The SSPH (Singapore Self-Propelled Howitzer) 1 Primus is a lightweight 155mm 39-caliber howitzer designed to meet the requirements of the Armed Forces of Singapore. SSPH 1 howitzer consists of a tracked chassis with a 155mm gun compliant with Joint Ballistic Memorandum of Understand and fully compatible with all NATO ammunition.

The 30 tons class SSPH1 howitzer is powered by a 550-hp diesel engine. It fires 3 rounds within 20 seconds and provides a burst rate of 6 rounds per minute. The fire control system uses the latest digital technology to effectively and precisely deliver artillery rounds on the battlefield.

The Primus howitzer maximum range is 19km firing unassisted projectiles, while that figure increases dramatically to 30 kilometers with rocket assisted projectiles.

The SSPH1 Primus self-propelled howitzer entered service with Singapore artillery units in late November 2003.

The chassis of Primus is based on the proven now BAE Systems, Ground Systems M109 155 mm self-propelled howitzer (SPH), which is the most widely used system of its type in the world.

It has been upgraded in a number of key areas and has a new power pack, which is similar to the one fitted to the Singapore Technologies Kinetics Bionix infantry fighting vehicle (IFV) that has been in service with the SAF for five years.

The use of common subsystems for Primus and the Bionix IFV offers a number of significant advantages including easier training and reduced logistics.

The driver's compartment is front left with the diesel power pack compartment to the right. This leaves the remainder of the chassis clear for the crew compartment, above which is the turret.

The Primus power pack consists of a Detroit Diesel Model 6V-92TIA diesel developing 550 hp coupled to an L3 Combat Propulsion Systems HMPT-500-3EC fully-automatic transmission.

This is more compact and powerful than that fitted in the most recent US M109A6 155 mm/39 calibre Paladin SPH. The engine compartment is fitted with a fire detection and suppression system.

According to ST Kinetics, the armour of the Primus provides protection against 7.62 mm armour-piercing rounds fired from a distance of 30 m and 90 per cent protection against 155 mm high-explosive fragments at 50 m.

A new all-welded steel turret has been fitted to the aluminium chassis, which is power operated and was developed by ST Kinetics. It is fitted with a locally-developed 155 mm/39 barrel with a muzzle brake and fume extractor. This meets the NATO Joint Ballistics Memorandum of Understanding.

When in the travelling configuration the 155 mm/39 calibre barrel is held in a travelling lock mounted at the front of the hull. The lock folds back onto the glacis plate when not in use.

The maximum range of the Primus is dependent on type of projectile and charge combination used, but is probably about 19 km with the old M107 high-explosive (HE) projectile and 30 km with an extended-range full bore base bleed projectile.

In addition to smoke, HE and illumination projectiles the locally developed 155 mm cargo round would also be fired. This round is already in service with the SAF and carries 64 top-attack bomblets, which are each fitted with a self-destruct mechanism.

A semi-automatic loading system is provided to increase the rate of fire and reduce crew fatigue. The fuzed projectile is loaded and rammed automatically, with the modular charges loaded manually.

It has a burst rate of fire of three rounds in 20 seconds, sustained rate of fire of two rounds for 30 minutes and a maximum rate of fire of six rounds a minute. The bustle-mounted magazine holds a total of 22 155 mm projectiles. A 7.62 mm machine gun is fitted on the turret roof for local and air defence purposes and two banks of three electrically-operated smoke grenade launchers are fitted.

A digital control system automates the complete projectile-loading process and gun laying operation. An ammunition inventory-management system keeps track of all onboard 155 mm ammunition projectiles and accounts for expenditure during firing.

The weapon is laid onto the target using an automatic fire-control system (FCS) that includes an onboard positioning and navigation system based on a ring laser gyro system.

This allows Primus to operate autonomously and enables it to react speedily to urgent drills or fire missions. It eliminates time-consuming operations such as alignment, surveying and manual laying, which leads to improved overall system accuracy.

The automatic FCS can receive target information from the battery or regimental command post. It takes 60 seconds to come into action and open fire, and 40 seconds to be re-deployed. A direct-fire sight is fitted that has a maximum range of up to 1.5 km.

Suspension is of the torsion bar type, and either side has seven dual rubber-tyred roadwheels with the drive sprocket at the front, idler at the rear and track return rollers. The upper part of the suspension is covered by a rubber skirt.

Primus has a crew of four, including the commander and driver. A number of enhancements are also underway, including an automatic fuze setter.