75 mm Howitzer Motor Carriage T18

United States of America (1942)
Self-Propelled Gun – 2 Mild Steel Prototypes Ordered, 1 Built

With the onset of the Second World War in 1939, the United States began rapidly developing new self-propelled guns to modernize their antiquated ground forces, which were only equipped with towed guns. In early 1941, the Firestone Tire and Rubber Company submitted their proposal for a fully enclosed self-propelled gun based on the chassis of the M3 Stuart Light Tank. This vehicle was designated as the 75 mm Howitzer Motor Carriage T18. Two mild steel prototypes were ordered in 1942, but only one vehicle was completed before the termination of the T18 project.

The Beginnings of American Self-Propelled Guns

In 1918, at the end of the First World War, the United States began developing domestic self-propelled guns. These vehicles were inspired by French designs of the time, such as the Canon 155 mm GPF sur affût-chenilles St Chamond, and based on Holt tractors. However, with the end of the war arriving sooner than expected, just a handful of these self-propelled guns were actually produced. These completed vehicles were used as a basis for future mechanized artillery development, but large-scale budget cuts in the early 1920s severely hindered any further experimentation.

American self-propelled gun development remained relatively stagnant for many years, with the United States’ first (and, until World War 2, only) mechanized artillery regiment, the 1st Battalion, 6th Field Artillery, being established in 1934. They were equipped with vehicle-towed 75 mm Pack Howitzer M1s, far from state-of-the-art. By the time World War 2 began, this was the only battalion of mechanized artillery in the United States Army.

As war began in Europe, a rushed re-militarization effort began in the United States. A single battalion of towed light howitzers would be nowhere near enough firepower for the upcoming global conflict, so field artillery battalions were restructured, and modern designs for future self-propelled guns were pursued. Of course, it takes a not-insignificant amount of time to develop and produce an entirely new vehicle, so an expedient solution was chosen. This resulted in the 75 mm Howitzer Motor Carriage T30, an M3 Half-track mounting a 75 mm Pack Howitzer M1A1. The T30 was rushed into service while development of a proper self-propelled gun continued.

75 mm Howitzer Motor Carriage T3

An early proposal for a self-propelled gun based on the Combat Car M1 was submitted in 1939. This vehicle, designated 75 mm Howitzer Motor Carriage T3, had a rather interesting design. The turret and upper hull of the Combat Car were removed and a short superstructure was constructed. The T3 HMC featured two guns: a 75 mm howitzer M1A1 in the right side of the superstructure and a .30 caliber machine gun located inside a modified M2A3 Light Tank turret on the top of the superstructure. Curiously, due to the lack of a proper gun mounting, a pair of doors could close around the howitzer to protect the crew. However, the doors had to be opened to traverse the gun, creating an opening in the casemate front. The vehicle’s armor was quite thin, at a maximum of only .625 in (15.9 mm) thick on the front of the machine gun turret and hull. Mobility was similar to the Combat Car M1, although the vehicle accelerated slower due to its increased weight.

The T3 HMC had a crew of three: gunner, loader, and driver. Even with a crew this small, the T3’s interior was still quite cramped. Issues reloading the howitzer and operating the machine gun were made apparent during testing. These poor crew ergonomics led to the T3’s eventual cancellation in 1940. With just a single prototype completed, the T3 Howitzer Motor Carriage was not considered successful. However, lessons learned during its development helped influence future self-propelled gun projects.

Development

In June 1941, after the cancellation of the T3 Howitzer Motor Carriage, guidelines for a new self-propelled gun were created. This new vehicle was to act as a close-support vehicle and would mount either a 75 mm or 105 mm howitzer. It was to be based on the chassis of the M3 ‘Stuart’ Light Tank. Almost immediately, the 105 mm howitzer was dropped as a potential armament. The limited size of the M3 chassis would make operating the gun difficult and the howitzer’s weight would cause the vehicle to be front-heavy. With the 105 mm howitzer off the table, two designs mounting a 75 mm howitzer were proposed and evaluated.

