Self-Propelled Mortar Carriers I

M21 81mm Mortar Carrier at tests

Half-track carriers were one of the most versatile designs of all armoured fighting vehicles to be used during the Second World War. The Japanese Army had this type of vehicle, as did the French Army, but it was the German and American armies which developed their half-track vehicles to serve in a whole range of roles, from mounting anti-tank guns and field guns to serving as carriers for mortars. One of the first types to be developed for the mechanised infantry battalions of the US Army was the M4, which entered service in October 1941. It carried an M1 81mm mortar in a fixed mounting to allow it to fire rearwards from the back of an M2 half-track vehicle. Unfortunately this layout was not favoured, probably because the carrying vehicle had to be manoeuvred into firing position instead of simply being driven forward to open fire on targets, like standard self-propelled guns such as the M7 ‘Priest’ with its 105mm gun. A modification was made so that the crew could dismount the mortar in order to fire it on a baseplate from prepared weapon pits. The modified mounting corrected the drawback and fitted the mortar to allow it to fire forward from within the vehicle. It was operated by a crew of six men and carried ninety-six rounds for the M1 mortar, which comprised mainly HE but with some smoke and illuminating bombs. Between late 1941 and December 1942, the White Motor Company of Cleveland, Ohio, produced 572 of these vehicles, which went on to serve in mainly the European theatre. The design weighed 7.75 tons, had an overall length of 19.72ft and could reach speeds up to 45mph on roads. It measured 6.43ft in width and 7.4ft in height and carried a .30in calibre machine gun for self-defence with 2,000 rounds of ammunition. Some vehicles were armed with the heavier .50in calibre machine gun, and the crew also had personal weapons.

Another variant was designated as the M4A1, and from May 1943 the White Motor Company built 600 of these vehicles. This was slightly larger and heavier weighing 8 tons but still carrying ninety-six rounds of ammunition for the M1 81mm mortar, which was mounted to fire forward. A crew of six operated the vehicle and weapons, which included a .30in calibre machine gun with 2,000 rounds mounted for self-defence. The M4A1 was 20.3ft overall in length, 7.44ft in height and 6.43ft in width. It could reach speeds of up to 45mph on roads. Together with its M4 counterpart, these mortar carrying vehicles served with armoured units such as the 2nd Armoured Division, nicknamed ‘Hell on Wheels’, from 1942 and later served across Europe after June 1944. Despite the successful development of these two types of mortar carrier, the Ordnance Department decided to re-evaluate the layout and develop a third type of mortar-carrying half-track based on a modified M3 half-track and conduct experiments with an 81mm mortar mounted to fire forward over the driver’s cab.

Field trials and firing tests proved this new layout to be superior to the M4 design in some respects, and in June 1943 it was standardised as the M21. The White Motor Company, with its experience in developing such vehicles, was awarded the contract to build the new design, and between January and March 1944 produced 110 units. Meanwhile, trials were continuing using an M4 half-track to mount a 4.2in (107mm) mortar for use with the chemical mortar battalions. Mobility and firing trials were conducted to assess the feasibility of this combination to lay smoke screens. The mounting was the same as that used on the 81mm mortar but the recoil forces of this heavier weapon proved too great for the vehicle’s chassis, the trials were suspended and the project dropped. Two other projects, known as T27 and the T27E1, using the M1 mortar mounted in the chassis of tanks, were examined, but these were terminated in April 1944. The T29 to mount an 81mm mortar into a converted chassis of an M5A3 light tank was another short-lived project which never got off the drawing board. The Ordnance Department then tried mounting the 4.2in mortar on the M3A1 half-track, and this proved much better. For some reason the design team appears to have reverted to mounting the mortar to fire rearward out of the vehicle and the configuration was designated T21. A change of design to mount the mortar to fire forward resulted in the designation T21E1, and even mounting the weapon into a the chassis of an M24 light tank was considered, but it was not pursued and the complete project was dropped shortly before the end of the war in Europe in 1945. Two other proposals for self-propelled mortar carriers were the T36 and T96 projects. The T36 suggested mounting a 155mm mortar in the chassis of an M4 Sherman tank and the T96 a 155mm mortar onto the chassis of the M37 gun carriage. They were good ideas but by the time these proposals were put forward the war was coming to an end and the projects were dropped.

The M4, M4A1 and M21 mortar carriers were based on the M2, M2A1 and M3 half-tracks respectively, of which some 60,000 of all types were built. They served in various roles, including self-propelled gun and anti-aircraft gun platform with quadruple-mounted .50in calibre heavy machine guns known as the M16. There were also communications vehicles in this range. The White Motor Company built the prototype of the M21 in early 1943 as the T-19 and, following successful trials, it was standardised in July the same year. It was accepted into service in January 1944 and among the units to receive the vehicles was the 54th Armoured Infantry Regiment of the 10th Armoured Division, which later saw heavy fighting during the Battle of the Bulge in December 1944. The M21 had a crew of six to operate the vehicle, mortar and the machine gun for self-defence, while frames on the side of the vehicle allowed mines to be carried which could be laid for defensive purposes in an emergency. The vehicle had a combat weight of 20,000lbs (almost 9 tons) with an overall length of almost 19ft 6in. The height was 7ft 5in and it was almost 7ft 5in at its widest point. The barrel of the M1 81mm mortar was supported with a bipod and a special baseplate mounting which allowed it to be fired from the rear of the vehicle. A total of ninety-seven rounds of ammunition were carried and included smoke, illuminating and high explosive rounds. A store of forty rounds of ammunition was kept in lockers either inside the hull where the crew could access it easily ready to use. A further fifty-six rounds were kept in storage lockers, twenty-eight rounds either side of the hull, which could be loaded into the rear of the vehicle to maintain levels of ammunition ready to fire. This arrangement was the same on the M4 and M4A1 vehicles. The mortar of the M21 could be traversed 30 degrees left and right; for greater changes the vehicle had to be manoeuvred to face the direction of the target. The mortar could be fired at the rate of eighteen rounds per minute to engage targets at ranges of almost 3,300 yards with the high explosive rounds. The barrel could be elevated between 40 and 85 degrees to alter the range. The .50in calibre machine gun was fitted on a pedestal mount to the rear of the vehicle and a total of 400 rounds of ammunition were carried. From there the firer could traverse through 360 degrees to provide all-round fire support. The vehicle was only lightly armoured up to a maximum 13mm thickness.

