The concept of a self-propelled gun (SDO) offers an optimal balance between the mobility of an artillery system and the complexity of its production. At the same time, not all samples of this kind were able to show the desired characteristics. So, in the early sixties in the United States, two self-propelled howitzers were tested at once, which could not demonstrate high mobility. A few years later, Lockheed proposed a new version of the LMS, which was distinguished by the use of the most daring ideas. It was believed that the M2A2 Terrastar could have a uniquely high mobility and maneuverability.
Recall that since 1962, LMS models XM123 and XM124 have been tested at American proving grounds. The two products had different artillery units, but were built on similar principles and received similar additional equipment. Initially, they had a pair of 20-horsepower engines and a hydraulic transmission, but such equipment could not provide high mobility. Removing one of the engines and installing an electric transmission also did not lead to the desired results. In addition, both SDOs had serious shooting problems.
Self-propelled gun M2A2 in the museum. Photo Wikimedia Commons
By the mid-sixties, the XM123 and XM124 projects were closed due to the presence of a number of unsolvable problems. For several years, the development of American LMS stopped. However, the situation soon changed. Lockheed specialists have found an acceptable way to dramatically increase the cross-country ability of land vehicles, including self-propelled guns. First, it was tested on an experienced all-terrain vehicle, and then introduced into the LMS project.
In 1967, Lockheed employees Robert and John Forsythe proposed a Tri-star wheel undercarriage design. Such a propeller was based on an assembly in the form of a three-beam cage, on which three wheels and several gears were present. It was assumed that such units would allow the wheeled vehicle to overcome various obstacles, including large enough and too complex for other equipment.
Experienced Terrastar all-terrain vehicles equipped with four Tri-star units were soon built and tested. The transmission provided the drive for all four products. During the tests, the high characteristics of mobility and cross-country ability on rough terrain were confirmed. The unusual propulsive device got a chance to get into new projects of ultra-high cross-country vehicles.
At the very end of the sixties, several proposals appeared at once on the use of the "Triple Star" on one or another technique. Among other things, it was proposed to build a new self-propelled weapon. It was assumed that the new model with an improved chassis would have the increased maneuverability required on the battlefield. Such a SDO could show the most serious advantages over the previous models of its class, and thanks to this, it could find a place in the army.
Howitzer M2A1 - the future M101A1. Photo US War Department
In the creation of a new LMS, Lockheed enlisted the support of the Rock Island Arsenal, which had already participated in the development of similar projects. Arsenal was supposed to provide the basic weapon and carriage, and Lockheed specialists were responsible for the development of new equipment and the subsequent assembly of the prototype. In the future, by joint efforts, they were supposed to conduct tests and, upon successful completion of the work, set up mass production.
The new project received the working designation M2A2 and the additional name Terrastar (another spelling is also found - Terra-Star). It is curious that the index of a promising SDO pointed to the basic model of weapons, but under its old name. The basic M101A1 howitzer was formerly designated M2A1. The additional name of the project, in turn, emphasized the continuity with the previous experienced all-terrain vehicle.
The existing M101A1 field howitzer of 105 mm caliber with a standard carriage was chosen as the basis for the M2A2. It was planned to remove some units from this product, and in addition, it was planned to install a number of new devices, including the most interesting ones. First of all, it was planned to replace the wheel travel and install a new power plant, according to its scheme, reminiscent of the units of the older LMS.
The swinging artillery piece of the gun remained the same. A 22-caliber rifled 105-mm barrel was used, which was not equipped with any muzzle devices. The breech of the howitzer was equipped with a semi-automatic horizontal wedge breech. The barrel was equipped with hydropneumatic recoil devices and mounted on a long cradle with a characteristic rear guide. Near the breech on the cradle, there were trunnions for mounting on a gun carriage. A spring balancing device was provided under the rear rail.
Tri-star block with cover removed. Lockheed Photos
The M101A1 carriage was quite simple; most of its details were transferred unchanged to the new project. The upper machine was a support of low height with devices for mounting a cradle and side vertical guidance sectors. The lower machine was in the form of a crossbeam with attachments for all devices, including wheel travel, beds and the upper machine. In the M2A2 project, some units were removed from the lower machine, and elements of the power plant appeared on its front. Unlike other samples based on the M101A1, there was no shield cover on the carriage of the new howitzer.
