X-1/X-1A/X-1B/X-1C/X-1D/X-1E

Specifications

Company- Bell Aircraft; Stanley Aircraft (wings) for X-1E
Type- Testbed for supersonic (Mach 1+) flight.
Goals- Investigate flight characteristics at greater than sonic velocities. Structural, physiological phenomena within transonic speed envelope. Exploratory aerodynamic heating tests; experimental reaction control system. High-speed wing performance.
Primary Testing Facility Research- Edwards AFB
Dimensions- Span- 28 ft, 0 in; Length- 35 ft, 8 in
Max Speed- Mach 2.44
Range- N/A
Max Altitude- 90,000 ft
Power Plant- One Reaction Motors XLR-11RM-5 (E6000-D4 rocket)
Thrust- 6,000 lbf
Weights- Fully Loaded: 16,487 lbs
Payload- N/A
Flights- 236 (157 X-1, 25 X-1A, 27 X-1B, 1 X-1D, 26 X-1E)
Number of Prototypes Built- 7 (3 X-1, 1 X-1A, 1 X-1B, 1 X-1D, 1- X-1E)
Project Tenure- 1946-1958
Project Status- Cancelled

Information

First Generation X-1

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 the 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 the 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, an NACA team was incorporated into the program. The first glide flight of the X-1 occurred on Jan. 19, 1946, at Pinecastle Field, Fla., by Bell test pilot Jack Woolams. 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 handing before it was returned to Bell in March 1946 for installation of the rocket engine. 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. The NACA would provide support, such as technical advice and data analysis. The 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. Not until December 1947 would word leak of the achievement, and it was not until March 1948 that the U.S. Air Force officially confirmed the achievement.

The 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 the NACA for a job as a company test pilot. A. Scott Crossfield joined the research efforts in April 1951, adding 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-1-2's 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 cancelled it. The 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 cancelled 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 and 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.

Next Generation X-1

The second generation X-1s used the same wing, horizontal tail, and XLR-11 rocket engine as the first generation aircraft, with a new cylindrical fuselage just over 4 1/2 feet longer than the original design. This was the maximum length that could be carried by a B-29 or B-50 launch aircraft. The cockpit design was also changed to a "stepped" canopy, allowing the pilot to enter from the top, instead of the side hatch on the original X-1. A more significant change was the fuel system. The new aircraft used a low-pressure turbopump, which eliminated the heavy spherical liquid oxygen (LOX) and alcohol tanks. The new, larger tanks now conformed to the fuselage shape. As a result, the airplane's calculated maximum performance had increased to Mach 2.47 at 70,000 feet.

Four of the second generation X-1s were originally to be built by Bell Aircraft: the X-1A for dynamic stability tests; the X-1B for air load research; the X-1C for armaments tests using a .50 cal. machine gun in the nose and a gun sight for the pilot; and the X-1D for heat transfer research. The X-1C was cancelled while in the mockup stage.

The X-1D (serial number 48-1386) was the first of the second generation aircraft to be delivered, arriving at Edwards Air Force Base, Calif., in July 1951. The first glide flight was made on July 24, by Bell test pilot Jean Ziegler. The flight was successful, but after touchdown the nose gear was damaged, requiring several weeks of repair. The aircraft was then turned over to the Air Force, and Lt. Col. Frank Everest was assigned as project pilot. Its first powered flight was scheduled for August 22, 1951. The launch was aborted, however, due to the loss of nitrogen pressure. Everest attempted to jettison the propellant, triggering an explosion and fire. The X-1D was jettisoned and destroyed on impact. The X-1D accident board theorized that a fuel leak had created an explosive mixture of air and alcohol, which was ignited by a spark from the X-1D's radio or an external power source.

The X-1A (serial number 48-1384) was not delivered to Edwards until January 7, 1953. Ziegler made the first glide flight on February 14, 1953. After a second glide flight six days later, he made the first powered flight on February 21. The Phase I contractor flights continued through April, when the aircraft was returned to Bell for modifications. The aircraft was returned to Edwards in October 1953, and began a series of flights at higher Mach numbers. Maj. Charles E. "Chuck" Yeager reached a speed of Mach 2.44 at 74,200 feet on December 12, only a few days before the 50th anniversary of the first powered flight. The aircraft began to roll to the left as it reached maximum speed. Yeager tried to counter with right aileron and rudder, but the X-1A began to roll to the right. The aircraft then tumbled out of control, throwing Yeager around in the cockpit, and rendering him unconscious. He finally revived at about 29,000 feet, with the X-1A in an inverted spin. Despite being groggy, Yeager was able to recover and land back at Edwards, without the help of chase planes.

