Artillery Up Close and Personal: The Trench Mortar

^{National Archives from Army}

^{U.S. troops use mortar to help establish a beachhead on the right bank of the Rhine in 1945.}

Of all the war‑changing weapons, this has to be one of the most unim‑pressive. It looks like a piece of plain pipe propped up on a couple of legs. And, to a large extent, that’s what it is.

When trench warfare developed on a large scale, all armies felt a need for something that would lob explosives down into their enemy’s trenches. That was not a brand‑new need, of course. Mortars, which throw shells on a high trajectory, had been among the earliest of firearms. Small mortars for close‑range work had been around since the 17th century, but nobody had ever used high‑trajectory weapons on the scale they were wanted in World War I. All kinds of contrivances, such as catapults, were tried. The Russians had a catapult that consisted of a pivoted wooden arm that threw hand grenades. Instead of a skein of rope, it was powered by a modern steel coil spring attached to the short lower portion of the arm.

The Germans observed the use of small mortars during the Russo‑Japanese War and began to build up their stock in preparation for the next war, which everyone in Europe assumed would happen sooner or later. By 1914, they had 2,000 Minenwerfers (mine throwers), as they called these small mortars. They came in a variety of calibers, from 3 to 9.8 inches. One type of shell had a tube at one end containing powder and a percussion cap. The tube was inserted in the short barrel of the small mortar and fired. This system allowed a small, portable gun to fire a comparatively heavy shell. Unfortunately, the shell, gy‑rating end over end, wasn’t very accurate. In the 1948 Israeli war for independence, the Israelis built a similar mortar from odds and ends and called it “Little David.” Little David was hailed as a masterpiece of ingenuity by people who had never heard of its German prototype. Most German Minenwerfers were more complicated than Little David’s ancestor, and heavier, too. All were muzzle‑loaders, but they had recoil mechanisms like the field guns. Many were rifled, with driving bands engraved to fit the rifling. Because the shells now flew point‑first, they could be fitted with ordinary percussion fuses. Previously, they had time fuses or a gadget called an “all ways” fuse – a rather dangerous device that would explode the shell no matter what part struck a solid object.

As it turned out, the 2,000 Minenwerfers were but a drop in a bucket of what was needed.

The British came up with a much simpler gun after gas was introduced. Called a Livens projector, it was merely an unrifled steel tube with a diameter of eight inches welded to a steel base plate. Groups of 25 projectors were dug into the ground, placed at a pre‑determined angle facing the German trenches. Each gun was loaded with a powder charge wired for electrical ignition and a drum of poison gas 8 inches wide and 25 inches long. All 25 guns were then fired simultaneously. The gas drums burst and the gas vaporized as soon as they landed.

It was just a short step from the Livens projector to the next British design, the Stokes or Newton‑Stokes trench mortar. Versions of the Stokes mortar were adopted by every country in the world soon after its introduction because it was light, accurate, and versatile. And above all, it was cheap and easy to make in great quantities.

The new trench mortar was a smoothbore steel tube that rested on a separate steel base plate. The barrel was propped up on two legs, making the whole weapon a kind of tripod. On top of the legs was an elevating gear, making possible fine adjustments. Although it was a smoothbore, the mortar’s projectiles flew point‑first and accurately, because they were stabilized by fins on their tails. The firing mechanism was simply a fixed firing pin at the bottom of the tube. To fire the mortar, the gunner merely dropped a shell down the muzzle and snatched his hand out of the way immediately. The shell slid down the barrel, and a powder charge, contained in what looked like a shotgun shell, struck the firing pin. The pin ignited the percussion cap on the end of the “shotgun shell” and the mortar round went sailing off to the enemy. Later, it became possible to vary the power of the propelling charge by adding increments in the form of rings of smokeless powder to the tail of the mortar shell. That gave the gunners two ways to vary the range – changing the elevation or adding increments to the propelling charge.

The trench mortar gave the infantry a weapon that could be carried by one or two men and was capable of firing a shell of significant size at the enemy.

The French in World War I also used a small, 37 mm cannon on a tripod, but that was a flat trajectory weapon mostly useful for countering machine gun nests, and its shell was far smaller than the trench mortar’s. Trench mortars come in a variety of sizes. The British in World War II used a 50‑mm (2‑inch) mortar that one man could carry and operate. The Japanese had a similar gun with a curved base plate. Some GIs called it a “knee mortar,” supposing that the curved base plate fit over the gunner’s extended leg. One or two American soldiers tried to fire it that way and ended up with broken legs. U.S. trench mortars in World War II were in calibers 60 mm, 81 mm, and 4.2 inches (106.6 mm). The 4.2 inch mortar was not limited to short ranges. It could send a shell 6,000 yards, or about 3 1/2 miles. It took the shell about a minute to go that far.

Using the 4.2 mortar, a good crew could put half a dozen shells in the air before the first one landed. The Warsaw Pact countries and some other nations used a 120 mm mortar. The Chinese and North Korean version was a superb weapon, as any veteran of the Korean War will affirm. NATO has since adopted a 120 mm mortar. Some modern mortars are rifled, a system that increases range and accuracy at the expense of simplicity and speed of fire.

The introduction of the trench mortar made it possible to lay unprecedentedly heavy fire on enemy positions. In World War I, the trench mortar was used extensively by German “storm troops” during Ludendorff’s 1918 offensive. In World War II, mortars were everywhere. And the trench mortar’s simplicity has made homemade mortars popular with guerrillas all over the world.