Category: Cold War US MBT Prototypes

United States of America (1984-1987)
MBT – Models Only

In 1984, the US military was considering the problems connected with a new range of vehicles, such as the new M1 Abrams main battle tank and M2 Bradley Infantry Fighting Vehicle (IFV). As part of the evaluation of trends in future vehicles, a commission looked into the potential for electric drive systems for a 40-ton (36.3 tonne) (tank) and 19.5-ton (17.7 tonne) (APC/IFV) platform.

The US Army’s Tank Automotive Command (TACOM) issued a contract to General Dynamics Land Systems for this project – to evaluate existing electric drive technologies to use in future vehicles. This was contract number DAAE07-84-C-RO16 divided into 2 phases – a third phase was added later under contract modification P00006.

The goal was roughly that of evaluating the ‘new’ (electric drive for vehicles predates armored vehicle) technology available across a variety of platforms for what it may offer for further development. What it actually generated was the realization that electric-drive fighting vehicles were not only possible but had some valuable features worth exploring, especially with regards to a series of heavy IFV platforms. However, like so many other studies, this work faded away and the design work was abandoned. To this day, in 2020, the M1 Abrams remains in service with a conventional power plant along with numerous other armored vehicles in the US inventory. Despite the billions of dollars spent, to date, the US military has yet to capitalize on the potential of electric-drive vehicles.

Phase I: A survey of existing technology (document JU-84-04057-002)
Phase II: Generation of concept vehicles with electric drive
Phase III: A parametric study and evaluation with selection of 3 recommended concepts for further consideration

General Dynamics had actually been looking into the potential of electric drive systems as early as 1981, producing electric-drive concept vehicles for various other vehicle projects. It also had possession of a 8 x 8 wheeled, 15-ton (13.6 tonne) Electric Vehicle Test Bed (EVTB) it had paid for itself in order to test and validate electric drive.

General Dynamics EVTB (also known as the Advanced Hybrid Electric Drive vehicle). Source: DiSante and Paschen, and Khalil

The timetable for the project was for Phase I to be concluded by the end of 1984. In the end, the report on this phase was finished in July 1984 and then published in January 1985. By this time the second phase was already underway with an expected conclusion date in the latter half of 1985 to be followed by another report and, starting in the middle of 1986, Phase III running through into the start of 1987.

Why Electric Drive?

The potential of electrical drive systems was experimented with on tanks as far back as WW1. An electrical transmission offered the designer a significant freeing up of the internal layout of an armored vehicle, as the drive motors did not have to be next to the engine, and the ability to deliver continuous, reliable power in preference to mechanical systems. This is primarily because an electrical drive system has far fewer moving parts and bearing surfaces than a mechanical system. There are also major advantages, not the least of which being volume. An electrical system could be smaller than the equivalent mechanical system and smaller volume meant more internal volume in a vehicle for other things and/or a reduction in the amount which needed to be protected by armor – that means less weight too. Electrical transmissions are also quieter due to the absence of gearing and driveshafts and offer the not insignificant potential to provide electrical power for the vehicle’s systems.

Study Concepts

Some 38 possible concepts across the 19.5 (17.7 tonne) and 40-ton (36.3 tonne) vehicles were considered over four basic vehicle considerations. Plans from various companies and one university submitted concept plans for the program namely: Westinghouse, ACEC (Ateliers de Constructions Electriques de Charleroi), Unique Mobility, Garrett, Jarret, and the University of Michigan. All of the options were to consider a scheme for a baseline vehicle.

Baseline 40-ton electric drive vehicle. Source: GDLS

Baseline Vehicle Description

The baseline vehicle for the EDMBT was very similar in external hull layout to the M1 Abrams, with the automotive elements placed under a raised engine deck at the back of the tank. It had a relatively conventional external shape except that all of the crew were in the hull. Seven wheels on each side were drawn mounted on what appear to be arms, suggesting that it probably kept the same style of torsion bar suspension as on the Abrams. The most noticeable difference though is the lack of a turret, as the vehicle adopted a crewless weapon mount on the roof. This is the only weapon carried on the vehicle and is shown as an automatically loaded 155 mm STAFF (Small Target Fire and Forget) cannon with an elevation range of -7 to +20. Fitted with a single 7.62 mm coaxial machine gun, the gun carries just 15 rounds in an unusual T-shaped bustle at the back. A further 18 rounds were to be carried in the front right of the hull, alongside the driver. No armor was described but, unlike the Abrams, it had a pronounced slope to the glacis. One important note from the drawing is the location of the primary fuel tank containing 420 liters at the front, which would have added to the frontal protection. Protection levels could therefore reasonably be assumed to be at least no less across the frontal arc of the hull as on the Abrams. It is important to remember though that the vehicle shown in the drawing (LK10833), whilst more than a mere doodle of a viable tank design, should only be taken as an illustration of a possible future tank. The power plant work could just as legitimately be refitted to the Abrams – the key part of the study was not this tank per se, but a study to evaluate these power systems for tank propulsion.

