Fuzes for mortar rounds.
Mortars provide infantry commanders with close and immediate fire support. Traditionally they are used to deliver suppressive fire, often against enemy infantry. Although high-explosive (HE) mortar rounds are the most common pattern used in action, mortars can also be used to deliver illumination or smoke rounds.
In the urban combat featured in so many of today's headlines from Iraq, mortars offer a high rate of fire, steep angle of fall and short minimum range. During the 2003 invasion of Iraq, some US Army mortar teams were engaging targets at ranges down to 300 metres.
Like the fuzes fitted to artillery projectiles, those used on mortar rounds can be of four types: point-detonating, point-detonating delay, time and proximity--though many designs incorporate two or more operating modes.
The environment faced by mortar fuzes is different from that with which artillery fuzes must cope. The muzzle velocity of a mortar is much lower than that of a gun, so the setback shock is much lower, but still sufficient to provide a signal to the fuze that the round has been fired,
Artillery rounds historically employ the forces resulting from projectile spin as part of the arming process, but this principle cannot be adopted for rounds fired from smoothbore mortars. Artillery fuzes use reserve power sources such as batteries, but mortar fuzes often use air-driven turbines to generate power.
The size of the threaded hole in the nose of mortar rounds is not fully standardised. Nato uses its own standards, as did the former Warsaw Pact. Even within Nato there is not a single standard, so fuze manufacturers must in some cases offer two variants of the same fuze. For example, BT Fuze Products (formerly Bulova Technologies) offers the M567 super-quick/delay fuse for 81 mm mortar projectiles, and the functionally identical M935 for use on 60 mm and 120 mm projectiles.
While smaller-calibre mortar rounds require custom-designed fuzes, the larger-calibre projectiles can use fuzes primarily designed for use on artillery rounds. An example of this approach is the Fuchs Electronics M9801, which was developed for use on 155 mm artillery rounds. Compatible with 105 to 203 mm projectiles, it can also be fitted to rounds fired from 120 mm rifled mortars.
The addition of mortar capability to an existing artillery fuze design can require modifications which result in a new model of fuze compatible with both roles. In developing its new DM84 multi option artillery fuze, a further development of the company's DM74 design. Junghans Feinwerktechnik reduced the activation energy of the fuze so that it could be used on projectiles fired from mortars with rifled barrels. (Other changes involved the integration of a second burst height and a prolonged delay time in the delay mode, and the elimination of manual setting.)
Quick and Superquick
The simplest and most common types of fuze are the quick and superquick point detonating variants, which are designed to detonate the round as soon as a hard surface is struck. These types of fuze are particularly effective in mortar rounds due to the bomb's high angle of impact. Most fragmentation from the bursting round is released in sideways directions. with much less going to the front and rear. At low impact angles much of this fragmentation is wasted, being directed either downward into the ground or upward into the air. At steeper impact angles it sweeps out a circular area around the point of impact.
Quick and superquick fuzes are also useful against personnel under light cover. Fitted to HE projectiles, they give attackers the ability to clear away the camouflage, earth cover and rubble on which the defenders are relying. Their effect is less predictable against targets sheltered by trees, since the fuze may function prematurely when it strikes the foliage. Depending on the density of the foliage and the nature of the target, this premature detonation may either improve the round's effectiveness, or degrade it. For the Mapam round, Ruag has adapted a fuze so that it will detonate upon striking surfaces as soft as snow, as indeed, if the round is set off upon reaching a harder surface, the fragmentation effect under one metre of snow or more will be substantially muffled.
As their name suggests, delay fuzes allow the round to penetrate the target before detonation, the degree of penetration being dependant on the hardness of the target and the impact velocity of the round. Delay fuzing can be used against earth and log emplacements and against some types of masonry and concrete structures.
In virtually all delay fuzes, the delay is measurable in milliseconds, but on at least one occasion in the past, a military use has been found for longer delays. During the Vietnam War, North Vietnamese engineers developed a slow-acting delay fuze for use on HE and fragmentation rounds fired from the Soviet-designed 82 mm mortar. Activated on impact, this used a chemical process to delay the explosion. Since US firepower had made it advisable for Vietnamese mortar teams to fire from maximum range, then move off before the first rounds impacted on the target, long fuze delays would have increased the effective duration of this sort of harassing fire.
