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The Bell X-1: Program changed history, established research aircraft concept

When most people think of the Bell X-1, they think of then-Capt. Chuck Yeager breaking the sound barrier on Oct. 14, 1947.

However, the X-1 program involved seven different variants of the aircraft, and multiple flights from the program’s first flight on Jan. 19, 1946, through 1958.

The first generation X-1 aircraft changed aviation history in numerous ways, and not simply because they were the first aircraft to fly faster than the speed of sound.

Rather, they established the concept of the research aircraft, built solely for experimental purposes, and unhampered by any military or commercial requirements.
Although subsequent X-planes were built for a wide range of purposes — technology or concept demonstrators, unmanned test missiles, and even as prototypes in all but name — the X-1s were built to go faster than an aircraft had ever flown before.

The X-1 resulted from technological challenges facing aircraft designers in the late 1930s and early 1940s. Aircraft had begun to experience both subsonic and supersonic airflow over their wings. This created a range of undesirable characteristics: compressibility, increased drag, trim changes, severe turbulence, and loss of control effectiveness. Wind tunnels were affected by the same aerodynamic problems, and their data proved to be unreliable in this regime. As a result, a few individuals — John Stack of NACA, Ezra Kotchner of the Army Air Forces, and Walter Diehl of the Navy — realized a specialized research aircraft offered the only feasible means of getting supersonic aeronautical data.

The Army Air Forces selected Bell Aircraft to build three X-1 aircraft.

The fuselage was the same shape as a 0.50 caliber machine gun bullet, which was known to be stable at supersonic speeds. The X-1 wings were straight, rather than swept back, and relatively thin for the time. The X-1-1 (serial number 46-062) had a wing with an 8 percent thickness/chord ratio. The X-1-2 (serial number 46-063) had a 10 percent ratio wing. The X-1 was powered by an XLR-11 rocket engine, which had four chambers and burned liquid oxygen (LOX) and a mixture of alcohol and water. In 1945, rockets were viewed with suspicion by some engineers. Both NACA and Navy preferred a jet-powered research aircraft, rather than one using a rocket, as the Army Air Forces had selected.

The X-1-1 was delivered by Bell in December 1945. At the same time, the Army Air Forces asked that NACA personnel oversee the instrumentation and data analysis of the X-1 flights. As a result, a NACA team was incorporated into the program.

The first glide flight of the X-1 occurred on Jan. 19, 1946, at Pinecastle Field, Fla., flown by Bell test pilot Jack Woolams because Muroc Army Air Field had been flooded. The X-1-1 was air launched from a B-29. Woolams made a total of 10 glide flights to test the X-1’s low speed handling before it was returned to Bell Aircraft in Buffalo, N.Y., in March 1946 for installation of the rocket engine, and modifications to prepare it for powered flight tests. The aircraft was delivered to Muroc in October 1946.

The first group of NACA engineers arrived at Muroc Field (now Edwards Air Force Base, Calif.), in September 1946 in preparation for the initial flights of the X-1-2.

Bell test pilot Chalmers “Slick” Goodlin made the first glide flight in the X-1-2 on Oct. 11, 1946. After a total of four glide flights, he made the first powered flight on Dec. 9, reaching a speed of Mach 0.79. By June 1947, Bell had proven the airworthiness of both X-1s up to speeds of Mach 0.8. The contract freed the company from responsibility above this speed.

On June 30, 1947, Army Air Forces and NACA representatives agreed on a two-phase flight program. The Army Air Forces would use the X-1-1, with its thinner wing, to conduct an accelerated program to reach Mach 1.1 as quickly as possible. NACA would provide support, such as technical advice and data analysis. NACA would then undertake a slower-paced, more detailed series of research flights at transonic (near the speed of sound) speeds, using the X-1-2 and its thicker wing.

Capt. Charles E. “Chuck” Yeager was selected as the pilot for flights to Mach 1.

He made his first glide flights on Aug. 6, 7 and 8, 1947. Yeager undertook his first powered flight in the X-1-1 on Aug. 29, reaching Mach 0.85.

Over the next six weeks, Yeager came closer to Mach 1, reaching Mach 0.997 on Oct. 10. For the NACA engineers, used to a more cautious step-by-step approach, Yeager and the Air Force seemed to be acting in haste. Still, on Oct. 14, Yeager reached a speed of Mach 1.06 at 43,000 feet, becoming the first man to fly supersonic. Air Force officials designated the flight and all data as Top Secret two hours later.

However, the story of Yeager’s Oct. 14 flight was leaked to a reporter from the magazine Aviation Week, and the Los Angeles Times featured the story as headline news in their Dec. 22 issue. The magazine story was released on Dec. 20. The Air Force threatened legal action against the journalists who revealed the story, but none ever occurred. The news of a straight-wing supersonic aircraft surprised many American experts who, like their German counterparts during World War II, believed that a swept-wing design was necessary to break the sound barrier. On June 10, 1948, Air Force Secretary Stuart Symington announced that the sound barrier had been repeatedly broken by two experimental airplanes.

