Assault Amphibian Personnel Carrier LVTPX-12

United States of America (1964-1982)
Amphibious Landing Vehicle – 15 Built

The United States Marine Corps (USMC) has, as core element of its role, the task of assaulting enemy-held coastlines. In order to fulfill this obligation, they required a form of transport that could get them from the landing ship off the coast to shore quickly and safely. It needed to be fast in the water as it was vulnerable to enemy fire with nowhere to hide and had to protect the occupants to get them onto the beach. It should also be able to deliver supporting firepower to support the Marines in their attack. All that sounds simple enough, but the combination of these demanding requirements was a complex juggling skill to balance the competing requirements.

A Modern Amphibian – 1964

The existing fleet of amphibians dated back to the Second World War and were obsolete. The need of the Marine Corps had not gone away so a new vehicle was needed. As a result the Bureau of Ships (a Department of the Navy) issued a preliminary specification on 23rd January 1964 for an Assault Amphibian Armored Personnel Carrier with a minimum requirement for a forward water speed of 8 mph (12.7 km/h) from tracks or 10 mph (16 km/h) from auxiliary power, a reverse (backing) speed in water for 3.5 mph (6.3 km/h). The vehicle was to carry enough fuel for 7 hours at 8 mph and to manage a forward speed on land of 30 mph (48.3 kmh).

The dimensions of the vehicle were not to exceed 26’ (7.9 m) long, a beam (width) of not more than 10.5’ (3.2 m), and a deck height of 8.5’ or less (2.6 m). Space was important and the inside of the vehicle had to provide a space of at least 14’ (4.3 m) long and 6’ (1.8 m) wide sufficient for a team of 25 marines with field equipment.

Weight was to be kept at a minimum but not at the expense of armor. The LVTPX-12 was to be an amphibious assault vehicle, so the gunner had to have an efficient view, and the protection had to be enough against 99% of 105 mm air burst shrapnel at 50 feet (15.2 m). Armament was to consist of either a 20 mm cannon or 7.62 mm machine gun mounted in a turret. The US Marine Corps followed this 1964 requirement with their own in March of that year for a new Landing Vehicle Tracked (LVT) to replace their LVTP-5’s.

Several ideas for construction were entertained, and the firm of Chrysler was one of them, and was awarded contract number 4777 for their work. Engineers at Chrysler looked afresh at all areas of amphibian vehicle design with work on the project divided into several phases, with Phase 1 running from June 1964 to May 1965. Three specific area required attention for the design: water speed, armor protection, and the transmission.

Planning

Various options for a layout with a single engine were considered including:

• Single engine and transmission forward
• Single engine and transmission aft
• Single engine forward, transmission aft
• Single engine aft, transmission forward
• Double engines anywhere along the length

Although, in theory, the troops could be unloaded through a front opening door, this exposed them to enemy fire, and consequently the choice, following advice from the US Marine Corps, was an engine and transmission forward design allowing the crew to disembark via a power ramp at the rear covered by fire from the vehicle.

A double or dual engine design permitted more flexibility, being able to put the engines anywhere in the vehicle making the trim much easier to manage. The only engine narrow enough to fit was the GM6-71T but each one produced less power than a 12V71T, just 460 hp. A pair of GM6-71T’s though would produce 800 hp which was plenty, but came at a higher price as together they were heavier and were not as efficient.

The only advantage of the dual engines therefore was an ease of control and with that, the idea was dumped. When consideration was given to transmissions, either Mechanical, Hydrostatic, or Electric, only two engines were under consideration: the 12V71T or the 8V71T.

Modelling

More than 100 separate models of various types and modifications of the design were produced over the course of testing, including models of just the bow and stern respectively to find the optimum shape. A single box shaped model was made of a 5 road wheel layout to which various shaped box and stern sections could be attached for trials. All these various designs meant that the length and exact water-trim level varied each time. Only the width, at 10.3’ (3.14m) remained the same, although the trials were conducted with a ⅕ scale wooden model rather than a full size pickup. Some of the trials also involved a beam reduction of 20% with a much thinner vehicle to see if that would deliver the water speed improvements demanded. The narrow design caused a lot of problems, the wheels had to have very little travel, just 9” (229mm), and the whole wheel and suspension arrangement was much more complex. This also made handling harder and less efficient. There was insufficient width for two propellers, so only a single propeller could be used, which would mean a loss of ground clearance too. The marginal improvements in performance in the water were not worth the compromises required and the idea was dropped.

