Barrel Distortion and First=Round Hits

Modern battle tanks are expected to have the capability of knocking out an enemy tank within a few seconds of its acquisition. While there are several means to combat adversary tanks, be it advanced technology “smart” munitioned artillery, air-to-ground attack, or third-generation antitank guided missiles fired from ground-mobile or air-launched platforms, it is an established fact that the tanks’ most deadly enemy was, and still is today, a well-trained, skilled tank gunner, using his main armament, fire control system and a wise choice of ammunition, working in good coordination with the tank commander and loader. However, to achieve first-hit probability within acceptable parameters and at a high rate of firing sequence, several aspects - some of which are well know, others partially or completely ignored - have to be taken into consideration, or adequately solved. High-hit probability is of special importance when a negative numerical ratio is encountered in combat. This will certainly be the case in a Warsaw Pact-NATO confrontation, and in circumstances prevailing in the Middle East. To redress this inequality in numbers, high technology solutions, such as sophisticated weapon systems, weapon mix integration and realistic training schedules have been developed. These are aimed to open the engagement at maximum range, using a combination of high-hit probability with lethal terminal effect to reduce numerical superiority of an advancing enemy until he closes in for the kill at shorter range, where flat trajectory firing will cause high attrition. In order to understand the problems of long range gunnery techniques, a glimpse into ballistic theories is required, as these have a crucial bearing on the behavior of the round from its firing to its arrival on target. Ballistic Considerations Ballistics theory is one of the most difficult fields of technology, involving critical measurements, but in order to simplify the matter, we shall discuss in short three aspects having immediate influence on our problem. They are: internal ballistics, external ballistics and, terminal ballistics. Internal Ballistics. This concerns the phenomenon of the firing process which leads to the movement of the round inside the gun barrel until it leaves at the muzzle. External Ballistics. This deals with the behavior of the round from the time it leaves the muzzle along its flight path until it reaches its final point. Terminal Ballistics. This applies to the hit and penetration sequence once the round hits its target, disintegrates on impact or comes to final rest. As the subject is obviously far beyond the scope of this short article, we shall touch only on a few aspects necessary for our examination. To achieve high-hit penetration and lethal effect, the armor-piercing round has to be propelled at a flat trajectory for maximum range - the result of high muzzle velocity. This is made possible by the high pressure created by burning explosive propellants inside the gun chamber. As the pressure reaches its climax, the round is moved along the barrel towards the muzzle. The speed at which the round leaves the barrel is called the muzzle velocity. There are various ways to increase this speed - usually by reducing the caliber of the projectile. The higher the speed by which the round is propelled, the flatter will be its trajectory over a measured distance, until the law of gravity will cause it to fall to earth. Theoretically, then, given a certain muzzle velocity, a known-weight projectile, fixed at zero aiming point, should hit its assigned 1 1

- -.. _...... __ Graph of internal pressure during firing. I -- I I An exaggerated view of barrel elasticity during firing. An exaggerated view of barn1 bend during firing.

Typical Internal Ballistics Drawings exaggerate the various factors in barrel distortion. target located within a certain distance. This method, known as combat range shooting - where the gunner simply places his aiming sight cross-hair on target and fires, was once common practice, and still is under certain conditions. Unfortunately, modern combat requirements are more stringent and, therefore, need elaborate systems to achieve the necessary results. While modern fire control systems, involving advanced technologies for range measuring, sensoring of climatic environments, gun and ammunition parameters and other intricate aspects of the firing process, are extremely well designed, even the most sophisticated system will leave room for error -both human and systematic -frequently resulting in non-hits of the first, second, and even third round fired. Some of these errors are usually accepted as within normal gunnery standards, based on average results in training. However, on a closer examination of some of the lesser known fields of ballistics, a much higher gunnery standard can be envisaged if viable solutions are adopted to overcome these problems. Barrel Distortion Returning momentarily to the subject of internal ballistics, a well known but underestimated phenomenon - the behavior of the gun barrel during firing -comes immediately to mind. The gun barrel is never absolutely straight. The bending of the barrel comes from several factors: -a downward bend normally results from its specific weight (an average modern tank gun protrudes some 5 meters and weighs several tons). Bending sideways is due to climatic influence on the gun barrel, such as lateral wind cooling, rain, sleet or snow on one side of the barrel. Although these deviations are minimal and extremely difficult to measure, they nevertheless have a crucial effect on the first-hit capability of the fired round. Active solutions involving advanced sensoring of wind variables, temperature, and barometric pressure, fed into integrated computer systems, only partly solve the acute problems presented by barrel distortion. Further complications are added by 12 the internal process taking place inside the barrel during the firing sequence. As the round is propelled forward under great pressure, the gun barrel widens elastically with the advancing projectile, causing the barrel to bend (similar to the behavior of a rubber hose as water passes through it). Once the round moves forward, the pressure subsides and the barrel returns to its normal state, but this causes it to bend again. The bending is circumferential and depends on the ammunition used. For example, a 105-mm APDS round causes the barrel to bend upwards, while HEAT rounds of the same caliber bend the barrel downwards. Again, rifled and smoothbore gun barrels vary in their bending behavior. To overcome barrel distortion in the field, various means have been developed. One of these is a visuaVoptica1 combination - the Muzzle Reference System (MRS) - providing a means whereby the gunner can align his sight with the axis of the muzzle. Originally working with a passive mirror system, a more advanced active device uses along-duration light source integrated into the gunner’s optics and the central FCS computer. The MRS is an expensive device, and it is also vulnerable to enemy fire under combat conditions. Furthermore, the MRS only partially compensates for barrel bend variables. A less expensive solution which provides better results is the thermal jacket or sleeve, which has become standard equipment on most modern MBTs and has been effectively combat-proven by the IDF in Lebanon. Although the thermal sleeves have been adopted by most armies for quite awhile, the importance of this factor on high first-hit probability has somehow been ignored, with more potential accredited to the active fire control system’s influence. The thermal sleeve, being a passive means, is nevertheless a dominant factor in the achievement of first-hit probability and it can reduce error considerably. Precision Measurements One of the reasons for ignorance is due to the lack of understanding of the highly sensitive parameters of gun distortion - either from environmental or ballistic phenomena - which can be calculated only measurements ly professional there remains between theore ratory criteria with precision sion -an extr consuming bus ironed out by 1 grating sophisti Such tests h patterns of ho operates under The results prc ment paramete and difficult ta conditions. Th have a crucial probability and considerable er not be compen sophisticated fii Precision m live-firing tests is a variation i along the gun 130°C, depenc rounds fired, temperature ch Temperature ly small and r normal conditic difficult to me test simulating Outside firirl5 LGJLJ I I P V G llulrlLru maximum barrel distortion at 1000 hours in the morning and 1600 hours on bright sunny days, although it was thought that solar radiation would influence the barrel vertically at noontime. However, the reverse radiation from the heated ground had an almost precise countereffect, rebalancing the barrel. Such precision measurements are almost impossible to achieve under laboratory tests. Although such elaborate measurements may seem somewhat exaggerated to the layman gunner, it has nevertheless been proved beyond doubt that errors up to one mil have been made due to external causes affecting the barrel on a normal bright day. If, during firing, rain beats down on the barrel, errors over 7 mil have been recorded. The obvious results would be complete misses on target, even though all other parameters might be perfectly observed within the normal firing sequence. - - - - -... - _ -. - -.. _. r _ _ _ _ _ _,______, while U. S. tanks like the M60, right, and Soviet T-72, below, use rigid insulating sleeves held on by metal bands. A snug fit is essential for accuracy, the author says. i t 1 13

