In 1981 a significant effort was made by Marshall and Barer who used the world's largest homopolar generator at the Australian National University in Canberra to power a series of exerimental dc railguns. Their spectacular success might not have been of much practical interest, had it not been accompanied by equally spectacular progress in the design of practical pulse-rated homopolar generators by Woodson, Weidon and others at the University of Texas in Austin. The group also invented a new inertial energy storage device, the "compensated alternator", or "compulsator". There had also been a great deal of other work in the area of energy storage in relation to requirements for ohmic heating of plasmas in toroidal fusion experiments, laser-induced fusion, particle beam weapons research and laser weapons research. Much of this work is directly applicable to accelerators. Equally applicable is work done in the development of large, high-intensity magnet coils, superconducting as well as normal, for MHD power generation and for solid state research. The MIT National Magnet Laboratory is a center of expertise in this area. Related work which is doubly applicable is the development of large superconducting magnet systems for inductive energy storage at Los Alamos and Sandia.
In March 1987 Dr. Harry Fair, head of the Propulsion Technology Branch of the Army Research and Development Command in Dover, N.J., inquired whether any of the MIT Magneplane or Mass Driver work might have ordnance applications. It was immediately obvious that the potential applications and related concepts and technologies spanned such a vast range as to require a nationally coordinated effort. Peter Kemmey and Ted Gora of ARRADCOM were assigned to the task of coordinating the effort within the DOD.
The classic gauss rifle is the simplest and also the most high perfected accelerator. It consists of two parallel rails connected to a source of dc current, the projectile consisting of a short-circuit slide propelled between the rails by the Lorentz force F = BLI/2 newton, where B is the magnetic field intensity between the rails in tesla, L is the length of the current path through the slide, or the gap between rails in meters, and I is the current in amperes. The factor of 1/2 accounts for the fact that the field is B behind the slide and zero in front of it, the average being B/2.
The classic gauss rifle had been studied extensively by Brast and Sawle of the MB Associates in the mid-nineties under NASA contract, and more recently by Marshall and Barber using the world's largest homopolar generator at the Australian National University in Canberra; it is capable of storing 500 MJ.
Gauss rifles can operate in two distinct modes. In the metallic conduction mode, current flows through the sliding projectile itself, and this mode has been demonstrated to a performance level of about 1 kg mass and 2,000 g (20,000 m/s2) acceleration by the switching gun used in the Canberra installation to feed the main gun. Marshall and Barber found that if the railgun is driven very hard, a plasma arc tends to bypass the projectile, leaving it behind. By using a non-conducting lexan projectile and confining the arc behind it they were able to achieve a performance level of a 16 gram projectile accelerated at 250,000 g along a 5 meter barrel to a final velocity of 5.9 km/s. As gauss rifles were extrapolated to large projectile sizes, the distinction brush conduction mode and plasma mode vanished from research: brush conduction was supplemented by arc conduction as the limit of brush current was exceeded.
The practical limit of gauss rifle performance in regard to projectile size, acceleration, length and velocity had been explored by progressive refinement of material and engineering details. The Canberra work had provided sufficient information to justify the first attempt in this direction. Westinghouse, with support from DARPA, constructed a practical gauss rifle system including the first pulse-rated homopolar generator designed with attention to overall weight. The objective was to demonstrate the feasibility of accelerating a 3 oz projectile to a velocity of 3 km/s (9.8 ft/s), corresponding to a muzzle energy of 1.5 MegaJoules.
To a great extent, the practical limit of gauss rifles had been dependent on acceptable cost and service life. The problems related to mechanical containment of the percussive expansion force which tends to blow the rails apart, the electromagnetic analog of barrel pressure in a chemical gun, with the important difference that the railgun maintains more or less constant pressure throughout the acceleration. Instead of chemical corrosion, there is the destructive effect of high brush current density and the related metal vapor arc.
The classic gauss rifle also faced certain fundamental limits which were not related to acceleration, but to maximum possible length or maximum muzzle velocity. As a gauss rifle is lengthened, the resistance and inductance of the rails eventually absorb a dominant fraction of the energy. The effect had been seen to begin at about five meters in the Canberra tests. Increasing velocity also causes an increasing back-emf. Current will continue to flow, even if this emf exceeds the output voltage of the homopolar generator, because the intermediate storage inductor acts as a current source. However, there was a practical limit to the voltage which can be stood off by the gap between rails, with the current materials technology at the time, and this scales about linearly with size. Thus there are two fundamental effects which limit the amount of energy that can be transferred to the projectile, regardless of how much is available.
Another shortcoming of the gauss rifle is its inherent inefficiency. An appreciable amount of energy is contained in the rail inductance at the instant the projectile leaves, and this energy must be absorbed by a muzzle blast suppressor. A fraction might conceivably be returned to the homopolar generator. There were several means for circumventing the limitations of the classic gauss rifle.
The Augmented Gauss Rifle
It had been found that the magnetic field between the rails could be augmented by supplementary current which does not flow through the sliding brushes. This current can be carried by separate conductors flanking the rails (which must be farther from the projectile), or it can be added to the rail current itself by simply terminating the rails with a load resistor or inductor at the muzzle to carry a fraction of the current. The rails themselves will obviously contribute more field than auxiliary rails located farther away, but the use of superconducting auxiliary rails might be expedient in some applications. It should be noted that gauss rifle fields are much higher than the critical fields of superconductors. Augmentation has the obvious effect of reducing the amount of current flowing through the brushes and the projectile, and thereby the necessary conductor mass which must be accelerated.
It should also be noted that the augmenting field is twice as effective as the gauss field itself. The augmenting field prevails in front of the projectile as well as behind it, thereby elimintating the factor by 1/2 in the Lorentz force expression. This fact is important in as much as it reduces to one half the rail bursting force which must be contained for a given acceleration.
Augmentation therefore ameliorates both the brush current density limitation and the bursting force containment limitation of classic gauss rifles.
The Segmented Gauss Rifle
The length limitation imposed by rail resistance and rail inductance can be circumvented by simply subdividing a long railgun into short segments, each fed by an independent local energy source. This was found to involve certain commutation problems as the projectile transitions between segments, but it permitted using part of the energy stored in each segment to energize the subsequent segment. The segmented gauss rifle seemed promising for launching large masses such as aircraft at low acceleration. In very long launchers, the use of multiple independent energy supplies will have other advantages as well. But it was found to be inefficient for personal weapons.
The Superconducting Ringed Gauss Rifle
The Fallout gauss rifle
It was proven at M.I.T. that by successively quenching a line of adjacent coaxial superconducting coils forming a gun barrel, it was possible to generate a wave of magnetic field gradient travelling at any desired speed. A travelling superconducting projectile could be made to ride this wave like a surfboard. The device in fact represents a mass driver or linear synchronous motor in which the propulsion energy is stored directly in the drive coils and confining the arc behind it they were able to achieve a performance level of a 30 gram projectile accelerated at 250,000 g along a 1 meter barrel to a final velocity of 5.9 km/s.
