Saturday 31 August 2019

A Bigger Howitzer on a Smaller Chassis

The American Ordnance Committee standardized the HMC T47 as the HMC M8 on May 15th, 1942. This vehicle was accepted into service even before a prototype was built and became the first American light SPG to enter production. The HMC M8 was a good vehicle, but had its drawbacks. The fighting compartment was cramped and there was no room to improve the armament. Experience in combat showed that 75 mm was too small of a gun even for a light SPG. This prompted the development of a new SPG, also based on the Light Tank M5 chassis. The result was a number of experimental SPGs, including the HMC T82.

An open top alternative

When the idea of building an SPG on the chassis of the Light Tank M3 was first explored in 1941 the 75 mm Pack Howitzer M1A1 was not the only option for armament. The second choice was to use the 105 mm M3 howitzer, a shortened version of the M2 howitzer. It was more powerful than the 75 mm M1A1, but also twice as heavy and twice as big. It was apparent that the gun turned out to be too heavy for a light tank chassis, and it was abandoned for over two years.

The initial mockup of the HMC T41 fighting compartment, early April 1942.

The first variant of an SPG with the 75 mm M1A1 howitzer was also not great. The American military expected the SPG to serve in a close support role for light tanks, and thus have the same protection. This resulted in the HMC T18, which had a number of advantages, but also many drawbacks. The thickness of the armour was even higher than on the initial tank, but the fighting compartment was too cramped and poorly ventilated. Development of a new SPG was approved by the Ordnance Department. This time the prospective Light Tank M5 was used as the chassis.

Inside the fighting compartment.

The first designs of the SPG that would later be indexed HMC T41 were formed by April of 1942. The designers attempted to tackle three of the T18's problems: cramped fighting compartment, bad ventilation, and poorly placed (nearly vertical) casemate armour. The approach to this design was fairly radical. The length of the fighting compartment was increased as far as possible. It was opened from the top. The gun was placed in the center, shifted slightly to the right. A large gun shield protected the opening from bullets and shrapnel. The driver was placed to the left of the gun.

This mount was simpler than on the T18 and had higher elevation angles. The fighting compartment was roomy for a vehicle of this class and offered good working conditions for the crew. There was one downside. The movement of the gun this far forward increased the wear on the front wheels.

The final variant of the T41. By this point the project was already cancelled.

Changes were introduced into the T41's design after a review of the model. The gun moved further to the right and the gun shield was altered. Now it was housed inside the hull and was large enough to protect the gunner's head. The prototype would have been built in this configuration.

The gun was shifted to the right so the driver would have a more comfortable working space.

However, the HMC T41 never made it to the prototype stage. A decision was made in April of 1942 to use the Light Tank M3 chassis instead of the M5. This was because M5 production was just starting, but the M3 was already in production and had seen battle. The T41 also had a competitor: the T47, where the gun was placed in a rotating turret. When the T47 was accepted into service as the M8, work on the T41 ceased.

Strangely enough, no one tried to put a 105 mm howitzer into the T41. Work on this topic began a year later and the designers had to start from scratch.

Miniature HMC M7 

At first it seemed that the choice of the HMC M8 was the correct one. The designers created an agile vehicle with good fire mobility that could keep up with the tanks. The debut of this new SPG was in Sicily, and the SPGs were fighting in Italy by the fall of 1943. It became clear that a 75 mm HE shell was often not enough. American infantry command started thinking about installing the 105 mm M2A1 howitzer into an SPG back in October of 1942, but work did not progress past discussions.

The first prototype of the HMC T82. Aberdeen Proving Grounds, December 1943.

Aberdeen Proving Grounds received an urgent task from General Barnes to develop an SPG on the Light Tank M5 chassis on November 5th, 1943. The armament would be the 105 mm M3A1 airborne howitzer. The proving grounds staff were up to the challenge. The 105 mm Howitzer Motor Carriage T82 was ready for trials in December. Not many changes were introduced into the base vehicle. The turret and turret platform were removed from the Light Tank M5 and replaced with an open top fighting compartment. Experience with the HMC T41 was considered, but the result was unique. The front of the SPG was closer to that of the HMC M7. The designers also abandoned the idea of using thick armour. The armour of the T82 was only 12.7 mm thick, which was enough to protect from rifle caliber bullets. 23 rounds of ammunition for the howitzer fit inside the SPG. Like in the T41, the cannon was shifted to the right. The resulting vehicle was lighter than the HMC M8: only 11,521 kg.

The vehicle received 356 mm wide metallic track links.

