A lightweight towed array sonar developed by SEA has been successfully integrated and trialled with the Manta XLUUV. This combination of technologies has significant implications for anti-submarine warfare, seabed warfare and underwater surveillance capabilities.
The 9-metre Manta uncrewed submersible built by MSubs in 2020 has been conducting a series of trials and experiments off Plymouth, run as part of the MoD’s Defence and Security Accelerator (DASA) programme. SEA worked with MSubs to add their Krait passive towed array to Manta and designed a low-risk launch and recovery process. During the trial, it was confirmed Manta could be operated with a functional 150m thin-line towed KraitArray attached. The sonar was integrated with the existing communications and sensor systems which provide position, time, speed and depth data to create a detailed acoustic profile of the underwater environment. A support vessel was used to simulate the noise of submerged targets for the KraitArray to track.
The Krait Defence System is a compact Anti-Submarine Warfare (ASW) concept developed by SEA primarily for small vessels. There are four elements comprising the passive towed array (KraitArray), an active sonar (KraitSense), an acoustic torpedo decoy system (KraitShield) and a lightweight ASW torpedo launcher (KraitStrike). Some or all of the elements of this modular system can be fitted to existing platforms such as OPVs. This is not comparable with the sophisticated high-end Thales Sonar 2087 and ASW systems fitted to the Type 23/26 frigates but perhaps should be considered as a low cost, bolt-on option that could give the Type 31s and OPVs a basic ASW capability in extremis.
The KraitArray is made from three 50m modules, giving a total length of 150m with a bend radius of 100m. The array can be lowered to depths of 300m and is equipped with up to 128 acoustic channels and 32 non-acoustic sensors. SEA say that KraitArray is already in production for undisclosed overseas customers.
It is likely that the array was just attached to the stern of the Manta XLUUV for the trials and not streamed from an internal reel. The Astute class submarines have a towed array over 120m in length on a towing cable that is at least 600m long and can quietly deploy and retract the array from an internal reel as required. (A backup ‘clip on’ array is also carried on the upper casing). The TA is part of the highly sophisticated sonar Type 2076 which includes active and passive arrays, powerful signal processing software and skilled human operators. The KraitArray is obviously a lot simpler but towed arrays are especially useful as they distance the sensor from the noise radiated into the water by the submarine’s machinery and water flowing over the hull. They can also place the sensor lower in the water column, closer to the source of the noise or into the thermal or salinity layers that channel sounds over long distances.
Although passive sonars do not require large power supplies, developing the towed array equipped XLUUV to a point where it can serve as an operational asset will involve overcoming several hurdles. Manta uses lithium batteries that can only be recharged from an external power source which constrains its range. The Boeing Orca XLUUV, the first of which was recently launched by the US Navy, has conventional diesel-electric propulsion, periodically surfacing to recharge its batteries using a diesel generator. This allows ORCA to be deployed on patrols for weeks and it may also carry active sonar needed to obtain fire control solutions. Later iterations could eventually be weaponised with torpedoes to prosecute targets independently.
Autonomous weaponised submersibles present particularly difficult command and control problems and for the foreseeable future at least, their value will be primarily for deployment on ISR missions. When a contact of interest is detected, to be of tactical use the XLUUV needs to pass the data on as soon as possible. This is likely to involve either relatively short-range underwater communication with another ‘data node’ UUV or submarine or the vessel must come close to the surface and pass data to satellites or maritime patrol aircraft.
There is a long list of potential missions for the towed array-equipped XLUUV that would help supplement the work of the RN’s overstretched SSN force. These range from simple patrols of UK waters to sanitise the approaches to ports looking for intruding adversary submarines to more long-range deployments in the North Atlantic to supplement ongoing ASW work. In future, XLUUVs could be deployed from the Type 26 frigates to sanitise waters ahead of the carrier strike group or carried by the MROSS for use in monitoring undersea infrastructure. Let us hope that the DASA initiative succeeds in accelerating this capability into service as soon as possible.