The emergence of increasingly capable unmanned surface vehicles (USVs) may enable minefields to be cleared to acceptable levels of risk more quickly than traditional MCM approaches while putting fewer people and valuable assets in harm’s way. The idea is simple: develop a set of inexpensive, expendable USVs that can sweep for mines while enduring high rates of attrition.
Not a New Problem
The idea of using vessels that could be put at acceptable risk to sweep naval minefields is not new. The U.S. Navy used it in France toward the end of World War II.2 In 1973, when the U.S. Navy cleared its mines from Haiphong Harbor under the terms of the Paris Peace Accord, it employed the USS Washtenaw County (LST-1166) to conduct “check sweeps” after other MCM operations were complete. The ship was filled with foam, and sailors were able to operate her topside amid copious padding.3 During the 1982 Falklands War, the Royal Navy tried a similar approach. The aptly named HMS Alacrity conducted check sweeps in Falkland Sound, between the two main islands, to determine whether those waters were mined.4
The key difference between these earlier acceptable risk cases and the present situation is that USVs make it possible to avoid putting personnel and warships at risk. The rate of attrition could be as high as one expendable USV per mine, because these USVs would be inexpensive and numerous.
The development of expendable USVs would involve limited technological investments, short timelines, and minimal risks. An expendable minesweeping USV only needs to know how to move among various predesignated waypoints until it is either damaged or finished with its run. If cost considerations permitted, USVs could be made more efficient and effective by using the Control Architecture for Robotic Agent Command and Sensing (CARACaS), developed by the Office of Naval Research, to incorporate swarm-like behaviors in which USVs have awareness of each others’ movements. The risks associated with third-party traffic would be low, since other vessels generally prefer to keep their distance from known minefields. Even if USVs were subject to some risk of collision, capture, or capsizing in heavy seas, some losses would be acceptable.
Aside from autonomy, expendable USVs must have the ability to collide with contact mines with enough force, at the right depths, to detonate them. Attaching an underwater rake to the bow of a USV could enable it to detonate contact mines at depths beyond its actual draft. Even if the USVs are smaller than the ships the mines are targeting, having them transit at moderate to high speeds could provide enough momentum to detonate even a reasonably insensitive contact device. (The miner will want the mine to be at least moderately responsive; otherwise, a target ship could potentially avoid damage by traversing the minefield slowly, as ships in a minefield typically do.)
Expendable USVs also must be able to detonate influence mines by generating multiple signatures associated with a warship. Putting an underwater speaker system on the USV, and constantly replaying the sounds emanating from a warship, would give it an appropriate acoustic signature. A magnetic signature could be generated by the use of strong permanent magnets or electric currents. If it is moving fast enough, even a USV of limited size could generate a substantial pressure signature.
Several distinct classes of expendable USVs could be developed. Small models (perhaps on the order of 7-meter rigid-hulled inflatable boats [RHIBs]) might be useful for clearing mines in near-shore, hostile environments to reduce risks for amphibious landings. Larger, more capable platforms—ranging in size from 11-meter RHIBs to the 40-meter antisubmarine warfare (ASW) continuous trail unmanned vessel (ACTUV) developed by the Defense Advanced Research Projects Agency—could be used in more open-water environments. Even if some purpose-built expendable USVs were as large as the ACTUV, they could be substantially cheaper; they also could use an ACTUV hullform without requiring its ability to autonomously track submarines.
In other cases, it may be viable to use converted civilian vessels that have been endowed with autonomy. Rustbucket commercial vessels that are being scrapped could be filled with foam and docked in little-used ports near prospective theaters. When needed, sailors could crew them into contested waterspace before turning the mission over to autonomous waypoint navigation, after which all personnel would disembark. The result would be minesweepers with large drafts and beams for detonating contact mines, as well as ample signatures for detonating influence mines. These large expendable USVs also could potentially continue their missions after multiple detonations. During the “Tanker Wars” of the 1980s, unmodified oil tankers sometimes withstood mine detonations.5
Concept of Operations
Expendable USVs would by their very nature employ a different concept of operations from traditional MCM and the LCS mission modules that have been developed in recent years. Most influence minesweeping has been conducted using gear towed well behind the minesweeping platform (a helicopter or surface vessel), so that the platform itself is not damaged by detonations. A USV is used as the towing platform for one of the emerging LCS modules, the Unmanned Influence Sweep System (UISS). Unfortunately, towed minesweeping gear is unable to generate the same pressure signature associated with a warship, so its effectiveness can be subverted by incorporating pressure sensitivity into mine detonation algorithms. (If the towing platform is a small surface vessel, the mine will detect its pressure signature in advance of the towed bodies, and will not detonate.) An expendable USV overcomes this hurdle by generating its pressure and other signatures within the hull.
The designs and concepts of expendable USV operations need to take into account the risk that damaged USVs could become a hazard to navigation. Converted civilian ships that are damaged could block waterways and might need to be programmed to move out of key waterspace once they are in imminent danger of sinking. In very shallow water, small USVs meant to clear the approaches to beaches could physically impede later traffic; as such, they may have select joints pre-weakened to ensure that they break into small pieces. Other USVs that are disabled but not sunk could be allowed to drift around in the minefield, potentially detonating additional mines as they do so. If necessary, firepower from other platforms could easily eliminate sunken or drifting USVs.
Moving rapidly throughout the minefield, a swarm of expendable USVs would provide wide-area coverage within relatively short timelines, potentially more effectively than other minesweeping operations. The MCM mantra, “Hunt when you can, sweep when you must” derives from the fact that minesweeping does not inspire the same degree of confidence as minehunting. A lack of detonations when the minesweeper passes through an area may indicate that influence mines are absent, or it could be that they did not detonate in response to the minesweeper’s actions because they are able to recognize that the minesweeper is not a target ship or because they incorporate mine counter-countermeasures (MCCM). One example of MCCM is the use of shipcounters, which keep an influence mine from detonating until it has detected the appropriate signatures on multiple occasions. Another is the use of probabilistic detonation, in which the mine has a given probability of detonating when it detects those signatures. These tactics can enable some influence mines to persist in the environment even after multiple iterations of minesweeping.