Israeli gunners greatly value thermal sleeves, seen here on Merkava. An effective thermal sleeve can reduce this error to 0.25 mil. Normally, the barrel warms up on firing the first round and starts compensatory distortion - reaching an optimal close to zero after the tenth round. The maximal error of 20 cm at 1,000 meters would apply - in contrast to about one meter error with the first round fired. Even under optimal conditions, however, errors cannot be completely ruled out. Other aspects such as the barrel jump, as the round leaves the muzzle, are difficult to measure and compensate, as jumps differ in elevation and deflection, with variable effect. The Thermal Sleeve Known modern thermal sleeves are manufactured from plastic or metal insulation material, and cover the barrel with a closely fitting protective cylinder tube. These are common in German, French and Soviet tank guns. The British method adopts a differrent concept, using a loosely fitting tarpaulin jacket. All thermal sleeves or jackets are designed to insulate the barrel from outside environmental effects. If the sleeve does not fit closely to the barrel, the remaining air space acts as an air trap which causes temperature changes with hot air rising upward -resulting in distortion. The plastic insulation is insufficient, as it leaves about 70 percent of the bend. Thermal sleeves, using methods by which the heat is conducted around the metal sheath, reducing barrel bend to a mere 30 percent, contribute considerably to first-hit probability. One major factor in the efficiency of the thermal sleeve is elimination of water collecting inside. Water condensation can cause severe changes in temperature and become a serious cause of error. Another problem is presented by the sleeve sliding directionally with the movement of the round. When fired, the round is propelled by over 500Gs, a tremendous force which causes the thermal sleeve to slide forward. Although modern sleeves use tightfitting bands and clamps, these may not be sufficient to withstand the force, and they may break under strain. Therefore, special arrangements have to be made to keep the sleeve tight over the barrel. Conclusion Integrated fire control systems have become the most effective means to improve the performance of any battle tank and achieve high-hit probability under combat conditions. The probability of achieving a hit on another tank depends on a variety of factors, which include range, type of ammunition and its condition, gun jump, barrel wear, parallax error (difference between reference points in the optics), barrel distortion, as well as turret cant angle, and environmental effects, such as ambient temperature, cross winds, rain, and air density (atmospheric pressure). We shall examine the influence of this factor in a subsequent article, as it is of great interest in long-range gunnery, in which the projectile, propelled at hypersonic speeds, is extrememly vulnerable to aerodynamic pressures encountered under conditions similar to those in high-speed flight. The little-known effects of barrel distortion have been examined in this article, familiarizing the reader with passive solutions to an acute problem with crucial effects on precision gunnery. There is no doubt that an effective thermal sleeve can contribute cost-effective solutions towards achieving high-hit probability under combat conditions. Israeli tank gunners, a very hard-to-convince lot, have adopted thermal sleeves as a must - to the extent that, in combat, damaged thermal sleeves were repaired by the crews in the field, using tape to close ruptured sections. While the thermal sleeve is of a passive nature, it blends well into the active and highly sophisticated technologies of the integrated fire control system. In reducing barrel distortion, an effective thermal sleeve can become a crucial factor in achieving first-hit probability. (Reprinted with pmission from DEFENCE UPDATE INTERNATIONAL, No. 51.)

DAVID ESHEL, now retired from the Israeli Defense Forces, was a founding member of the IDF Armor Corps in 1948 and served for 26 years in various staff and combat assignments. A graduate of the French Armor School at Saumur and a former lecturer at the IDF Command and Staff College, he is now editor of Defence Update International. Among his book credits is the recently published “US. Rapid Deployment Forces.” 14