NACA now began flying the X-1-2 on research missions.

On Oct. 21, 1947, NACA pilot Herbert H. Hoover made a glide flight. Hoover followed this mission on Dec. 16 with a powered flight to Mach 0.84. In January 1948, a second NACA research pilot, Howard C. Lilly, joined the program. The initial NACA flights in the aircraft sought data on turns and pull ups, side slips, and elevator effectiveness at subsonic speeds. It was not until March 4, 1948, that Hoover reached Mach 1.029. Hoover became the second man to reach Mach 1, on the first NACA and the first civilian supersonic flight. Lilly flew at Mach 1.1 on March 31.

Robert A. Champine replaced Hoover and Lilly on the X-1 program in November 1948, undertaking studies of wing pressure distribution, stability and control, and stabilizer effectiveness. John H. Griffith continued these research efforts when he replaced Champine on the X-1 program. Griffith flew the X-1-2 through October 1950, when he left NACA for a job as a company test pilot.

A. Scott Crossfield joined the research efforts in April 1951, followed by Joe Walker in August.

The research usefulness of the first generation X-1 aircraft was nearing an end.

The second generation X-1 aircraft, then under development, would be able to reach twice the speed of sound.

After Yeager’s Mach 1 flight, the X-1-1 had been used by the Air Force to acquire data on stability and control, wing and tail loading, high-altitude flight, and pilot familiarization. After a final flight by Yeager on May 12, 1950, the X-1-1 was retired and given to the Smithsonian Institution. The X-1-2 continued flying, but technical problems brought its work to a close. The X-1-1 and X-1-2 both used a fuel system pressurized with nitrogen. The X-1-2s nitrogen tanks were nearing the end of their fatigue life, risking a possible explosion. Consequently, NACA officials grounded the X-1-2, which later returned in a much-modified state as the X-1E.

The X-1-3 (serial number 46-064) represented the final example of first generation X-1 series. The X-1-3 was externally identical to the other two aircraft. The fuel system in the X-1-3 did not rely on nitrogen pressure, however, but rather on a turbopump. This eliminated the need for the heavy nitrogen tanks, and resulted in a calculated maximum speed of Mach 2.4, a full Mach number higher than the X-1-1 or X-1-2 could reach. Funding cuts and turbopump development problems, however, delayed the aircraft a full three years. The Air Force had also contracted with Bell Aircraft to develop the second generation X-1A, X-1B and X-1D. Interest in the X-1-3 faded, and the Air Force canceled it. NACA, wanting its own Mach 2 aircraft to experiment with, picked up the Air Force’s canceled X-1-3.

The X-1-3 was delivered to Edwards in April 1951. Bell test pilot Joseph Cannon successfully made a glide flight in the aircraft on July 20. On Nov. 9, 1951, a captive flight was made by the X-1-3 aboard the B-50 launch aircraft. This was to be a rehearsal for the first powered flight, as well as a test of the jettisoning system.
Engineers canceled the jettisoning tests, however, when nitrogen pressure fell. The B-50, with the fully fueled X-1-3 still attached, landed back at Edwards safely, and preparations began to jettison the LOX. As Cannon pressurized the LOX tank, however, a dull thud was heard, followed by a hiss. A small cloud of white vapor escaped from the X-1-3’s center section. Then, a violent explosion occurred, with yellow flames and black smoke engulfing both the X-1-3 and the B-50. Cannon escaped from the X-1-3, but spent nearly a year in the hospital recovering from severe burns on his legs, arms, and body. The fire and subsequent explosions destroyed both the X-1-3 and B-50.

Variants
Later variants of the X-1 were built to test different aspects of supersonic flight. One of these, the X-1A, with Yeager at the controls, inadvertently demonstrated a very dangerous characteristic of fast (Mach 2 plus) supersonic flight: inertia coupling. Only Yeager’s skills as an aviator prevented disaster. Later Mel Apt would lose his life testing the Bell X-2 under similar circumstances.

X-1A — Bell Model 58A
Ordered by the Air Force on April 2, 1948, the X-1A (serial number 48-1384) was intended to investigate aerodynamic phenomena at speeds greater than Mach 2 and altitudes greater than 90,000 feet, specifically emphasizing dynamic stability and air loads. Longer and heavier than the original X-1, with a stepped canopy for better vision, the X-1A was powered by the same Reaction Motors XLR-11 rocket engine. The aircraft first flew, unpowered, on Feb. 14, 1953, at Edwards, with the first powered flight on Feb. 21. Both flights were piloted by Bell test pilot Jean “Skip” Ziegler.

After NACA started its high-speed testing with the Douglas Skyrocket, culminating in Scott Crossfield achieving Mach 2.005 on Nov. 20, 1953, the Air Force started a series of tests with the X-1A, which the test pilot of the series, Yeager, named “Operation NACA Weep”. These culminated on Dec. 12, 1953, when Yeager achieved an altitude of 74,700 feet and a new airspeed record of Mach 2.44. Unlike Crossfield in the Skyrocket, Yeager achieved that in level flight. Soon afterwards, the aircraft spun out of control, due to the then not yet understood phenomenon of inertia coupling. The X-1A dropped from maximum altitude to 25,000 feet, exposing the pilot to accelerations of as much as 8g, during which Yeager broke the canopy with his helmet before regaining control.

