U.S. Navy war plans depend on nuclear-powered attack submarines (SSNs), especially where access is denied to U.S. and allied surface ships and aircraft. This would be especially true in the South China Sea if the Navy were called on to repel an invasion of Taiwan. U.S. submarines possess a level of undersea superiority assured in no other domain on the far side of the globe. But, despite this technical and tactical superiority, U.S. SSNs are not adequately armed or equipped to conduct an extended sea-denial campaign, especially against the large, concentrated, unconventional force of more than 5,000 boats and ships China would likely use to invade Taiwan.1
The submarines’ superiority depends in part on high-quality, high-cost heavyweight torpedoes whose inventory—on ships and in depot reserve—is insufficient to face such a flotilla. Operational wargames have identified this inventory as the critical shortfall of the force.2 A new weapon and a new way of thinking about submarine tactics are imperative. Expanding SSNs’ arsenals to include not only the heavyweight torpedoes currently carried but also very lightweight torpedoes would allow for more-flexible engagements against smaller and unconventional vessels.
The Mk 48
The Navy’s attack submarine force is built around the Mk 48 heavyweight torpedo. Introduced in 1972, the Mk 48 was designed to allow SSNs with enough stealth and speed to defeat the Soviet Navy in the open ocean by closing with and destroying even the most capable Soviet capital warships and antisubmarine warfare (ASW) platforms. During the Cold War, this would have been done in short, limited engagements to win an attritional naval war against the Soviet battle fleet as well as to prevent the launch of nuclear weapons from Soviet ballistic-missile submarines.3
Weighing 1,678 kilograms, the Mk 48 torpedo delivers a 295-kilogram high-explosive warhead. Jane’s Naval Forces has estimated its maximum operating depth at 800 meters and says it possesses a range of up to 50 kilometers and a maximum speed of 55 knots.4 Though it has never been fired in anger, it is a highly versatile, accurate, and reliable weapon capable of destroying or disabling most submarines or armored surface vessels with a single hit. Wire-guided, the Mk 48 can be steered to a target while the submarine provides the most up-to-date targeting information, significantly improving the probability of a disciplined operator scoring a hit.5 The Mk 48 remains one of the most capable and credible weapons in the U.S. arsenal. It has undergone several upgrades to its propulsion and targeting systems in the past 50 years to keep it relevant to the modern force. The latest variant is the Mod 7 Common Broadband Advanced Sonar System torpedo.6
Despite all its capabilities, however, the Mk 48 is a weapon for a different war. U.S. SSNs of course need to maintain the capability to destroy capital warships and submarines, but much more will be asked of them in the next conflict.7 The Mk 48 is far from obsolete, but U.S. attack submarines will have critical new requirements placed on them in a maritime war against China, and the heavyweight torpedo is not the ideal weapon to meet all of them.
Limitations of the Mk 48 Weapon System
In a war to defend Taiwan, attack submarines will be key assets that will have to penetrate deep into the South and East China Seas to conduct sea-denial operations against China’s fleet.8 If that fleet included only People’s Liberation Army Navy (PLAN) warships, the current SSN deployment loadout of heavyweight torpedoes might be sufficient. Challenges arise, however, when considering that China’s invasion force would also employ vast numbers of China Coast Guard, maritime militia, and merchant marine vessels—more than 5,000 ships and craft, from fishing boats to heavy cargo ships and passenger ferries.9 The enormous quantity of shipping used would take tremendous firepower to repel.
The current U.S. fleet is not equipped to do so. A Virginia-class SSN typically deploys with 20 to 26 Mk 48 torpedoes, a number limited by space in the torpedo room.10 A proficient crew can launch its first four weapons in about two minutes. To reload each tube takes 10 to 20 minutes—up to an hour for all four—depending on the arrangement of the room, an amount of time that will increase for follow-on engagements as weapons need to be moved from more distant stow positions.
Unless it is counterattacked, an SSN typically will linger through an engagement to wire-guide a torpedo and maximize the hit probability. During the reload time, the submarine is most vulnerable to counterattack. Unless she hits all potential targets in the area in the initial engagement, she would need to withdraw and reattack several hours later. Under this model, the SSN could be counted on for four to five engagements, destroying perhaps two to three ships each time, for a total of 8 to 15 kills, before she must return to the depot to reload.
With these significant limitations, assuming eight SSNs are available in theater during the first two weeks of battle, they could kill no more than about 120 ships.11 While this would be a dominating number when set against the PLAN’s count of capital warships and submarines, it represents a mere dent in China’s total maritime force. It would not provide a meaningful defense of Taiwan against a full-scale invasion.12 All these factors weigh heavily on every commanding officer, who must not waste high-value weapons on low-value targets. The inventory shortfall has been recognized since at least 2001, but no solution has yet been implemented.13
The ability of U.S. submarines to approach and attack capital warships with heavyweight torpedoes is without match. The problem with such a plan is that it fails to account for China’s center of gravity—its ground invasion force—matching instead the U.S. Navy’s center of gravity, the SSNs, against China’s surface combatants, a supporting element. During the weeks U.S. submarines would spend hunting down and destroying the PLAN battle fleet, the invasion would continue, enabled by logistical support from hundreds or thousands of other boats and ships. Therefore, U.S. attack submarines need to carry more weapons—enough to significantly disrupt the Chinese invasion and logistics seatrain.
