The genesis of the AAF’s research airplane took place in Ezra Kotcher’s office at Wright Field, on November 30, 1944, when Robert J. Woods, chief engineer and co-founder of the Bell Aircraft Corporation, dropped by for a chat. Kotcher, of course, had long been searching for a contractor to take on his project and thus what began as a simple dialog on the problems of high-speed flight quickly evolved into a very earnest discussion concerning Bell’s interest in building the airplane. The basic requirements, at least as set forth by Kotcher, were certainly straightforward: employing a rocket engine with at least a two-minute high-speed endurance capability, the aircraft would have to be able to attain 800 mph at 35,000 feet; while it would not be encumbered with all of the usual military specifications, it would have to be overstrength for the purposes of safety; and Bell would only have to guarantee its safety and controllability up to a speed of Mach 0.8. Woods was not inclined toward the use of rocket propulsion and, at least at this point, it was not an absolute requirement. Almost without blinking an eye, he committed his company to the project right then and there.

The requirements may have been straight-forward but, as Robert M. Stanley assembled a small design team to undertake preliminary studies, they quickly discovered what it felt like to be set adrift on an uncharted sea. Two of them, design engineer Benson Hamlin and aerodynamicist Paul Emmons, traveled to various research facilities around the country in search of useful data and expert advice. They found little of either. Hamlin later recalled commenting to Emmons on the train ride back to the Bell facility in Buffalo, New York, that they were basically free to design the aircraft "any way we please, and no one can criticize us."

The design team’s freedom, however, was not absolute. In order to insure that the airplane would not break apart during its turbulent transition from subsonic to supersonic flight, both the AAF and the NACA insisted that it be designed for an ultimate load factor of 18 g's. That represented a figure about 50 percent higher than for any existing fighter aircraft. Ben Hamlin later recalled that this load factor was nothing more than an "ignorance factor." "I protested in vain," he noted, "and lost another battle. It proved to be loads and loads of ignorance, that factor did." To complicate matters further, after much debate within the Langley ranks, the NACA determined that the rugged airframe should be configured with what, for the day, were extremely thin wings. Based on evidence that thin wings, with higher critical Mach numbers, retained more effective lift in the transonic region, the NACA ultimately decreed that Bell should produce two sets of wings which would be interchangeable on each of the test aircraft: one set with an eight-percent and the other with a ten-percent thickness-chord ratio. Thus the Bell team had to come up with a design for extremely thin wings which would support 18 times the weight of the fuselage and its contents without breaking--a very formidable task.

Although Kotcher had, most emphatically, expressed a preference for rocket propulsion, Bell was free to employ any type of power plant--or combination of power plants--which would provide the desired performance. During the course of the design team’s investigation, a pure turbojet system was quickly ruled out because the highest speed attainable with even the most powerful engines then under near-term development would have only been about Mach 0.9 at sea level and the thrust would have fallen off very substantially from that at altitude. Next, they turned to the jet-rocket combination which would employ a turbojet for takeoff, climb to altitude and return to base, and a four-chamber, 6,000-pound thrust Aerojet liquid-fuel rocket engine for high-speed runs at altitude. This resulted in an excessively large airplane because, as Bob Stanley and Bell design team member Roy J. Sandstrom later reported:

This left them with only one potentially viable alternative. They determined that, although the fuel consumption of a pure rocket system would be high, the airplane’s rate-of-climb would also be high --averaging better than 20,000 feet per minute between sea level and 35,000 feet with a climbing speed of 500 mph. Thus, they concluded, the volume of fuel required for the climb phase was relatively low and the amount required to accelerate from climbing speed to the targeted test speed would also be less than with the combined turbojet and rocket system. Thus, by a process of elimination, they finally arrived at the all-rocket propulsion configuration which Kotcher had advocated all along.

