The Propulsion Disabler – A Strategic Weapon

31 December 2021

Introductory Note

This post’s name has been changed. Propulsion Disablers were introduced in Clio two years ago with the speculation that they might prove to be transformative in naval warfare. In the interval since (intellectual) experience indicates this may be true. They appear technically feasible. No specialist in undersea warfare has suggested that PDs cannot be built.  In addition, it seems clear that PD-armed minefields would be useful not only in strategically offensive sea control (global blockade) but also for defensive sea control on a lesser scale.  The obvious case of the latter, defense of Taiwan, is now addressed. The two-sided strategic potential of the PD means it is a weapon that can make strategies work. Though armed with conventional explosives, it is a “strategic” weapon. On the contrary, it may be possible to produce a workable PD that can discriminate with great accuracy enemy ships from its own and friendly ships and almost all others.

Purpose

Propulsion disablers (PDs) are small torpedo-like devices that destroy a ship’s screws and rudders. The purpose of this essay is to describe: 1) the strategic, operational (theater-level) and tactical opportunities that PDs offer the US Navy; and, 2) the threat that PDs will likely pose to the surface ships of the Navy and those of friends and allies. Submarines are addressed in only one case.

Origins of the PD Idea

PDs were originally conceived as an ideal weapon to implement an offensive global blockade strategy. They are now viewed also as highly useful for defensive purposes. PDs are a weapon that can make strategies work. In this sense, though conventional, they are “strategic” weapons.

The PD is the product of strategic thinking, not technological development. The propulsion disablement idea has been around in principle for a long time. I first encountered it during the Cold War. The crisis intermingling between Sixth Fleet and the Fifth Eskadra led many to wonder: Why don’t we have a way to put our opponent’s ships out of action short of sinking them? What’s new today is the technical possibility of actually doing that. The idea that PDs might also threaten US/Allied surface ships is born of a simple maxim: If I can do it to him, he can probably do it to me; so I should think hard about my defenses.

Assumptions

It is assumed that production of PDs is technically feasible today, or in the near future, by the US and by its adversaries. Because the pace of innovation in the current era is so rapid, no attempt is made to estimate how quickly effective PDs might arrive (nor am I remotely qualified to offer an opinion on that subject). However, their eventual appearance seems nearly certain and because of their attractiveness, the interval is likely to be short. The most likely form falls under the USN’s category of an underwater unmanned vehicle (UUV).

Ships depend on their mobility to accomplish their reason for being. In the case of civil ships that reason is mainly the movement of cargo (among many others). In the case of naval ships that reason is mainly lethality. Depriving a naval ship of its mobility has essentially the same result as sinking it: the ship loses its lethality against targets beyond the range of onboard weapons. It also makes the ship a stationary target, vulnerable to seizure.  (This combination of consequences may require that the definition of “mission kill” may need modification/expansion.)

If a PD could deprive a ship of its mobility with minimal—ideally zero—damage to the platform itself or its crew, there would be unprecedented civil, military, and political consequences.

Consequences of PD Employment

Unlike a sunk ship, a PD-ed ship would present no irreversible loss of an expensive asset that expresses national sovereignty. Thus, there might be a clouded, ambiguous casus belli. In a situation where a PD was delivered by stealthy means, it might not be possible to identify with certainty the state or even non-state actor that “fired” it.

Although PDs are inherently less violent than torpedoes, they are not benign. A ship, naval or civil, helpless before an unforgiving sea would present its owner (and the disabling party) with complex choices that need careful analysis. The owner would first be concerned with rescuing the crew and then with recovering the ship and cargo, if any, but might lack the means to do either or may simply choose not to. In that case, those tasks would fall to the disabler who, as a matter of moral, political, and likely legal obligation, could not be indifferent to the fate of those he has put at risk. 

In the case of offensive blockade, disposing of, say, many tens or even hundreds of disabled or seized ships would have to be part of a blockader’s plans. The adversary’s civil ships would have to be towed to safe harbor, their crews interned or repatriated, and cargoes seized or returned to their rightful owners (assuming the ship’s or other documentation permits the owner(s) to be identified). In the case of navy ships, the platform would be confiscated and the crews would become prisoners of war. This would be likely be a lengthy and resource consuming process. (I am indebted to K.J. Moore for raising the problem posed by disabled ships. He is of course not responsible for my treatment of the matter.)

