Thoughts on...FUTURE TANK DESIGNS
by Richard M. Ogorkiewicz

ARMOR MAGAZINE
JULY—AUGUST 1968

The past few years have seen a remarkable increase in the number of countries producing battle tanks. Not so long ago there were only three: the United States, Britain and the Soviet Union. Now, however, there are thirteen and between them they have produced a new generation of battle tanks.

All this activity leaves little doubt about the importance which armies throughout the world continue to attach to tanks. It also sets the scene for some time to come since the tanks which are now being produced will remain in production for another two or three years and in service for many more. But what will follow them?

There will certainly be a need for another generation of battle tanks, if tanks are taken for what they basically are. namely a means of making heavy weapons more mobile or, in other words, mobile ground weapon platforms. Indeed, as such, tanks are going to be needed as long as there are ground forces. What is far less certain is the form which they will take in the future.

Some indication of the shape of things to come is given by the MBT70, the main battle tank being developed by the United States and Germany for service in the seventies. However, this vehicle only represents a particular solution and is not necessarily indicative of the form which the majority of future tanks might or should take. Therefore, it is more sound not to rely entirely on a single solution but to consider the general trends in tank design.

Tanks are carriers of weapons intended to destroy battlefield targets of which the most difficult are hostile tanks. Hence, the primary criterion of their effectiveness is their ability to kill hostile tanks. This, in turn, involves the ability of their weapons to perforate armor. Therefore, any attempt at identifying trends in tank design might well start with the growth of the armor piercing capability.

How the ability of tanks to perforate armor has grown over the years is shown in Figure I, where the penetration of tank weapons has been plotted against their year of introduction. The penetration shown is at normal impact and at 500 meters, which corresponds approximately to the average range of tank engagements during World War II. Since then the expected range of engagements has increased but the performance of APDS projectiles, which are typical of current high velocity ammunition, falls off relatively little with range while that of HEAT projectiles or missiles is independent of range. Therefore, penetration at 500 meters provides a reasonable basis for comparing the armor piercing performance of different tank weapons over the past fifty years.

When the results shown in Figure 1 are examined, it is evident that the armor piercing capability of tanks has risen steeply during the past thirty years. This is due to the combined effect of two developments. One is the fitting of tanks with progressively larger weapons which has in- creased penetration in proportion to the caliber of the weapon. The other development is improved effectiveness of armor piercing weapons in relation to their size, which has increased the tanks' ability to penetrate armor still further. In consequence, the armor piercing capability of tanks is now very considerable and there is no evidence that a limit has been reached.


Figure I—Growth of the armor-piercing capability of tank guns over the years as it is represented by penetration at normal impact at 500 meters. Vertical scale represents millimeters of penetration.

APDS versus HEAT

Recent advances in armor penetration have been achieved almost entirely with APDS and HEAT ammunition. Of the two, HEAT ammunition offers superior penetration. But, its relative performance needs to be qualified because shaped charges can penetrate armor without doing much harm. This became evident during World War II, when shaped charge weapons were first introduced, and has been brought out again in Vietnam.

Thus, shaped charge projectiles or missiles must be capable of perforating a significantly greater thickness of armor than that which they are expected to defeat if they are to cause lethal damage behind the armor. In other words, the thickness of armor which HEAT projectiles or missiles perforate with lethal effect is less than the thickness which they can just penetrate.

Moreover, the effectiveness of HEAT projectiles and missiles can be reduced by grids or similar devices installed in front of armor. Such devices, which set off shaped charges away from armor, have to be located well in front of the armor. However, when this can be done these protective arrangements can reduce penetration significantly.

Alternatively, the penetration of shaped charge warheads can be reduced considerably in relation to the weight of the additional protection by resorting to slabs of low density polymeric materials.

GUNS versus GUIDED MISSILES

Comparisons between APDS ammunition on the one hand and HEAT projectiles and missiles on the other bring out other facets of the weapon systems of which they are a pan. A leading comparative factor is complexity which is generally greater with the latter ammunition types. This is particularly true of guided missiles which are the most attractive means of delivering HEAT warheads but their reliability is significantly lower than that of the simpler and more robust gun systems.

Notwithstanding, the probability of scoring a hit with a gun falls off rapidly with range while that of missiles does not. Consequently, the overall effectiveness of a HEAT guided missile system is greater at long ranges than that of a gun firing APDS shot while the converse is true at short ranges. The crossover point is not easy to define but most recent estimates put it at between 2000 and 3000 meters.

What really matters, however, is not the exact range at which guided missiles become more effective than guns, or vice versa, but what this range is in relation to the range at which tanks most frequently sight each other. Clearly, if the range at which missiles become more effective than guns is well above the range at which tanks sight each other then guns are superior overall. This appears to be the case.

At any rate, this conclusion has been reached in Germany where a 120mm gun firing APDS is now favored for the MBT70. A similar conclusion was reached earlier in Britain, as shown by the 120mm gun of the Chieftain battle tank. But the German conclusion is more telling as it is the more recent and has been reached in face of competition from the 152mm gun/launcher firing Shillelagh missiles which is favored by the United States for the MBT70.