The Air Force decided not to make any additional high Mach flights with the X-1A, due to the instability. Rather, the X-1A would be used for very high altitude flights by Maj. Arthur Murray. A series of flights were attempted during the spring and summer of 1954, but only four were successful. On two of these flights, including the record-setting flight of August 26, 1954, which reached 90,440 feet, similar instability occurred. This was not as severe as on Yeager's flight, due to the lower speed and higher altitude. The loss of control was traced to a decrease in directional stability, which allowed inertial coupling to occur. The X-2 and X-3 research aircraft were also affected by inertial coupling.

The X-1A was turned over to the NACA for high-altitude/high-Mach flights in September 1954, and were sent to Bell Aircraft for modifications. The X-1B (serial number 48-1385) made several Air Force pilot checkout flights, and was then turned over to the National Advisory Committee for Aeronautics on December 3, 1954. Like its sister ship, the X-1B underwent modifications for its research role.

The X-1A returned to Edwards in mid-1955, and made its first NACA flight on July 20. Joe Walker reached a speed of Mach 1.45, and landed safely. The next NACA X-1A flight was scheduled for August 8. During the countdown, an explosion occurred in the aircraft. The B-29 launch aircraft could not be landed with the damaged X-1A aboard, so the rocket plane was jettisoned and destroyed on impact.

The X-1A was the fourth rocket plane destroyed by explosions. The others were the X-1D, the X-1-3, and the X-2 #2. The investigations following each crash found no common factor in the accidents. After the X-1A crash, the debris was brought in from the desert and laid out on a hangar floor. The X-1B which had returned from modifications only a week before the loss of the X-1A, was parked next to the debris. When the X-1B's liquid oxygen (LOX) tank was examined, tricresyl phosphate (TCP) was found in both the LOX tank and the LOX lines to the engine. TCP was used to treat the leather gaskets in the tank and LOX lines. Tests indicated that when the leather gaskets were in contact with LOX, the TCP was impact sensitive. The shock of the tanks pressurizing would, under certain conditions, cause it to explode. A reexamination of the four accident indicated that TCP explosions were responsible in each case. The leather gaskets were removed from the X-1B and the surviving X-2, and no further explosions occurred.

With the cause of the explosion identified, work on the X-1B resumed. A total of 300 thermocouples were installed on the aircraft for data on aerodynamic heating. The first NACA flight in the X-1B was made on August 14, 1956 by John B. McKay. By January 1957, a total of two checkout flights and four heating flights had been made by McKay. The data was considered representative of what future Mach 2 aircraft might encounter.

The aircraft then undertook a series of research flights for high-Mach number stability and control data. These were made beginning in May 1957 and continuing through August. A total of seven flights were made in the series. The X-1B was then modified for tests of a reaction control system (RCS). Several hydrogen peroxide rockets were mounted on a wingtip, the aft fuselage and the tail to provide control when the dynamic pressure was too low for conventional aerodynamic controls. This was to gain experience for the upcoming X-15 program.

McKay made the first RCS checkout flight on November 27, 1957. Two more flights were made in January 1957, before rains closed the lakebed until spring. The plan was to resume flights at low aerodynamic pressures. A preflight inspection found cracks in the LOX tank, which would require a new tank be built. This would have been too expensive, and the aircraft was grounded. The RCS flights were transferred to an F-104, while the X-1B was sent to the Air Force Museum.

X-1E

The NACA X-1E was the last of the X-1 series of aircraft, and its construction benefited from the lessons of earlier research aircraft programs. This included the explosions which destroyed the X-1D and the X-1-3 research aircraft, the poor performance of the X-3, and the destruction of both X-2s.

