Recall that from 1962 on the American test sites, the LMS of the XM123 and XM124 models were tested. Two products had different artillery units, but they were built according to similar principles and received similar additional equipment. Initially, they had a pair of 20-strong engines and hydraulic transmission, but this 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 LMS had serious problems when shooting.
By the mid-sixties, the XM123 and XM124 projects were closed due to a number of unsolvable problems. For several years, the development of the American DLS stopped. However, the situation soon changed. Lockheed's specialists have found an acceptable way to drastically increase the terrain of ground vehicles, including self-propelled guns. At first it was tested on an experienced all-terrain vehicle, and then introduced into the project the DLS.
In 1967, Lockheed employees Robert and John Forsyths proposed a tri-star (“Triple Star”) wheeled undercarriage design. Such a propulsion unit 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 will allow the wheeled vehicle to overcome various obstacles, including quite large and too complex for other equipment.
Soon, experienced Terrastar ATVs were built and tested, equipped with four Tri-star units. The transmission provided the drive of all four products. During the tests, high mobility and maneuverability on rough terrain were confirmed. An unusual mover got the chances to get into new projects of ultra-high maneuverability.
At the very end of the sixties, several proposals appeared on the use of the Triple Star on a particular technique. Among other things, it was proposed to build a new self-propelled gun. It was assumed that the new model with an improved chassis will have increased permeability required on the battlefield. Such an LMS could show the most significant advantages over previous models of its class, and thus find a place in the army.
In creating a new LMS, Lockheed gained the support of the Rock Island Arsenal, which was already involved in the development of similar projects. Arsenal was supposed to provide the base gun and the carriage, and Lockheed experts were responsible for the development of new equipment and the subsequent assembly of a prototype. In the future, by joint efforts, they had to carry out tests and, upon successful completion of the work, establish mass production.
The new project received the working designation M2A2 and the additional name Terrastar (also found other writing - Terra-Star). It is curious that the index of promising LMS pointed to the basic model of weapons, but under its old name. The basic howitzer M101A1 in the past was designated as M2A1. The additional name of the project, in turn, emphasized the continuity with the previous experienced all-terrain vehicle.
As the basis for the M2A2, we chose the existing M101A1 field howitzer of the 105 mm caliber with a regular gun. From this product it was planned to remove some units, 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 the new power plant, which in its scheme resembled the aggregates of older DLS.
The swinging artillery part of the gun remained the same. Used threaded 105-mm barrel length 22 caliber, not equipped with any muzzle devices. The brigade of the howitzer was completed with a semi-automatic horizontal wedge gate. The barrel was equipped with hydropneumatic recoil devices and mounted on a cradle of great length with a characteristic rear guide. Near the breech on the cradle there were pins for mounting on the gun carriage. Under the rear rail provided spring balancing device.
The M101A1 carriage was quite simple; the main part of its details without changes was transferred to the new project. The upper machine was a support of small height with devices for mounting the cradle and the side sectors of the vertical pickup. The lower machine had the form of a transverse beam with fastenings for all devices, including wheel travel, beds and the upper machine. In the M2A2 project, some units were removed from the lower machine, and power plant elements appeared on its front end. Unlike other samples based on M101A1, there was no shield cover on the carriage of the new howitzer.
Manual guidance drives were saved. With their help, the gunner could move the barrel within the horizontal sector 23 ° to the right and left of the longitudinal axis. Elevation angles varied from -5 ° to + 66 °. On the left side of the cradle there were attachments for sights. The standard sights of the base howitzer provided shooting both with direct fire and with mounted trajectories.
Layout left existing sliding bed welded construction. They were pivotally connected to the lower machine and could be fixed in the reduced position for carrying. On the back of the bedding there were coulters for laying in the ground when shooting. 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, in the back of the right frame placed the power plant. According to known data, a small power internal combustion engine was used, which transmitted power to hydraulic pumps. The pressure was transferred through the hoses to a pair of hydromotors installed in front of the bottom mast machine. Directly on the carriage were placed two mechanical gearboxes that ensured the transfer of engine power to propulsion. The engines themselves were mounted on the crankcase of the gearboxes.
To the right of the power plant was the driver's seat. Control levers were placed next to it to monitor the operation of 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 thrusters. The synchronous change of this parameter allowed changing the speed and moving straight ahead. The difference in the speeds of the two motors introduced the LMS into a turn.
Terrastar howitzer on tests. Photo by Militaryimages.net
Instead of the standard wheel track, the LDS M2A2 received the original Tri-star undercarriage. On the transverse axis of the gearbox, a special design with three wheels and its own means of power transmission was fixed. Howitzer received two such devices - one instead of standard wheels.
From the inside, next to the carriage, the Tri-star product had a flat casing of three-beam shape, in which there were gearing elements. The shaft entering the casing was connected to the central gear. In each of the "rays" of the casing there were two gear wheels of small diameter: one was intermediate, and the second was connected to the wheel axis. Thus, a single shaft from the engine or gearbox could ensure the simultaneous rotation of the three wheels in one direction. In addition, under certain circumstances, the drive shaft provided the rotation of the whole structure around its axis.
Tri-star propulsion for 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. From 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 slab, a large diameter pipe passed.