French views, on the other hand, coincide with the American preference for guided missiles. Having confined itself to HEAT projectiles for the 105mm gun of the current AMX30 battle tank, the French Army is now developing missiles as the armament of is future battle tanks. These missiles have been given the generic name of A.C.R.A. (Anti-Char Rapide Autoguide) which gives an indication of their principal characteristics, namely supersonic speed and automatic guidance.

Higher speeds and more sophisticated guidance systems will undoubtedly characterize other future missiles. Also, these missiles are likely to have more effective shaped charge warheads. Improvements in their manufacture can increase armor penetration at optimum stand off distance from about four to more than five times the charge cone diameter.

Improvements in the performance of guns firing APDS shot might be expected to come, in the first instance, from further increases in muzzle velocity. At present the muzzle velocity of APDS shot ranges from about 4400 to 5000 feet per second (fps) but higher velocities are quite feasible.

In fact, as many as thirty years ago German researchers working with Mauser rifles showed that muzzle velocities of up to 9150 fps were attain- able with powder propelled projectiles. A few years ago this velocity was actually attained at the Canadian Armament Research and Development Establishment with a modified 76mm gun. Admittedly, velocities in excess of 9000 fps have only been achieved with very light projectiles. But a practicable ratio of projectile to powder charge weight could be obtained at muzzle velocities of 6000 to 7000 fps.


Figure 2 – Trend in the armor protection of tanks as represented by the horizontal thickness of the frontal hull plates. The vertical scale represents horizontal thickness in millimeters.



Figure 3 – Graph relating vehicle weight increase (vertical scale in tons) to the horizontal thickness of frontal hull armor.

Unfortunately, when projectiles strike armor at very high velocities the mechanics of penetration change from those which apply to present day APDS shot and less effective use is made of the kinetic energy imparted to the projectile by the gun. Therefore, increases in muzzle velocity beyond the current maximum of about 5000fps are far less attractive as a method of achieving still greater penetration with any given size of gun than might appear at first sight.

Higher velocities might still be desirable as a means of making projectile trajectories even flatter and thereby increasing hit probability. However. an alternative approach to greater hit probabilities is generally preferred. This is to use more sophisticated fire control systems featuring laser range-finders and ballistic computers. However, more sophisticated fire control devices rob APDS firing guns of some of their advantages of simplicity and reliability. Moreover, more sophisticated fire control systems will benefit not only APDS shot but also other types of projectiles. They might do so to such an extent in fact, that medium velocity guns firing HEAT projectiles might become a more attractive alternative than they are at present

ARMOR PROTECTION

The rise in the armor piercing capability of tanks illustrated in Figure 1 is interrelated, of course, with increases in the armor protection of tanks. The increases in armor protection over the years can be characterized by plotting the horizontal thickness of the upper frontal hull plates of different tanks against their year of introduction. The upper frontal hull plates have been chosen because they are critical and, at the same time, they are more constant in thickness than the front of the turret, so that they are easier to define.

The results of such a plot are shown in Figure 2. The data provides an interesting comparison with the penetration data shown in Figure 1. Evidently, the thickness of armor rose rapidly during World War II just as the penetration capability of tank guns did. But. whereas penetration has continued to rise steeply, the thickness of armor has not increased substantially from that introduced in 1944 and 1945. As a result, the thickness of tank armor is becoming less than that which tank weapons can perforate.


FIGURE 4—Plot of nominal ground pressure in pounds per square inch (vertical scale) against vehicle weight. The broken line corresponds to the best designs.

Any attempt to increase further the thickness of armor runs up against the problem of vehicle weight. The actual relationship between thickness and weight is illustrated in Figure 3. This shows a plot, covering the designs of the past twenty-five years, of the thickness of the upper frontal hull plates versus vehicle weight. The best that can be expected is indicated by the broken line which corresponds to the equation:

T = 5 * W

where

T = horizontal thickness of upper frontal hull armor in millimeters
and
W = weight of tank in long tons.

Thus, a tank with a horizontal thickness of hull armor of 100mm might be expected to weigh at least 20 tons and one with 200mm 40 tons and so on.

WEIGHT AND GROUND PRESSURE

Having established the relationship between armor thickness and vehicle weight, the next problem to consider is the effect of vehicle weight on the pressure exerted by tanks on the ground. This ground pressure has a most important influence on tank mobility.

The essence of the problem is that ground pressure increases with vehicle weight. In other words. track size cannot be increased in direct proportion to the weight of tanks. Thus heavy tanks inevitably have a higher ground pressure than light tanks.

This fact is still not generally recognized, even by some authors of articles in ARMOR who ought to know better. For instance, the author of an article on "How Heavy the Thunderbolt" in the May-June 1966 issue of ARMOR tried to show that a heavy battle tank need not have a higher ground pressure than a light reconnaissance tank. What he ignored was the fact that his argument was based on a fallacious comparison between well-designed heavy tanks and lighter tanks poorly designed from the track point of view. The relative position would have been quite different had a well-designed light tank been considered.

