Sound Barrier: The Plane

The airplane that eventually would break the sound barrier was the Bell Model 44, originally known as the Bell XS-1, but later referred to as just the X-1. A gound breaking design, careful consideration was given to all aspects of the aircraft.

Autographed photo of the X-1 to Bob Cardenas from Chuck Yeager in the SDASM Collection.

The Shape:

The Bell design team consisted of Robert Stanley, Benson Hamlin, Paul Emmons, Stanley Smith and Roy Sandstrom.  The first task for the team was to assemble adequate data from which to draw up a suitable airframe.  Hamlin the design engineer and Emmons, project aerodynamicist, left Buffalo and toured the research institutions in the United States that might have developed suitable data that would be able to give the Bell team advice.  At Aberdeen Proving Grounds Army scientists suggested that the two men visit the Ballistics Laboratory at Wright Field, so Hamlin and Emmons left for Ohio.  The two Bell engineers had earlier concluded that the only objects they knew that moved at supersonic velocities all the time were bullets.  They wondered why a .50 caliber bullet had the shape it did.  

The .50 Caliber bullet shape can clearly be seen in this photo which shows SDASM's Bell X-1 reproduction under construction.

At Wright Field they asked ballistics experts the question, and what a .50 caliber bullet’s transonic drag values were.  The ballisticians did not know the drag, but did explain the shape.  What mattered was the dispersion pattern.  In tests where researchers fired bursts of bullets from a machine-gun at a target, bullets having an ogival shape produced the best pattern.

The Engine:

The second question decided for Bell was that centring around the Aerojet 6,000lb-thrust ‘Rotojet’.  The Rotojet would utilise red fuming nitric acid and aniline as propellants. These two hypergolic, meaning that whenever they mixed, they reacted violently, thus obviating the necessity of providing an ignition system for the engine.  Its development was not moving smoothly due to technical complications and, like Bell, the AAF recognised the hazardous characteristics of using hypergolic fuel combination on a manned aircraft.  The Aerojet 6,000lb-thrust engine dropped from consideration.  

Testing of the XLR11 engine.

Fortunately, a substitute for the Aerojet engine was already at hand: another 6,000lb-thrust RLX11 engine developed for the Navy by Reaction Motors, Inc. of Pompton, Plains, New Jersey.  The RMI engine consisted of four separate rocket cylinders, each of which produced 1,500lb-thrust.  The company designation of the engine was 6000C4: 6,000lb-thrust, four cylinders.  The propellants were liquid oxygen and alcohol diluted with water. 

The Wing:

It is interesting to note that the XS-1 contract stipulated a straight-wing configuration, for, by this time, the AAF already knew of the potential advantages of wing sweep as a means of alleviating compressibility shoc.  

The X-1s wings were small but sturdy. 

The reason why the AAF did not employ sweep on the XS-1, or at least let the contractor choose between a swept and nonswept configuration, was that the Air Technical Service Command still considered the straight wing configuration as the most viable configuration in the future and desired the XS-1 to provide data applicable to conventional aircraft design, as typified by the Lockheed P-80 straight wing fighter. 

The Tail:

One of their most important decisions concerned design of the horizontal tail surfaces.  John Stack and other Langley engineers concluded that the XS-1’s horizontal stabilizer should have a lower thickness: chord ratio than the wing, for if the wing encountered severe compressibility effects, the horizontal tail would not experience the same problems simultaneously,  By using a thinner airfoil section on the tail surfaces, the critical Mach-number of the tail-the velocity where the drag occurred-would be higher than the critical Mach number of the wing (the pilot could still maintain some control).  

Detail view of the X-1's all moving tail.

To ensure adequate longitudinal control during transonic flight, Stack, Gilruth and others further suggested that the horizontal tail be all-moving.  In other words, rather than have a fixed horizontal stabilizer surface with a movable elevator, NACA recommended that the stabilizer itself be adjustable by the pilot while in flight.  For ordinary subsonic flight, therefore, the pilot could control the aircraft through the elevator.  If the need arose, he could move the stabilizer as well as the elevator, in effect making the horizontal tail all-moving. The stabilizer was controlled with an electric motor by the pilot.

This image of SDASM's X-1 reproduction under construction shows the 4 holes for the XLR11 engines 4 exhaust tubes.

Air-Launching or Ground Takeoff:

One question that raised some disagreement among Bell engineers was whether to design the XS-1 for air launching.  From the start, Benson Hamlin wanted to design the airplane with skids, so that a launch airplane could carry it aloft, launch it and the pilot could fly the mission and then come in for a landing on the skids, Hamlin believed that air launching was the best way of getting high performance, since the rocket airplane would not need to consume rocket propellants except during the brief climb from launch height to the 35,000 foot test altitude.

The X-1s landing gear is evident in this view.

(Robert) Stanley and others agreed that the skid landing gear seemed the best technical approach, but the formidable Robert J. Woods was strongly opposed.  Woods was interested in the X-1 as a possible first step towards development of a manned rocket-propelled interceptor, and wanted to include a retractable landing gear so that the airplane could take off from the ground.  Woods did not favour the launch aircraft idea, stating that it would not produce an operational-type airplane.  The controversy remained unresolved until arbitrated by the company president, Lawrence D. Bell who decided both in favor of a retractable landing gear and Stanley’s arguments in favor of air launching the airplane. 

Next page in this exhibit. 

San Diego Air & Space Museum

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