The traditional pattern of time fuze is based on a mechanical timing system, but these are giving way to electronic time (ET) fuzes based on solid-state electronics. The US does not currently have an operational ET fuze, but uses the Diehl/Junghans M776/M772. This German design requires a waiver from US safety standards.
ATK (Alliant Techsystems) is developing the Electronic Time Fuze for Mortars (ETFM), which is due to enter service with the US Army and US Marine Corps. Two variants of the ETFM are being developed--the XM784 for use on 60 mm and 120 mm rounds, and a larger-diameter XM785 for use on 81 mm rounds. These are expected to replace the current M776 & M772 mechanical time fuzes. ATK is using a modular design approach in which a single fuze design will fit both sizes of housing.
The ETFM uses a new dual-safe safe & arm (S&A) device that eliminates pull wires and makes extensive use of commercial off-the-shelf surface-mount electronics. The fuze is hand settable without the use of tools and incorporates a backlit LCD suitable for day and night use. Since the timing is via a crystal-controlled electronic time base, the new design will be more accurate than the mechanical time fuzes it is due to replace.
Earlier in 2004 the company provided a batch of 40 fuzes for design verification tests conducted at minimum and maximum charge weights, and at extremes of operational temperatures. These trials demonstrated a 94 per cent proper fuze function rate. Under a second test series, which will probably take place before this article is published, 200 fuzes will undergo design validation ballistic tests under a full range of environmental and ballistic test conditions.
Proximity fuzes are used to airburst HE rounds. The exact triggering height depends on the nature and radio frequency reflectivity of the terrain in the target zone. Wet or marshy soil reflects the energy from the fuze better than dry ground does, so will increase the bursting height. Foliage heavy enough to reflect radio energy will increase burst height by about the height of the foliage, but light foliage will have little effect.
Junghans Feinwerktechnik recently introduced the PX 581 Optronic Proximity Mortar Fuze, which was developed as part of a collaborative venture with the Finnish company Noptel. It uses a laser transmitter to emit ranging pulses of light at a frequency of 500 per second. The receiver section of the fuze measures the decreasing range to impact and contains intelligent decision-making logic that is programmed to detonate the round at the desired altitude. This technique gives a more accurately controlled height of burst, says the company, and the fuze does not react to clouds, mist, snow or rain. Unlike RF-based proximity fuzes, the PX 581 is unaffected by jammers designed to disrupt fuze operation. The PX 581 can be fitted to 60 mm, 81 mm, and 120 mm projectiles.
Proximity fuzes are less useful in urban combat, since they are liable to function prematurely before the descending round has reached street level. However, they can be used for targets such as observation points or fire positions located on top of buildings.
Multi-option fuzes allow the user to select delay, impact and proximity modes. In many armies, fuzes of this type may eventually replace traditional fuze types, but their cost, and the number of older fuzes stockpiled in military inventories, may result in them serving alongside the older models for some time to come.
Technological improvements can increase the effectiveness of existing patterns of fuze. For example, the KDI/L3 Communications M734A1 version of the M734 Multi-Option Fuze used on all 60 mm, 81 mm, and 120 mm HE rounds offers improved electronics that give a more consistent and accurate height-of-burst over terrain with various reflection coefficients. It also includes an apex sensor intended to limit the time of fuze radio frequency radiation, and to eliminate accidental detonation while the round is on the ascending part of its trajectory.
Radio frequency jamming is a countermeasure that can be used to cause premature functioning of proximity-fuzed rounds, detonating them at a harmless altitude. The M734A1 can detect jamming and gracefully desensitize the fuze electronics to prevent premature fuze operation. Once the fuze is out of the jammer range, it will return to full proximity mode if the predefined height of burst has not been passed.
The M783 is a point detonating/delay version of the M734A1. Removal of the proximity sensor electronics results in a more economical fuze suitable for training mission and other scenarios that do not require proximity functioning. Common parts are used wherever possible in order to minimise the unit cost of producing M734A1 and M783 fuzes. The M783 replaces the earlier M776 / M772 mechanical time fuzes.
With the development of 'smart' mortar rounds, a new level of complexity can be added to fuzes. The Talley Defense Systems XM984 Extended Range Mortar Cartridge rocket-assisted 120 mm mortar round is designed to carry a payload of 54 M80 submunitions over ranges of up to eleven km. The design incorporates an embedded dual-function time fuze developed by BT Fuze Products. Located between rocket motor and payload, this ignites the rocket used to boost projectile range. Towards the end of the round's flight it initiates the expulsion charge used to release the submunitions.