The first, designated 75 mm Howitzer Motor Carriage T17, was based on the chassis of the M1E3 Combat Car. This chassis was chosen because of its sizable internal space. However, rather predictably, the T17 was canceled because it did not use the requested M3 Light Tank chassis. The vehicle never left the drawing board. This left just one capable design; the Firestone Tire and Rubber Company’s proposal, the 75 mm Howitzer Motor Carriage T18.

While it might seem unusual that the Firestone Tire and Rubber Company was contracted to produce an armored fighting vehicle, they had a long history of producing various other goods, including tank parts, for the American military. They produced tank tracks, M5 Light Tank turrets, artillery shells, and 40 mm Bofors anti-aircraft guns, to name just a few. Therefore, it was not completely unexpected for Firestone to attempt to develop an entire armored vehicle by themselves.

In October 1941, a wooden mock-up of the T18’s superstructure was produced by Firestone and fitted to an early M3 Stuart chassis. Suitably impressed and ready to suggest improvements, the Ordinance Committee approved the production of two mild steel pilot vehicles. The first pilot was delivered in May 1942, when testing could finally begin.

The 75 mm Howitzer Motor Carriage T18’s Design

At a Glance

The T18 Howitzer Motor Carriage, from the lower hull down, was identical to a standard early production M3 Stuart. Both tanks shared the same lower hull design, suspension, drivetrain, engine, etc. However, the most striking visual change was the T18’s large cast casemate. The Stuart’s upper hull and turret were removed, replaced by the boxy fighting compartment designed to protect and contain the 75 mm main gun and three crew members.

Firepower

The T18 was, much like the T3 HMC before it, armed with the 75 mm Pack Howitzer M1A1. The howitzer was fitted to a modified version of the M3 ‘Lee’ Medium Tank’s 75 mm gun mount and located in the front right of the superstructure. Within the vehicle, 42 rounds of 75 mm ammunition could be carried. For the gunner, an M1 periscopic sight was installed on top of the mount. Gun traverse limits were 15° to either side and between 20° to -5° vertically. The M1A1 howitzer could fire an assortment of rounds, including the M48 High-Explosive shell, the M66 High-Explosive Anti-Tank shell, and the M64 White Phosphorus shell. The M66 HEAT shell would have given the T18 HMC a fighting chance in an engagement with enemy armor. However, with a velocity of just 1,000 ft/s (305 m/s), this shell would have been quite hard to aim at any targets beyond close range. The M66 HEAT shell could penetrate a maximum of 3.6 in (91.4 mm) of armor. This gave the T18 HMC’s howitzer similar penetration to the M4 Sherman’s 75 mm M3 gun. The M1A1 howitzer’s maximum rate of fire was about 8 rounds/min, but even a trained T18 crew would probably not have been able to maintain that volume of fire. Limited by the spatial confines of the vehicle, the crew’s achievable rate of fire would probably have been no higher than 6 rounds/min.

To increase the firepower of the T18 HMC, two .30 caliber M1919A4 machine guns were placed in the vehicle’s sponsons. The machine guns were unable to traverse. Therefore, the only way to aim them was by turning the entire vehicle. The machine gun mountings and mounting locations were quite similar to those of the M3 Stuart. A maximum of 4,900 .30 caliber bullets could be carried within the vehicle. With its armament loadout, the T18 HMC could effectively fight as a direct-fire assault gun, neutralizing infantry with its machine guns, demolishing obstacles with high explosives, and even fighting tanks with its HEAT shell.