40-ton (36.3 tonnes) vehicle Concepts

With four (five including one minor amendment) configurations being considered, the design task was simplified by the specification of the engines to be used. Although the AD-1000 advanced diesel engine generating 1,000 hp was selected, other options were considered across the 19.5 ton (17.7 tonnes) and 40-ton (36.3 tonnes) projects for alternative forms of power. However, in the end, other than the possibility of switching to a petrol-turbine the existing diesel engines were the only technology mature enough to be considered.

Each design was identified by concept number followed by a design number, for example ‘I-3’’ was Configuration 1 Design 3, whereas II-4 was Configuration 2 Design 4, and so on. Vehicle concepts selected to go forward from theoretical design to a drawing stage were all allocated a drawing number starting AD-8432-xxxx.

For the 40-ton (36.3 tonnes) concept, just two candidates were identified for further study – these were I-3 and IV-2. I-3 was designed by Garret and used a larger version of the same system as I-10 for the 19.5-ton (17.7 tonne) vehicle. The second was IV-2 from Unique Mobility which used scaled-up versions of the dual-path AC permanent magnet system it had proposed for the 19.5-ton (17.7 tonne) IV-2 concept.

Garret Concept I-3 40-ton (36.3 tonnes) Application

The drive system for the 40-ton (36.3 tonnes) vehicle application was the same as that of the Garret I-10 19.5 ton (17.7 tonne) vehicle, namely that it used two different paths for the delivery of automotive power, one mechanical and one electrical. The electrical system alone delivered power for speeds from 0 to 15 mph (24 km/h) and, when more power was needed to go above that, the mechanical system was unlocked and coupled to the electrical system. The control unit then controlled the power between these two units.

The electrical power was provided by a permanent magnet AC generator driven by the engine rectified to DC and then inverted in order to provide power to the traction motors. The generator was an oil-cooled Garret-type rated at 400 hp and rotated at 18,000 rpm with 93.5% efficiency. The oil-cooled rectifier for this system operated at 685 Volts DC at 98% efficiency and connected to a 284 Volt AC inverter operating at 96% efficiency.

The traction motors used rare-earth metal magnets made from neodymium which removed the problem of the cobalt-type magnets as the US had adequate stocks of neodymium. The cost of 400 of these power units for the 19.5 ton concepts was estimated to be 1985 US$145,000 per unit (just under US$350,000 in 2020 values), but for the 40-ton (36.3 tonnes) concept, the cost would be around 1985 US$240,000 (over US$575,000 in 2020 values) as it used two traction motors for each final drive.

The Garret traction motors delivered 192 hp each and were able to operate at 200% for up to 30 seconds and deliver power to the final drive units which operated at a 4:1 reduction ratio.

Cooling was an important factor in all of the systems and calculations for the Garret systems (both I-10 for the 19.5 ton and I-3 for the 40-ton) were made. For the 40-ton (36.3 tonnes) vehicle, a maximum heat rejection of 8,737 BTU/Min (9,218 KJ/ Min) was needed.

Analysis by GDLS across the 40-ton (36.3 tonnes) drive systems showed that there would be 855 hp available. The Garrett system was the better of the two for the 40-ton (36.3 tonnes) vehicle and was capable of forward acceleration from 0 to 20 mph (32.2 km/h) in under 7 seconds and reverse acceleration from 0 to 10 mph (16.1 km/h) in under 5 seconds.

Conclusion

When this study was being done, the M1 Abrams was still a relatively new tank in service with the US military. The Soviet Union was still the major enemy to worry about with potential hordes of tanks able to swamp the armies of NATO in Europe still a constant threat in the minds of the NATO Generals. Lacking the option for a quantitative advantage over the Soviets, a qualitative advantage was sought and part of that grand quest was the goal for a tank with greater protection and more firepower than any Soviet contemporary. Just as the M1 Abrams had entered service to provide that advantage, the plan was simply to make an even better vehicle. Here, a turretless design with an autoloader that offered a small target and was capable of destroying any Soviet threat, and which also had the design flexibility offered by an electric drive, was seen as a promising approach. This vehicle was certainly not the only concept at the time to try and shed the weight of a turret on the Abrams or to increase its mobility and firepower. However, no electric drive main battle tank was ever produced along these lines, as the need for such an expensive system expired along with the Soviet Union.

Of the 38 possibilities for a drive system and layout for a 19.5 ton vehicle just three systems had been identified as being suitable for investigation or development; the Belgian ACEC DC system, the Garret AC permanent magnet drive, and the Unique Mobility dual-path AC permanent magnet drive system. Yet, for this heavier, 40-ton (36.3 tonnes) concept MBT design just two ideas made the cut, the Garret (I-3) using a larger version of the system proposed and selected as a potential system for the 19.5 ton (17.7 tonne) vehicle (I-10), and the Unique Mobility concept (IV-2), once more using a scaled-up version of its system proposed for the 19.5-ton (17.7 tonne) (IV-2) concept. Clearly from a logistics point of view and likely from a cost point of view as well any system selected for this 40-ton (36.3 tonnes) project should really have as much in common with the system on the 19.5-ton (17.7 tonne) project as well. Both projects, however, came to nothing and were dropped.