Manual guidance drives were retained. With their help, the gunner could move the barrel within the horizontal sector by 23 ° to the right and left of the longitudinal axis. The elevation angles varied from -5 ° to + 66 °. On the left side of the cradle there were mounts for sighting devices. The standard sights of the base howitzer provided both direct fire and hinged trajectories.
The carriage was left with the existing sliding frames of a welded structure. They were pivotally connected to the lower machine and could be fixed in a reduced position for transportation. On the back of the bed there were coulters for resting on the ground when firing. In the M2A2 project, the left frame remained unchanged, while on the right it was planned to mount several new devices and units.
First of all, the power plant was placed in the rear of the right frame. According to known data, a low-power internal combustion engine was used, which transmitted power to hydraulic pumps. Through the hoses, the pressure was transmitted to a pair of hydraulic motors installed in front of the lower carriage machine. Two mechanical gearboxes were placed directly on the carriage, which ensured the transfer of engine power to the propellers. The engines themselves were installed on gearbox housings.
To the right of the power plant was the driver's seat. Next to it were placed control levers to control the operation of the hydraulic motors. With the help of a pair of levers, the driver could control the pressure at the inlet to the motors of the two propellers. Synchronous change of this parameter made it possible to change the speed and move straight. The difference in the revolutions of the two motors introduced the SDO into a turn.
The Terrastar howitzer is being tested. Photo Militaryimages.net
Instead of the standard wheel travel, the M2A2 SDO received an original running gear of the Tri-star type. A special design with three wheels and its own power transmission means was fixed on the transverse axis of the gearbox. The howitzer received two such devices - one each instead of the standard wheels.
On the inside, next to the carriage, the Tri-star product had a flat three-beam casing, in which the gear elements were located. The shaft entering the inside of the casing was connected to the central gear. In each of the "rays" of the casing there were two small-diameter gear wheels: one was intermediate, and the second was connected to the axle of the wheel. Thus, one shaft from a motor or gearbox could provide synchronous rotation of three wheels in one direction. In addition, under certain circumstances, the drive shaft provided rotation of the entire structure around its axis.
The Tri-star propeller for the self-propelled howitzer was equipped with large-width wheels with low-pressure tires. It was assumed that this would reduce the specific pressure on the ground and further improve the permeability. On the outside, the axles of the three wheels were connected by a three-beam plate. For greater rigidity in the center of the structure, between the gearbox and the plate, a large diameter pipe passed.
An additional undercarriage element was placed on the back of the right frame. A single wheel with a low pressure tire was located on a caster. The use of one more "triple star" on the bed was considered inappropriate. The rear wheel support could rise up when the gun was transferred to the firing position.
The original undercarriage was large and affected the overall dimensions of the howitzer. In addition, the weight of the item has increased markedly. The total length of the LMS M2A2 Terrastar in the stowed position reached 6 m, the width increased to 3.5 m. The height remained at the same level - less than 1.8 m. The weight from the initial 2, 26 tons increased to 2.5-2.6 tons The artillery unit remained the same, and therefore the updated howitzer had to show the same characteristics as before. The initial velocity of the projectile, depending on its type, was at the level of 470 m / s, the firing range reached 11, 3 km.
LMS in firing position, rear view. Photo Wikimedia Commons
In the stowed position on a flat surface, the M2A2 Terrastar howitzer was supposed to stand on five wheels at once. Each "triple star" of the main wheel travel was supported by two lower wheels, and the beds were supported by their own rear wheel. When driving under the same conditions, the torque was distributed simultaneously between all six driving wheels of the carriage. Four "lower" ones, standing on the ground, provided movement. The new SDO, like its predecessors, had to go barrel forward.
The original propulsion device had to show its advantages when hitting an obstacle or when driving over rough terrain. If there was a large obstacle in the way of the Tri-star, its forward movement would stop. At the same time, the hydraulic motor continued to work, as a result of which the entire structure had to turn around the standing wheel. During such a turn, the wheel located at the top moved forward and downward, getting the opportunity to stand on an obstacle. Receiving torque from the engine, the wheels could jointly drag the SDO onto an obstacle.