PD Capabilities

The Navy has long had in place a wide variety of UUV programs guided by Master Plans dating from the 2000’s. However, as publicly described, these plans do not give priority to PDs nor to defense against them. Existing technologies (e.g., miniaturization, computing power, extended battery storage, exotic propulsion means, etc.) and, critically, a warhead a small fraction of the size of torpedoes designed to sink ships—all suggest that a PD UUV would be small. Many might be carried in the space occupied by a 3,000-pound torpedo. Their cost would also be a fraction that of a Mk48 (said to be in the range of $10 million each) and so tactics based on their use in large numbers would be affordable. (For example, specially designed, very small PDs could be expended in defense against swarms of small, possibly robot, enemy surface attackers.) They would be passive, constructed of stealthy, mainly non-ferrous materials and so difficult to detect. They would be capable of considerable range in both mobility and target detection, especially of large surface ships. 

PDs would also be smart. They would employ a high-fidelity library of the sonic signatures of the naval and civil ships of the adversary, the ships of friends and allies, and above all, US Navy ships, all collected in peacetime. PDs would distinguish with high accuracy between friendly and enemy ships and those of third parties, and between categories of enemy shipping, allowing excluded targets like ferries, passenger ships, and the like to be avoided. It is assumed that many of the adversary’s ships utilize common power trains simplifying their identification. (The PD threat may cause adversaries to explore and perhaps acquire ships with non-conventional means of propulsion or novel sonic signatures like hydrofoils.)

Emerging technologies are likely to enhance PD capabilities, while efforts to reduce, mask, or simulate the detectable signatures of traditional surface ships are less likely to keep pace. PDs would mainly be delivered by air or submarine, though surface ships would also be armed with them for use in offensive blockade. 

PD Minefields

A highly valuable use of the PD would be as the warhead for stationary mines. If technology allows, PD minefields could be osmotic, like the semipermeable membranes of the living world. It would allow the passage of friendly ships of all types but deny passage of enemy ships by detecting, attacking, and disabling them. PD minefields would give a new dimension to defense against amphibious landings, protection of ports and harbors, convoy routes, and at-sea locations where ships must assemble.le. 

The PD mine would be essentially un-sweepable because it could attack and disable the enemy minesweeper long before the sweeper could sweep it. (Obviously, PD minefields would be incomplete if they did not also defend against the adversary’s submarines. Attention here is confined to surface ships. Others will need to assess the PD as an ASW weapon.) 

Other Forms of PD Employment

Aircraft might deliver PDs against many enemy civil ships fairly rapidly over a wide area. Because of their size, it cannot be excluded that small numbers of PDs could be delivered by cruise or ballistic missiles. In the latter case, the missile would not have to hit its target—the golfer’s hole-in-one—it would only have to hit the green or just the frog’s hair. Submarines might deliver many tens of PDs from modules already under development for other uses, or of new specialized types.

PDs might be employed singly against civil ships, e.g., container ships, tankers, LNG carriers, etc. Against warships they might be employed singly, depending on their stealth, or perhaps in swarms. Swarms would seek to saturate defenses, overwhelm countermeasures, and increase the probability that multi-screw ships could be completely disabled.

Opportunities for Use of PDs by the US and Allies

PD devices can be used for global blockade which is sea control for offensive purpose. They could also be highly useful in sea control for defensive purposes. In the defensive case PDs would be key weapons to defeat an adversary’s attempt to use of the sea to achieve military or politico-military objectives, as China might seek to do vs. Taiwan. Let’s first examine global blockade, then Taiwan.

PDs for Global Blockade – Strategic Offense

PD devices have potential for offensive blockade versus China, Russia, and lesser adversaries. The immediate aim would be to deny the adversary any use of the sea, civil or military. (For details see the posts Blockade: Military-Economic Warfare vs. China and Blockade: Military-Economic Warfare vs. Russia.) The ultimate aim would be to coerce the adversary into ending combat and accepting a return to the status quo ante.