The potential advantages of an electric drive have still not yet been fully exploited by the US military or other tier 1 militaries around the world. With the prospect of freeing up additional internal volume, allowing a new configuration layout, and offering improved performance, a new generation of electric-drive AFVs is possible but unlikely as militaries opt to stick to traditional tried and tested propulsion systems.

Sources

GDLS. (1987). Electric Drive Study Final Report – Contract DAAE07-84-C-RO16. US Army Tank Automotive Command Research, Development and Engineering Center, Michigan, USA
DiSante, P. Paschen, J. (2003). Hybrid Drive Partnerships Keep the Army on the Right Road. RDECOM Magazine June 2003
Khalil, G. (2011). TARDEC Hybrid Electric Technology Program. TARDEC

United States of America (1962-1963)
Main Battle Tank – None Built

By 1962, the 105 mm Gun Tank M60 was still a new tank in service with the US Army but, just like any current system, was already being considered for future replacement, redevelopment, and upgrading. The latest generation of Soviet tanks were well armed and armored, and much smaller than their American counterparts. A new tank would be needed to deal with this increasingly dangerous threat. During the height of the Cold War in 1962, the Armor Association of the United States Army held an open competition for the design of a new tank. Of the designs submitted, some were clearly better thought-out and practical in terms of production, cost, and combat effectiveness than others.

Nonetheless, the competition formed part of one means by which the US Army could assess new and novel ideas for the potential next generation of tanks. One such tank, perhaps the most outlandish of the top four finishers, came from the fertile mind of Everett Philip Ridlon of Hibbing, Minnesota. Ridlon, an electrical engineer by trade. He submitted a quad-track tank with a crewless turret propelled by a hybrid-drive system based on the M60.

Suspension

Probably the most obvious thing about Ridlon’s design is the suspension. Six wheels on each side divided into groups of three with a strong angling at the front and back respectively. Assuming the raised wheel at the front of the lead unit and rear of the rear-most unit were the drive sprockets this provided a strong degree of redundancy in the design so that should one unit become damaged by enemy fire or a land mine or accident, the vehicle would not be immobilised. Each wheel was held on a single arm providing a good degree of movement and is reminiscent of the suspension arms of the M60. If it was just like those on the M60, then the arms would be hydraulically damped in their movement.

Hull

The design of this vehicle was not going to need a new tank hull, as Ridlon simply planned to reuse the lower hulls from the M60. He proposed stripping out of the original drive components and fitting a new engine and the motors. On each side in the middle, where the two track units were closest to each other, the road wheels would be on the ground, creating a large empty space above them. This meant that the new upper hull of the M60 donor tank was going to need to bulge out across the side to improve the ballistic protection in that area.

Within the hull would sit the two crew along with the myriad of engines and motors proposed. In amongst all of this would be compartmentalised storage for fuel, compressed air, hydraulic fluid, water, fire extinguishers, and other items which were seen as being able to add to the protective structures around the crew.

Armor

The lower hull would be that of an M60, but the upper half would be remade to feature a large curved section across the top half bulging out at the sides to make use of the low section above the ends of both track units on each side. Further, Ridlon wanted the armor to be made in sections so that, as it was hit by enemy shells, the outer sections would break away on impact. To accomplish this, he wanted the outer sections of the armor to be made ‘soft’, with ‘hard’ armor on the inside, in what he describes as a “live” system. Further, he stated “the outer armor is composed of ribbed interlocking plates which give greater depth of armor and less weight as well as catch the projectile higher on the sides and thus disperse impact energy over a larger surface area”. The whole plan was not practical in that sense, but it could be considered as modular as each damaged section could at least be replaced.

Firepower

The drawing of the turret and main armament is almost comically poor, with an impossibly small turret described as a ball-type turret. In this turret were to be two machine guns and either an automatic cannon or an automated one. The turret, as drawn, certainly appears far too small to accommodate any men but, if it is considered to be a remote turret, and a ball-type turret at that, it seems a little less ridiculous. Those turret-mounted weapons were not the end of the arsenal the tank would carry, as Ridlon also proposed rocket tubes should be placed in the upper hull, capable of attacking ground or airborne targets.

