M21 Mortar Motor Carriage

The M21 Mortar Motor Carriage (MMC) was a World War II United States Army Self-Propelled Artillery mount on a half-track chassis. It was equipped with one 81 mm M1 mortar and an air-cooled M2 Browning machine gun in a coaxial mount. It was produced by the White Motor Company in 1944. It was produced in relatively small numbers (110).

It mainly served on the Western Front in Normandy, Southern France, and the Battle of the Bulge in World War II. It was deemed to have been outclassed in 1945 when the 81 mm M1 mortar was found to not be powerful enough to provide effective artillery support.

Specifications
The specifications for the M21 are similar to the specifications for the M3 Half-track. The vehicle was 6.32 m long, 1.96 m wide, and 2.26 m high with a wheelbase of 135.5 in. The suspension for the front wheels were leaf springs and track's suspension used a vertical volute spring. Powered by a White 160AX, 128 hp, 386 in3, 6 cylinder petrol engine with a compression ratio of 6:3:1, the M21 could reach up to 45 mph on a road. The fuel capacity was 60 USgal and the vehicle had a range of 150 mi. It had a power-to-weight ratio of 15.8 hp/ton with the vehicle weighing 9 tons.

Design
The M21 had a different layout from the M4 and M4A1 MMC because it was based on different halftracks (the M4 was based on the M2 Halftrack while the M21 was based on the M3 Halftrack). The major difference was the location and direction of the mortar (the M4's mortar was rear-facing and was near the back, while the M21's mortar was front-facing and was closer to the front). Other changes were the differences between the halftrack that they were based on.

Development
The mortar in the M4 and M4A1 MMCs was mounted so the mortar's tube was facing to the rear. This caused problems when the troops that operated the mortar mounted the vehicle, so the US 2nd Armored Division relocated the mortars so the troops could fire from the front. The Ordnance department followed suite and a new 81 mm MMC, the T19 MMC, was created. The new T19 went through its developmental trials, which were completed in July 1943, and the vehicle was later standardized as the M21 MMC. Unlike the M4 mortar carriage which was based on the M2 Halftrack, the M21 was based on the M3 Half-track with a longer and roomier rear.

Service history
The M21 served on the Western Front in Normandy, southern France, the Battle of the Bulge, the Battle of Belgium, Operation Market Garden, and during the Invasion of Germany from the west. The M21 served with the US 3rd, 1st, and 7th Armies during the campaign in France, and the 2nd Armored Division, which developed it. In addition, 57 examples were leased to Free French forces.

Usage
In a standard US tank battalion there was one mortar company equipped with either M21s or older M4 MMCs. The M21 was used to mainly to illuminate targets at night or provide indirect fire support to the infantry. The usage was uncommon because the White Motor Company was only able to produce 110 M21s in 1944.