Protection

The T18 was a reasonably well-protected vehicle. While the cast armor of the casemate was flat, it compensated with pure thickness. The front of the casemate was an impressive 2 in (50.8 mm) thick, which would have offered reasonable protection against 37 mm rounds from a distance. The sides and top of the casemate were 1.25 in (31.8 mm) thick and the rear was just 1 in (25.4 mm) thick. As for the lower hull of the T18 HMC, the armor was unchanged from the M3 Stuart the vehicle was based on. The lower side of the T18 was the same thickness as the casemate side, 1 in (25.4 mm). The heavily sloped upper front plate and cast lower front plate offered .625 in (15.9 mm) and 1.75 (44.5 mm) of protection respectively. Finally, the rear armor of the T18 was 1 in (25.4 mm) thick, while the floor armor ranged from .5 in (12.7 mm) thick at the front of the tank to just .375 in (9.53 mm) thick at the back. Overall, this armor layout was reasonably thick for its time, protecting the vehicle against many of its common threats frontally from a distance.

However, this armor profile had a few disadvantages. Despite its thickness, the T18’s casemate armor was completely vertical. While this design decision increased the available space inside the vehicle, it limited the actual protection the armor could offer. Sloped armor can deflect and deform armor-piercing rounds, helping prevent a penetration. Completely flat armor, however, offers no such benefits. Incoming armor-piercing rounds maximize their penetrative effects. Furthermore, the weight of the casemate’s heavy frontal armor placed significant strain on the vehicle’s suspension. When observing pictures of the T18, the overloaded suspension becomes apparent quickly. The vehicle had a noticeable frontal tilt, as the vehicle’s rather forward center of mass placed much more strain on the forward bogie than it did the rear. Similar issues of front-heaviness plagued other uparmored American tanks, such as the Assault Tank M4A3E2 ‘Jumbo’ based on the M4 Sherman chassis.

Mobility

The T18 HMC mounted the same Continental W-670-9A engine as the M3 Light Tank it was based on. This was a gasoline engine capable of producing 250 net hp at 2,400 rpm. Automotive testing of the T18 HMC was successful, revealing only slight differences in mobility between the T18 and a standard M3 Light. Both vehicles could reach the same top speed of 36 mph (58 kph) and had similar automotive characteristics. However, the slight difference in mobility was due to the T18’s increased weight of 14.88 tons (13.5 tonnes). For comparison, the standard M3 weighed only 14 tons (12.7 tonnes). Because of the weight disparity, the vehicles also had different power-to-weight ratios. The T18’s was 16.8 hp/ton (18.5 hp/tonne), while the Stuart’s was 17.86 hp/ton (19.69 hp/tonne). This difference was quite small and likely caused the T18 to accelerate slightly slower than the M3 Stuart. Regardless, having mobility only slightly worse than a very speedy light tank is still quite impressive and the T18 proved that it would have been able to maneuver around quickly and responsively.

Crew and Ergonomics

The T18 had a crew of just three, consisting of a gunner, driver, and commander/loader. To enter and exit the vehicle, two roof hatches were provided. While the T18 wooden mock-up had only one hatch, a second was added to the pilot at the request of Aberdeen Proving Ground.

Crew conditions inside the vehicle were likely poor. The driver’s only vision source was a single forward-facing periscope, severely limiting his ability to gauge his surroundings while driving. The vehicle did not have any pistol ports to peer through or a commander’s cupola, either. The only other source of precious situational awareness during combat was the gunner’s sight, which could only traverse as far as the gun could. The commander/loader did not have any source of vision at all, a very serious drawback. Understandably, the T18 would have been extremely vulnerable to flanking attacks during combat that it could neither see nor defend against. The vehicle’s lack of a dedicated commander combined with the limited vision of the crew would have resulted in a blind vehicle operated by overworked personnel.

Additionally, ventilation of the main gun was an issue. With no ventilation fans of any type, and a limited internal casemate volume, the vehicle surely would have filled with dangerous fumes when the main gun was fired continually. The only way to ventilate the crew compartment would have been to open the roof hatches, which created another problem. Driving around un-buttoned in the middle of combat is not generally considered to be a good idea, especially in close-quarters fighting. Crews would have been stuck between a rock and a hard place. Either they could try and ignore the gasses created by the howitzer or they could compromise their protection by opening the roof hatches. However, for the long-range indirect fire duties that T18 crews would have invariably found themselves participating in, opening the hatches would have been a much smaller issue. Far from the frontline and in much less imminent danger, opening the hatches to increase crew visibility and casemate ventilation would have been a no-brainer.