Automotive

Ridlon, somewhat preciently for a US Main Battle Tank, proposed the use of a very small ‘gas turbine’, that is, a turbine-type engine running on petrol. This engine was not to directly drive the tank though, but was to drive a series of small high-speed homopolar generators. Each of these generators would be spread around the vehicle to minimize the chance of a single one becoming damaged and incapacitating the vehicle. Ridlon envisaged this system being duplicated for all military vehicles, as the humble Jeep would need just a pair of these small generators, a truck three and eight for a tank. Ridlon proposed eight small turbines working together to deliver power to thirty-two motors which powered the four sprockets which drove the tracks. The idea was that, by increasing the number of possible drive options, it would be impossible to be crippled by the loss of any one drive unit, motor or generator. The chances of all of those elements being made to work without something breaking seems highly optimistic even though it is the best part of his design considering elements of protective redundancy in the drive units to avoid being crippled and vulnerable to enemy fire. Rearranging the automotive elements of the tank to multiple small motors and generators would have made significant changes to the internal layout possible but that was beyond Ridlon’s skills as a designer, which perhaps explains why the drawing was so poor and the ideas on armor so poorly conceived.

Conclusion

Ridlon’s design took third place in the Armor Association’s competition, behind the Forsyth brothers’ coupled-tank and Eischen’s MBT, yet is drawn and described very crudely. The design appears utterly impractical with multiple complex systems, yet was held in high regard by the Armor judges. The question is why?

Perhaps it was a combination of novelties of the ball unmanned ball turret, the hybrid drive, the compartmentalization or some or all of those, but whereas the Forsyth design was a competent and well-thought-through design, this vehicle was simply impractical and an example of fantastical thinking for the time. There was no likelihood this vehicle would ever have been built and its inclusion in third place seems surprising given other better thought out designs. Ridlon did better than his tank design did, by 1970 he was teaching at a technical college before retiring in 1992. He died of lung cancer in 2011.

Illustration of Ridlon’s Main Battle Tank, based on his original sketch, produced by Andrei Kirushkin, funded by our Patreon campaign.

Sources

Armor Magazine January-February 1963
Hibbing Daily Tribune, 12th June 2011 ‘Everett Philip ‘Babe’ Ridlon

United States of America (1948-1954)
Medium Tank – 6 Built

In the early 1950s, the United States Military began a design program to develop tanks that would replace those currently in service. The faithful M4 Sherman had begun to show its age and was in the process of being replaced by the M26 Pershing and the upgraded M46 Patton.
At their core, however, these tanks were still very much vehicles of World War II era and did not make use of newer technologies that had begun to appear. One of the tanks to spring from the design program was the Medium Tank T42. Other tanks to spring from this program included the Light Tank T41 and Heavy Tank T43. These would become the 76mm Gun Tank M41 Walker Bulldog and the 120mm Gun Tank M103 respectively.

The wooden mockup of the proposed T42. Photo: Presidio Press

Design and Development

At a meeting at Detroit Arsenal on the 28th of September 1948, specifications outlined by the United States military for a new Medium Tank were put forward. On the 2nd of December, the designation of Medium Tank T42 was secured.
The Military’s Specifications were thus:

• A weight of approximately 36 tons
• Better armor protection than the M46 but equivalent armament
• Main armament stabilization in elevation and azimuth
• An automatic loading system
• A concentric recoil system (Hollow tube around the barrel. A space-saving alternative to traditional recoil cylinders)
• Blister mounted  .30 Cal (7.62mm) machine gun on each side of the turret
• Coaxial .50 Cal (12.7mm) and .30 Cal machine guns.

A number of these initial features were based on the Prototype Light Tank T37. This included the blister mounted machine guns. Other features included a similar chassis length with five road wheels, powerplant and transmission, mudguards/sand shields over the tracks, and a turret ring diameter of 69 inches. This was the same diameter that was introduced in 1941 with the M4.
Construction of a mock-up was approved in March 1949 and reviews of this model were held in October and December, with a number of suggestions put forward to improve the design. The similarities to the T37 began to disappear. Ground contact length was increased from 122 to 127 inches (3.09 to 3.22 meters), the turret ring was widened to 73 inches and finally, the turret mounted blister machines guns were deleted.

Hull

The hull of the T42 was a combination of two parts. The forward bow portion was a single homogeneous steel casting, while the rear was a welded assembly of steel armor plates. The two halves were welded together in the middle of the tank. The casting of the upper glacis plate was 4.0 inches (101.6 mm) thick, sloped at 60 degrees.
The T42 eliminated the archaic feature of a bow machine gun and accompanying crew member. As such, the driver was alone in the hull. Room left over by the absent crew member was taken up by a 36-round ammunition rack.

One of the first T42 Prototypes. Photo: US Archives

Mobility

The T42 did retain the engine and transmission of the T37. This consisted of the Continental AOS-895 gasoline engine (AOS: Air-cooled, Opposed, Supercharged) rated at 500 horsepower, and the General Motors CD-500 cross-drive transmission. This gave the tank a top speed of 41 mph (66 km/h). The driver operated the vehicle with the Manual Control joystick, often known as the ‘Wobble Stick’.
The tank was considered underpowered, however. Tests were mounted by placing the powerplant in the hull of an unused Medium Tank T40 chassis and running it against a late model M4A3. These tests took place at the Aberdeen Proving Ground on the 7th of November 1950. The T42 proved to be only marginally more mobile than the M4, reinforcing the opinion that the tank was underpowered.
The tank ran on a five road-wheels with the drive sprocket at the rear. There were three return rollers unevenly placed along the return of the track. The wheels were attached to torsion bar suspension.