In 1951, the Air Force expected to begin flights with the X-1D, while the NACA was to soon receive the X-1-3. Both aircraft could reach speeds in excess of Mach 2, but were lost in explosions before they were able to produce data. The X-1A and X-1B would not be delivered for two more years, leaving only the NACA's X-1-2 (serial number 46-063) still flying. The Air Force had already retired the X-1-1 (serial number 46-062), giving it to the Smithsonian. Soon after, the NACA discovered that the high-pressure nitrogen spheres in the second X-1 were likely to fail due to metal fatigue. The aircraft made its 54th and final flight on Oct. 23, 1951.

At this juncture, the NACA decided to flight test the characteristics of very thin wings at transonic speeds. Accordingly, NACA engineers proposed the X-1-2 be modified with the new airfoil and a new turbopump fuel system (which eliminated the high-pressure nitrogen spheres). The

At this juncture, the NACA decided to flight test the characteristics of very thin wings at transonic speeds. Accordingly, NACA engineers proposed the X-1-2 be modified with the new airfoil and a new turbopump fuel system (which eliminated the high-pressure nitrogen spheres). The High-Speed Flight Station (now the NASA Dryden Flight Research Center, Edwards, Calif.) made some important modifications. The side hatch was replaced by a upward opening canopy, similar to that on the D-558-II. The new canopy allowed addition of an ejection seat, and gave the pilot easier access in and out of the cockpit. Technicians also installed the new turbopump fuel system. Meanwhile, Stanley Aviation Company built the new thin wings, a mere 3 3/8th inches thick at the wing root, fitted with over 200 pressure openings for aerodynamic data, and another 343 strain gages for structural loads and aerodynamic heating measurements. Similar in thickness to those on the X-3, the wings of the emerging aircraft - called the X-1E - would supply the Mach 2 flight data which the X-3 had been unable to provide due to its poor performance.

Once the modifications were completed, several months of ground tests were required before the aircraft could take flight. NACA research pilot Joe Walker was selected as project pilot. Walker made the first X-1E glide flight on Dec. 15, 1955. The XLR-11 engine and the turbopump had a number of problems which forced Walker to shut them down. The X-1E did not fly again until April 1956, but, despite some problems, it made good progress. On the X-1E's sixth flight, on June 7, Walker reached Mach 1.55, in what was the airplane's first supersonic flight since being modified. Walker raised this to Mach 1.74 on June 18, Mach 2 on Aug. 31, and Mach 2.1 on Sept. 14.

This speed buildup was followed by three flights that were marred by engine and turbopump problems. It was not until April 1957 that flights resumed. The X-1E suffered another setback on May 15, 1957, when after a Mach 2 flight, the aircraft was severely damaged in a landing accident. Repairs to the X-1E continued through the summer, and the aircraft did not fly again until Sept. 19, 1957. The X-1E ended its flights for the year on a high note, however, when on Oct. 8, the aircraft reached Mach 2.24. This was the highest speed it would achieve.

The X-1E's research flights indicated the aircraft had marginal directional stability at high Mach numbers. The X-1E, despite all the modifications, still had the original bullet-shaped X-1 fuselage, so this would be expected. To improve directional stability, two ventral fins were added to the aft fuselage. When flights resumed in May 1958, Walker tested the aircraft's stability and control with the fins.

Walker left the X-1E program after 21 flights, and NACA research pilot John McKay took his place. McKay made two checkout flights in the X-1E during September 1958. The X-1E now was used to test engine modifications and a new fuel. The rocket chamber pressure was increased, and nozzle extensions added to increase engine thrust. (Similar nozzle extensions were also added to the D-558-II and X-2.) McKay flew a pair of research flights in October 1958 to test these engine changes. His Nov. 6, 1958, X-1E flight was a low-altitude/low-Mach test of a new propellant called "U-Deta." This was a mixture of unsymmetrical dimthydrazine and diethylene triamine which replaced the alcohol/water mix. The engine modifications and the new propellant were expected to boost the X-1E's maximum speed to nearly Mach 3.

After McKay's November 6 flight, which was his fifth flight and the 26th by the X-1E, the aircraft was grounded for inspection and installation of a new ejection seat. During the inspection, X-rays showed a significant crack in the fuel tank wall. The cost of repairing the crack, along with the impending arrival of the X-15, meant the end of the X-1E program. A number of the research projects intended for the X-1E were transferred to F-104s, which had similar performance. The X-1E is on display in front of the Dryden headquarters building.

** Information provided by NASA Dryden Research Center **