Testing

Testing of the models was conducted at the Ship Hydrodynamics Laboratory at the University of Michigan, and involved not just the various LVTPX-12 configurations, but also the LVTPX-11, LVTPX-2 and LVTPX-7. The model tests showed that very small scale models gave unreliable results and no model smaller than ⅕ scale should be used for testing. They did however, confirm the final hull shape required and the most efficient method of propulsion in water.

Water Propulsion

Two options for propulsion of the LVTPX-12 were considered; propulsion in water by the tracks alone, and propulsion by other means, such as bow or stern propellers. Additional consideration was given to the use of hydrojets too.

For the tracks driving the vehicle in water, 5 different types of grousers ranging from 1.12” (28.5 mm) to 1.25” (31.8 mm) wide were tested to find the most efficient type.

The grouser tests showed that increasing the height of the grouser without increasing the width did not increase the efficiency of the vehicle in water, as it simply increased the friction instead. Wider grousers, likewise, did not improve performance either, although grouser number one was marginally better than all of the others. It was also found that, just like with the LVTP-5, the space between the hull and return portion of the track had to be as small as possible and blocked or the returning track would rob the forward track of motive power as it was effectively driving the vehicle in the opposite direction. An attempt of the LVTPX12 model to alleviate this with a completely covered side track made the problem even worse; large side plates did not help and neither did cutting holes in them. If a fender was to be used it would have to be small and only on the forward portion of the track and were essential to meet the design speeds wanted.

Precisely the opposite was true using the propellers to drive the vehicle. Even these small front fenders were harmful to water speed. Therefore, the LVTPX-12 would either be a track driven short-front fender design or a non-fendered propeller driven machine.

One peculiar finding was the use of a stern plane. Contrary to a first impression, the addition of a large flat stern plane placed flat along the back of the tracks, full width, actually improved performance in the water even though it actually interrupted the flow of water. The engineers at Chrysler did not know why this was but found that this had first been suggested in 1860 by Arthur Rigg in Great Britain to improve the efficiency of paddle wheels on ships.

Having looked at the stern plane, the engineers looked at bow vanes and the first attempt involved a design copying that on the LVTP-7. This plane was supposed to prevent submergence of the LVTP-7 at speeds of up to 10 mph, although even in a calm sea the real vehicle struggled to manage 7mph without serious swamping and even less in Sea-State 2 (2 foot waves). The same bow vane fitted toot the LVTPX-12 did not improve performance in the water at all and was actually a hinderance based on a loading of 59,000lbs (26.8 tonnes). The outcome was that a bow plane was pointless and the vehicle was simply better served with a boat shaped bow. Stern shape was even simpler. Because the only bow shape acceptable had to be boat shaped it extended the vehicle to the maximum 26’ (7.92 m), the stern shape could not be boat-shaped. Any angle less than 15 degrees was equally bad and the conclusion was simply to make it square to maximize space instead.

On the question of propellers, the single 27” (686 mm) Kort propeller was found to be highly satisfactory for water propulsion, but caused other problems. It had to be fitted as low as possible for maximum propulsive effort, but it reduced the ground clearance to just 16” (406mm), although a secure stowed position for it was provided. The significant advantage of a single propellor was that it could easily be used for steering the LVTPX-12, but during sharp turns increased the risk of capsize. To solve this capsizing issue, the solution was two propellers with one on each side. Although they would be vulnerable to damage as they projected outside of the width of the vehicle, they would not affect the ground clearance and even should both fail the vehicle would use its tracks to manage 6mph. Controllable steering for a twin propeller design would necessitate the use of controllable pitch propellers.