Turret

The turret was a completely new design, but not too dissimilar to that of the T41 (M41 Walker Bulldog) in length and shape. It was completely cast in construction. The lengthy turret bustle was used to store the radio set and also housing for the ventilator fan. The turret was manned by 3 crew members; Commander, Gunner, and Loader.
The outside of the turret was dominated most noticeably by the armored housings of the stereoscopic rangefinder lenses. Also known as ‘Frog’s Eyes’, this type of gun system continued to see use after the T42 on vehicles such as the M48 Patton III and M103. Atop the turret, on the right, was the commander’s vision cupola with an AA mount for a .50 cal. machine gun. The loader’s hatch was to the right of this.

Armament

It was suggested by a British liaison officer that the Ordnance QF 20-Pounder was far superior to the 90mm Tank Gun M3A1 used on the M46 Patton. Despite its use on Britain’s own Centurion and, being a more powerful weapon, the 20-Pounder was deemed unsuitable for use on a Medium Tank by the US.
The US instead opted for a newly developed 90mm gun, the T119. This gun was a vast improvement over the M3A1. Firing it’s APDS (Armor-Piercing Discarding-Sabot) round, it could punch through 11.1 inches (282mm) of homogenous steel armor, angled at 30 degrees, at a distance of 1000 yards (914.4 meters).
The main armament was complimented by the coaxially mounted Browning M1919A4 .30 cal. (7.62mm) Machine Gun and Browning M2HB .50 cal. (12.7mm) Heavy Machine Gun.

Nomenclature Change

On the 7th of November 1950, the United States Ordnance Committee instigated a change in nomenclature for tanks in the US Military. It was decided that weight designations (Light, Medium, Heavy) were no longer suitable due to changes in the way tanks were developed and employed on the battlefield, and the varying calibers now available. The caliber of the gun replaced the weight designation. For example, the T42 changed its designation from ‘Medium Tank T42′ to ’90mm Gun Tank T42’.

The Korean Tank Panic

Six prototypes were constructed and finally delivered to the Aberdeen Proving Grounds for testing on 30th December 1950. However, by this time, the Korean War had been raging for a full six months. To put it mildly, this caused a bit of a panic among the hierarchy of the US Military, and a crash program was launched to find a suitable tank to field in the conflict. The death warrant for the T42 was signed when the US Army Field Forces (AFF) declared the tank unfit for production. This did not stop the Ordnance Department though, who continued to work on the tank in the hopes that it could yet become the US Army’s next Medium Tank. Prior to this, in November, work had begun on the T42’s replacement, designated as the 90mm Gun Tank M47. Characteristics of the vehicle were outlined in January 1951.
The immediate answer to the panic was found by returning to the current tank in service, the M46 Patton. It was found that most of the T42’s issues were with its hull and the turret was found to be perfectly serviceable. As such, a program began to mount the T42 turret on the hull of the M46.
The M46 was slightly modified to accept the new turret. This modification took the form of expanding the hulls turret ring to match the turrets at 73 inches. This combination was tested with the use of an M46 hull. This vehicle was designated the M46E1. Only one was produced for tests purposes.
To bring the M46 hull up to the requirements for the M47, the angle of the 4-inch (101.6mm) upper plate was increased to 60 Degrees from the vertical. The air filter in the upper hull front was also removed, giving a better contour to the armor profile. This configuration was accepted and serialized as the Medium Tank M47 Patton II. However, it arrived too late to serve in the Korean War. The tank was declared obsolete in 1957 in the US Military but went on to see service in the arsenal of other countries military’s. In the US, the M47 was replaced in service by the 90mm Gun Tank M48 Patton III.

An early production M47. Photo: Presidio Press

The 90mm Gun Tank T42.

The 90mm Gun Tank M47 Patton II, the combination of the T42’s turret and the hull of the M46 Patton.

