The Corporal M2: America's First Nuclear Guided Missile
The Private solid-fuel program was completed in April after the testing of 41 Model A and F projectiles, and it achieved its goal of providing basic information about launching, stability, control, and verification of performance calculations. Victory in Europe was announced on May 8 and construction was begun at a new 40-mile-wide by 100-mile-long Army Proving Ground at White Sands, New Mexico, that June. The missiles for the final phases of ORDCIT would be tested here. The WAC “baby” Corporal, an unguided liquid-fuel, 0.4-scale version of a full-scale tactical missile, was first launched that September. The rocket was 16.2 feet long, weighed 690 pounds, and was powered by a 1,500-pound-thrust liquid-fuel motor that used a combination of red fuming nitric acid as oxidizer and an aniline-alcohol mixture for fuel. A Tiny Tim solid-fuel rocket gave it a boost to provide flight stability during launch. WAC Corporal Models A and B provided much-needed basic information about the performance and design of liquid-fuel motors, as well as answered questions about the aerodynamics, structural integrity, and balance for larger missiles.
Corporal E, a full-scale prototype for the 75-mile-range tactical missile, had its initial test on May 22, 1947. The ramjet-powered third phase of ORDCIT was scrapped after JPL decided that a liquid-fuel rocket was more satisfactory for immediate development. By this time, postwar budget cuts had reduced government funding and the missile program had slowed dramatically. It was downgraded from weapons development to a research project. This was not altogether misplaced, considering the number of technical problems that would require solution.
Douglas Aircraft produced the Corporal E airframes, which were 30 inches in diameter by 39 feet, 8 inches in length. JPL built the engines, which used the same propellants pioneered in WAC Corporal B, but stored them in separate tanks connected to a multibottle pressurization system. Fully fueled, the missile weighed 9,250 pounds. Flow to the motor was started by burst-diaphragm valves. The motor generated 20,000 pounds of thrust for a maximum duration of 60 seconds and was cooled by the flow of fuel around the engine bell. The system needed complete redesign after round 3 to improve cooling characteristics owing to burnout in the motor’s throat region. The new motor had a remarkably light weight of 125 pounds and was a resounding success. Along with the motor came a redesigned 52-jet injection system.
A rudimentary guidance system supplied by Sperry Gyroscope provided attitude control around three axes during the missile’s vertical ascent and the powered transition to the missile’s desired trajectory. The autopilot received internal inputs from two gyrosyn gyroscopes to control roll and pitch and an A-12 vertical gyro for yaw. An early pneumatic control system proved unsatisfactory, and after considerable delay it was replaced with an electropneumatic design in round 5. Electric servos in the tail were used to adjust four movable fins. Because insufficient pressure was exerted at low speed for control, carbon vanes placed inside the jet and mechanically connected to the fins were used for launch adjustments. Telemetry and radar tracking produced by Gilfillian Brothers apprised ground control of the missile’s trajectory. Following the testing of round 4, the operational guidance system was placed in development.
Corporal E also saw implementation of a new launching system that was carried over to the semimobile tactical missile. Four 10-foot-long, spring-loaded steel struts were placed equidistant around the small launch pad and provided support at a point one-fifth of the way up the missile’s body. After the missile had risen approximately four inches on its trajectory, the struts automatically retracted, allowing for an unobstructed launch. This reduced stress on the lower missile body, allowing for the installation of additional inspection hatches and improved servicing characteristics.
In September 1949, the Russians exploded their first atomic bomb and Congress removed its restriction on tactical atomic weapons. This freed Army Ordnance to field a nuclear-armed missile. Because of delays in the Hermes program, Colonel Holgar N. Toftoy chose Corporal in December 1950 for a crash program to enter the service. Although the warheads initially considered for Corporal were conventional, chemical, or biological, the final choice of nuclear was based on Corporal’s accuracy. Without guidance, Corporal could produce a circular-error probability, or CEP, of only 10 miles radius. With terminal guidance, it was hoped that accuracy would be increased to a highly theoretical 300 yards. The atomic warhead selected to arm Corporal was the W7.
The choice of the W7 warhead was based in part on a 30-inch diameter that was identical to the diameter of Corporal. Utilizing powerful new explosives and a 92-lens implosion system, the weight of the warhead was kept to 1,500 pounds. The penalty for the use of additional lenses was the need for more detonators and a more complex and powerful detonating system. A special innovation of the W7 warhead was an electromechanical screw system used to automatically insert the nuclear capsule into the explosive assembly in flight. This provided a margin of safety in the event of a launch accident. The W7 employed composite plutonium or alloy cores that made the most efficient use of the limited quantities of fissile material available at the time and provided yields in a range from 2 to 40 kilotons. Manufacturing of W7 warheads was begun in 1952, and 300 were produced for the Corporal program. Corporal was never tested with a live warhead.