The Bofors Gun – Part III: Modern developments

The basic model of the m/48 is the Bofors 40L70 Type A, which requires an external source of electrical power. Otherwise identical to the Type A, the 40L/70 Type B has an on-carriage electrical generator over the front axle. Both these basic weapons are obsolescent in the face of modern battlefield aircraft, but they can be updated by the fitting of a variant of the Weibull upgrade package described in the entry for the 40L60. The 40L70 upgrade package was developed by Weibull in association with the Swedish defence matériel administration, and trials confirmed that this package could ensure the arrival of 30% of the fired projectiles within 13 ft 1.5 in (4 m) of a 225-mph (360-km/h) target’s centre, and the arrival of 80% of the projectiles within 26 ft 3 in (8 m).

Another upgrade package is the Inisel Felis, developed in Spain on the basis of the CETME Linca prototype system. This is a towed optronic system that can be used with initial target designation from an associated radar, in Spanish service the Contraves Italiana LPD-10 equipment. Felis comprises a high-definition TV camera with autotracking capability, a laser rangefinder, a portable target designator, and an optional radar interface unit. The Felis system has three possible operating modes. In the first radar data is used to prime the optronic system and initiate the automatic tracking sequence. The second involves the autonomous portable designator being used to provide initial target designation. In the third the TV camera undertakes a preprogrammed scanning pattern until target acquisition is achieved. The Felis system can be used with the 40L60 gun, but is optimised for use with the 40L70 produced under licence in Spain by SA Placencia de la Armas.

Added capability
For all its capabilities in its basic form, the 40L70 is limited to clear-weather daylight engagements, and this is a decided tactical limitation in the types of weather prevalent in Europe and some parts of Asia. Bofors also appreciated that the higher performance of modern attack aircraft also posed problems insoluble with the simple sights and impact-fused ammunition of the original Types A and B, and thus set about the development of a more advanced system with day and night capability. This matured as the 40L70 BOFI Fair Weather single AA gun system, and is essentially an advanced development of the Type B with the Bofors Optronic Fire-Control Instrument and proximity-fused ammunition. The variant weighs 12,125 lb (5500 kg) in travelling order, and can accept target data from an off-carriage radar or fire-control system, but is itself based on a laser rangefinder, an image-intensifying sight with x7 day magnification and x8 night magnification for a maximum target detection range, under ideal conditions, of 7,655 yards (7000 m), and a fire-control computer. The gunner, seated in a cab to the left of the weapon, tracks the target with his optronic sight and the angular data derived from this tracking movement are fed into the computer, together with range and range-rate data from the laser rangefinder, to allow the generation of aiming and firing instructions for the gunner. An updated fire-control solution is continuously computed during firing.

The BOFI system can also be enhanced in capability through linking with an off-carriage optical target indicator. This allows a dedicated target-detection operator to find a target, whereupon he aims his sight at it and pulls a trigger. This alerts the gunner, who has merely to press a button in his cab for the optical target indicator’s data to be fed by landline into the BOFI system and automatically to train the mounting onto the correct bearing and elevation so that the gunner can acquire the target without delay.

Further development of the BOFI concept has produced the 40L70 BOFI All Weather with the addition of a J-band pulse-Doppler radar for all-weather day and night capability. This model has a travelling weight of 12,566 lb (5700 kg). The radar is comparatively light, but has first-rate operational capabilities, including automatic target acquisition and tracking. For acquisition the radar operates in fixed-frequency moving target indication mode, but for tracking switches to frequency agility for better accuracy. The system is used to its best advantage with the Target Data Receiver unit of the basic BOFI system. This allows the receipt of data from an off-carriage radar or fire-control system. As soon as the gunner receives such data, he presses a button which feeds the data to the radar, which moves with the gun mounting to the indicated bearing and elevation before beginning its automatic target-acquisition phase. As soon as the target has been acquired, the radar switches to tracking mode, and a light indicates to the gunner that smooth tracking is being secured. Angular and range data are fed to the computer, which produces a continuously updated fire-control solution. The system continues to work on the target’s predicted position should electronic countermeasures or ground masking interfere, though the gunner can at any stage switch to the optronic mode.

Italian development
The 40L70 has been produced under licence in several countries, almost invariably as an identical copy of the baseline Swedish weapon. However, in Italy the basic weapon has been developed slightly as the Breda 40L70. This is essentially the Bofors 40L70 with a few Breda-designed improvements and an optional automatic feeding device. Modifications to the gun boost the cyclic rate from 240 to 300 rounds per minute, and the addition of the automatic feeder has the advantages of reducing the crew to an on-mounting two men, and increasing the practical rate of fire to a figure closer to the cyclic rate. The feeder has its own electrical power source, and accepts ammunition in groups of three from a three-layer magazine on the traversing platform. The magazine is loaded by means of prepared ramps with 144 rounds from four-round clips, and one man is sufficient for this task even when the gun is firing. There is also a 32-round feeder for the Breda weapon. The travelling weight of the Breda equipment is 11,684 lb (5300 kg), and the equipment’s travelling dimensions include a length of 23 ft 10.5 in (7.28 m), deployed width of 23 ft 10.5 in (7.28 m) and height of 8 ft 8.5 in (2.655 m).

Ultimate model
The latest version of the 40L70 is the Bofors Trinity self-propelled 40-mm single AA gun system, which is essentially a development of the 40L70 BOFI concept based on a modular assembly of the gun, ammunition system and fire-control system for use in shipboard and mobile land applications on a tracked chassis. Such chassis are the Steyr 4K 7FA armoured personnel carrier or on an 8×8 wheeled chassis like the MOWAG Shark multi-role vehicle for good cross-country mobility.

