The M21 Mortar Motor Carriage (MMC) was a

self-propelled artillery Self-propelled artillery (also called locomotive artillery) is artillery equipped with its own propulsion system to move toward its firing position. Within the terminology are the self-propelled gun, self-propelled howitzer, self-propelled mo ...

mount on a

half-track A half-track is a civilian or military vehicle with regular wheels at the front for steering and continuous tracks at the back to propel the vehicle and carry most of the load. The purpose of this combination is to produce a vehicle with the cro ...

chassis used by the

United States Army The United States Army (USA) is the land service branch of the United States Armed Forces. It is one of the eight U.S. uniformed services, and is designated as the Army of the United States in the U.S. Constitution.Article II, section 2, cla ...

during

World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...

. It was equipped with an 81 mm

M1 mortar The M1 mortar is an American 81 millimeter caliber mortar. It was based on the French Brandt mortar. The M1 mortar was used from before World War II until the 1950s when it was replaced by the lighter and longer ranged M29 mortar. General data ...

and an air-cooled

M2 Browning machine gun The M2 machine gun or Browning .50 caliber machine gun (informally, "Ma Deuce") is a heavy machine gun that was designed towards the end of World War I by John Browning. Its design is similar to Browning's earlier M1919 Browning machine gun, w ...

. It was produced by the

White Motor Company The White Motor Company was an American automobile, truck, bus and agricultural tractor manufacturer from 1900 until 1980. The company also produced bicycles, roller skates, automatic lathes, and sewing machines. Before World War II, the compa ...

in 1944. Only 110 examples were produced. It mainly served on the Western Front in

Normandy Normandy (; french: link=no, Normandie ; nrf, Normaundie, Nouormandie ; from Old French , plural of ''Normant'', originally from the word for "northman" in several Scandinavian languages) is a geographical and cultural region in Northwestern ...

, and

Southern France Southern France, also known as the South of France or colloquially in French language, French as , is a defined geographical area consisting of the regions of France that border the Atlantic Ocean south of the Marais Poitevin,Louis Papy, ''Le midi ...

, and then later in the

Ardennes The Ardennes (french: Ardenne ; nl, Ardennen ; german: Ardennen; wa, Årdene ; lb, Ardennen ), also known as the Ardennes Forest or Forest of Ardennes, is a region of extensive forests, rough terrain, rolling hills and ridges primarily in Be ...

during the

Battle of the Bulge The Battle of the Bulge, also known as the Ardennes Offensive, was the last major German offensive (military), offensive military campaign, campaign on the Western Front (World War II), Western Front during World War II. The battle lasted fr ...

. It was deemed to have been outclassed in 1945 when the 81 mm M1 mortar was found to have insufficient power to provide effective artillery support.

Development

The M4 was developed to meet the need for a specialized mortar carrier based on the

M2 Half Track Car The M2 half-track car is an armored half-track produced by the United States during World War II. Its design drew upon half-tracks imported from France in the 1930s, employing standard components supplied by U.S. truck manufacturers to speed pr ...

, which the M21 soon replaced. The initial M4 was designed to carry a mortar. In emergencies, the mortar could be fired from the vehicle. The improved M4A1 had a reinforced floor so that the mortar could be fired from the vehicle, but the mounting faced to the rear and lacked traverse. This caused problems when the troops who operated the mortar mounted the vehicle, so the US 2nd Armored Division relocated the mortars so that they could fire from the front.Doyle (2003), p. 289 The Ordnance Department followed suit and a new 81 mm MMC, the T19 MMC, was created, which was based on the larger

M3 Half-track The M3 half-track was an American armored personnel carrier half-track widely used by the Allies during World War II and in the Cold War. Derived from the M2 half-track car, the M3 was extensively produced, with about 15,000 standard M3s and mor ...

. Developmental trials of the T19 were completed in July 1943, and the vehicle was later standardized as the M21 MMC.Zaloga (1994), p. 43.

