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Railgun Pulsed Power Program

Army Electric Gun Program

Pulsed AC Rotating Machines

Research into pulsed alternators at UT-CEM can be traced back to the late 1970’s with the invention of the compulsator (compensated pulsed alternator), a new type of low impedance electrical machine designed for very high power operation. Although it was conceived at UT-CEM to power laser flash lamps for fusion research at Lawrence Livermore National Laboratories (LLNL), the compulsator concept has been successfully applied to electric launchers for the U.S. Army and NASA, and high frequency microwave sources. Iron-core prototypes built for LLNL in the early 1980’s proved the operating principles of the concept. This involves the use of a compensating shield or winding in the alternator to lower its internal impedance to a very low value, while simultaneously tailoring the very large (multi-mega amp) current pulse to suit the required loads.

Iron-Core Compulsator

In the mid-1980’s, the first compulsator built for the U.S. Army electric gun program was successfully demonstrated. Named the "Iron-Core Compulsator" (ICC), the device is a 6-pole, rotating field machine which stores 40 MJ of kinetic energy at 4,800 rpm, and can repetitively deliver ten 1 MJ, 2 ms pulses. The ICC generates an open circuit voltage of 2 kV and operates at a peak current of 1 MA, resulting in a peak power rating of 2 GW. After fulfilling its stated contract goals by firing a burst of railgun shots at 10 Hz, the ICC has become a reliable CEM laboratory power supply for testing other components including high power switching for newer compulsators, and advanced railgun barrels.

Small Caliber

After seeing the proof of principle demonstration by the ICC, the U.S. Army then pursued the development of more compact, lighter weight pulsed power generators beginning in 1988. UT-CEM proposed and was contracted to build two single phase, self-excited, air-core compulsators. The air-core nature of the new machine concepts allowed required use of very high strength composites for construction of the rotor, which enabled it to be spun much faster - both storing more energy and making it much lighter. The first of these machines was built during the Small Caliber Rapid-Fire Railgun Program. The compulsator for this system is a 2-pole, rotating armature topology. While rated at half the power of the ICC, the small caliber compulsator weighed only 8% as much - so the power density was improved by a factor of six. The new machine demonstrated the viability of the SEAC concept and developed several new components; including the high speed composite rotor, the SCR based output switch and field rectifier.

9 MJ Range Gun

On a larger scale, the U.S. Army desired the capability to fire a large bore railgun at a test range and demonstrate the pulsed power supply at full scale. UT-CEM therefore began working on the 9 MJ range gun in 1988. Designed to launch 2 to 4 kg packages at velocities up to 4 km/s, the range gun system uses a much larger version of the small caliber compulsator as the pulsed power supply. Design features of this machine include 230 MJ stored energy, 6 kV and 3 MA peak output ratings, and 9 shot repetitive fire capability. Unfortunately, funding constraints combined with revised electric gun program focus has meant that the machine was shelved at 95% complete level.

Cannon Caliber Electromagnetic Launcher

The early 1990’s brought continued interest from the U.S. Army, but also from the Marine Corps. The Cannon Caliber Electromagnetic Gun System (CCEMG) is a joint project which seeks to demonstrate an EM gun system designed from a system/mission prospective. Presently undergoing lab testing, the CCEMG uses a second generation single-phase air-core compulsator which represents a factor of 3 increase in energy (3.5 J/g multi-shot) and power density (to 1,500 kW/kg) over the range gun system. The machine powers a rectangular bore railgun to accelerate 185 g launch packages to muzzle velocities far exceeding the conventional state of the art for this caliber. Rated at 4 kV and 850 kA, the compulsator stores 40 MJ and can deliver 15 shots without recharging the rotor.

Focused Technology Program

Presently, the U.S. Army is funding the Focused Technology Program to develop the next generation of compact compulsator power supplies, which will be consistent with their concept of the Future Main Battle Tank (FMBT) to be fielded by 2015. The FTP machine concept represents a fundamental departure from the previous air-core compulsators by evolving to a rotating field topology and a multi-phase operating modes. These changes, combined with light weight composite stator structures will result in great increases in specific performance over the CCEMG machine, and will ultimately allow integration of the system into a tank vehicle chassis. The compulsator will be a part of the all-electric tank which includes electric vehicle drive and suspension, and electric armaments. In this concept, the flywheel energy can be used as a flywheel battery to provide power for vehicle acceleration and regenerative braking, and will also produce electrical power for the electromagnetic armaments.

Sponsors
Iron Core Compulsator was sponsored by U.S. Army ARDEC. Small Caliber EM Launch System was sponsored by the Joint Services Small Arms Program at ARDEC. The 9 MJ EM Range Gun System was sponsored by U.S. Army ARDEC. The Cannon Caliber EM Gun program was sponsored by U.S. Army ARDEC and the U.S. Marine Corps. FTP is being sponsored by the U.S. Army Research Laboratories.