Fate

While the T18 offered some advantages over its predecessors, including thick frontal armor and the usage of a standardized chassis, the project was doomed from the start. A month before the first pilot vehicle was delivered in May 1942, the Ordinance Department canceled the T18 program. Even without a physical vehicle, it was clear that the T18 had many intrinsic issues that made it unfit for service. The vehicle’s flat armor, front-heaviness, lack of vision, and poor gun traverse limits were cited as the main reasons for vehicle’s rejection. The fate of the prototype following this decision is unknown. A popular theory states that the pilot was kept on display at Aberdeen Proving Ground until it was destroyed in 1947. However, this remains unproven and the current location of the prototype, if it survives, remains a mystery.

Legacy

The 75 mm Howitzer Motor Carriage T18 was just a single stepping stone in the development of a 75 mm American self-propelled gun. Before the vehicle was even canceled, new development requirements were put forth by the Ordinance Department in December 1941. Reflecting the lessons learned from the T18 program, these requirements requested a self-propelled gun design based on the M5 Light Tank chassis and utilizing sloped frontal armor.

In an attempt to satisfy these conditions, two designs were proposed in April 1942. These were the T41 and T47 Howitzer Motor Carriages. The T41 was an open-topped turretless design on the M5 chassis and the T47 was a proposal mounting a new open-topped turret in place of the M5’s standard turret. The T47 was considered to be the best design and, as a result, the T41 was canceled almost immediately. The T47 was continually improved and developed, resulting in the now-familiar turret with the large barrel flash deflector and direct vision hatches in the front of the hull. This new turret combined with the slightly-modified hull of the M5 Light Tank was standardized in May 1942 as the 75 mm Howitzer Motor Carriage M8 ‘Scott,’ a vehicle that would see widespread service with the United States as a successful infantry support weapon.

 

 

75 mm Howitzer Motor Carriage T18 Specifications
Dimensions (L x W x H)	14’10” x 7’4” x 7’1″ 4.53 x 2.24 x 2.16 m
Weight	14.88 tons (13.50 tonnes)
Armament	75 mm M1A1 Pack Howitzer (42 rounds) 2 x .30 caliber M1919A4 Machine Guns (4,900 rounds)
Armor	Casemate Front: 50.8 mm Side: 31.8 mm Rear: 25.4 mm Top: 31.8 mm Hull Upper front: 15.9 mm Lower front: 44.5 mm Side: 25.4 mm Rear: 25.4 mm Engine deck: 12.7 mm Floor: 12.7 mm to 9.53 mm
Crew	3 (gunner, driver, commander/loader)
Propulsion	Continental W-670-9A, 250 hp, 16.8 hp/ton
Top Speed	36 mph (58 km/h)
Suspension	Vertical volute spring
Total Production	1 mild steel prototype completed, 2 ordered

Sources

Stuart – A History of the American Light Tank by R. P. Hunnicutt
M7 Priest 105mm Howitzer Motor Carriage by Steven J. Zaloga
M3 & M5 Stuart Light Tank 1940-45 by Steven J. Zaloga
M3 Infantry Half-Track 1940-73 by Steven J. Zaloga
British and American Tanks of World War II by Peter Chamberlain and Chris Ellis
Aerodynamic Data For Spinning Projectiles by H. P. Hitchcock
Work on Sabot-Projectiles by the University of New Mexico under Contract OEMsr-668, and Supplements, 1942-1944 by J. W. Greig
Holt SPGs
Saint-Chamond SPGs
Firestone in World War Two
Tank Archives: T18 HMC: Quick Howitzer
Light Tank M3 Stuart