A better form of steering for the LVTPX-12 was by means of electrically driven 7.5” (191 mm) diameter hydrojets fitted within the sponsons of the machine. These small hydrojets extended just 39” (991mm) along the inside of the sponson weighing just 87 lbs (39.5kg) each.

It was recommended, however, that later some kind of reactive steering should be incorporated within the track drive. For the initial recommendation from Chrysler, the choice was to use twin propellers and vary the pitch to steer the machine. This saved weight and space and allowed for 13 hours of water operation at 8mph.

The Final Designs

After all of the development work, there were 5 main possibilities for the LVTPX12 recommended by Chrysler, all with the same basic dimensions, 26’ long by 10.5’ wide by 8.5’ high (7.9m x 3.2 m x 2.6m), and all of which met the requirements from the Navy, although the report from Chrysler was clear that only concepts 1 or 2 were ideal:

• Concept 1: Track Propelled, 12V71T engine
• Concept 2: Auxiliary Propelled, 12V71T engine
• Concept 3: Auxiliary Propelled, 8V53T engine
• Concept 4: Auxiliary Propelled, 8V71T engine
• Concept 5: Auxiliary Propelled, twin AC-350C engine

The final designs were based upon a final vehicle weight (unladen) of 45,000 lbs (20.4 tonnes) although some of them went as high as 53,670 lbs (24.3 tonnes) and 26 feet (7.9 m) long. Bow fenders would wrap 150 degrees around the front sprockets with the ability to retract for operation on land. The design would have stern baffles with a contravane extending 4” (101.6 mm), also retractable for land use.

Side skirts were also to be added. They improved water speed and it did not matter what they were made of, just so long as they were smooth and extended to the level of the bottom of the hull. The type 3 bow shape with type 1 grouser were also to be used.

Original requirement was 8 mph (12.8 km/h), but the design was calculated to be able to achieve 10.7 mph (17.2 km/h), although testing only went as high as 9.55 mph (15.4 km/h).

Protection

The hull was to use a modern generation of high hardness steel developed by Chrysler Defense Engineering providing the ballistic protection required making the vehicle lighter than it would be with an aluminium hull, as it permitted the hull to be semi-monocoque. Consideration had been given to a dual-hardness steel hull, but although this could save 1090 lbs (494 kg) in weight, the costs involved were considered too high to be justifiable. The same went for the idea of using titanium or ceramics within the armor; the costs simply did not justify the small additional benefits.

The top of the hull, including the cargo hatch, was to be made from military-grade steel (MIL-S-12560), 0.375” (9.5mm) thick with a nylon blanket backing. The sides, front and stern were made from BHN-500 steel, 0.31” (8mm) thick, and the bottom from military-grade steel (MIL-S-12560) too, 0.375” (9.5mm) thick, with structural elements made from either US Steel’s T1 high strength alloy or Cor-Ten low alloy steel as they were far more resistant to corrosion. The flooring inside the vehicle was to be aluminium paneling as were the fenders and external baffles

Protection had been required to defend against shell fragments and small arms fire only, and the use of a steel hull met these requirements. As for armament, Chrysler exceeded the requirements for either a 20mm or a machine gun by adding both. The 20mm Hispano-Suiza cannon and machine-gun were to be mounted coaxially, forwards on the hull in a small 360º rotating turret with 12º of depression. Just 325 rounds of ammunition for the cannon could be carried, but, along with 700 rounds for the 7.62mm machine-gun, provided adequate firepower for the vehicle. Spent casings from the cannon were ejected overboard via an ejection port, but spent cases from the machine-gun were simply to be collected internally. Additional firepower could be given by the mounted troops for whom small arms ports were provided.

Suspension

During trials, the multiple small wheels had been shown to have far too little travel, so the vehicle to be built was to have 6 large rubber-tyred road wheels on each side instead. These were supported on torsilastic springs cantilevered out from the sides of the vehicle. Drive was delivered to the sprockets at the back, pulling the rubber padded single pin track. The front idler was compensated to account for track movement during amphibious driving and was used to tension the track too.