Blockade would be global in scope (not localized as in Maritime Interception Operations) and total (third parties would enter defined exclusion zones at risk of being PD-ed). Blockade would not be the sole action at sea but would be prosecuted as a complement to other strategic tasks.

PD capability would provide useful payoff at all levels of planning for blockade and across all phases from prewar to planning for the post-war. In acute crisis, where threats might be ambiguous, the US NCA would not face a binary choice between sinking the adversary’s ship or letting it sail on. In war, PDs would be ideally suited for blockade enforcement. Blockade runners would be disabled, and blockade-breaking defeated with minimal side effects.

This is no minor matter. Enforcing blockades has been and remains fraught with moral, legal, and political problems. The propulsion disabler would likely transform blockade operations. Consider the historical example of Lusitania. An artist’s rendering of her sinking is below. This picture would have looked very different if she’d been hit not by a German torpedo but by a German PD. Lusitania was a British-flagged ship but had aboard several hundred US citizens, many of whom were among the 1200 who lost their lives when she went down. As a result of those unprecedented losses, American public opinion turned against Germany and stayed decisively so until the US entered World War I two years later. If she had been PD-ed, Lusitania would have gone dead in the water, likely then to be towed to Liverpool, and the war might have taken a different direction.

PDs today would reshape blockade and likely reshape naval warfare in general. No one needs reminding that throughout history new weapons have changed the ways navies have been employed. The propulsion disabler may prove to be such a weapon, both to use in blockade enforcement on offense, and—of equal importance—for the navies of the United States and its allies to defend against.

These observations are obviously hypothetical, used here to illustrate a point: a propulsion disabler would have given a radically new dimension to the submarine war against the SLOC a hundred years ago, just as it would change blockade enforcement today. There is little reason to expect that the PD will remain hypothetical. Whether the US fields one or not, adversaries almost certainly will.

At the operational level in war, PDs might prove almost as effective as torpedoes in defeating the enemy because they would render target ships essentially useless and burden the enemy with retrieving ships and crews. 

At the strategic level mass use of PDs could yield considerable leverage. Consider the case of a hypothetical war with China: if a half-dozen Chinese warships and several dozen civil ships were disabled, the rest might then be kept in port, producing the effects of a successful blockade (See the post Blockade: Military-Economic Warfare vs. China). 

Operationally, PDs’ low-cost, widespread deployability—air, surface, and subsurface—and likely efficiency would make them desirable. Politically, their ability to immunize the blockader against blockade’s highly undesirable side-effects make them necessary.

PDs for Defense of Taiwan – Strategic (Self)-Defense

PDs would play a central role in Taiwan’s self defense, holding promise to defeat China’s threat of amphibious invasion and, to a lesser degree, its threat of economic blockade. This would sharply reduce the need for the US to play a direct military role in supporting Taiwan. That action has been criticized as being of questionable legitimacy—the parallel has been drawn with the decision of the European powers not to intervene on behalf of the South during the US Civil War—and because it would be tantamount to war with China.

Taiwan’s action would be purely and manifestly defensive, posing no threat of any kind to China. Taiwan would adopt a broad defensive PD-mine strategy with the following shape:

• Taiwan would acquire significant stocks of PDs  (purchased from the US/possibly domestically produced using Taiwan-produced computer chips and other electronics) 
• Taiwanese forces would practice PD mine deployment and maintenance in peacetime. 
• PD mines would be placed exclusively within Taiwan’s territorial waters through which Chinese ships must pass if they are to accomplish their mission(s). In some areas like port and harbor approaches and planned convoy routes many PD mines might be permanently installed. 
• In times of severe crisis Taiwan would deploy many thousands more in all areas of expected Chinese attack. Should the crisis be satisfactorily resolved, deployed PDs would be recovered for reuse.
• Taiwan’s description of the minefields’ ability to discriminate between unfriendly (i.e., PLAN ships), friendly ships, and ships of third parties would be carefully crafted. Its message would emphasize that only unfriendly ships need fear attack, and then only if they entered Taiwan’s territorial waters with hostile intent.