The Medium Tank T69 with an Oscillating Turret mounted on the Hull of T42 Prototype vehicle no. 3.
All three illustrations are by Tank Encyclopedia’s own David Bocquelet

Further Developments

The Aberdeen trials began with automotive tests. In an effort to improve the T42’s mobility problems, the prototype vehicles sent to Aberdeen were fitted with the upgraded AOS-895-3 engine and CD-500-3 transmission. Thanks to the substitution of some steel parts with aluminum, this incarnation was 500 pounds (227 kg) lighter than its predecessor. The overall performance was improved, finally meeting the original specifications and surpassing the results of the previous test using the T40 hull. The Army Field Forces were still not impressed enough. Considering the T42 still much too underpowered, they continued to refuse its adoption. By this time, their attention had shifted to the M47 and T48 (later M48) development program.
The trials did not start well for the T42. Pilot vehicle number 1 was completely destroyed by a catastrophic fire caused by a loose pin in the final drive which tore a hole in the fuel tank. This resulted in fuel spraying over hot components of the engine. The tank was a fireball in seconds. Pilot vehicle number 2 arrived at Aberdeen in April 1951 to continue the now delayed automotive tests. This vehicle was later modified to allow the installation of a new transmission, the XT-500. This necessitated a modification the rear hull. This took the form of replacing the sloping rear hull plate with a vertical one. The XT was more efficient and had a lower production cost of the CD model, with just 60 percent of the overall part-count.

The T42 with CD Transmission on the left, and XT Transmission on the right. Note the changes to the rear of the tank. Photos: Presidio Press
Despite the deletion of the blister machine guns on the turret, developers were keen to employ extra machine guns somewhere on the vehicle to compensate for the lack of a bow mounted weapon. One solution was the mounting of machine guns on the mudguards just above the idler wheels. This took the form of an armored box. The box would contain one Browning M1919A4 machine gun, 680 rounds of .30 caliber (7.62mm) ammunition, a pneumatic charger, firing solenoid and a compressed air bottle. The system would be operated by controls in the driver’s position. The guns were fixed in traverse and elevation. Though not practical to aim, it was found that the weapons provided a good suppressing fire over an area. Further development was suggested, mostly to add a degree of traverse in the guns, but it went no further. The whole concept was later completely dropped.
During the spring of 1953, in an effort to keep the T42 project alive, a plan was formed to turn it into an option for a lighter more economical tank. Modifications planned were a steel elliptical hull and flat track suspension (track return supported by road wheels, as used on tanks such as the Soviet T-54). Had these plans materialized, the vehicle would have received the designation of 90mm Gun Tank T87, or Medium Tank T87. May 1953 marked the end of this project and the whole T42 program in general. The project was officially ended in Autumn 1954.

One of the later T42 Prototypes. Photo: Presidio Press

T69, the Only Variant

One of the original specifications for the T42 was not researched until almost a year after the tank was declared unfit for service. This specification was that an autoloader would be added when such a device was available. It was found that trying to add an autoloader in a conventional turret was impractical as the breach of the gun would have to return to a 0-degree elevation angle for it to line up after every shot. For this reason, it was decided that an oscillating turret would be the best option. Oscillating turrets are divided into two parts. A lower collar attached to the turret ring, and an upper portion with the gun fixed in place. The upper portion pivots on trunnions under hydraulic power providing the gun elevation and depression. With the gun fixed in place, the autoloader has a straight path to ram in the shells.
The new turret was mounted on the hull of T42 prototype number 3 which was modified with the XT-500 transmission. The tank received the designation of 90mm Gun Tank T69, also known as Medium Tank T69. The tank took part in a number of trials but, like the T42, it was not accepted for service. It was found that the vehicle bore no advantage over conventional designs.

The T69 at aberdeen proving grounds for evaluation. Photo: Presidio Press

Survivors

No whole T42 survives today. The only way the vehicle does survive is through the uses of its parts. A single hull survives as the T69, which currently resides in storage at National Armor and Cavalry Museum, Georgia, USA. The turret can, of course, be found all over the world where there is an M47 on display.

United States of America (1951-1958)
Medium Tank – 1 Built

In the early 1950s, the United States Military began a design program to develop tanks that would replace those currently in service. The faithful M4 Sherman had begun to show its age and was in the process of being replaced by the M26 Pershing and the upgraded M46 Patton.
At their core, however, these tanks were still very much vehicles of World War II era and did not make use of newer technologies that had begun to appear. One of the tanks to spring from the design program was the Medium Tank T42. This tank would form the basis of the T69 project.
The unique feature of the T69 among other medium tanks then in development was its oscillating turret and autoloading system. The T69 project followed on from the T71 Light Tank project, which featured a 76mm autoloading gun in an oscillating turret. It also ran parallel to the 120mm armed T57 and the 155 mm armed T58 Heavy Tank projects. Both of which also featured autoloading systems and oscillating turrets. These two were based on the hull of the M103 Heavy Tank.

The Medium Tank T69, with an Oscillating turret, based on the hull of the T42 Medium. Photo: Presidio Press

The Medium Tank T42

The T42 was originally designed to replace the M46 Patton. Starting life in 1948, the T42 was based on the T37 light tank prototype, but had increased armor protection and carried a T139 90mm gun (which would later be serialized as the 90mm Tank Gun M41) in a brand new turret. It did, however, retain the same basic dimensions and the five road-wheel running gear.

