installation while aircraft 37974 was modified to air-launch mode with rocket motor only. The carrier aircraft selected was a US Navy-operated Boeing P2B-1S Superfortress, ex-US Air Force B-29-95-BW 45-21787, later becoming NACA 137. Douglas recognised the safety and performance advantages accruing from air launching and studied modifying one Skyrocket by removing its turbojet engine and replacing the engine with increased fuel for the rocket engine. Thus modified with air launching, Douglas El Segundo plant chief engineer, Edward H Heinemann and his team estimated that it could attain airspeeds between Mach 1.46 and 1.6. NACA believed this modification highly desirable, for results from the swept-wing aeroplane could then be directly compared over the same speed range covered by the straight-wing Bell X-1. Furthermore, the supersonic behaviour of the D-558-II’s NACA 63 series airfoil section could be compared with the supersonic behaviour of the unconventional biconvex airfoil then designed for the projected Bell X-2 swept-wing supersonic research aircraft.

Hugh L Dryden, NACA’s Director of Research, firmly supported the Heinemann plan and proposed that Douglas modify the second Skyrocket to all-rocket air-launch configuration when NACA returned it to Douglas for installation of its rocket engine.

By September 1950, the third aircraft, its modifications limited to the fitting of retractable mounts for the air-launch hooks, was ready to make captive flight from the fuselage of the Superfortress. To Douglas and NACA launch crews, the plane was known more informally as ‘Fertile Myrtle’. On September 1950 with test pilot William ‘Bill’ Bridgeman, a tall, gregarious airman, D-558-II was launched at 24,850ft at a speed of 225mph from the ‘mothership’ piloted by El Segundo test pilot George Jansen (who had enthusiastically volunteered for the position). Bill Bridgeman had trained as a Navy pilot, flying Consolidated-Vultee PBY Catalina flying-boats from Pearl Harbor and Australia for the first 18 months of the war. Likewise, George Jansen also had a multi-engine flying background, serving during the war as a B-24 Liberator pilot with the 44th Bomb Group, the Eight Balls. After the war he joined Douglas as a test pilot and flew many of the initial performance investigations on the AD Skyraider and XA-2D Skyshark.

Although the flight plan called for a series of level flight speed runs and co-ordinated turns, Bridgeman just flew down to a landing on the lakebed, for the airspeed system malfunctioned due to frozen moisture in the lines. Prior to handover to the Agency, three more flights were made on turbojet power and two using the turbojet-rocket combination, again piloted by Bridgeman. On December 15, 1950, the Skyrocket was handed over to the Agency, becoming NACA 145. This third D-558-II was flown 66 times between December 22, 1950, and August 28, 1956, by NACA, USAF and USMC pilots performing a variety of tests including evaluation of high-speed handling with wing slats fully or partially opened, with slats replaced by a chord extension over the outer 32% of the wing panels, and with external stores. At the completion of its NACA test programme the aircraft had made 87 flights.

Below: Seen against the floor of the Mojave desert, the Skyrocket displays the clean lines facilitated by an integral rocket motor devoid of the need for air intake to disturb the air flow across the fuselage. Note the wing fences and the extended pitot tube. (Photo, Av News Files).

For the rest of this feature please see the May 2006 issue.