MTLS and ASROC – killing the submarine without a helicopter

Detecting and destroying submarines will always be a demanding task. By far the best solution for surface ships to destroy enemy submarines is using a helicopter but aircraft cannot fly in all weathers and must undergo maintenance. Warships hunting submarines need every possible tool at their disposal and here we look at what the options are when the helicopter is unavailable.

By tube

The Magazine Torpedo Launch System (mTLS) was designed in 1983 by J+S Ltd. The system first entered service on-board HMS Norfolk in June 1990. Previously RN escort vessels carried the Ship Torpedo Weapons System (STWS) which was a trainable launcher capable of loading 3 torpedoes and bolted to the upper deck. mTLS is fitted internally to the Type 23 frigates in the magazine, forward of the hangar. Twin 324mm glass reinforced plastic torpedo launchers on shockproof mountings, point out of the magazine on each side of the ship. Circular pneumatic doors cover the mouth of each tube and external steel shutters cover the doors when the ship is not in an operational posture. A roof-mounted handling system is used to move the torpedoes from racks into the tubes. The mTLS is integrated with the ship’s sonar and combat management system which can pass targeting data to the torpedoes prior to launch. Weapons discharge is controlled from the operations room, although there are manual back up control panels in the helicopter hangar. When commanded to fire, the mTLS doors open and a Sting Ray ASW torpedo is discharged overboard by compressed air, a small drogue parachute is deployed behind the weapon to reduce the impact of hitting the water. (Main image above)

SEA (previously J+S Ltd) continue to support the mTLS system in Royal Navy service. The MoD will award a £7million contract for a ‘technical refresh’ of the control system in April 2020 for completion in 2024. The procurement of 8 upgrade sets has led some to speculate that perhaps mTLS will be migrated to the 8 Type 26 frigates.

A weapon of desperation?

There is a school of thought that considers ship-mounted torpedoes almost useless. By the time a submarine is within sufficient range to be engaged with weapons launched from a ship, and it is more than likely to have sunk the ship. The Sting Ray lightweight torpedo has an effective range of about 11 km and is designed to search for its target, typically in a descending spiral. Heavyweight torpedoes fired by submarines usually have a range of up to 50km and have wire guidance so they can be directed onto their target using the submarine’s sensors. They may be fired down an initial bearing and close at high speed, only using active sonar in the terminal phase, reducing the time to detect and evade them. (For example, the Russian Type 65 heavyweight torpedo has an officially given range of 50 km at 93 km/h or 100 km at 56 km/h.)

The lightweight ASW torpedo is a ‘fire and forget’ weapon and, although fed initial target data before launch, must locate an evading submarine using the limited detection envelope of its own sensors. In more confined littoral waters where there is less space or depth to manoeuvre, the submarine’s torpedo range advantage is less clear cut. The greater background noise and more difficult sonar conditions of the coastal waters make it more likely the submarine and the surface ship come into closer proximity before either may detect the other. If conducting ASW in the littorals, the case for retaining the TLS is stronger.

Many Russian and Chinese warships mount relatively crude short-range anti-submarine mortars evolved from the Squid depth charge throwers of WWII. The ability to dump large amounts of explosive into the water around the ship can act like a basic underwater close-in weapons system that may countermine an incoming torpedo or at least confuse its guidance. This blunt approach will also temporarily deafen ships sonars and potentially that of nearby submarines, although to whose advantage that may be could vary. The RN relies on more sophisticated soft-kill in the form of the S2170 Surface Ship Torpedo Defence (SSTD) system. On detection of a torpedo, it launches noise-making decoys and advises the OOW on the best speed and course for the ship to avoid being hit.

Torpedo Launch System proposed by SEA to some of the bidders for the Canadian Surface Combatant competition, subsequently won by the Type 26 frigate design.

It is unclear if the Australian Hunter class or Canadian CSC Type 26 variants will be fitted with a Torpedo Launch System. It is interesting to note the Australian Hobart class destroyers, although optimised for air warfare, carry the US-made Mk32 Mod 9 TLS, modified to launch the French-made MU90 lightweight ASW torpedo. The equivalent RN Type 45 destroyer has no way to deliver a torpedo other than a helicopter, although their Type 42 predecessors mounted STWS. The US Navy still equips its latest Arleigh Burke Flight III destroyers with deck-mounted triple torpedo tube – STWS equivalents.

Since the RN still considers it worth investing in upgrading mTLS for its ageing Type 23 frigates there is clearly a recognition that the delivery of ordnance ‘sans helicopter’ remains a relevant requirement. The publicised specification for the Type 26 frigate does not include mTLS, although it was included in the design options under consideration as late as 2014. It is still possible mTLS may be fitted but it is unclear at this point.