All mines would be programmed to give top priority to Chinese mine sweepers—a crucial step militarily and, especially, politically. Chinese minesweepers would necessarily be violating Taiwan’s sovereign waters when they are attacked by PD. A Chinese minesweeper-Taiwanese PD encounter would constitute a casus belli around which Taiwan (and its US-led allies) could expect to rally support.

To defeat a Chinese amphibious invasion Taiwan would deploy PD minefields in defense of the most likely beaches to be attacked. PD-mines would be programmed to concentrate on bona fide amphibious ships and landing craft, ignoring as much as possible accompanying militia and other the ships/craft meant to confuse and dilute the weight of Taiwan’s defensive fire of all kinds.

If PDs disabled attacking Chinese amphibious ships, loaded with equipment and personnel, the Chinese would face a serious problem they seem unlikely to be able to resolve. Any ships or tugs sent to retrieve or otherwise support disabled amphibs would themselves be disabled. Thus, relief efforts would only add to the size of a force that has become hostage to the Taiwanese enemy. Taiwan would then deal with its Chinese hostages—or not—as time and humanitarian imperative require and its surviving capabilities permit. 

The prospect of such a highly visible and humiliating defeat would be a likely deterrent to any of China’s plans for invasion. Were an invasion nonetheless undertaken and even to achieve a degree of success in putting troops ashore, China would be aware that its efforts to resupply its forces over captured beaches would be subject to PD attack (assuming depleted PD minefields can be reseeded).

More generally, China would not be able to exploit any military victories it may achieve over Taiwanese forces. This includes the case of its possible economic blockade of Taiwan, where PDs would aid in mounting a defense but to a limited degree in an immediate sense. That is because China has multiple means of interdicting surface ship traffic in/near Taiwanese waters. These include aircraft and cruise missiles (including sea launched). These do not, as in the case of amphibious landing, depend on China’s use of the sea surface. 

Successful military blockade does not however translate into political gain for China. Assume that a Chinese sea blockade on its own or as one component of a wider military campaign succeeds in strangling Taiwan’s economy. PD minefields, deployed by remaining Taiwanese forces and supported (clandestinely) by the US and its allies, would prevent China from bringing in and supporting occupying troops. No matter how such troops arrived, whether by sea or by air, they would have to be resupplied by sea.

Political Effects

The US and its allies could play a silent back-up role. As much as possible, the PD minefield strategy would be, and would be seen to be, Taiwan’s effort at self-defense. This would underline the case for Taiwan’s autonomy if not outright sovereignty. The ability and willingness to defend itself is the first attribute of a sovereign entity. (It is assumed that Taiwan’s “postwar” political status would still be subject to some level of negotiations with the mainland, in which an undefeated Taiwan would enjoy a position of strength.)

A key point about defeating Chinese resupply efforts: Like the defense against Chinese amphibious invasion, all action would be by Taiwanese forces operating in Taiwanese waters. These facts would likely be significant in a public relations and political campaign to build support for Taiwan and to force China to terminate combat operations on favorable terms. (The effect on the CCP’s leadership of a defeat of the nation’s effort to use military force to recover its “renegade” province is unknowable. Given that defeat was inflicted by a very small nation using a very small high-tech weapon—the PD—such effects would likely not be favorable.)

Events of this magnitude involving China’s attack on a longstanding US ally in all but name could too easily result in a wider US-China war. If so, China would be subject to global blockade, described earlier. The blockade would be focused first and foremost on interdicting China’s effort to support its forces in Taiwan. Interdiction might well prove possible to do—but perhaps unnecessary if Taiwan’s PD-mine defense had already accomplished that objective on its own.

Presumably China’s planners have thought through these possibilities. If amphibious landings are infeasible, and sea blockade produces a fruitless—and dangerous—victory, why pursue them? The answer to this question cannot be definitively predicted. But the propulsion disabler would be an important, possibly decisive, factor weighing against China’s choice to attack. 

The Threat PDs May Pose to USN Surface Ships

The first obligation of the planner is to defend his own vulnerabilities. Our adversaries are surely as aware of the merits of PDs as are students of naval warfare in the West. They may be capable of producing large numbers of PDs from home-grown robotic and computer technologies, as well as strength in mass production of modern electronic devices. These factors suggest this potential should be taken seriously.