Unfortunately, the system which had long been under development for the AAF, the 6,000-pound thrust Aerojet "Rotojet" rocket engine, employed red fuming nitric acid and aniline as propellants. The two compounds were hypergolic, meaning that violent spontaneous combustion occurred whenever they came into contact with each other, and this raised very serious safety concerns among those who were attempting to design a man-rated airplane. Because of this and the fact that development of the Aerojet engine had fallen behind schedule, Kotcher finally settled on a 6,000-pound thrust rocket engine which had originally been under development for the Navy by Reaction Motors, Inc. Fifty-four inches in length and weighing just 208 pounds empty, the XLR-11 was a four-cylinder engine (1,500 pounds of thrust per cylinder) which employed liquid oxygen (the oxidizer) and water-diluted ethyl alcohol (the fuel) as propellants. These propellants were not spontaneously combustible and thus, in comparison with a wide range of other exotic combinations considered, they were relatively safe and easy to work with. The engine was regeneratively cooled by circulating the super-cold propellants through cooling jackets around the combustion cylinders before they were injected into them. According to the original design requirements, a turbine-driven pump system was to be employed to force the propellants through the cooling jackets and into the cylinders. Though the engine was not throttleable, the pilot would have the option to ignite or shut down each cylinder individually so that he could operate at 25-, 50-, 75-, or 100-percent power.

Unfortunately, development of the turbine-driven propellant pump was plagued by so many problems that, at Bob Stanley’s insistence and over John Stack’s strong objections, it was sidelined in favor of a system in which high-pressure nitrogen gas would be employed to force-feed the liquid oxygen and alcohol into the engine. This meant that the propellant tanks would have to be heavy, high-strength steel containers in order to withstand an internal pressure of 350 psi which would be required to force the liquid oxygen and ethyl alcohol into the rocket cylinders against the high pressures produced therein by the combustion of the two liquids. Not only would the tanks be heavier, but the almost spherical shapes which were now required would be far less efficient in terms of volume than the low-pressure cylindrical aluminum tanks which would have been used with a turbine-pump system. Propellant storage capacity was further reduced by the fact that the nitrogen used to pressurize the propellant system would also have to be stored in a group of heavy high-pressure tanks. A total of 12 of these 4,500 psi nitrogen containers, along with three heavy-duty--and a pair of smaller--pressure regulators which would be used to reduce the original nitrogen source pressure to usable levels, would be required to pressurize the propellants, and operate the landing gear and flight controls, as well as to pressurize the cockpit. The impact of all of this on the aircraft's design was critical: the vehicle's landing weight was increased by one ton, while fuel capacity was reduced from 8,160 pounds to an estimated 4,680 pounds and, instead of 4.2 minutes, engine burn time was limited to just 2.5 minutes.

While this created a number of headaches for the design team, it at least served to resolve a very contentious debate over whether the aircraft should be designed for air launch or ground takeoff. The NACA and a number of Bell personnel had argued vehemently in favor of ground takeoffs. Ostensibly, the NACA favored this approach because it would provide useful data on the widest possible range of conventional flight operations. Equally important, however, was the fact that a decision to conduct air-launch operations with a rocket-powered vehicle would virtually guarantee that the airplane could never be tested at a busy flying field such as Langley which was located adjacent to highly populated areas. The prospect of offsite operations at some remote location suggested to NACA managers that they might have difficulty exerting direct control over the test program. Those Bell management personnel, such as Bob Woods, who also argued for ground takeoffs did so because they were looking toward the future development of the craft into a rocket-powered interceptor. Key members of the design team, most notably Bob Stanley and Ben Hamlin, countered the ground-takeoff arguments by citing safety considerations and the need to conserve rocket propellants for actual high-speed work at altitude as compelling reasons for air launching the craft. Much to the chagrin of Woods and the NACA, the weight increase and reduction in fuel capacity caused by the high-pressure fuel feed system settled the issue; the research airplane would be air launched at relatively high altitude from the bomb bay of a specially modified B-29.

