Military

152mm Tank Cartridges

In the early 1960s, the Army embarked on a three-part tank development program that utilized a radical new concept of a guided, low velocity, 152mm projectile, with a combustible cartridge case. The program included the M551 Sheridan vehicle, a modified M6OAl tank, and the MBT-70 German-American tank program. The concept was based on commonality of ammunition (with the obvious benefits) and a tank killing capability that would "meet the threat" of the 1970s.

In response to calls to replace the M41 light tank and the M56 self-propelled tank gun, the Army initiated a program in 1961 to develop an armored vehicle that could perform both the airborne and cavalry roles. The vehicle, named the Armored Reconnaissance/Airborne Assault Vehicle (AR/AAV) XM551, or Sheridan, attempted to overcome the difficulty of designing a chassis with the ability to absorb the shock of recoil yet was light enough for airborne operations. In order to decrease the initial velocity (and hence the amount of recoil), the XM551 was designed with the 152-mm Shillelagh missile that was fired at subsonic speeds. After firing, missile velocity was increased using a solid -fuel motor.

First built by the Allison Division of General Motors Corporation in 1966, the M551 Armored Reconnaissance Airborne Assault Vehicle, Full Tracked, 152 mm (Sheridan) was developed as a replacement for the M41 light tank and the airborne M56 Scorpion self-propelled antitank gun. Intended as an airborne reconnaissance and assault vehicle, the Sheridan was "air droppable" by use of the Low Altitude Parachute Extension System (LAPES). Its gun-launcher fires both conventional munitions and Shillelagh missiles.

From the start, development of the system encountered major problems, particularly with the ammunition. While acknowledging that the Army had a firm Southeast Asia requirement for the SHERIDAN with its conventional 152mm ammunition, both the Combat Developments Command and the Continental Army Command expressed concern about deficiencies i n those munitions and maintained t h a t the problems should be overcome before designation of the system as Standard A. The SHERIDAN Project Manager argued that failure to classify the system as standard could jeopardize production funds, dissipate the momentum built up in the program, and lead to program stretchout and increased costs. On 21 May 1966, DA approved Standard A type classification for the General SHERIDAN weapon system, which included the SHILLELAGH missile system components. Excluded from the type classification approval were the 152m conventional ammunition, trainers, grenade projector, and the gunner's passive periscope.

The vehicle was approved for production, despite numerous technical problems and incomplete test results. In 1969, the M551 was deployed to Vietnam to meet an "urgent" field requirement. Ammunition fires, rounds that went off prematurely, and numerous maintenance problems gave the vehicle a poor reputation. Troops rode on top of the vehicle rather than inside because of the danger of mines and ammunition fires. Although the PM and other program personnel visited Vietnam to try to stay abreast of the problems, the vehicle continued to have problems.

By 1970, 1650 vehicles had been produced, the program had cost over $1 billion, and most of the vehicles were in storage awaiting modifications to make them acceptable for fielding. The vehicle was modified by 1975 and became the M551AI with the addition of a laser range finder. Also in 1975, a major product improvement program (PIP) was initiated in order to eliminate the numerous technical and maintenance problems that had plagued the vehicle since fielding. Most systems have been upgraded and improved since the Sheridan was introduced in the 1960s. For example, the M551AI has enhanced thermal-imaging and targeting sights. But in 1978, the Army decided to withdraw the vehicle from the active inventory, 6 with a plan to use some 330 vehicles as non-firing training tanks at the new National Training Center.

The M6OAlEl was an upgrade to the old reliable M6OAl tank, modified to fire the 152mm Shillelagh as part of the new tank concept. The plan was to mount the new weapon system on the M60 hull with a modified turret gun stabilization and fire control system. All M60s would become 152mm tanks to provide an interim Shillelagh capability until the MBT-70 was fielded. Between the idea and delivery, however, something went wrong with the M60AlE2's gun stab and fire control systems which Chrysler is still trying to fix. The problem became a major scandal in congressional hearings and in October 1968 the FY 70 buy was cancelled. Initial deliveries began in April 1968, were completed in December of that year and consisted of 243 turrets to be retrofitted to existina M60A1 hulls and 300 complete M60AE2 tanks.