Design

The M21 had a different layout from the early M4 and M4A1 MMC. 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). The mortar was able to traverse 30° either side and elevate from 40° to 80°. If required, it could be de-mounted from the vehicle and fired from the ground. In addition, there was a pedestal mount at the rear for a .50 cal (12.7 mm) M2 heavy machine gun.

Specifications

The specifications for the M21 are similar to the specifications for its parent vehicle, the

. The vehicle was long, wide, and highNess (2002), p. 207. with a wheelbase of .Berndt (1993), p. 152. The suspension for the front wheels were leaf springs and track's suspension used a vertical volute spring. Powered by a White 160AX, , , 6-cylinder

petrol engine A petrol engine (gasoline engine in American English) is an internal combustion engine designed to run on petrol (gasoline). Petrol engines can often be adapted to also run on fuels such as liquefied petroleum gas and ethanol blends (such as ''E ...

with a

compression ratio The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. A fundamental specification for such engines, it is measured two ways: the stati ...

of 6.3:1, the M21 could reach up to on a road.Norris (2012) The fuel capacity was and the vehicle had a range of . It had a

power-to-weight ratio Power-to-weight ratio (PWR, also called specific power, or power-to-mass ratio) is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measuremen ...

of 15.8 hp/ton with the vehicle weighing nine tons.

Service history

The M21 served on the Western Front, seeing action in

and in

southern France Southern France, also known as the South of France or colloquially in French language, French as , is a defined geographical area consisting of the regions of France that border the Atlantic Ocean south of the Marais Poitevin,Louis Papy, ''Le midi ...

, before later being used during the

, the

Battle of Belgium The invasion of Belgium or Belgian campaign (10–28 May 1940), often referred to within Belgium as the 18 Days' Campaign (french: Campagne des 18 jours, nl, Achttiendaagse Veldtocht), formed part of the greater Battle of France, an Military o ...

Operation Market Garden Operation Market Garden was an Allies of World War II, Allied military operation during the World War II, Second World War fought in the Netherlands from 17 to 27 September 1944. Its objective was to create a Salient (military), salient into G ...

, and 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.Doyle (2003), p. 391 In addition, 57 examples were leased to

Free French Free France (french: France Libre) was a political entity that claimed to be the legitimate government of France following the dissolution of the Third Republic. Led by French general , Free France was established as a government-in-exile ...

forces. By 1945, it was declared obsolete, due to the insufficient power of the 81 mm mortar as an artillery piece. Standard US tank and armored infantry battalions had a mortar platoon equipped with either M21s or the older M4 MMCs. The M21 was used mainly to illuminate targets at night or provide indirect fire support to the infantry. The usage was uncommon because the White Motor CompanyZaloga (2013), p. 26. was only able to produce 110 M21s in 1944.Ness (2002), p. 193.Berndt (1994), p. 24. In 1955, the United States leased 23 M21s to the newly formed

Bundesheer The Austrian Armed Forces (german: Bundesheer, lit=Federal Army) are the combined military forces of the Republic of Austria. The military consists of 22,050 active-duty personnel and 125,600 reservists. The military budget is 0.74% of natio ...

following the signing of the

Austrian State Treaty The Austrian State Treaty (german: Österreichischer Staatsvertrag ) or Austrian Independence Treaty re-established Austria as a sovereign state. It was signed on 15 May 1955 in Vienna, at the Schloss Belvedere among the Allied occupying p ...

that re-established Austria as a sovereign state. In 1968, all but one of the M21s were returned to the US Army. The remaining M21 was stored outside until 2003, after which it was restored to running condition and placed on display in the tank hall at the

Heeresgeschichtliches Museum The Museum of Military History – Military History Institute (german: Heeresgeschichtliches Museum – Militärhistorisches Institut) in Vienna is the leading museum of the Austrian Armed Forces. It documents the history of Austrian ...

Operators

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References

Citations

Bibliography

* * * * * * * * * * {{WWIIAmericanAFVs Half-tracks of the United States World War II armored fighting vehicles of the United States World War II half-tracks Military vehicles introduced from 1940 to 1944