During their generally disappointing tour of research institutions in the United States, Hamlin and Emmons had made a stop at the AAF Ballistics Laboratory at Wright Field. They knew that bullets traveled at supersonic speeds and wondered, specifically, how and why the shape for .50 cal. bullets (which were known to travel at speeds as high as 2,491 mph) had been determined. They discovered that the ogival shape of the bullet's nose had been selected because, in testing, it had produced the smallest dispersion pattern. Here at least was a configuration which had proven to be stable at supersonic speeds. Feeling that they were on to something, and with a paucity of other useful precedents, they decided to pattern the cylindrical shape of the fuselage after the bullet.

While not unaware of the potential advantages which might be offered by employing swept wings to delay the onset of compressibility, Kotcher had opted, early on, to go with a more conventional straight-wing planform and the Bell design team ultimately elected to go with NACA 65-108 (eight-percent thickness-chord ratio) and 65-110 (ten-percent) airfoil sections. The benefits of swept wings, which had been postulated by NACA aerodynamicist Robert T. Jones in early 1945 and which were soon to be confirmed as German research archives were examined, were still nothing more than theoretical calculations at the time of Kotcher’s decision. No one in this country had any experimental data on the characteristics of such airfoils and thus both Kotcher and the NACA concluded that applying them to the design of what was already an extremely unconventional airplane would introduce unnecessary additional risk into the program. Moreover, Bell was already faced with the daunting challenge of designing and fabricating the eight- and ten-percent wings. To overcome the problems inherent in building extremely thin wings capable of sustaining 18g loads, the Bell team eventually decided to employ exceptionally thick wing skins which tapered from one-half inch at the wing roots to a conventional thickness at the tips. The thick, milled aluminum skins were not only designed to add structural integrity and rigidity to the wings, they would also presumably maintain their smooth contours as turbulent flow developed in the transonic flight regime. In response to a NACA instrumentation requirement, Bell cut 240 pressure orifices in the skin of each of the left wings and installed 12 strain gauges within each so that pressure distribution and air loads data could be acquired. Finally, based on Stack’s and Gilruth’s original suggestions, two different sets of adjustable horizontal stabilizers were fabricated--one sized at a six-percent thickness-chord ratio to be flown with the eight-percent wings and the other, an eight-percent section, which would be flown with the ten-percent wings. If required to overcome rapid trim changes in the transonic flight regime, these adjustable stabilizers could be moved through a 15-degree arc at a rate of one degree per second. Should this rate prove to be inadequate, Bell made provision for it to be increased up to as high as three degrees per second.

In a fashion not unlike that demonstrated by the American aircraft industry throughout the recent war, the Bell team--supported by technical data and advice from the NACA--worked fast and effectively as it solved all of the foregoing as well as a host of other perplexing problems. The official contract for final design and construction of three XS-1 (for Experimental Sonic-1; the designation was later simplified to X-1) airplanes, at a total cost of $4,278,537, had been issued on March 16, 1945. Less than ten months later, on December 27 of that year, the first aircraft (serial number 46-062) was rolled out of the Bell plant in Niagara Falls, New York. As it rested on the ramp that day, the bullet-shaped, saffron-colored airplane’s simple, extremely clean lines bespoke its sole mission: speed.  According to Bell specifications, it had been designed for an empty weight, including instrumentation, of 6,511 pounds and a maximum gross weight--with 5,120 pounds of propellants and 301 pounds of pressurized nitrogen--of 12,050 pounds. Minus its nose-mounted pitot boom, the fuselage was 30 feet 11 inches long and the wingspan was 28 feet, pro-viding a net wing area of 102.5 square feet. Bell predicted that, due to current fuel storage limitations, the airplane could achieve a top speed of 916 mph at an altitude of 50,000 feet. The recently redesignated Project MX-653 was poised to enter a new phase.

It should be noted, however, that the Bell XS-1 had already achieved one of the fundamental purposes of the research program. Its design had stimulated the development of new wind-tunnel techniques at Langley which enabled researchers to begin to circumvent the choking problem. Thus, even before the XS-1 commenced its transonic flight research program, the NACA was able to acquire reliable transonic flow data up to about Mach 0.9 and make reasonably confident wing-flow predictions up to a Mach number of 0.93. Beyond that Mach number, of course, the flight re-searchers who were about to engage in the test program would be on their own.

Next Chapter