On May 28, 1954, Maj. Arthur W. Murray piloted the X-1A to a new record of 90,440 feet.

The aircraft was transferred to NACA during September 1954, and subsequently modified. The X-1A was lost on Aug. 8, 1955, when, while being prepared for launch from the airborne RB-50 mothership, an explosion ruptured the plane’s liquid oxygen tank. With the help of crewmembers on the RB-50, test pilot Joseph A. Walker successfully extricated himself from the plane, which was then jettisoned. Exploding on impact with the desert floor, the X-1A became the first of many early X-Planes that would be lost to explosions.

Bell X-1-3, aircraft #46-064, being mated to the B-50 mothership for a captive flight test on Nov. 9, 1951. While being de-fueled after this flight, it exploded, destroying itself and the B-50, and seriously burning Bell test pilot Joe Cannon. X-1-3 had completed only a single glide-flight on July 20. (NASA photograph)

X-1B — Bell Model 58B
The X-1B (serial 48-1385) was equipped with aerodynamic heating instrumentation for thermal research (more than 300 thermal probes were installed on its surface). It was similar to the X-1A except for having a slightly different wing.

The X-1B was used for high-speed research by the U.S. Air Force starting in October 1954, prior to being transferred to NACA during January 1955. NACA continued to fly the aircraft until January 1958, when cracks in the fuel tanks forced its grounding. The X-1B completed a total of 27 flights. A notable achievement was the installation of a system of small reaction rockets used for directional control, making the X-1B the first aircraft to fly with this sophisticated control system, later used in the North American X-15. The X-1B is now at the National Museum of the United States Air Force, where it is displayed in the Museum’s Maj. Gen. Albert Boyd and Maj. Gen. Fred Ascani Research and Development Gallery.

X-1C — Bell Model 58C
The X-1C (serial 48-1387) was intended to test armaments and munitions in the high transonic and supersonic flight regimes. It was canceled while still in the mockup stage, as the development of transonic and supersonic-capable aircraft like the North American F-86 Sabre and the North American F-100 Super Sabre eliminated the need for a dedicated experimental test vehicle.

X-1D — (Bell Model 58D)
The X-1D (serial 48-1386) was the first of the second generation of supersonic rocket planes. Flown from an EB-50A (s/n #46-006), it was to be used for heat transfer research. The X-1D was equipped with a new low-pressure fuel system and a slightly increased fuel capacity. There were also some minor changes of the avionics suite.

On July 24, 1951, with Bell test pilot Jean “Skip” Ziegler at the controls, the X-1D was launched over Rogers Dry Lake, on what was to become the only successful flight of its career. The unpowered glide was completed after a nine-minute descent, but upon landing, the nose landing gear failed and the aircraft slid ungracefully to a stop. Repairs took several weeks to complete and a second flight was scheduled for mid-August. On Aug. 22, 1951, the X-1D was lost in a fuel explosion during preparations for the first powered flight. The aircraft was destroyed upon impact after it was jettisoned from its EB-50A mothership.

X-1E — Bell Model 44
The X-1E was the result of a reconstruction of the X-1-2 (serial 46-063), in order to pursue the goals originally set for the X-1D and X-1-3 (serial 46-064), both lost in explosions during 1951.

The cause of the mysterious explosions was finally traced to the use of Ulmer leather gaskets impregnated with tricresyl phosphate (TCP), a leather treatment, which was used in the liquid oxygen plumbing. TCP becomes unstable and explosive in the presence of pure oxygen and mechanical shock. This mistake cost two lives, caused injuries and lost several aircraft.

The changes included:
• A turbopump fuel feed system, which eliminated the high-pressure nitrogen fuel system used in ‘062 and ‘063. Concerns about metal fatigue in the nitrogen fuel system resulted in the grounding of the X-1-2 after its 54th flight in its original configuration.
• A re-profiled super-thin wing, based on the X-3 Stiletto wing profile, enabling the X-1E to reach Mach 2.
• A ‘knife-edge’ windscreen replaced the original greenhouse glazing. An upward-opening canopy replaced the fuselage side hatch and allowed the inclusion of an ejection seat.
• The addition of 200 pressure ports for aerodynamic data, and 343 strain gauges to measure structural loads and aerodynamic heating along the wing and fuselage.

The X-1E first flew on Dec. 15, 1955, a glide-flight controlled by Air Force test pilot Joe Walker. Walker left the X-1E program during 1958, after 21 flights, attaining a maximum speed of Mach 2.21. NACA research pilot John B. McKay took his place during September 1958, completing five flights in pursuit of Mach 3 before the X-1E was permanently grounded after its 26th flight, during November 1958, due to the discovery of structural cracks in the fuel tank wall.

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