Ideas about coupling vehicles together and of an articulated body for fighting vehicles are as old as tanks themselves. Coupling two vehicles together offers the designer some significant benefits for mobility and design for a variety of roles, one of which was with amphibian tanks.
Amphibian tanks, which can operate on land or in water, have unique challenges to overcome. They have to be buoyant enough to float and yet armored sufficiently to protect the troops and then they still have to deal with the difficult transition from water to land. A coupled or articulated vehicle has the advantage of being long enough to straddle this boundary between water and land and potentially, of being able to cross bigger obstacles than a single-hulled vehicle.
With that in mind, the US Navy started work as early as 1966 studying the effects, benefits, and costs associated with coupling amphibian vehicles together. That study followed work between 1956 and 1958 focusing solely on improving water speeds for amphibian vehicles although due to time and financial constraints it was limited only to wheeled vehicles. Although those tests were inconclusive the one bright light from them was the testing of the ‘Sea-Serpent’ (not to be confused with the amphibian flame-thrower vehicle of the same name). Sea-Serpent was the name given to a multi-vehicle amphibian train of up to eight vehicles coupled together which managed a combined speed double that of one of the single vehicles.
Building on this finding for such a novel vehicle, in 1966, Davidson Laboratory (DL) looked at the problem from the point of view of tracked amphibians and concluded that it was feasible to couple together up to five LVTP-5s and that this ‘amphibian-train’ had only twice the water friction of a single vehicle. As such, this 5 vehicle train could increase speed by up to 50% in the water alone. Furthermore, the transition zone was traversed more easily by this 5 vehicle train, as were obstacles. Such an increase in mobility constituted a significant military advantage and clearly this success meant there would be further work in this area to expand on this.
The DL experiments did not use actual LVTP-5 but instead accurately shaped and weighed models in a test tank with them connected together by means of simple pin connections at the height of the center-of-gravity (CoG) of the vehicles.
One problem identified during these 1966 tests was that the increased vehicle speed produced an increased sized bow-wave with a consequent increased risk of swamping of the lead vehicle (as there is no bow wave for any of the following vehicles). To counter this, a freeboard shield would be required to be fitted to the lead vehicle of any LVTP-5 train regardless of length (Tests conducted during the 1970’s with coupled M113s also showed this additional freeboard shielding used). Additionally, as a larger freeboard shield was required for any length of LVTP-5 train, it was also found this had the side benefit of itself reducing drag in the water by 5%. In calm water, it required zero freeboard at 7.1 mph (11.4 km/h) and 12 inches (305 mm) at 7.6 mph (12.2 km/h). At 8.3 mph (13.4 km/h) however, she would need 20 inches (508 mm) of freeboard – the same amount as required unladen at 10 mph (16.1 km/h).
Vehicles used in an amphibian train had to be carefully positioned to reduce drag and the ideal gap between vehicles was found to be 1 foot (300 mm) compared to the original 6 feet (1.8m). This close-coupling reduced drag in the water by 15% and thereafter all testing was done with little or no gap between vehicles. Even with the drag reductions and freeboard changes though, the entire train was still effectively limited to a safe speed of 9.2 mph (14.8 km/h) in the water to provide a margin of safety for rough water.
Additionally for rough water the coupling, DL recommended that it “must be capable of absorbing some degree of shock, if wave or surf operation is contemplated”. They had found that, at a pitch angle of more than 25 degrees, their train broke apart. Obviously, in a real-world situation, this would have proven extremely hazardous.
The 1966 experiments ended with strong recommendations to pursue this modeling to the prototype stage with a train made from real LVTP-5 tested with a view to developing an effective type of coupling and box shields. It is not known, however, if any LVTP-5’s were ever modified in tested in this coupled configuration despite the potential benefits.
A Project Reborn
The work in 1966 had shown the potential for coupling for amphibians but also highlighted the lack of knowledge in coupled-vehicle systems. As a result, in 1972, money was put aside by Congress for work on cybernetically coupled technologies with positive feedback controls to tray and leverage this coupling technology. The following year (1973), two M113’s were modified and coupled together to form the M226 Cybernetically Coupled Research Vehicle (CCRV). That work showed, once more, great potential. Combined, the two vehicles exhibited greater mobility off-road than an individual vehicle, particularly when it came to obstacle crossing, and used a more controlled system for the coupling compared to the rather crude pin-attachment in the 1966 experiments. This time, instead of an uncontrolled pin vulnerable to excess pitch (in the water) and excess yaw (on land or in the water), the M113 CCRV connector limited movement to a maximum articulation of +/-45 degrees and a yaw of +/-30 degrees.
