Weapons: Where will the Mk 45 go next? | ADM July 2009

Julian Kerr | Sydney

The Mk 45 Mod 4 naval gun system, originally manufactured by United Defence (now part of BAE Systems), is a development of the Mk 45 Mod 2 system equipping numerous US and other nations' warships, as well as the Anzac class frigates of the Royal Australian and Royal New Zealand navies.

Following the retirement of the last of the US Navy's (USN's) battleships, the Mk 45 5inch gun became the largest gun in service with the USN.

However, the effective range of the Mod 2 is only 24 kms, which would force ships in the ground support role unacceptably close to the shoreline and the threat of land-based missile and gun batteries.

To meet the USN and US Marine Corps' surface fire support requirements it was therefore decided to upgrade the Mk 45 to fire extended range guided munitions.

This development began in 1994 and was originally scheduled to reach initial operating capability in the early 2000's.

The Mk 45 Mod 4 is a component of the Mk 34 Mod 1 Gun Weapon System, a fully-integrated subsystem of the Aegis combat system that interfaces with the Naval Fires Control System (NFCS), an automated mission-planning system for naval surface fire support.

The other two major components of the system are the Mk160 Gun Computer System (GCS) which accepts target data from the Aegis combat weapon system sensors and/or operator-entered data for indirect targets, and the Mk46 optical sight.

This is a two-axis, gyro-stabilised sensor platform containing a high performance colour daylight imaging sensor, thermal imaging sensor and eye-safe laser rangefinder.

The Mk 45 Mod 3 was to have been the existing Mod 2 but with a new adaptable control system.

This improvement was overtaken by the more radical proposals leading to the Mod 4.

Design changes
The Mod 4 design incorporates a 62 calibre barrel 40 inches longer than the 54 calibre barrel of the Mod 2; strengthened gun and mount subsystems; a lengthened recoil stroke to accommodate the higher firing energy of longer-range projectiles; enhancement of the Advanced Control System; and a reduced signature, low maintenance gun shield.

Barrel length was limited by a requirement that existing train and elevation drives not be stressed.

At the same time the USN decided to incorporate design improvements to update the Mod 2 EP2 gun loading panel and improve maintenance, operator interface, and training.

Mechanical receiver regulators (used for train and elevation pointing) were replaced with updated electronic controls.

Muzzle velocity for a conventional round was marginally increased from 2,650 to 2,700 feet a second and system weight without fluids and its four-flight lower hoist has risen from 22,226 to 24,389 kg.

The gun is manned by a gun captain, panel operator and four ammunition operators, all below deck.

Rate of fire with conventional ammunition is 23 rounds a minute, rising to 40 rounds a minute in burst mode.

Crucially, the Mod 4 also supports the higher breech pressures and associated requirements for longer range munitions.

While the Mod 4 can project a 31.75 kg base-bleed Cargo projectile out to 38 km, it is designed to fire higher-energy extended-range guided munitions (ERGM) with ranges of 76 kms (threshold) to 116 kilometres (objective).

This would enable ships with Mod 4 guns to stand 46 kilometres offshore and still reach up to 70 kms inland.

This objective suffered an embarrassing blow when USN funding for Raytheon's EX 171 ERGM development program was withdrawn in March last year.

The rocket-boosted ERMG, guided by a GPS/Inertial Navigation System, was designed to deliver a unitary warhead at ranges in excess of 100 kms but despite protracted development the munition failed to meet test objectives.

An alternative projectile, Alliant Techsystems' Ballistic Trajectory Extended Range Munition (BTERM), completed its final round of engineering demonstration flights at White Sands in September 2008.

Two of the three rounds travelled more than 92 kilometres but fell well outside the designated aim point, and the USN has reportedly terminated its interest in the projectile.

Extended range munitions?
Currently there is no USN Extended Range Munition (ERM) program, although there is an ongoing navy Analysis of Alternatives (AoA) evaluating options to replace the cancelled ERG program.

If a 5-inch precision guided munition is pursued by the USN as a result of the AoA, the requirements are expected to be similar to those for the previous ERM competition.

BAE Systems and Lockheed Martin announced in mid-2008 that they were teaming to produce such a 5-inch ERM projectile building on their success with the USN's Long-Range Land Attack Projectile (LRLAP) program.

LRLAPs are precision guided projectiles with a range of about 133 kms being developed for the Advanced Gun System (AGS), a fully automated, 155-mm, stabilised gun mount that will equip the USN's DDG-1000 class destroyers.

Much of the proven technology used with LRLAP is now being applied to the 5-inch ERM development.

Although the project is currently internally funded and no development schedule has been disclosed, this new initiative may ultimately provide the means for the long-range capability of the Mk 45 Mod 4 to be utilised.