As is the case with artillery fuzes, future designs of the mortar fuze will require Safe & Arming (S&A) systems which are smaller and more reliable than current mechanical designs, and easier to interface with electronic sub-systems. In next-generation fuzes, S&A units will probably be based on Micro-Electro-Mechanical Systems (Mems) manufactured at relatively low cost using microelectronic fabrication techniques, yet able to produce useful levels of force and displacement while consuming limited amounts of electrical power.
Under the American Department of Defense's SBIR programme, Tanner Research is working on a Mems-based Low Cost Mortar Fuze with integrated safe and arm mechanisms. Working in conjunction with fuze manufacturing companies, Tanner hopes to incorporate a smart, programmable Mems S&A device into a mortar fuze, with the goals of reducing size and weight, increasing reliability and safety, improving timing accuracy, and reducing component and system cost. "Our device will be fabricated using commonly available fabrication equipment to ensure low-cost production and ease of manufacturing and technology transfer", says the company. "Our S&A design will enable the integration of sensors and electronics with a fuzing device in a compact volume [fraction of a cubic inch]."
Speaking at the February 2002 meeting of the US industrial committee of ammunition producers (ICAP), Eric L Guerrazzi, president and CEO of KDI Precision Products warned that the deployment of Mems might be unlikely before the end of the decade. "There are several developments on the horizon that can revolutionise fuzing. One that everybody is familiar with is Mems. It's a buzz word that people will say 'well you can have a little S&A on chips and you can have it tomorrow', but in reality it's going to be seven to ten years before that stuff is ready for timeline. The funding that is available will pretty much dictate how long that timeline is. The sooner it gets here the better because it drives the fuzing prices drastically lower."
Guerrazzi was critical of the lack of joint procurement by the US services. "I think a lot of dollars could be leveraged for common solutions. Everything is moving to electronic fuzing, and sooner or later there will be common core modules of Mems, S&A to sense environments and acceleration and electronics that could do everything else. The services would benefit if the money to pursue that 'far-horizon' stuff were pooled."
One problem faced by fuze manufacturers is that mortars and their ammunition are seen as low-cost weapons. Guerrazzi told the meeting that the European Nato countries were "not sophisticated buyers" of fuzes. "They will generally take a low-cost supplier over one with more sophisticated performance. You get some points for having something in the US inventory but not a lot. It's difficult to deal, particularly if you are a small business, with the terms and conditions on their contracts and the guarantees that they want. It's just not fun."
Typical Fuzes for Mortar Rounds Operating Manufacturer Nation Fuze model mode(s) SAE Alsetex France Alta 81 SQ, I ATK USA XM784/785 ETFM ET F B Borletti Italy FB 267A SQ, I, D F B Borletti Italy FB 338 MT BT Fuze Products USA M567 SQ, I, D BT Fuze Products USA M935 SQ, I, D Chartered Electronics Singapore EF 792 P, PD Esperanza Spain Model 53 SQ, I Fuchs Electronics South Africa MTF-01 ET Fuchs Electronics South Africa M9801 PT, PD, SQ or D Fuchs Electronics South Africa M9813 ET Fuchs Electronics South Africa M9815 (Cobra) SO, PD, P Junghans Germany DM111A4 SO, PD, D Feinwerktechnik Junghans / Noptel Germany/ PX581 optical Finland proximity KDI/L3 Communications USA M734 A1 P, I, D KM Fuze Engineering Israel M25 P Kongsberg Norway PPD 323 P, PD Micro Technologic Switzerland F975 SQ, I, D Norinco China M-12 SQ, 1, D Norinco China MP-6 SQ, I Reshef "technologies Israel Lambda M-760 T, PD--EMI back-up Reshef Technologies Israel M797 EPD/ optional D Reshef'technologies Israel Alpha M787 P, PD Snem France FM 40 SQ, I Various contractors USA M745 PD Notes: P = Proximity, PD = Point Detonating, EPD = Electronic Point Detonating, SQ = Super-Quick, I = Impact, D = Delay, ET = Electronic Time, T = Time, MT = Mechancal Time, EMI = Electro-mechanical Impact