Trials showed that this concept was successful. There were no extraordinary complaints about the gunnery, which resulted in a deeper exploration of the T82 concept. Specialists from the Tank-automotive Center in Detroit shifted to work on the light SPG in November of 1943. After study of the Aberdeen prototype they developed their own design. It was ready by January of 1944.

T82 hull as designed by the Heil Company and Tank-Automotive Center.

However, the vehicle was not built in Detroit. Like with the GMC M18, the development was outsourced. The chassis was built by Massey-Harris, one of the producers of the Light Tank M5. The gun mount was built by the Heil Company from Milwaukee. This company was founded in 1901 and made a name for itself in an area that was far removed from tank building. By the 1930s it was a leading producer of garbage trucks, which it continues to produce to this day. However, it turned out that the equipment for building garbage trucks was sufficient for converting light tanks into SPGs.

Equipment and stowage diagram.

The T82 that was built at Aberdeen Proving Grounds arrived in Milwaukee to be converted. Heil Company would be responsible for finishing this "diamond". The final touches were put on the casemate and fighting compartment, as specified by the Tank-Automotive Center.

The second experimental HMC T82 on the Aberdeen Proving Grounds, November 1944.

A contract for two prototypes was signed with Heil on March 17th, 1944. The tank received secondary armament in the form of the Browning M2HB .50 caliber machinegun. It was installed on a pintle mount in the fighting compartment. A radio was installed in the left sponson. The gun was moved forward by 7.5 cm, which made the design of the casemate simpler. The gun received two T17E4 telescopic sights and an M1 panoramic sight. Since the weight of the vehicle increased, the tracks were widened to 406 mm. The suspension was also changed. The ammunition capacity increased. The vehicle now carried 34 rounds of ammunition in the fighting compartment and 24 rounds in racks to the rear.

This vehicle was given the name Little Joe.

Two pilot prototypes were built by mid-summer 1944. The dry weight of the vehicle grew to 12,701 kg, and fully loaded it weighed 14,515 kg. Massey-Harris replaced the engine and transmission elements. The result was a very balanced vehicle, combining powerful armament and good mobility. The vehicle was reminiscent of a miniature HMC M7. The comparison was not only visual. For example, the driver's hatch was taken from the M7 entirely. The crew consisted of 4 men. The final variant of the SPG had its MG mount in the front, to the right of the gun. It was necessary to place it here in case the machine gun was needed to fire at enemy infantry instead of aircraft.

The SPG received new widened tracks.

Heil Company and the Tank-Automotive Center took crew comfort seriously. Despite the small dimensions of the vehicle the fighting compartment was large and roomy. However, there were serious issues in the design of the M3 howitzer. Like its larger ancestor, the M3 had separate aiming. However, the M2 had a secondary flywheel that allowed the gun to be aimed by one man, but the M3 did not. As a result, the crew had two gunners. This is why there were two sights. The commander of the HMC T82 also had to double as a radio operator.

The small fighting compartment made installation of a gun guard rail impossible, meaning that the loader risked injury during firing.

The same vehicle from the rear.

Both experimental HMC T82 vehicles arrived at the Aberdeen Proving Grounds on August 15th, 1944. After that they were supposed to be sent to Fort Knox, home of the Armor Board, and Fort Benning, home of the Infantry Board. However, the tankers refused the second SPG. The very similar HMC T76 on the chassis of the Light Tank M24 arrived at Fort Knox shortly before, and it was accepted as the HMC M37 after trials. The T76 was noticeably superior. It was armed with the more powerful M2A1 howtizer and had a larger fighting compartment that housed a crew of 7. The Light Tank M24 also offered a more modern chassis.

HMC T82 from above, showing tarp supports.

Trials of the T82 were delayed due to the Aberdeen Proving Grounds being overloaded with work. They began only on November 27th, 1944, and continued until March 17th, 1945. The SPG had a top speed of 56 kph, lower than required (64 kph). Overall, the results of the trials were satisfactory. The SPG was deemed to be suitable for the task it was designed for. Despite the lower top speed, mobility was the same as on the Light Tank M5A1. The gun mount was also good. Drawbacks included weak protection and danger posed by the long recoil of the gun. New driving controls also made the driver's job more difficult.

A pilot batch of T82s was ordered for battlefield trials.

The fighting compartment was cramped despite the work that went into making it comfortable. However the HMC M8 was even more cramped.