Behind on the right frame placed an additional element of the chassis. On the self-orienting support there was a single wheel with a low-pressure tire. The use of another “triple star” on the bed was considered inexpedient. Rear wheel support could rise up when translating guns in the firing position.
The original chassis was large and had an impact on the overall dimensions of the howitzer. In addition, significantly increased the weight of the product. The total length of the M2A2 Terrastar LMS reached 6 m in the stowed position, the width increased to 3,5 m. The height remained at the same level - less than 1,8 m. The mass from the original 2,26 t increased to 2,5-2,6 t. should have shown 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.
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 based on the two lower wheels, and the beds were supported by its own rear wheel. When driving in the same conditions, the torque was distributed simultaneously among all six driving wheels of the mast. Four "lower", standing on the ground, provided the movement. The new LMS, like its predecessors, had to drive the barrel forward.
The original propeller was supposed to show its advantages when hitting an obstacle or when driving over rough terrain. If there was a major obstacle in the way of the Tri-star, its forward movement stopped. At the same time, the motor continued to work, as a result of which the entire structure had to be turned around the standing wheel. During such a turn, the wheel at the top moved forward and down, getting the opportunity to stand on an obstacle. Receiving torque from the engine, the wheels could jointly drag the LMS onto an obstacle.
Overcoming the pits and ditches looked different. The front lower wheel should fall down, ensuring the rotation of the entire propulsion unit. Further, the whole structure had to climb another slope, like any other obstacle.
In other words, depending on the characteristics of the landscape, either the wheels or the entire Tri-star device are spinning. The front thrusters of the M2A2 cannon, which had a drive, were supposed to provide movement and overcome obstacles. The rear wheel rotated freely and was only responsible for maintaining the bedding at the required height above the ground.
The right carriage frame with power plant. Engines and pumps removed under the new casing. Photo of Wikimedia Commons
It was proposed to use existing towing vehicles when transporting M2A2 SDO over long distances. At the same time, the own power unit of the howitzer was not used. However, this did not prevent the use of the chassis for a certain increase in maneuverability in comparison with the wheel travel of the base howitzer.
The transfer of Terrastar into a combat situation was not very difficult. After arriving at the firing position, the calculation was to stop the engine, raise the bed and fold the back support with the wheel. Then it was necessary to separate the beds and perform other operations in preparation for the shooting. The principles of shooting have not changed.
The prototype M2A2 Terrastar advanced self-propelled gun was built in 1969 year. When assembling it, available components were used, probably from different howitzers. Thus, the involved artillery part of the M101A1 howitzer was made by the Rock Island arsenal as far back as 1945 year (at that time this weapon was designated as M2A1). The carriage, in turn, collected in 1954-m. After another decade and a half, the carriage was rebuilt according to a new project, turning the standard howitzer into a prototype.
Field tests conducted by the Rock Island arsenal and Lockheed showed that the new version of the DLS has the most significant advantages over the previous ones. Thus, a power plant of sufficient power and hydraulic transmission in combination with the used chassis allowed the howitzer to reach speeds up to 30-32 km / h on the highway. On rough terrain, the speed dropped several times, but at the same time there was a very high mobility.
It was found that the self-propelled howitzer, despite the limited engine power, has good maneuverability. Without difficulty or with minor difficulties, bumps or pits with a vertical dimension of the order of half a meter were overcome. In fact, the M2A2 gun was not afraid of obstacles whose dimensions were less than the distance from the surface to the Tri-star propulsion axis. Thus, in comparison with the previous LMS, mobility on the battlefield has improved significantly. There were obvious advantages over towed systems, since Terrastar did not need a towing vehicle.
However, it was not without problems. First of all, the carriage for the DLS was too complicated to manufacture and operate. In addition, the complexity of the "triple star" adversely affected the reliability of the whole structure. These or other breakdowns occurred regularly, as a result of which the LMS lost its course and needed to be repaired. In addition, the power units and chassis non-optimally consumed the engine power, which could make it difficult to overcome some obstacles.
Quickly enough, the military studied the proposed weapon and made conclusions. Despite the presence of a number of advantages over existing artillery systems, the M2A2 Terrastar was considered unsuitable for use. Not later than the beginning of the seventies, the Pentagon ordered to stop the further development of the project. The product has lost the chance to enter the series.
However, the developers did not abandon their project. The existing self-propelled gun was left in trial operation as an experimental sample. Over the next few years, Lockheed and Rock Island Arsenal conducted various tests, refined the structure and studied its capabilities. The last experiments were carried out only in 1977 year - a few years after the refusal of the military to adopt.
After completing the tests, the only available prototype of the Terrastara was transferred to the Rock Island Museum of Arsenal. The experienced M2A2 is still on display in the open air. Next to these products are 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 use the new howitzer, as a result of which the third SDO project could not ensure the re-equipment of the army. In this case, it was not only about the closure of the project, but also about the cessation of work in the whole area. The concept of a self-propelled gun again failed to realize with obtaining all the desired results, and the US Army decided to completely abandon it. After M2A2 Terrastar new LMS were not developed.
On the materials of the sites:
Forsyth RW, Forsyth JP Marginal-Terrain Amphibian. San Francisco, Calif. Society of Automotive Engineers, 1968.