The differences which exist between good and poor designs is indicated by the vertical spread of points in Figure 4, which shows a plot of ground pressure against vehicle weight covering tanks designed during the past thirty-odd years. The best designs are clearly those which give the lowest ground pressure for any given vehicle weight. Points which represent such designs rise with increasing weight and the dotted line drawn through them corresponds to the equation:

P = 5 + (W / 8)

where

P = nominal ground pressure in pounds per square inch (psi)
and
W = vehicle weight in long tons

Thus, according to the above equation, the lowest ground pressure that might be achieved with a tank weighing 48 long tons (54 short tons) is 11 psi. But a 16 ton tank can have a ground pressure of as little as 7psi.

As a result, because ground pressure increases with vehicle weight, the heavier the tank the more likely it is to run into difficulties off the road. For instance, a relatively light, well-designed battle tank with a ground pressure of 10psi, or less, will have an adequate performance in most types of terrain. But a heavy vehicle with 13psi, which is the ground pressure of some of today's heavy battle tanks, can get bogged down even in agricultural soils.

Because of the need to keep the ground pressure down, in order to maintain an adequate level of mobility, the weight of tanks must clearly be down also. In turn, weight limitations mean that tanks can no longer be provided with enough armor to make even their fronts immune to all hostile weapons.

This does not mean that tanks are no longer viable, as some ill-informed journalists seem to think. But it docs mean that the employment of tanks must be based, more than ever, on fire and movement rather than the passive attributes of armor protection.

The constraints imposed by weight also mean that tank designers must give up trying to achieve the impossible, namely trying to make tanks invulnerable. Instead they should concern themselves more with deciding just how many hostile weapons will have to be allowed to perforate lank armor. If too many, the task of an enemy will obviously be made too easy and tanks will have too little freedom of movement on the battlefield. If too few, the tanks will be heavy and their capability of being where and when they are needed will suffer.

SURVIVAL AND SILHOUETTE

Whatever the weight of tanks and the thickness of armor, their probability of survival on the battle-field can be increased significantly by reducing their silhouettes to a minimum, either permanently or temporarily.

The best example of the first approach is provided by the Swedish S tank, which has no turret.

The consequent reduction in height offers advantages which are still not fully appreciated even, once again, by the authors of articles in ARMOR. For instance, the author of the article "M60A1 Name Enough" in July-August 1965 issue of ARMOR claimed that the height of tanks was of little consequence, mainly because tanks in firing positions try to expose only their turrets. This ignored, however, not only the fact that defilade positions are not always conveniently available but, what is more, that tanks frequently have to leave cover to advance against the enemy. When they do the probability of a low, turretless tank, such as the S lank, being hit by, for example, a typical hostile tank gun at 1000 meters is 30 percent less than that of the best of the conventional turreted tanks.

An example of the second approach is provided by the MBT70, which has an adjustable hydro-pneumatic suspension with large travel. In consequence, it is able to take up firing positions behind cover with little more than its periscopes showing over it and must expose its turret only when it has to rise to fire its main armament. The ability to do this will be particularly valuable to tanks on the defensive. This will increase still further their chances of success against attacking hostile tanks.

In view of the current interest in adjustable suspensions, it might be worth adding that the idea of raising and lowering the whole tank by means of its suspension is at least 24 years old. This capability was planned for the E10 light tank which was being designed in Germany toward the end of World War II.

The probability of survival on the battlefield can also be increased by tanks being more agile, as a result of being fitted with engines giving higher power-to-weight ratios.

The German Leopard and the French AMX30, which are in advance of the other battle tanks in this respect, already have 20 bhp per long ton. The MBT70 has even more: in fact, as much as 30 bhp per long ton. This very high power-to-weight ratio has been made possible by the development of the variable compression engine which has a high output in relation to its size. But, unfortunately, this brings in complexity where it is least desirable in an engine, namely in the pistons.

One advantage which is not likely to accrue from higher power-to-weight ratios is a significantly higher sustained speed of tanks off the road. This, for all the claims being made for hydro-pneumatic suspensions, will remain severely restricted by the ride characteristics of tanks and the vibration tolerance of their crews.

Therefore, tanks are not simply going to become faster moving targets and thereby more difficult to hit. This is sometimes disingenuously suggested in attempts to show that greater agility is not going to improve their chances of survival. Quite clearly any possible increase in speed will not alter greatly the probability of tanks being hit once a hostile weapon has been aimed at them.

But higher power-to-weight ratios will improve their acceleration, which will enable them to dash more quickly from cover to cover. This will also increase their average cross-country speed. In general, therefore, tanks will be able to expose themselves for shorter periods of time during which they can be observed, aimed and fired at.

The advantages which accrue from greater agility are not. however, automatic but are dependent to a very large extent on the tactics pursued by tank units. In particular, slow-motion head-on assault tactics of the kind seen all too often in the past can nullify most of the advantages of agility which tanks possess. The future effectiveness of tanks depends on the way they are employed as much as on their design.