Control for the system was by means of a simple joystick with positive feedback in all cases, except for pitching where the feedback caused some problems. A simple movement of the driver’s joystick would control the hydraulic arms on top of the vehicle to move the two hulls relative to each other as well as providing stiffness preventing too much pitch and yaw.
The conclusion of the trials in 1974 had proven the experiments in 1966 to be partially correct. A coupled vehicle did exhibit manoeuvrability benefits over the single hull system. Some specific recommendations and findings about the coupled-vehicle system came out of the project including the reduction of water resistance for amphibious vehicles and assisting amphibious assault vehicles in breaching defences.
Irmin Kamm, from the Stevens Institute of Technology and one of the lead authors for TACOM on cybernetically coupled vehicles had already worked on amphibian trains coming up with the Sea-Serpent back in 1969 and then, on the M113 CCRV project. He continued his work on the coupled-vehicle concept with amphibians in 1979 with an evaluation of the concept of a ‘Coupled-LVT’ (LVT – Landing Vehicle Tracked). He was clearly advocating for more work in this area as, in 1979, he concluded that study saying “the advantages [of a coupled vehicle] exceed the drawbacks, it is recommended that existing vehicles be coupled and tested to establish their operational capacity”. The end-user was being clearly identified as the US Marine Corps just like before but the requirements had changed a little. Instead of a coupling permitting +/-45 degrees and a yaw of +/-30 degrees as on the M113 CCRV, this coupled LVTP would have a maximum pitch of just +/- 30 degrees.
Instead of the A-frame drawbar connected to a ball and two hydraulic actuators, this time, the system would be simpler. That older system had to be connected manually but with a new cone-into-cone system, the vehicles could connect or detach on sea or land without having to stop and get out. Each vehicle would simply have a ‘male’ convex cone on the front and a ‘female’ concave cone on the rear, with both located at the center of gravity line. This system was much more versatile. Instead of a master-and-slave unit, as with the M113 CCRV, this time any vehicle so equipped with a coupling cone could connect to any other on sea or land adding a new level of capability to the idea.
Just as before, control over pitch and yaw was performed by either lead or following vehicle by means of a joystick (although a steering wheel was also considered in the study) and the engines would be synchronized to produce the same output. Transmission and braking was to be changed manually though.
With two LVTP-7’s coupled together in this system, the expected performance increases were impressive at a price of additional weight of controls and fittings and a reduction in the internal space in each vehicle. Another note was that the coupled vehicles presented a slightly larger target to enemy fire although from the front the area exposed would obviously be the same. Coupled together, the vehicle would also get a new name. It was to be the LVTP-77.
The name ‘LVTP-77’ appears here for the first time as it is two LVTP-7’s coupled together and rather than sum the two 7’s to make LVTP-14, akin to what was done adding together the ‘113’ part of the two M113’s which were coupled forming the M226, here they simply added a second ‘7’ to make LVTP-77. Looking at the figures for the LVTP-77, they clearly demonstrate that the coupled amphibian is more stable and faster in water, and in fact, faster on land too, with better obstacle crossing, but there was one other concept investigated by Kamm as an alternative. This was an articulated vehicle made from multiple-units comprising a single vehicle rather than two or more vehicles combining.
This articulated vehicle would have two sections of 14 feet (4.27m) and 19 feet (5.80 m) respectively fore and aft with a single 890 gross horsepower engine and weighing 54,000lbs (24.5 tonnes). The fore-section contained the engine and transmission and the rear section contained all of the crew, and an auxiliary engine.
The 1979 study had concluded that a coupled amphibian was not only possible and beneficial but also desirable, as was an articulated LVT. The recommendation going forwards was to design, install and test a suitable coupling on either a pair of LVTP-7’s or LVTPX-12 vehicles.
