Novel Tank Guns? The Electromagnetic Rail Gun and the Liquid Propellant Gun Are Still a Generation Away

Much progress has taken place recently in the development of tank guns. An outstanding example of this is the high pressure 120-mm smoothbore guns which were first successfullv develoDed in Germanv they would have to fire APFSDS projectiles with a considerably higher muzzle velocity than the 1,650 m/s (5,400 ft/s) which is typical of current 120-mm smoothbore tank mns. I by Rheinmetall an> are now also being produced in the United States under the XM 256 designation. The Rh 120 already arms the I 5 Leopard 2 tanks adopted by West 2 g Germany, the Netherlands, and Switzerland and is to arm the new 3 Japanese TK-X tank. The 120-mm z XM 256 is now being fitted in the 2 F latest, current production version of the US M1 tank, the MlA1. Other n 7 similar 120-mm smoothbore guns, g g capable of firing the same am- n n munition, are being adopted in France and several other countries. 0 In fact, guns of this kind are set to $ redace the 105-mm M68 or L7 rifled LT c P 2.0 1.o le5 0.5 i / Figure 1 Muzzle velocities higher than this could certainly be achieved with solid propellant guns. More than twenty years ago, the Canadian Armament Research and Development Establishment (now the Defense Research Establishment Valcartier) had fired projectiles at up to 2,790 m/s (9,150 ft/s) from a modified 76-mm gun. Such high velocities may still be confined to experimental guns, but velocities of the order of 1,900 to 2,000 m/s (6,200 to 6,560 ft/s) are attainable with future tank guns. Unfortunately, further increases in muzzle velocity would require disproportionately large increases in the weight of the propellant. As it is, at 1,650 m/s, the weight of the propellant is already as high as that of the projectile, and if the velocity were increased to 1,900 or 2,000 m/s, the weight of the propellant would have to be doubled. In consequence, the size and weight of the ammunition would increase considerably. In addition, the higher muzzle velocities would result in a further increase in the rate of bore wear, which is already a major problem with tank guns. The alternative to higher muzzle velocities is an increase in the caliber of tank guns, and guns of 140- or 145-mm have been discussed. The achievement of the required MUZZLE VELOCITY KM/SEC bility of tank guns by an increasein caliber instead of muzzle velocity involves less technical risk and avoids further deterioration in the life of gun tubes. But larger caliber - &ns as the standard tank guns of The new generation of 120-mm tank guns represents a major advance on the currently used 105- mm guns and should be able to cope with the armor of enemy tanks for a the non-Communist world. Oe5 lVo lS5 *." 2*5 3.0 increaseinthearmor-piercingcapa-Graph illustrates the tremendous increase in propellant weight needed to increase veloci*- good many years. However, tank armor is improving. Consequently, future enemy tanks will be more difficult to defeat. There will come a time, therefore, when even more powerful guns will be needed and it is not too early to start thinking about them. Solid Propellant Guns The guns with greater armor-piercing capabilities which will be needed in the future could well be a further evolutionary development of conventional solid propellant guns. They could even be of the same 120-mm caliber as the guns