The T42 prototype. Photo: US Archives
The T42, to the worry of the US Military, was still halfway through development when the Korean War Commenced in June 1950. This gave rise to the infamous “Korean Tank Panic”. As a quick solution to this problem, it was decided to take the turret of the T42 and mount it on the M46 hull. This spawned the Medium Tank M47 Patton II.
The T42 itself would never make it to full-scale production, having never met all of the Military’s needs and expectations. A few of the tanks would be kept for experimentation and further development. This led to its use as the base hull for the T69.

Birth of the T69

The T69 was born out of the idea from the United States Ordnance Committee that an automatic loading system would be added to the T42’s turret should one be designed and become available. Preliminary experiments with a loading system inside this turret were not successful due to the limited space and the need to line up the breach with the loading system after every shot.
Further studies by the Rheem Manufacturing Company found that it would indeed be feasible to mate the T139 90mm gun with an autoloader if the equipment was mounted in an oscillating turret. Oscillating turrets, made famous by the French and their AMX-13, were a new feature at this time. These turrets have a fixed gun in a two-part turret. The lower half, or ‘collar’, is connected to the turret ring and provides horizontal rotation. The upper part, or ‘body’, carries the gun moving up and down on a set of trunnions providing vertical traverse. Turrets of this design allowed the use of autoloader mechanisms as the gun was fixed in place, meaning the loader did not have to be re-aligned with the breach after every shot.

Profile shot of the T69. Photo: US Archives
A new contract was drafted with Rheem who then proceeded to draw up plans and prepare mockups of the turret and loading system. Work began on the turret in summer 1951. However, there were lengthy delays due to the late arrival of equipment. A total of six different designs for the turret were evaluated by APG (Aberdeen Proving Grounds) and tested by personnel supplied by the AFF (Army Field Forces) before one was selected. A number of turrets for ballistic tests were then built for APG to test the armor protection. Only after this would development finally continue in the summer of 1955.
The turret was mounted on the second T42 pilot vehicle modified to carry the XT-500 transmission. This combination was then designated the 90mm Gun Tank T69, otherwise known as Medium Tank T69.

Hull

The hull of the tank was made up of two parts. The front half was a long rounded casting of steel homogeneous armor, it was 4 inches (101.6 mm) thick and angled at 60 degrees. The rear was welded steel armor plate. The two halves were welded together in the center.

Engine

The T42 hull was powered by the Continental AOS 395 gasoline engine, (air-cooled six-cylinder supercharged 8.2-liter engine) rated at 500 horsepower. This ran through a General Motors CO-500 cross-drive transmission, later upgraded to the XT-500 (this required changes to rear of the engine compartment, resulting in a vertical rear plate). Together, this gave the vehicle a top speed of about 41 mph (66 km/h). This engine was retained for the T69. The driver’s position was located at the front left of the hull with an ammunition rack to his right. The driver steered the vehicle via the Manual Control Stick, often known as the “Wobble Stick”. The Manual Control was a single joystick that controlled left and right movement, as well as controlling forward and backward speeds.

Turret

The body of the turret was a single cast piece with the 90mm gun protruding from a long ‘nose’. The angles of the casting provided numerous deflective surfaces against incoming rounds. This body was attached to a fully cast collar by trunnions, forming the fulcrum point of elevation and depression. Maximum elevation was 15 degrees, maximum depression was 9 degrees. This motion was actuated by a hydraulically powered mechanism, though should it fail manual operation was possible. The collar was then attached to the 73-inch turret ring.
Turret crew consisted of the Gunner, Loader, and Commander. The Loader sat to the left of the gun, with the gunner on it’s right. The Commander was situated at the right rear of the turret underneath a rotating vision cupola.

Another profile shot of the T69. In this photo, the turret is partially raised to about half of its maximum elevation, and the roof is open. Note the hydraulic bar propping up the roof. Photo: Presidio Press
Access into the turret was rather easy. There was a hatch on the left of the turret roof for the loader, and another atop the Commander’s cupola at the rear right. The traditional hatches in the turret roof were not the only point of entry, however. If needed, the entire turret roof had the ability to raise up via hydraulics and could rise to almost a full 90 degrees. This allowed full access to the interior of the turret, easy removal of the gun and loading system, and quick ammunition resupply. In case of emergency, it also allowed for a quick exit of the turret. This was operated by a control in the Loader’s position.
Other features on the turret consist of an AA mount for a Browning M2HB .50 Cal. (12.7mm) Heavy Machine gun on the commander’s cupola and a ventilator in the left rear. On each side of the turret, positioned just above the fulcrum point were the ‘Frog’s Eyes’, the armored housings for the lenses of the stereoscopic rangefinder. The same can be found on the M47, M48 and so on.

An interior photo of the T69s turret taken recently at the NACM. 1: Gunners position. 2: Escape hatch. 3: 90mm Gun. 4: Recoil guard. 5: Ammunition cyylinder. 6: Ramming and extraction system. Photo: Rob Cogan.