In May 1980 General Vessey, the Vice Chief of Staff, approved a recommendation from ODCSOPS to withdraw M60A2 tanks from the active Army and convert them with an M48A2 turret. ODCSOPS proposed to call the hybrid the M48A7. The M60A2 had been armed with a low-velocity 152-mm. cannon and missile launcher assembly which fired the Shillelagh missile as its primary round and a conventional shell as its alternative. The M48A5 turret mounted a high-velocity 105-mm. gun. By the end of 1981 both CONUS and USAREUR maneuver units had turned in all of their M60A2s-540 vehicles, the entire Army inventory-to the Anniston Army Depot in Alabama for refitting.

Changes in tactics and doctrine were in process to keep pace with the combat potential of the remarkable new tank and weapons system. Despite these giant strides in the development of armor vehicles and weapons, a program for yet another new main battle tank for the 1970 Army was already under way in a co-operative effort of the United States and West Germany. After three years of development, the first prototype of this joint undertaking- the MBT-70, described as the most advanced military tracklaying combat vehicle in existence- was unveiled in October 1967.

The joint effort with the Federal Republic of Germany, under which the main battle tank 70 (MBT-70) has been developed, was modified to a co-operative program in the middle of 1969. Changes in the U.S. design were recommended to decrease production costs and increase the reliability of the vehicle; sacrifices in combat effectiveness were minimal. In December 1969, the new program was reviewed in the Office of the Secretary of Defense and in the following month the recommended changes were approved. The design changes have thus reduced the estimated unit cost of the tank by almost 30 percent. With the modification of the program and of the co-operative features of the engineering effort, the tank received a new designation, XM-803; the composite nomenclature MBT-70/XM-803 was to be used until February 1971. Concerned about expense, Congress withdrew funding for the MBT-70 in December 1971, thereby canceling the program program, under claims that the MBT-70 was overly sophisticated and unnecessarily complex.

The primary anti-tank round was the high explosive, anti-tank, multipurpose round (XM409) Its objective is to be capable of defeatingheavy tanks at battle ranges and also to provide soft target capabilities (personnel, unarmored vehicles, etc.) at all usable ranges. Also under development as part of the Shillelagh Weapon subsystem were the white phosphorus round (XM410), which was primarily for screening, marking, and incendiary use, and the target practice/training round (XM411). In addition, development of three more ammunition rounds was initiated late in the Sheridan program. These were the high-explosive round (XM657) which is to be used as an interim round until the XM409 is acceptable; the beehive round (XM617); and the canister round (XM625) which was to be used until the beehive round is developed. The beehive and canister rounds were for use primarily in an antipersonnel role.

A product improvement test of the combustible cartridge case, 155-mm, XM157, was conducted at Aberdeen Proving Ground to obtain information on flarebacks and relate this information to the possible safety hazard of exposing a cartridge case to an environment which may produce a flareback. The 152-mm gun-launcher, M81, installed in a pilot model of the M551 General Sheridan vehicle was used in the flareback study. Photographic and visual observation along with strategically positioned thermocouples provided most of the data. Firings were conducted to study flareback as influenced by different ignition sources and wind conditions and its effect, in direct exposure, on the combustible cartridge case. Supplementary laboratory tests supplied information on the ignitability of the cartridge case, XM157. The data from these firings and laboratory tests indicate that flarebacks are not likely to ignite stowed rounds; sufficient heat is available from a flareback to ignite a cartridge case in the rear of the gun but only under specific conditions. The probability of occurrence is low. Time did not permit testing at all possible ambient conditions; but it is felt that the conditions of the tests resulted in flarebacks of greater severity than would be expected to occur under service conditions.

Conventional (non-missile) 152mm ammunition was issued as fixed (complete and permanently assembled) rounds. The propelling charge is not adjustable. A complete round of ammunition consists of all components required to fire the weapon once. These components consist of a primed cartridge case containing the propelling charge and a projectile, fuzed or unfuzed depending on type. The complete round is loaded into the weapon as a unit.