Transmission

The transmission for the LVTPX-12 design was a complex problem. The engine would have to deliver in excess of 600h p (recommended to be 800 hp) through the tracks to power the vehicle, which would still only deliver a top speed of 16 mph (25.7 km/h) on land. Various solutions over different types of transmission were considered, all of which were going to be more suitable than using the transmission from a tank like the LVTP-5 had done.

Chrysler concluded that for the final design a new transmission should be developed by a Phase II contractor of a type recommended by Chrysler. Chrysler were prepared to develop this new transmission but not out of their funds. They were clear it would have to be a government-funded and owned project only.

Crew

The vehicle itself was to feature just three crew members. A driver positioned in the front left, an assistant driver in the front right and the commander acting as the gunner stood who centrally with his head in the small turret at the front. Seating at the back was provided for 25 troops on benches, although, had the LVTPX-12 been accepted for service, it is likely this area would have been adapted for a variety of other uses too from mortar carrier to recovery vehicle all on the same platform. Access for personnel was via either the large cargo hatch above the troop compartment, the large rear powered ramp, or crew hatches. Two hatches were at the front, with one each for the driver and co-driver, and there were two emergency escape hatches in the vehicle with one on each side of the hull.

Outcomes

Chrysler’s model LVTPX-12 development team had been tasked with producing an amphibian vehicle capable of 8 mph and yet was calculated to be able to manage over 10 mph. In all areas, this design surpassed the LVTP-5, providing for improved armament and performance. The problems of making an amphibian APC were obvious during the development. A large amphibian was simply ill suited to both land and sea operations. Either too heavy for the sea or ill protected for the land. Big, bulky and cumbersome, these vehicles, packed with troops, were to be the amphibious assault vehicles for the US Marine Corps. In comparison with the LVTP5-A1, the LVTPX12 was considered by Chrysler to be a better design and still manageable within the budget constraints imposed by the military.

The US military continued with development of the amphibian as fighting in Vietnam had shown the extreme vulnerability of these vehicles to mines in particular and the requirements of 1964 had changed from high speed beach assault to more emphasis on land operations. The LVTPX-12 design was therefore modified to a more modest vehicle and subsequently the LVTPX-12 was manufactured as a prototype first in September 1967 and later as a batch of 14 more prototypes by Food Machinery Corporation (FMC), which were finished by 1969. It was still the ‘Landing Vehicle Tracked LVTPX-12’, but not for long.

The engine and transmission had been relocated and it was only an LVTPX-12 in name. At least three prototypes and possibly as many as 10 were manufactured under the LVTPX-12 name, but are easily mistaken for the LVTP-7, as the design was modified in favor of a smaller size and weight and refined further. The finished vehicles were smaller, but the work on the original LVTPX-12 was not wasted. It produced valuable experience and lessons for the military.

One prototype vehicle which underwent tests around June 1969 was reported as being made from aluminium instead of steel as originally planned. This switch to aluminium is also confirmed by Hunnicutt, who states that the LVTPX-12s made for trials were made from 5083 aluminium, just like the M113 APC. The Chrysler prefered engine was changed for the smaller 8V53T diesel producing just 400 hp and the torsilastic suspension was changed to torsion bars.

The LVTPX-12 program might not have been successful in itself, but it continued into what was to become the LVTP-7 program instead. By this time, very little of the original form of the LVTPX-12 remained. Gone were the boat shaped front, the central front turret, and side egress doors, and the visual similarities with the LVTP-7 make identification and tracking of the vehicle from this point almost impossible.

Following successful trials of the LVTPX-12 at Aberdeen (Maryland), Yuma Proving Grounds (Arizona), Fort Greely (Alaska), and in Panama it was accepted for service, and the LVTPX-12 name was almost completely dead by 1969, when it was officially redesignated LVTP-7 in this new form and it entered service with the US Marine Corps in 1972, replacing the LVTP-5 completely by 1974.

Epilogue

The fate of these prototypes is not known, although two did get modified into test beds of the LVTRX-2 recovery vehicle with a 30,000 lb (13.6 tonne) winch fitted to the vehicle roof. The weapons cupola was removed at that point. Other experiments were the LVTCX-2 as a Command Variant and LVTEX-3 as an Engineering variant. A final variant planned but never built was the LVTHX-5 with a turret mounted 105mm gun, but all of these had little to do with the original LVTPX-12 and were now firmly in the realm of the LVTP-7.

