An agreement to move forward with the ASW USV project’s design phase was agreed by the Dutch Naval Design consortium and the MoD’s Materiel and IT Command (COMMIT) in April this year.
While not yet a funded programme of record, the RNLN sees USVs as a potential ASW force multiplier that would work in conjunction with the new ASWFs (deploying a low frequency/active passive variable depth sonar) and their embarked NH90 helicopter (equipped with sonobuoys and the HELRAS active dipping sonar).
Presenting to the Defence Leaders Combined Naval Event (CNE) 2024 conference in Farnborough, UK, on 22 May, Commander Paul Dröge, Branch Head Underwater Warfare Technology in COMMIT, said that interest in unmanned ASW had germinated from an integrated sensor and payload study undertaken by Nevesbu, TNO and MARIN. This activity combined operations research and analysis, modelling and simulation, and initial concept design and engineering work, with both USVs and unmanned aerial vehicles identified as potential offboard ASW sensor carriers to increase the surveillance coverage offered by new ASWF frigates.
“We are looking at an active sonar for this unmanned surface vehicle. There are broadly speaking two variants possible. First of all, a dipping sonar as used in maritime helicopters like the L3Harris HELRAS used in our navy’s NH90 helicopters [or] the Thales FLASH dipping sonar widely used by NATO nations in their [naval] helicopters”
“The second option [for a USV] is a miniaturised low frequency active towed sonar system [such as the GeoSpectrum Towed Reelable Active Passive Sonar]. Both are viable options, each with their pros and cons.”
Commander Paul Dröge, Branch Head Underwater Warfare Technology in COMMIT
Based on factors of system automation and weight, concept development work led by COMMIT has focused on a compact <12 m USV design – sized to fit inside the ASWF mission bay – configured with an active dipping sonar. Designed to operate in North Atlantic conditions up to Sea State 5 (including launch and recovery), the USV concept of operations would be based on a ‘sprint and dip’ cycle, with the craft to be capable of 20 kt ‘sprint’ transits up to Sea State 5.
Endurance has been set at a minimum of 96 hours, Commander Dröge told the CNE 2024 event. “That’s because if you go less than four days, your time from launch and recovery and transit to the operating area [means] a big chunk of your time is going to be spent busy with handling the system and not doing operations.”
“Of that 96 hours, we have said 50% in transit, and 50% stationary [operating] the sonar.”
Launch and recovery will require a cradle-type system. The maximum weight of the USV has been set at 10 tons (including payload and fuel).
Other key requirements for the USV include good situational awareness, a high level of systems reliability, and robust connectivity. “Also, platform functions must be reliable, and able to cope with emergencies such as fire or an engine failure,” Commander Dröge said, adding that redundant propulsion was mandated “to make sure that the USV can return to the mother ship if one of the engines fails.”
The USV concept design has positioned the dipping sonar in a central moonpool. “We have chosen [this central position] because it limits the effects of vehicle movement [pitch and roll] on the sonar,” explained Commander Dröge. “Also, this position protects the sonar against environmental affects during transit and stowage.”
The initial USV design – 11.2 m in length, 2.5 m in beam and displacing 9.2 tons – has proved the basic feasibility of the concept to meet the user needs. “It fits the requirement almost [although] the redundancy is not completely achievable,” said Commander Dröge.
Additional study work has considered the possible incorporation of a motion compensation system. Various options exist, including a lower stiffness hoist cable for the sonar transducer, a mechanical heave compensation system, advanced processing, or combinations of all three. The other option, said Commander Dröge, was to simply accept the motion impacts. “Let’s be honest, the hull-mounted sonar of a frigate is also moving up and down and we don’t [mitigate] that,” he said.
Work to further mature the design is now being taken forward under a cooperative contract signed by COMMIT and the Dutch Naval Design industry cluster on 10 April. Led by Damen Naval, the Dutch Naval Design team also includes MARIN and DEMCON. It is understood that COMMIT’s aspiration is to have a prototype ready by the end of 2025.
Speaking at the same CNE 2024 event, Captain Andre van der Kamp, Head of Naval Projects in COMMIT, said that the ASW USV project was not at this stage a funded acquisition programme. “We’re writing the requirements now, and I expect at the end of the year we will send out the letter to parliament about what it is we want,” he said.