It remains to be seen whether PDs will prove to be just a new form of undersea threat to be answered with traditional ASW measures, or whether they may be transformative. It is hard to imagine an asymmetric capability more attractive to China or Russia: a fairly simple, inexpensive way, possibly difficult to defend against, to neutralize the surface ships of the world’s most powerful navy. Nor one where the disparity in costs were so great: many thousands of PDs produced at a fraction of the cost of a carrier strike group and whatever may be prove necessary for its defense against PDs.

Possible Scenarios for Use of PDs by Adversaries

Consider three cases involving China:

(1) In peace, China successfully uses a PD against a US warship on a Freedom of Navigation operation near a Chinese-claimed area of the South China Sea. China denies all responsibility. The US searches for an appropriate response and is preoccupied with retrieving the ship.

(2) In a crisis at the brink of war, the Chinese do not fire explosives at an approaching CVSG. Rather they use PDs against the carrier. A successful attack would be a US nightmare: 110,000 tons of useless steel, drifting helplessly and displaying US impotence on worldwide television screens—a scene repeated over weeks until the ship can be towed away for repair—assuming China does not PD the tug. On the grounds of prudence, the US withholds commitment of the rest of the carrier force.

(3) In war, PDs will likely find a place in a mix with kinetic and explosive weapons. They may be the weapon of choice because of their unprecedented military advantage: putting ships out of action and at the same time forcing the opponent to deal (or not) with his disabled ships.

Suggested Actions

• Develop PD capabilities for strategic offensive and strategic defensive uses as outlined here. It’s also critical to give counter-PD a high priority in Navy planning for the defense of the carriers and the rest of the surface Navy.
• Evaluate the potential of PDs in an antisubmarine role, including the possibility of antisubmarine PD mines. As necessary, consider the possibility that US submarines may face a PD threat.
• Direct the Intelligence Community to search for signs of PD development in all of our adversaries’ actions, including in their open military writings.
• Ensure that intra-Navy research and analysis addresses PD/counter-PD. (See the note below on some relevant Navy efforts.) The Navy should also request the Joint Non-Lethal Weapons Directorate to similarly adjust its focus. The JNLWD’s “Strategic Plan, 2016-2025” includes a category “Stop Large Vessels.” But the category had no content in 2018, and the Directorate’s general perspective is offensive with little corresponding attention to defense against an adversary’s non-lethal attack.
• Suggest to OSD national options to respond if a US Navy ship (or perhaps even a civil ship or ships of allies) were PD-ed. This should be done as a precautionary minimum to avoid being caught flatfooted by a surprise PD attack. Even today it may be possible that the Chinese could produce a primitive PD warhead for a stationary mine placed on the perimeter of claimed territorial waters. Should evidence come to light that an adversary has or may soon possess operational PD capabilities, policy decisions on the matter would be urgent and mandatory.
• Assess the international and domestic legal implications of PDs and take legal and policy actions as necessary.

Conclusion

The US should develop and field PDs as quickly and with as little fanfare as possible. It seems highly likely that our adversaries will do so independent of US action. Even if that likelihood is estimated to be small today, the probability of the eventual appearance of PDs is high, and the possible consequences of their introduction could prove revolutionary. This combination of probability and consequence dictates a serious need to think through immediate and long-term measures both to exploit PDs for US strategic offense/defense and to counter them in defense of US and allied surface ships.

Note: A comprehensive overview, if now a bit dated, shows the direction of Navy thinking about mine warfare which would be a major —perhaps the major—means of exploiting PDs, especially for blockade, is Joshua J. Edwards and Captain Dennis M. Gallagher, USN, “Mine and Undersea Warfare for the Future,” Proceedings, August 2014 (Vol. 140/8/1,338). Currently a wide variety of Navy UUV efforts are underway, including some recent and soon-to-be-deployed hardware. None is focused on PD/counter-PD, as far as is publicly known. The Coast Guard has shown specific interest in using a small torpedo—the Surface Ship Torpedo Defense (SSTD) currently being evaluated—for what is a PD in all but name. Employment of swarms of small underwater devices is in early stages of technical evaluation of their feasibility, independent of a conception for their tactical use.

Bradford Dismukes, San Francisco, December 31, 2021