There are limits, however, to how extensively a miner can implement these MCCM tactics before they become self-defeating. Influence mines with excessively high shipcounts or low probabilities of detonation may not detonate until after a conflict is over. Influence mines that are highly selective in terms of the signatures they require to detonate may find that the signatures of their actual targets do not meet these criteria. Ships are periodically degaussed (demagnetized) to reduce their magnetic signatures, and they can diminish their acoustic, pressure, and other signatures by moving slowly. Moreover, the miner’s knowledge of individual ships’ or classes’ signatures will generally be imprecise. Highly selective influence mines, or those imbued with an excess of MCCM, may not detonate at all. If influence mines are to pose an effective threat, most of them will need to have limited selectivity, as well as low shipcounts (if any) and/or high probabilities of detonation. These last two points can be addressed by having USVs repeatedly cover the same tracks within the minefield, meeting the detonation criteria several times in any given area. This protracts timelines, but it does not render the concept of expendable USVs impractical. Moreover, repeatedly combing the waterspace in this fashion could help to eliminate drifting mines, should an adversary use them to create a non-persistent threat that is otherwise difficult to counter.
A key question about expendable USVs is whether they will be eliminated by adversary attacks before completing their missions. Like all surface and air-based MCM platforms, they have limited survivability in the face of attack and are essentially incapable of self defense. No MCM platform can survive without some degree of protection from other platforms: other ships and aircraft will need to suppress enemy fires while the MCM operation is ongoing. However, expendable USVs would have several advantages over existing and emerging MCM platforms in terms of attrition rates. Most MCM platforms follow a “mowing the grass” model in which they slowly move in highly predictable patterns through a minefield. A swarm of faster-moving expendable USVs could reduce this predictability, coordinating with one another to periodically shift their tracks while also monitoring the extent of their collective efforts by location. The smaller platforms would be hard to target effectively by virtue of their size. An adversary who used guided weapons to overcome this would diminish its arsenal and would be using weapons that are more expensive than the targets. The largest expendable USVs—converted civilian vessels filled with flame-resistant foam—could survive a hit, at least long enough to clear one or more additional mines. Their vulnerabilities would be mitigated by their numbers; the loss of some to enemy targeting would not prevent others from completing the mission.
Naturally, ensuring that numerous expendable USVs would be available in a contingency would require a well-orchestrated logistical chain. Stockpiling the smaller, purpose-built platforms in a series of warehouses throughout a theater would enable some to self-deploy to various locations where they might be needed. If distances were too great to permit self-deployment, they could be transported by civilian ships within permissive waterspace. Former civilian merchant ships that have been converted to large, expendable USVs could self-deploy from ports throughout the theater, lingering in permissive waterspace until required.
Costs and Limitations
An inherent attribute of mine warfare is that the resources required for MCM are far larger than those for minelaying. Although expendable USVs may be more costly than the mines they eliminate, the relevant comparison is their cost relative to other approaches to MCM. The life-cycle costs of acquiring and maintaining more sophisticated systems, deploying them, and training sailors to use them are far larger. Moreover, expendable USVs can reduce timelines relative to other approaches to MCM, preventing strategically critical delays. Their costs may be defrayed by ancillary benefits. In peacetime, they could also be used for other purposes, such as collecting intelligence, characterizing the physical environment, or launching small unmanned aerial vehicles. Regardless of any possible additional benefits, it will be critical to focus on keeping costs down throughout the acquisition process, avoiding the cost growth that has doomed many past programs.
Clearly, expendable USVs are not a panacea, and complementary approaches to MCM are needed. Effective intelligence, surveillance, and reconnaissance (ISR) against minelaying operations could enable them to be interdicted in select circumstances and can provide valuable insights regarding minefield or even individual mine locations. Other MCM platforms—including LCS mission modules, exquisitely capable marine mammals, and possibly novel variants of traditional systems—can be used to clear permissive environments where time permits, or even to operate clandestinely in select circumstances. Also, expendable USVs may not be able to counter antisubmarine minefields that include tethered contact mines well below the surface and/or influence mines that are sensitive to unique submarine signatures. However, they can play a key role in enhancing MCM capabilities against most antisurface mine threats.
These are challenging times for the United States and its allies in naval mine warfare. Numerous potential adversaries have mining programs that could impede naval operations while also damaging or sinking U.S. warships. At the same time, the U.S. Navy has a dearth of emerging and legacy MCM capabilities. In this context, it makes sense to develop relatively inexpensive, expendable minesweeping USVs to enable more rapid, cost-effective mine clearance. Limited investments in such capabilities today could enable the U.S. Navy to enter hazardous waters more quickly and deter potential aggressors who might employ mines.
1. Government Accountability Office, Defense Acquisitions: Assessments of Selected Weapon Programs, GAO-16-329SP, March 2016, www.gao.gov/assets/680/676281.pdf .
2. Tamara Moser Melia. “Damn the Torpedoes”: A Short History of U.S. Naval Mine Countermeasures, 1777-1991 (Washington, DC: Naval Historical Center, 1991), 59.
3. Moser, “Damn the Torpedoes,” 109.
4. Lawrence Freedman, The Official History of the Falklands Campaign, volume 2: War and Diplomacy (London: Routledge, 2005), 364.
5. For example, the SS Bridgeton; see Moser, “Damn the Torpedoes,” 121.