The programme was initiated in 1981 after Bofors had undertaken a thorough investigation of current and future air defence requirements and capabilities against targets as diverse as high-performance aircraft, low-altitude missiles, and partially-concealed hovering helicopters. The result is the extremely ambitious Trinity, a modular system which weighs about 11,023 lb (5000 kg) in its land-based form with radar, yet is capable of decisive engagements with individually aimed rounds to maximum ranges of 6,560 yards (6000 m) against fixed-wing aircraft, 5,470 yards (5000 m) against rotary-wing aircraft, and 3,280 ft (3000 m) against missiles.

The Trinity equipment uses completely new subsystems that are nevertheless firmly rooted in Bofors’ experience with its previous 40-mm weapons. The core of the system is the new ordnance, which is based on the 40L70 but fitted with a Solothurn muzzle brake to reduce the recoil that would otherwise adversely affect accuracy with the new ammunition, which produces some 25% more muzzle energy than the standard range of 40-mm ammunition.

Bofors did at one time consider a multi-barrel arrangement for the Trinity equipment, but this was not used for reasons of weight and reduced accuracy. Indeed, so accurate is the ordnance of Trinity that the system is designed to fire a 10-round burst in a pre-programmed pattern blanketing the target. The cyclic rate of fire is 330 rounds per minute, and the feed system comprises 100 rounds located as one round in the breech, five vertical rows of nine to the left of the gun and six vertical rows of nine to the right of the gun. This feed system is well able to accommodate two types of ammunition, the switch from one type to another being accomplished at the touch of a button.

High-capability ammunition and fire control
The new ordnance can fire the complete range of standard 40-mm ammunition, but to match the exceptional capabilities of the barrel Bofors has developed a new Trinity 3P round. This is based on the PFHE round of the standard 40-mm guns, but uses a new type of dense propellant so that a heavier projectile can be fired with increased muzzle velocity, to provide greater range and enhanced accuracy. The whole round weighs 6.173 lb (2.8 kg), and the projectile weighs 2.425 lb (1.1 kg) complete with a 4.94-oz (140-g) bursting charge of Octol and more than 1,000 0.118-in (3-mm) tungsten alloy pellets dispersed to a lethal radius of 24 ft 7 in (7.5 m).

In itself this Trinity 3P is a formidable round, but its capabilities are boosted by the use of a new Philips Elektronikindustrier programmable fuse, which is set while the round is in the feed system so that the bursting parameters for each round can be programmed during each burst of fire. It is this combination of barrel accuracy and programmable fuse that allows the use of a pre-programmed detonation pattern for the projectiles in any 10-round burst, allowing the operator to blanket a target with detonating projectiles and so maximise kill probability.

To combine the capabilities of ordnance and ammunition for greatest effect, the Trinity system has a microprocessor-based fire-control system built up on the basis of real-time digital processing using different combinations of sensors. At the heart of this system, apart from the data-processing subsystems, are a tracking radar, with two different types being offered; a modular optical sight system with day and night capability; a Rosemount meteorological data system; a Litef gyro reference system for platform data; an Intertechnique thermal imaging system; an Ericsson laser rangefinder and a Lear Siegler muzzle velocity radar. The two tracking radars each possess a tracking range of between 220 and 21,870 yards (200 and 20000 m) against targets with any speed between 0 and 2,684 mph (0 and 4320 km/h). Greater capability can be provided by integrating the system with a longer-range 2D or 3D surveillance radar. In land application this can be either an on-mounted equipment or separate but data-linked, with an optional IFF facility in either case. In naval applications the surveillance function can be provided by a warship’s main search radar, with remote control possible via the ship’s fire-control system.

Deployment options
The basic mounting comprises a powered turntable, giving 360° traverse, that accommodates the tracking radar and other sensors, the feed system and the ordnance, the latter being capable of elevation in an arc from -20° to +80°. In land applications the gunner’s enclosed cab is generally located to the left of the ordnance, with the retractable antenna for the optional search radar at the rear of the mounting, though the system could be given additional battlefield capability by locating the gunner under armour inside the carrying vehicle. In naval applications the gunner can be located either on the mounting or below deck. A typical land application weighs 8,157 lb (3700 kg) without search radar and gunner’s cab. A typical shipboard application, with a gunner’s cab and also integrated into a higher-level fire-control system, weighs some 9,590 lb (4350 kg), of which only 441 lb (200 kg) are attributable to under-deck hardware.

As with the 40L60 gun, the 40L70 has also been widely adopted for naval use, in this instance mostly with Breda or Bofors mountings. The Breda mountings are covered in Part IV, and two types of Bofors single mountings have been produced for some 30 navies. Both are fitted with an electro-hydraulic power system, and can be used in local or remote control. The mountings can be traversed through 360° at the rate of 85° per second, and elevated in an arc between -10° and +90° at the rate of 45° per second in powered mode. Each type has a reversionary manual system for local control, in which mode the gun is gyro-stabilised and used with a reflex sight. Weather protection can be provided by an optional plastic cupola, and the mountings weigh 6,173 or 6,614 lb (2800 or 3000 kg), though a later version designed for completely automatic use with a 100-round feed system weighs 7,275 lb (3300 kg). The mountings have a maximum horizontal range of 13,125 yards (12000 m) and an effective slant range of 4,375 yards (4000 m), and the rate of fire is between 300 and 330 rounds per minute.