/ r GUN BARREL currently being Produced- But if FIGURE 2. Illustration shows major components of the regenerative liquid propellant they were again of that caliber, gun. 16 ARMOR The Magazine of Mobile Warfare guns are inevitably more difficult to mount in tanks. They are, therefore, looked upon with greater favor by the designers of guns than by the designers of tanks. However, if they were to prove the best means of providing greater armor-piercing capability, tanks would have to be and can be designed to carry them. Either approach obviously presents difficulties, and these have led to increasing interest in possible alternatives to conventional, solid propellant, guns. The principal candidates for succession are two novel types of guns, namely liquid propellant guns and electromagnetic guns. Liquid Propellant Guns Liquid propellant guns are almost forty years old, but their development has been very intermittent. It started in the late 19408, following the lead taken in Germany during WWII in the application of liquid propellants to rockets. Its first outcome was a number of experimental liquid propellant guns which were built and tested in the US during the 1950s. At about the same time, work on liquid propellant guns also began in Britain and led to the design of an 83.8-mm liquid propellant tank gun - which is preserved at the Royal Military College of Science as a teaching exhibit. However, by 1960 the development of liquid propellant guns was virtually abandoned, largely because of the difficulties encountered. A major difficulty was the toxic and highly corrosive nature of the bipropellants which were used at the time. For example, one of the two components of a bipropellant that was used was red fuming nitric acid! As if'the hazardous nature of their propellants were not enough, liquid propellant guns also failed to produce consistent results. Moreover, they were complex and appeared to offer no significant performance advantages over solid propellant guns. As a result, there was little further interest in liquid propellant guns until the early 1970s. What was mainly responsible for the revival of interest which then took place was the development of new. General Electric photo shows the firm's test fixture used for its research on regenerative liquid propellant guns. single-compound monopropellants that were much safer and far easier to handle than the earlier liquid propellants. The new monopropellants were pioneered by the US Navy, but during the mid-1970s an attempt was made to exploit their advantages in what was originally the High Mobility-Agility Program and subsequently the Armored Combat Vehicle Technology Program. In fact, it was planned that the 75-mm solid propellant ARES gun, which was being developed for the HIMAG test bed vehicle, would be succeeded by a high velocity 75- mm liquid propellant gun. However, the development of the latter got no further than test fixtures and was terminated after two consecutive explosions in 1976. Apart from being rushed, the development of the 75-mm liquid propellant gun ran up against the fundamental difficulties associated with the combustion of liquid propellant which is bulk-loaded, as it was in this case. Bulk-loading means that all the propellant is pumped into the chamber of the gun before it is ignited and was favored until then because it involved less mechanical complexity than the alternative method of regenerative injection of the propellant. The basic feature of regenerative injection is that it does not involve pumping the propellant into the chamber of the gun, but into a reservoir separated from the chamber by a piston with injection orifices. Combustion in this case is initiated by an ignition train which pres-surizes the chamber and, consequently, forces the piston back, causing some of the propellant to be injected through it from the reservoir into the combustion chamber, where it ignites. Then. as that pro- ARMOR The Magazine of Mobile Warfare 17 pellant bums, the pressure in the chamber rises, forcing the piston to inject more propellant into it. Regenerative injection was tried in the 1950s and was used, among others, in the British 83.8-mm liquid propellant tank gun. But it was then abandoned because of the mechanical complexity it involved. However, in 1974 it was taken up again, this time by General Electric. Since then, General Electric has used its own resources to conduct a commendably systematic program of development of the regenerative injection monopropel-lant gun concept. The program involved hundreds of firings from 8-, 25, 30- and finally 105-mm gun fixtures and the successful results it produced started another revival of interest in liquid propellant guns. A very important consequence of this revival has been the award by the US Army of a contract to General Electric to demonstrate the viability of an artillery 155-mm liquid propellant gun, which is now being developed. Work on liquid propellant guns is now also going on in Germany and France and, once again, in Britain. It will be some time before we shall see what emerges. But there is a good chance that, after the two earlier false starts, the development of liquid propellant guns will lead to success this time. Success is likely not merely because it might be a case of third time lucky but because the most promising of the current development programs are bypassing the problems which defeated the earlier attempts to develop liquid propellant guns. This is particularly true of the regenerative injection monopropellant gun which is being developed by General Electric and which does not have to contend either with the hazardous propellants of the earliest liquid propellant guns or with the combustion problems of the bulk-loaded 75-mm gun of the mid-1970s. Moreover, the internal ballistics of liquid propellant guns are now better understood than they were thirty, or even ten years ago. These differences between the past and present development programs need to be recognized as they are important to any assessment of the current efforts to develop liquid propellant guns. For this reason, they are described here in some detail.