Strangely, although the LVTPX-12 was ‘dead’ by 1969, the name crops back up again in 1982 with Congress allocating money to design and construct 2 sets of hydropneumatic suspension units for the LVTPX-12 along with clear armor inserts, suggesting the vehicle was still serving, performing a continuing role for testing and evaluation.

At least one of the original prototype vehicles survives at the Allegheny Arms and Armor Museum, Pennsylvania.

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Illustration of the Assault Amphibian Personnel Carrier LVTPX-12 produced by Jarosław Janas, funded by our Patreon Campaign.

Specifications LVTPX-12 Track-Propelled Version
Dimensions (L-w-H)	26 x 10.6 x 8.6 feet (7.9 x 3.2 x 2.6 meters)
Total weight, battle ready	51,990lb (23.6 tonnes, combat laden) with 10,000lb (4.5 tonnes) payload
Crew	3 (Driver, Assistant Driver, Commander) + 25 troops
Propulsion	GM 12V71T 800hp
Maximum speed	> 10 mph (16.1 km/h) in water, 30 mph (48.3 km/h) on land. < 5 mph (8 km/h) in reverse in water
Armament	1 x 20mm Hispano Suiza cannon and 1 x 7.62mm machine-gun
Armor	Welded steel up to 8mm thick

Specifications LVTPX-12 Auxiliary-Propelled Version
Dimensions (L-w-H)	26 x 10.6 x 8.6 feet (7.9 x 3.2 x 2.6 meters)
Total weight, battle ready	53,670lb (24.3 tonnes, combat laden) with 10,000lb (4.5 tonnes) payload
Crew	3 (Driver, Assistant Driver, Commander) + 25 troops
Propulsion	GM 12V71T 800hp, 8V53T 400hp, or 8V71T 530hp
Maximum speed	> 10 mph (16.1 km/h) in water, 30 mph (48.3 km/h) on land. < 5 mph (8 km/h) in reverse in water
Armament	1 x 20mm Hispano Suiza cannon and 1 x 7.62mm machine-gun
Armor	Welded steel up to 8mm thick

Specifications LVTPX-12 (LVTP-7) Prototype
Dimensions (L-w-H)	26 x 10.6 x 8.6 feet (7.9 x 3.2 x 2.6 meters)
Total weight, battle ready	48,500 lbs (22 tonnes)
Crew	3 (Driver, Assistant Driver, Commander) + 24 troops
Propulsion	GM 8V53T 400hp diesel
Maximum speed	40mph (64.3 km/h) land 8.4mph (13.5 km/h) water
Armament	1 x 1 x .50 cal. (12.7 mm) Browning M2 Heavy machine gun
Armor	Welded 5083 aluminium

Sources

Final Engineering Report on the LVTPX12, Vol.1 Technical Study. (1965 ). Chrysler Corporation, Detroit.
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.
The Stability of the amphibious Craft LVTPX-12 in Waves and Surf. (1968). Robert Patterson. Massachusetts Institute of Technology, Department of Naval Architecture.
Evaluation of LVA Full-Scale Hydrodynamic Vehicle Motion Effects on Personnel Performance. (1979). William Stinson. Naval Personnel Research and Development Center.
Bureau of Ships Preliminary Specification for Assault Amphibian Personnel Carrier (LVTPX12), (1964). Bureau of Ships
US Marine Corps FY82 Exploratory Development Program. (1982). Marine corps Development and Education Command
LVTPX-12 Accepted. (November 1967). J.H. Alexander. Marine Corps Gazette Vol. 51, Issue 11
Special Test for LVTPX-12. (June 1969). Marine corps Gazette Vol.53, Issue 6.
Amtracs: US Amphibious Assault Vehicles. (1999). Steven Zaloga, Terry Handler, Mike Badrocke. Osprey New Vanguard No.30
Bradley: A history of American fighting and support vehicles. (1999) R.P. Hunnicutt, Presidio Press

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