Overcoming pits and ditches looked different. The front lower wheel had to fall down, ensuring the rotation of the entire propeller. Further, the entire structure had to rise to another slope, like any other obstacle.
In other words, depending on the terrain, either the wheels or the entire Tri-star assembly were spinning. The front propellers of the M2A2 gun, which had a drive, had to provide movement and overcoming obstacles. The rear wheel rotated freely and was only responsible for maintaining the beds at the required height above the ground.
The right frame of the carriage with the power plant. The motors and pumps have been retracted under a new casing. Photo Wikimedia Commons
When moving the LMS M2A2 over long distances, it was proposed to use existing tractors. At the same time, the howitzer's own power plant was not used. However, this did not prevent the use of the capabilities of the chassis for some increase in cross-country ability in comparison with the wheel travel of the base howitzer.
Transferring Terrastar to a combat position was not very difficult. After arriving at the firing position, the calculation had to turn off the engine, raise the beds and fold the rear support with the wheel. Then it was necessary to part the beds and carry out other operations to prepare for firing. Shooting principles have not changed.
A prototype of the promising self-propelled gun M2A2 Terrastar was built in 1969. When assembling it, available components were used, probably from different howitzers. So, the involved artillery unit of the M101A1 howitzer was manufactured by the Rock Island Arsenal back in 1945 (at that time this gun was designated as M2A1). The carriage, in turn, was assembled in 1954. After another decade and a half, the carriage was rebuilt according to a new project, turning a standard howitzer into a prototype.
Field tests carried out by the Rock Island Arsenal and Lockheed have shown that the new version of the LMS has the most serious advantages over the previous ones. So, the power plant of sufficient power and the hydraulic transmission in combination with the used chassis allowed the howitzer to reach speeds of up to 30-32 km / h on the highway. On rough terrain, the speed dropped several times, but at the same time, a very high mobility remained.
It was found that the self-propelled howitzer, despite the limited engine power, has good maneuverability. Bumps or holes with a vertical dimension of about half a meter were overcome without difficulty or with minor difficulties. In fact, the M2A2 gun was not afraid of obstacles, the dimensions of which were less than the distance from the surface to the axis of the Tri-star propeller. Thus, in comparison with the previous LMS, mobility on the battlefield has improved significantly. There were obvious advantages over towed systems, since the Terrastar did not need a tractor.
Museum specimen, rear view. Photo Wikimedia Commons
However, it was not without its problems. First of all, the carriage for the LMS was too complicated to manufacture and operate. In addition, the complexity of the "triple star" negatively affected the reliability of the entire structure. One or another breakdown took place regularly, as a result of which the LMS lost its speed and needed repair. In addition, the power units and chassis were not using the engine power optimally, which could make it difficult to overcome some obstacles.
The military quickly studied the proposed weapon and made conclusions. Despite the presence of a number of advantages over existing artillery systems, the M2A2 Terrastar gun was considered unsuitable for adoption. No later than the beginning of the seventies, the Pentagon ordered to stop further development of the project. The product has lost its chances to enter the series.
Nevertheless, the developers did not abandon their project. The existing self-propelled gun was left in trial operation as an experimental model. Over the next few years, specialists from Lockheed and the Rock Island Arsenal conducted various tests, finalized the design and studied its capabilities. The last experiments were carried out only in 1977 - a few years after the military refused to accept it into service.
After the completion of the tests, the only prototype of the Terrastar was transferred to the museum at the Rock Island Arsenal. The experimental M2A2 is still on display in an open area. Next to these products are the prototypes of the LMS XM123 and XM124, created in the early sixties. Thus, the museum was able to collect all the samples of self-propelled artillery developed by the United States.
The military decided not to accept the new howitzer into service, as a result of which the third SDO project was unable to ensure the rearmament of the army. At the same time, it was not only about the closure of the project, but also about the termination of work in the whole area. The concept of a self-propelled weapon again failed to be realized with all the desired results, and the US Army decided to finally abandon it. After the M2A2 Terrastar, new LMSs were not developed.