Coupling the LVTP’s
With the M226 CCRV, it was simple. It was just a test rig so it did not really matter if the combat capabilities, such as the troop space, were severely hampered. For the actual project with LVTP-7’s or others, the rear ramp and emergency exits had to be kept clear, as would the cargo hatch. Unlike the M226 CCRV, the coupling had to be remotely connectable and disconnectable on land or water and each vehicle had to be able to operate as either the master or the slave. For an articulated LVT, the vehicle was to be limited to 33’ (10.06 m) and had to be equipped with a rear exit ramp and top cargo hatch. The restriction of just one driver was not a problem, but the articulated vehicle was also supposed to be able to dump one part of itself and be able to operate independently on land if needs be. Hence, the reason for the auxiliary engine in the aft unit of the articulated LVT design.
There were 12 separate coupling systems suggested for the coupled LVTP, but only a single viable design for the articulated LVT. The types of connections considered were ball joint, off-center ball joint, symmetrical yaw, symmetrical yaw with independent pitch control, dependent pitch and yaw, dependent pitch and yaw with coupling frame, yaw – no roll, pitch only, turntable, trunnion mount, trunnion mount- internal coupler, and, split pitch and yaw. Only the split pitch and yaw type was suitable for both remote coupling and further development.
The articulated LVT was perhaps more promising as it did not need to concern itself with the problems of remote coupling. In keeping with articulated and coupled vehicles though, the turning radius was large, 35’ (10.7 m), over twice as big as a single LVTP-7 but substantially less than either the coupled M113CCRV or the LVTP-77.
All of the pitch and yaw controls were completely contained within the vehicle with steering by the simple means of yaw control from the driver. The front unit’s 850 ghp engine would be supplemented by a 100 hp auxiliary engine in the back providing some very limited mobility in the absence of the lead unit or if it became damaged by enemy fire. In the normal situation though, this 100 hp engine was used only for driving the pumps for moving the actuators for the coupling.
Power in the water was to be provided by water jets enclosed in the fore sections powered from the main engine. Although it would be more efficient in water-drive terms to have them in the rear section, it was a more efficient use of weight and space to have them in the front. Steering in the water was by means of water deflectors in the jets which moved in conjunction with the yaw of the vehicle for efficient steering. The articulated vehicle would only need one driver who was required to be in the rear although was ideally to be in the front section for visibility. The coupled vehicle concept, on the other hand, would require a driver in both sections to ensure smooth operation and obviously would not need to couple up remotely. In an emergency, it could decouple from the front unit by means of an explosive bolt – in this was the driver did not have to get out at all.
Having conducted various analytical work on the concepts for arrangements of the LVTP-7 and LVTPX-12 models, trials were carried out and once more confirmed the efficiency of coupling an amphibian. Unlike the coupled LVTP-5, which does not appear to have gone further than the concept stage, there was work carried out between 1979 and 1980 on the LVTP-7 project.
The Articulated LVT
The other part of the project which seems to have gained no traction was the Articulated LVT. Despite being better in many regards than the M226 and LVTP-77, simpler, no remote coupling or uncoupling, a single driver, etc., the project simply did not get anywhere yet was a fairly straightforward design. Two sections, engine at the front with the crew at the back, it could pitch through +/- 30 degrees and yaw to +/-34 degrees. Two small turrets on the aft portion contained cupolas for the commander and driver with the secondary weapon (unspecified but likely a .50 caliber machine-gun) in the commander’s turret. The troop compartment behind this was large with enough space for 6-8 Marines provided with a large rear ramp. The front section allowed the vehicle to climb obstacles the LVTP-7 could not and propel it through the water just as well as the LVTP too. An added advantage was that this fore-section would also be the first part coming out of the water to draw enemy fire but, being completely unmanned, did not matter. Instead, it actually served to provide substantial protection for the aft section and in the event of the fore section being damaged the auxiliary engine could simply move the vehicle forward and use the fore section as a large shield until it could be discarded. No armor is actually specified anywhere in the design, but it is a reasonable assumption to assign values comparable to the LVTP-7.
Floating in the water, the fore section with the water jets tilted upwards sharply directing the water jets down whilst the main vehicle was relatively level behind it. Poking out just above the water from this fore section was another weapon, remotely operated and forming the primary weapon for the design specified only as an ‘automatic weapon’.