Links, Resources & Further Reading

Presidio Press, Patton: A History of the American Main Battle Tank, Volume 1, R. P. Hunicutt
An original government report on the T69, Read HERE.
National Armor and Cavalry Museum (NACM)
NACM Curator, Rob Cogan

Illustration of the T69 Medium Tank prototype by Tank Encyclopedia’s own David Bocquelet. The color is speculative as there are no known original colored photos. As such, the standard US Olive Drab paint scheme was chosen.

Armament

The T69 was armed with the T178 90mm gun. This gun was essentially the same as the T139 but was mounted upside-down. This meant that the vertically sliding breach slid up towards the turret roof instead of down towards the floor, avoiding collision with the loading mechanism. The mounting lugs were also modified so that the gun’s concentric recoil mechanism (hollow tube around the barrel. A space-saving alternative to traditional recoil cylinders) could be mounted in the forward part of the turret, in the nose. There was a fume extractor towards the muzzle of the gun, just behind the muzzle-break. This was a relatively new feature on tanks at the time. Firing an AP (Armor Piercing) shell, the gun could penetrate 6.2 inches (157.48 mm) of armor at 1,000 yards. A coaxial Browning M1919 .30 Cal. (7.62mm) Machine Gun was mounted on the left of the main armament. When not in action, the turret would be traversed almost fully to the rear. The gun would then be placed in a travel lock mounted on the left rear of the engine deck.

A head-on shot of the T69, showing the 90mm gun, coaxial .30 cal (7.62mm) mg to its left, and the .50 cal (12.7mm) on the commander’s hatch. Photo: Presidio Press.

Autoloader

The T178 gun was fed by an 8-round autoloader mechanism. The system was mounted longitudinally on the centerline of the turret. It consisted of a magazine with an integral ramming system. The magazine took the form of a conical 8-tube revolving cylinder, like a scaled-up version of something found on a Smith & Wesson Revolver for example. The chambers of the cylinder were reloaded manually by the Loader and could be loaded with up to three different types of ammunition. AP (Armor Piercing), HEAT (High-Explosive Anti-Tank) or HE (High-Explosive) for example. The gunner could select which ammunition type he needs to fire via a control panel in his position.

A cross-section of the T69’s turret showing the autoloading apparatus. Photo: Presidio Press
When engaged, the cylinder was lifted into line with the breach, the hydraulic rammer then pushed the round forward into the breach. Upon withdrawal of the rammer, the cylinder indexed (rotated) forwards one chamber. The cylinder assembly then dropped back down to its stationary ready position low in the turret. Once fired, the empty shell was then passed along a chute to an ejection port in the turret bustle that automatically opened upon recoil of the gun. Once the shell was clear, the port automatically closed when the gun returns to battery (recovers from recoil). The rate of fire could be as fast as 33 rounds per minute. This was when firing just one ammunition type when interchanging between various types, rate of fire was reduced to 18 rounds per minute.
As well as the eight rounds in the cylinder, 32 rounds were held in the bow to the right of the driver. In the T42, this rack held 36 rounds. It was found, however, there was little clearance between the autoloading assembly and the turret ring for the loader to have access to this row of four extra rounds. It was the responsibility of the Loader to replenish the Cylinder when all rounds were spent.

A rear view of the T69 with the turret open. Note the shell ejection port in the turret bustle. Photo: Presidio Press

Fate

The T69 was tested at Aberdeen Proving Grounds from June 1955 to April 1956. The tests were dogged by a high rate of component failure which prevented in-depth study of the automatic loading system and operation of the oscillating turret. The tank was deemed unsatisfactory for service, but various tests on the vehicle would continue. Lessons learned would pave the way for future technologies and developments. The T69 Project was finally officially terminated February 11th, 1958.
The T69 was not the last experiment with oscillating turrets and autoloaders by the US Military. The project would be followed by the T54. Not to be confused with the infamous Soviet T-54, these were a series of prototypes based on the M48 Patton III hull. They were intended as a means to develop a turret for the M48 that could carry the 105mm Tank Gun T140. A variant of this project, the T54E1, carried the gun in an oscillating turret and used an autoloading system.

A photo taken in the early 1980s showing the tank at the Aberdeen Proving Grounds. Photo: preservedtanks.com
The T69 did survive, however. It was preserved at the Aberdeen Proving Grounds for many years, but it has since been removed from the site with the closure of the museum in late 2010. It was moved to Fort Benning and is currently a part of the collection of the National Armor and Cavalry Museum (NACM), Georgia, USA. The museum will open to the public in a few years. Recently, the tank was stripped of its old weathered paint given a fresh coat of protective Red-Oxide primer. In late 2017, the vehicle was given a fresh coat of Olive Drab paint.


The repainted T69 at the National Armor and Cavalry Museum. First picture shows it in the red oxide, the second shows it in its new paint job. Photos: NACM and Rob Cogan