The M21 was fitted with a White 160AX six-cylinder petrol engine which developed 147hp at 3,000rpm to give speeds of up to 45mph on roads. Fuel capacity was 60 gallons and this allowed an operational range of 200 miles on roads. The front wheels were operated by a standard steering wheel and the tracks were fitted with double sets of twin bogies as road wheels, larger ‘idler-type’ wheels at the front and rear of the track layout and only one return roller. The open top of the vehicle could be covered by a canvas tarpaulin during inclement weather and this could be thrown off quickly when going into action. Although only few in number, together with the more numerous M4 and M4A1 mortar carriers, the three designs provided excellent mobile fire support to infantry units wherever required. All three designs were equipped with radio sets to communicate and receive orders as to where to deploy if needed to fire against targets. Some units of the Free French Army were supplied with some fifty-two examples of the M21 self-propelled mortar vehicles, which were used during the European campaign.

One armoured unit, the 778th Tank Battalion, recorded of the mortar carriers attached to D Company in December 1944 that the fire support they provided was ‘instrumental on several occasions in assisting the advance of the infantry by placing fire on enemy gun positions and strongpoints that could not effectively be fire upon by other weapons’. The account continues by stating how ‘the two … mortar platoons, from advantageous positions on the west side of the Saar River placed harassing fire on the city of Bous, on the east side of the river. The platoon fired an average of 350 to 400 rounds per day into the city’. Continuing in their support of D Company, the mortar carriers fired from elevated positions at Bisten from where they suppressed German positions. Another armoured unit, the 746th Tank Battalion, was provided with fire support from mortar carriers and the unit recorded how these vehicles were able to ‘fire support to [cover] advance infantry elements in many instances when tank fire cannot be employed successfully’. This account continues by recording how self-propelled mortar carriers ‘were attached to an infantry regiment and further attached to one battalion and the assault company thereof. By following closely behind the advancing infantry, the mobile mortars lay down covering fires within their maximum range before displacing to the next bound. In some actions, the mortar carriers have backed down the axis of advance from one bound to another.’ Yet despite the mortar carrier’s effectiveness in supporting advances at very close quarters and keeping up with the advance, by the end of the war some officers in armoured units dismissed their usefulness. There were plans to develop the M21 vehicle to carry the larger 4.2in calibre mortar but it never entered service.

During its rearmament programme the German Army investigated the possibility of using half-tracked vehicles, and the way in which they could be developed into a variety of roles to support troops on the battlefield. By the time Poland was attacked, the German Army was equipped with several versatile designs of armoured half-tracked vehicles, mostly serving in the primary role of transporting troops on the battlefield and a secondary role as communications vehicles. Production of these designs continued so that several months later, when the blitzkrieg was launched against Western Europe in May 1940, the fleet of half-track vehicles was even larger. The two most widely-used types were the SdKfz 251 and the smaller SdKfz 250, which went on to prove itself to be no less versatile than its larger counterpart. In fact, by the end of the war in 1945; the SdKfz 250 had been developed into no fewer than twelve different configurations.

The German Army was quick to realise that light armoured half-track vehicles could be used on the battlefield as flexible workhorses. Of all the designs to enter service, it was the SdKfz 251 series, weighing 8.7 tons in its basic APC version and capable of carrying ten fully-equipped infantrymen as well as the driver and co-driver, which would prove invaluable in many campaigns, including North Africa. From the very beginning it complied with the requirements calling for an armoured vehicle capable of transporting infantrymen on the battlefield. Known as the Gepanzerter Mannschraftstran-portwagen (armoured personnel carrier) when it was first proposed in 1935, the vehicle quickly took shape and in 1938 the prototype was ready for field trials. It was produced by the companies of Hanomag and Bussing-Nag, which built the chassis and hulls respectively, and the vehicle was given the title of Mittlerer Schutzenpanzerwagen (medium infantry armoured vehicle) with the designation of SdKfz 251. The first vehicles were in service in 1939 and some were used during the campaign against Poland. Production was low at first, in fact only 348 were built in 1940, but there were enough numbers to be used during the campaign in the west in 1940. The SdKfz 251 was fitted with a Mayback HL42 TKRM six-cylinder water-cooled petrol engine which developed 100hp at 2,800rpm to give road speeds of up to 34mph, which was more than sufficient to keep up with the tanks in the armoured divisions.

The APC version was 19ft in length, 6ft 10in in width and 5ft 9in in height. The vehicle could cope with vertical obstacles up to 12in in height, cross ditches 6ft 6in in width and had an operational range of 200 miles on roads. Armour protection was between 6mm and 14mm, but the rear crew compartment where the infantry sat had no overhead protection, which exposed the troops to the elements and also the effects of shells exploding overhead. Two machine guns, either MG34 or MG42, were fitted to allow one to fire forwards from behind a small armoured shield and the weapon at the rear was fitted to a swivel mount to provide fire support for the infantry as they exited the vehicle. Being open-topped, the infantry could jump over the sides to leave the vehicle or exit through the double rear doors. The machine guns, for which 2,000 rounds of ammunition was carried, could be taken from the vehicle when the infantry deployed.