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only of accelerating small pellets but also real-size projectiles. Most of the progress to date has been achieved with one particular type of electromagnetic launcher, namely the rail gun. This consists basically of two parallel conducting rails which are connected to a power source and, at the same time, constitute the sides of the accelerator, or launcher, bore. When the launcher is used, current flows down one rail and in the opposite direction along the other, passing from one to the other through an armature that slides between them. This generates magnetic fields around the rails which, together with the current flowing through the armature, produce a force that accelerates the armature and, with it, any projectile. The principle of operation of the rail gun is evidently simple, but in practice it requires very high currents and a considerable amount of electrical power. For instance, a 1 kilogram projectile launched at 3,000 m/s, which would have the kinetic energy approximately equal to that of an APFSDS projectile fired from an existing 90-mm tank gun, might require the generation of as much as 2 gigawatts (2 billion watts) during the launch period. If this amount of power had to be generated continuously, it would require a large power station! Fortunately, guns are not fired continuously and the power which needs to be generated can be reduced to more manageable levels by building up energy during the time when projectiles are not being launched and then drawing on it during the short projectile launch periods. In practice, this means that the power produced by a prime mover is first stored in the form of the rotational kinetic energy of a flywheel, which is then drawn on periodically and converted into short pulses of electrical energy. The conversion of mechanical into electrical energy takes place in homopolar generators, whose rotors also act as flywheels. The homopolar generators can, therefore, store the energy supplied by prime movers and, when connected by switchgear, produce the short pulses of high current electrical energy required by the rail launchers. Some idea of the power that might actually be required can be obtained by considering the launching of the 1 kilogram projectile mentioned earlier at the modest rate of six rounds per minute. This would require a prime mover with a constant output of about 3,800 horsepower, which is more than twice the maximum power of the most powerful tank engines in use today. Moreover, even if the power demand were brought down to the level of tank engines - either by a lower rate of fire or by higher component efficiencies - their use to power electromagnetic guns would hardly be acceptable as it would deprive tanks, temporarily at least, of their mobility. The power required by an electromagnetic gun calls, therefore, for a separate, dedicated, prime mover. The only suitable candidate for it is a helicopter-type gas turbine, the weight of which could be only 320 kilograms (706 lbs). However other components of the electromagnetic gun system would be considerably heavier. This applies not only to the homopolar generator, but also to the inductor, which is necessary to shape the energy pulse delivered to the rail launcher, and the launcher itself. In consequence, an electromagnetic gun would be two to three times as heavy as a comparable solid propellant gun. Nevertheless, electromagnetic guns offer a number of important advantages. The most important of them is their ability to launch projectiles with much higher velocities than other types of guns. This implies greater armor penetration for a given size projectile, or the achievement of a given penetration with smaller caliber projectiles than with other types of guns. However, there is a limit to the extent to which caliber can be traded sensibly for velocity. The reason for it is that increases in velocity beyond a certain level provide little further increase in penetration. The high projectile velocities offered by electromagnetic guns also imply flatter trajectories and shorter projectile flight times, which should lead to a greater probability of hitting maneuvering targets and might allow considerable simplification of fire control systems. The smaller caliber of electromagnetic guns would also mean that more projectiles could be carried in a tank and the smaller projectiles would be easier to handle by automatic loaders. In addition, there would be no spent cartridge case to extract and automatic loading would be greatly simplified by the open-breech nature of electromagnetic launchers. What is more, instead of propellants, electromagnetic guns would use nothing more vulnerable than kerosene or diesel fuel. But electromagnetic guns also suffer from several serious disadvantages. Apart from their weight and bulk, the most serious of them is the rapid erosion of the launch rails caused by the high current arcing between them and the armature. Major problems are also posed by the switchgear, which has to carry very large current surges. Some of the disadvantages of electromagnetic guns may be eliminated, or at least reduced, by further development, which might make them more suitable for tanks than they appear to be at present. After all, electromagnetic launchers are only a few years old and there is plenty of scope for further development. But this is bound to take time and it would be unreasonable to expect electromagnetic tank guns to become practicable in the near future, even under the most favorable circumstances. In the meantime, it is necessary to continue the development of solid propellant tank guns and to roduce at least one more genera- - ion of them.

RICHARD M.

is a consulting engineer who has served as armor advisor to many nations. He participated in the US Army's Armored Combat Vehicle Program and has performed studies for the Defense Advanced Research Projects Agency. The author of many books on armored vehicle design, he has also written over 75 articles for ARMOR Magazine. ARMOR The Magazine of Mobile Warfare 19