The positive reviews from the Aberdeen Proving Grounds piqued the interest of the Infantry Department. On January 27th, 1945, orders were received to deliver the second prototype to Fort Benning. The SPG arrived on February 3rd and went through a small set of trials. The results were hardly exceptional. Despite good mobility and successful firing trials the commission deemed the T82 as unsatisfactory.

Nevertheless, the infantry was satisfied with the HMC M8 despite its less effective weapon. Work on the HMC T82 ceased on June 21st, 1945. No prototypes survive to this day.

A fast mortar

In addition to self propelled howitzers, American units often had self propelled mortars. The first experiments with these vehicles began in the late 1920s. A tractor on the Light Tank T1 chassis was used. This project did not progress past the experimental stage.

A self propelled mortar was designed on the chassis of the M2 halftrack, which gave birth to the 81 mm Mortar Carrier M4. Counting the improved M4A1 and M21 over 1000 units were built. This vehicle became the most popular self propelled mortar in the US Army. Despite a number of issues the vehicle remained in service for a long time. Similar designs were built in Israel.

81 mm Mortar Carrier T27. Aberdeen Proving Grounds, March 31st, 1944.

The M4's drawbacks were the same as any halftrack SPG: high silhouette, thin armour, poor mobility in mud. An order was given in November of 1943 to work on a fully tracked mortar carrier. The Ordnance Committee initiated work on two vehicles on the Light Tank M5A1 chassis: the Mortar Carrier T27 and T27E1. Work on both was done by Chevrolet. This was a reasonable decision. Like Cadillac, the developer and manufacturer of the Light Tank M5A1, Chevrolet was a part of General Motors.

There was plenty of room for a mortar and ammunition inside the 81 mm Mortar Carrier T27.

The project documentation was ready by January 20th, 1944. After examining it the Committee approved the development, but the 81 mm Mortar Carrier T27E1 was cancelled. Instead the 4.2 Inch Mortar Carrier T29 was proposed. This would have been the same T27, but with a 107 mm chemical mortar.

The same vehicle from the side. The superstructure installed instead of the turret platform roof is visible.

The 81 mm Mortar Carrier T27 was completed by March 4th, 1944. The production process was not complicated. The turret and turret platform room were removed and replaced wit a small superstructure. It was partially open from above, and this roof was hinged and could be opened further. The fighting compartment housed 2 crewmen, an 81 mm mortar, and 75 rounds. The aiming arc was impressive: 46 degrees horizontally and from 40 to 80 degrees vertically. In addition to the mortar a .50 cal Browning M2HB was installed on a pintle mount. An extra mount was installed in the rear of the fighting compartment for AA work. The mortar could be removed and used separately. Its base plate was stored on the front plate. The mass of the vehicle was 14,288 kg and top speed was 64 kph. 

Inside the fighting compartment of the T27.

The 4.2 Inch Mortar Carrier T29 was finished by the end of March and was very similar to the T27. It's not hard to tell them apart. The base plate of the 4.2 inch mortar was much larger and the longer barrel protruded from the fighting compartment. The ammunition capacity was less: 60 rounds. The range was also reduced to 35 degrees horizontally and 40 to 65 degrees vertically. The mass increased to 14,402 kg, but the mobility was not changed.

4.2 Inch Mortar Carrier T29, Fort Knox, May 1944. The mortar barrel and larger base plate on the front can be seen.

Trials of the T27 on the Aberdeen Proving Grounds began in late March of 1944. Firing from the mortar and machinegun was tested. Two machineguns were installed for the duration of the trials. AA fire proved ineffective because of the narrow range, but the mortar crew's working conditions were acceptable. The T27 received a passing grade. It was shown to the representatives of the artillery branch, which also approved it.

The same vehicle from the top. The machinegun is installed on the AA mount.

The 4.2 inch Mortar Carrier T29 arrived at the Aberdeen Proving Grounds some time later. 144 rounds were fired during trials, and the results were mixed. The vibrations caused by firing damaged the engine cooling system. Nevertheless, it was still demonstrated to the artillery commission on April 14th. Orders were given to send both the T27 and T29 to Fort Knox.

The crew's seating was spartan.

The results of trials in Fort Knox were less positive. The T27 had good feedback, but the T29 was worse. Complaints were made about the protection and crew comfort of both vehicles, although the T27 was slightly roomier. On August 28th, 1944, both vehicles were rejected by the Armor Board. The mortar carrier program was closed on December 21st, 1944, but development died down even earlier. The halftrack mortar carriers never received a replacement on a tank chassis. Post-war mortar carriers were built using tracked APCs.

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