ASCROC-VLA-Engagement

By missile

Improving sonar and processing technology has increased the detection range of both passive and active sonar but finding and then holding a submarine contact is always a challenge for a surface ship, especially if the submarine is aware it has been detected. A weapon that can react instantly 24 hours a day in virtually all weathers and can prosecute the submarine with little warning at distance from the ship is a very attractive option. The further away from the ship a can engaged the better, even if just reacting to a fleeting sonar contact. There is often great tactical value in forcing the submarine to evade and lose contact with its target, even if not destroyed. Putting a weapon in the water may force it to turn away or even break the wire guiding a torpedo it may have launched.

The Anti-Submarine Rocket (ASROC) first went to sea with the US Navy in the 1960s. A rocket fired from the ship can quickly deliver a lightweight homing ASW torpedo out to around 20km away from the ship. A similar Australian-developed system called Ikara was in service with the RN in the 1970s and 80s fitted to HMS Bristol and eight converted Leander class frigates. ASROC has been refined over the years, although the basics are much the same. Modern US Navy combatants carry the RUM-139 ASROC which is vertically launched from the ubiquitous Mk41 VLS. The ASROC designation is no longer used and they are known as the Vertically Launched Anti Submarine Missile (VLA). The Type 26 frigate will have 24 strike-length Mk 41 cells, offering the RN opportunity to purchase VLA.

Lockheed Martin has manufactured over 1,000 VLA missiles since 1993 and the latest RUM-39C version carries the Mark 54 Lightweight Hybrid Torpedo (LHT). LM are keen to stress that other lightweight torpedoes could be readily accommodated. The costs of integrating Sting Ray onto VLA are unknown but it seems unlikely to be an especially complex or costly challenge. Sting Ray is fractionally lighter and shorter than the Mk54, although have the same NATO-standard 324m diameter. Commonality with existing RN torpedo stocks would be preferable from a logistics and training perspective, the latest Sting Ray MoD 1 is also widely believed to have superior performance to the Mk54. Confusingly the UK is purchasing a stock of MK54s to arm the 9 Poseidon P-8A maritime patrol aircraft it is buying from Boeing. This is to avoids the delay and costs of integrating Sting Ray with the Poseidon’s systems. It also simplifies the sharing of weapon stocks between US, UK and Norwegian aircraft likely spend much of their time operating jointly from RAF Lossiemouth and Reykjavik.

MK41 VLS and VLA Mk54

(Left) Simplified diagram of a missile launch from a MK 41 cell. (Center) A VLA contained in its canister that is lowered into a VLS cell. (Right) The main elements of the standard US Navy VLA – booster rocket, airframe, parachute and Mk54 torpedo.

The VLA Extended Range (VLA-ER) was a project that aimed to provide four to five times more range by adding a wing glider kit to the torpedo. VLA-ER was abandoned in 2010 but High Altitude ASW (HAASW) glider kits are being used by the P-8A Poseidon to drop Mk54 torpedos from high altitudes. Japan has developed the Type-07 VLA which is compatible with Mk41 VLS, but flies at supersonic speeds and has a range in excess of 30km.

Without the ability to detect submarines the weapons to prosecute them are useless. The development of towed array sonar dramatically increased the detection range using passive methods but a new generation of quieter submarines has seen a return to reliance on active but shorter-range sonar. Future unmanned technology offers the surface ship a way to increase detection range. The Type 26 has a large mission bay and is likely to be able to embark networked UUVs or USVs which could carry sensors to dramatically extend awareness of the underwater space.

With eight years until the first Type 26 frigate is at sea, there is plenty of time to consider the options and allocate funding but at present, there is no commitment in the MoD equipment plan to purchase any weapons for the Mk41 cells. This could be considered a form of heresy for a navy in desperate need of more hulls, but as a hypothetical question, could it be better to cancel the 8th Type 26 frigate and spend the money on a full outfit of missiles to fill the Mk41 cells? Would 7 frigates that could rely on a good stock of VLA, Tomahawks and anti-ship missiles be of more use than 8 ships that are fitted “for but no with” weapons that never materialise because there is never the budget available?

For the Type 26 frigate, its helicopter will remain the best submarine killing option, especially the Merlin with its dipping sonar and sonobuoys to localise contacts. Bad weather, maintenance issues, low fuel or crew fatigue may all mean the helicopter could be unavailable at the critical moment. VLA, or possibly TLS, back up is clearly essential to counter the 24/7 submarine threat, especially for the Type 26 who’s primary purpose is to protect other ships from submarines.