The Vietnam War left us with iconic imagery: UH-1 “Hueys” flying at treetop level. Soldiers grasping their M-16s. Flak jackets and helmet straps. Blasts of napalm. One item unfortunately missing from this list is an 81-mm mortar topped with a “piggyback” .50-caliber machine gun.
The Navy began developing the Mk 2 Mod 0 81-mm mortar in the early 1960s to arm its smaller patrol craft. (See “Armaments and Innovations,” Naval History, August 2018.) By the end of the Vietnam War, it was used on a dozen classes of Navy and Coast Guard vessels.1
The same combination of light weight, firepower, and versatility that proved valuable in Vietnam could make the mortar a useful sea-control weapon today, to provide:
- Defense from small boats and trawlers
- A standardized launcher for countermeasures and drones
- An antiship option for personnel ashore
- Armament for vessels of opportunity
Mortar Basics and Advantages
Mortars are small artillery pieces typically employed by ground forces as indirect-fire antipersonnel weapons. Table 1 summarizes their key features compared to other shipboard defenses. The U.S. military primarily uses unguided munitions in its 81-mm and 120-mm mortar systems, but more advanced mounts and projectiles exist. Shells with laser, GPS, or infrared guidance are available, and specialized antiarmor and air-burst versions also exist. DefendTex has developed an 81-mm drone that can provide intelligence, surveillance, and reconnaissance or lethal payloads.2 The Marine Corps has even tested the Advanced Capability Extended Range Mortar (ACERM), which has a guidance system and a 10,000-yard range. The Army is considering an advanced mortar turret that can launch a 40-inch-long, 40-pound projectile 20,000 yards.
Table 1. Shipboard Defensive Systems.
Layered defense. Many ships rely on the Mk 38 25-mm cannon to engage small surface targets at up to 2,500 yards. Today’s 120-mm mortars can engage targets at three times the range of the Mk 38, while weighing less than one-seventh of a Mk 38 mount.3 An 81-mm mortar can more than double the range at less than one-tenth the weight. Mortars also offer the potential for superior armor penetration—ten times that of a 25-mm shell. Pairing a mortar with a 25-mm cannon would force enemy boats to run a multilayer gauntlet.
Mortars also offer an alternative to missiles that balances range and magazine capacity. Point-defense missiles that are effective against antiship cruise missiles could be used against small boats, but their magazines are limited. Smaller missiles, such as the AGM-114L Hellfire Longbow, outrange current mortars, but at 100-plus pounds each (not including the launcher), they are much heavier than 28-pound 120-mm or 10-pound 81-mm mortar shells.3 Small crew-served missiles such as the Javelin cannot match the mortars’ range. Even advanced mortar rounds are a fraction of the cost of missiles.
A standardized launcher. High-fidelity self-defense countermeasures remain aspirational rather than operational: beacons that perfectly mimic specific ships on radar; chaff that jams all communications; buoys that can confuse or deceive enemy sonar techs. Someday, these or other countermeasures will require delivery from ships in harm’s way. Mortars could provide this capability, depositing their payloads miles from the ship, and they are small and cheap enough to distribute to every ship in the fleet.
Mortars also could launch drones. Drones can be launched vertically from almost any ship, but vertical launch comes with a range penalty. The Insitu ScanEagle, for example, uses a horizontal launcher, while the Skyfront Perimeter launches vertically.4 Both are similar in size and payload, but the ScanEagle has more than ten times the endurance.5 Drones that fit inside existing mortar tubes are already available.
Landing parties. Commandant General David H. Berger’s strategic vision orients the Marine Corps toward sea control. The Corps has experimented with the High-mobility Artillery Rocket System in this role, but it is too large to fit in an MV-22 or any landing craft smaller than a landing craft, air cushion.6 A mortar-based system would give dismounted units an option to strike enemy vessels at sea with precision-guided munitions out to the horizon, in addition to their standard uses.
Vessels of opportunity. The Navy’s fleet is growing, but whatever the final number turns out to be in 2035, today’s fleet is only 290 ships. What can the Navy do if war comes before the fleet maxes out?
The Coast Guard could fill part of this gap. It already has approximately 50 patrol-type cutters that are comparable in size to or larger than a Cyclone-class patrol craft.7 And the Navy could use commercial vessels for logistical support or even as limited combatants (See Ensign Joseph McGraw, “Modest Proposals: Solutions for the Surface Navy in the Absence of a Robust Industrial Base,” U.S. Naval Institute Blog, May 2020). Cutters and armed commercial ships could focus their primary armament on limited mission areas (antisubmarine warfare, minesweeping, etc.) and use the mortars to cover other threats. Mortars are readily available, inexpensive, and versatile—strong candidates for defending these assets.
Mortars face three main hurdles for integration into the fleet: recoil, training, and perceptions.
A typical 81-mm mortar has a 10,000-pound recoil force. A larger baseplate could spread the force, and other systems exist that could reduce the recoil force. Considering the weight and range advantages of mortars over typical gun mounts, mortar-based systems can still be competitive.8
Mortars pose unique dangers to their operators. Both Army and Marine Corps mortarmen receive weeks of training, and sailors would need it as well. Sending sailors to the other services’ schools would teach them to use mortars effectively and enhance joint force integration.
But the mortar also must battle navalists’ perceptions of a high-end fight. Even in a vision of battlefields full of hypersonic missiles firing from beyond visual range in fleet-on-fleet actions, mortars can play a role. They are useful in other scenarios as well: If the next war looks more like a maritime counterinsurgency, with the enemy blending into commercial traffic, mortars have deep magazines that can hold those targets at risk, while preserving more sophisticated weapons for higher-end threats.
Give it a Shot
The Navy could take an incremental approach to integrating mortars onto ships:
- Task the Naval Surface Warfare Center at Indian Head, Maryland, with testing a variety of available mortar rounds for effectiveness against maritime targets, potential for countermeasure launching, and suitability for shipboard use.
- Encourage the Marine Corps and the Naval Research Lab to pursue an anti-armor version of the ACERM.
- Fire existing Marine Corps and Army mortars from ships during a SinkEx. Assess their effectiveness on the target and their structural impact on the ship.
Mortars will not shoot down hypersonic missiles, they will not replace every cannon in the fleet, and they will not decimate enemy surface action groups. What they could do is give commanding officers options to engage low-end targets, distract the enemy, and improve situational awareness. Considering their low cost, light weight, and long development history, the Navy should investigate their potential for in today’s fleet.
1. Bob Stoner, “Ordnance Notes Mk 2 Mod 0 and Mod 1 .50-Caliber MG/81mm Mortar” (2002).
2. “SOFIC 19—DefendTex Drone-40,” Soldier Systems Daily, Soldier Systems, 24 May 2019.
3. U.S. Army, “120mm M120A1 Towed.”
4. “Perimeter 8: Skyfront—Longest Endurance Hybrid Drones,” skyfront.com/perimeter-8/.
5. Insitu Inc., “ScanEagle,” 2017.
6. Kyle Mizokami, “The Marines Want a Rocket Launcher That Fits in an Osprey,” Popular Mechanics, 30 October 2017.
7. U.S. Coast Guard, “The Cutters, Boats and Aircraft of the U.S. Coast Guard.”
8. Joseph Trevithick, “Army Shows Off Awesome Automatic Mortar System That’s Still Too Expensive To Field,” The Drive, 12 April 2018.