Putting the LVTP-77 Concept Into Reality
Tests took place at the USMC camp, Camp Lejeune, North Carolina in 1979 and 1980 with LVTP-7 being tested firstly with three new designs of bow planes to cope with the anticipated larger bow wave. These new bow planes were 54” (1,372 mm) long, 42” (1067 mm) long, and 42” (1,067 mm) with a 6” (152.4 mm) lip. The vehicles were tested for speed at Courthouse Bay and at the Battalion Maintenance Pier followed by open water tests in the open ocean off Onslow Beach, all at Camp Lejeune, compared to an unmodified vehicle and a ‘jury-rigged’ coupling between two LVTP-7s.
The results showed that the increased bow planes did not directly affect speed by reducing drag, but did reduce swamping which allowed for a small increase in speed. Regardless, however, the coupled LVTP-77 was faster by about 0.5 mph (0.8 km/h) in all conditions even without a bow plane and the method of coupling was inadequate with the vehicles too far (about 24 inches / 610 mm) apart, something known to increase drag. Had the coupling been properly constructed and fitted no doubt further improvement could have been made.
The tests concluded that a new bow plane should be designed and made for the LVTP-7A1 to reduce the swamping experienced at even 6.5mph in calm waters and further work should be done on the coupled LVTP idea. The gap between the vehicles was going to be a problem though, the requirement for the ramp to still lower guaranteed the vehicles had to be too far apart to make complete use of the extra efficiency when in the water.
Either way, the coupled LVTP7 idea was dropped. The benefits were obvious, but simply did not outweigh some of the associated problems and the money to develop the system was not available. As a result, the whole project was terminated. It is assumed that the two LVTP-7’s modified with the coupling were simply returned to USMC service straight after the trials. The articulated LVT was even less successful. Despite being the logical solution to the coupling development, it never got past the proposal stage. The LVTP-7 was serviceable and available and budgets, seemingly, were better spent elsewhere rather than on a relatively small improvement to an existing vehicle.
Illustration of the LVTP-77 Cybernetically Coupled Amphibian produced by Tank Encyclopedia’s own David Bocquelet
|Total weight, battle ready||103,968lbs (47.2 tonnes) to 106,500lbs (48.3 tonnes)|
|Crew||Minimum 1 (driver), ideally 2 (one in each vehicle)|
Specifications (Articulated LVT)
|Dimensions||33 ft x 14 ft x 19 ft (10.06m x 4.27 x 5.79 meters)|
|Total weight, battle ready||54,000lbs (24.5 tonnes) fore section 15,200lbs (6.9 tonnes), and aft section 38,800lbs (17.6 tonnes)|
|Crew||Minimum 1 (driver)|
|Propulsion||main engine LCR-V903 890hp, auxiliary engine 100hp|
|Armament||primary automatic weapon in fore section controlled remotely, secondary weapon on commander’s turret on aft section|
Cybernetically Coupled Research Vehicle. (1974). Ronald Beck and Irmin Kamm, Stevens Institute of Technology, USA
50 years of the International Society for Terrain-Vehicle Systems: Ground Vehicle Mobility, Modeling and Simulation ar TACOM-TARDEC ‘A Brief History’ (2012). Dr. Peter Kiss and Dr. Sally Shoop Editors. International Society for Terrain-Vehicle Systems, Germany
The Water Performance of Single and Coupled LVTP7’s, with and without bow plane extensions. (1980). Irmin Kamm and Jan Nazalwicz. Ship Research and Development Center, Office of Naval Research, Department of the Navy.
Drag Studies of Coupled Amphibians. (1966). R.L. Van Dyck and I. R. Ehrlich. Office of Naval Research, Department of the Navy.
Department of Defense Appropriations for Fiscal Year 1973. (1973). United States Congress Appropriations Committee
Analysis of Obstacle Negotiation by Articulated Tracked Vehicles: The State of the Art. (1981). Peter Brady. Naval ship Research and Development Center, Office of Naval Research
High Speed Wheeled Amphibian: A Concept Study. (1969). C. J. Nuttall and Irmin Kamm. Davidson Laboratory, Stevens Institute of Technology, USA
An Evaluation of the Coupled LVT Concept. (1979). Irmin Kamm, Peter Brown, and Peter Brady. David Taylor Naval Ship Research and Development Center, Stevens Institute of Technology, USA