Development of the Compact Rapid Attack Weapon System
A massive cross-strait invasion might include as many as 5,000 vessels transporting and sustaining an army of several hundred thousand.14 To defeat an armada of that size, U.S. SSNs need a weapon they can carry by the hundreds and launch by the dozens. It must be a weapon they can expend en masse without a commanding officer having to conduct a complicated risk/reward assessment for every shot fired.
The requisite weapon has been under development intermittently for three decades. As the Cold War wound down in the early 1990s, the Office of Naval Research (ONR) was tasked to develop a specialized very lightweight torpedo as a naval defense system that could target and destroy an incoming torpedo. The Navy funded it for the limited purpose of enhancing the torpedo defense of U.S. capital surface ships. The result was a design with a 6¼-inch diameter, but as the Cold War ended and the fear of a fleet engagement dissipated, the project was mothballed before it could come to maturity.15
In 2010, interest in the antitorpedo defense system was renewed, again with capital-ship defense in mind. Then–Chief of Naval Operations Admiral Gary Roughead declared it an urgent operational need, and the Penn State University Applied Research Lab (PSU ARL), in partnership with ONR, got to work. The program history that follows comes primarily from the author’s interview of Tony Kittell, a program engineer at PSU ARL who has worked on this and associated projects for the past 16 years.
PSU ARL began developing a Compact Rapid Attack Weapon (CRAW) prototype, starting from a modification of the ONR design: 9 feet long with a slightly bigger 6¾-inch diameter. Though the preliminary design was to be launched from surface ships, the size was chosen as the largest that could fit in a Virginia-class submarine external decoy launcher. In the ensuing years, PSU ARL developed a fully functional prototype, and in 2014, CRAW demonstrated early operational capability from the aircraft carrier USS George H. W. Bush (CVN-77). Eventually, CRAW was deployed as a defensive system on five aircraft carriers, only to be defunded again in 2019.16
In 2020, the Undersea Defensive Warfare Systems Program Office (PMS 415) expressed interest in seeing CRAW developed for employment on the Virginia class, still primarily as a defensive measure. Virginia-class boats have 14 external 6-inch launchers—seven each, port and starboard—which house and launch acoustic jamming countermeasures and acoustic decoy devices.17 The desired tactic was to pair acoustic decoys or jamming countermeasures with CRAW as an antitorpedo, hard-kill defensive measure.
But, while the Navy has begun to recognize the offensive potential of these weapons, the ideas have not gained enough traction. CRAW development has matured and is ready for production, with an initial batch of a few dozen under contract with Raytheon for defensive early operational capability in 2026.18 There is currently no intent to expand the arsenal of CRAWs on SSNs above and beyond the external launchers, limiting them to 7 to 14 per platform under the current design. But the Navy should be thinking bigger.
Implementing CRAW
Very lightweight torpedoes (VLWTs) could revolutionize submarine approach-and-attack tactics. Mk 48 torpedo updates have led to incremental improvements in these tactics, but there has been no landmark transformation. And those incremental changes do not account for the tactical environment in the South and East China Seas and the Taiwan Strait that is so different from that of the North Atlantic.19
Using CRAW as a defensive antitorpedo weapon is essential, but it is only the beginning. CRAW could be a valuable offensive weapon in the western Pacific environment if the submarine force’s philosophy can bend to embrace it. With aggressive battle tactics and production ramped up to permit a CRAW loadout numbering in the dozens, each SSN would pose a tremendous threat to an enemy amphibious force as well as its battle fleet. This would disrupt a large-scale invasion and interdict enemy logistics lines that employ many small- and medium-sized surface craft.
To do that, the Navy first would need to prioritize the continued development of CRAW as both an offensive and defensive weapon. The PSU ARL prototype warhead system is ready to meet this multimission need. But the Navy must fund it far beyond the handful of weapons in the pipeline.20
Beyond the raw capabilities of the torpedoes themselves, the submarine force must develop innovative ways to load many onto each ship. Carrying up to 14 in the external launchers is a good start, but it could be dramatically expanded. Given their size (6¾-inch diameter, 9 feet long), an SSN could carry 10 CRAWs for every heavyweight torpedo. A canister launch system could, in principle, allow up to two stacks of five inside each torpedo tube, enabling a multitarget, rapid-fire, and ready-reload capability. An SSN might be able to fire ten or more weapons at as many different targets in a two-minute engagement and still have dozens of weapons left for follow-on engagements. Instead of 26 heavyweight torpedoes and 14 CRAWs, the SSN could carry 10 heavyweights and 160 CRAWs internally (plus whatever self-defense weapons are loaded in the external launchers), substantially improving its potential effectiveness against the otherwise overwhelming fleets of medium and light craft China could employ in an amphibious invasion.21
Though initial production run details and costs are not publicly available, Penn State estimates each VLWT might cost roughly an order of magnitude less than the $5 million Mk 48.22 If that is correct, the cost of a full submarine loadout would remain about the same—$5 million for each Mk 48 and $5 million for each set of ten CRAW VLWTs.
New Tactics
A dud Mk 48 or a miss represents 4 or 5 percent of a Virginia-class submarine’s torpedo firepower. A single CRAW dud (or miss) would represent only about half a percent. Recognizing this could lead to a transformational shift in offensive thinking, permitting innovative tactics well outside those currently considered feasible.
With a maximum salvo size of four weapons, a significant reload time, and only 26 torpedoes on board, at present an SSN commanding officer must discriminate carefully in choosing targets to engage as well as allow for duds along the way. He or she might even bypass a valuable engagement to preserve the submarine’s weapon inventory to hunt for a higher value target—capital ships and ASW-capable combatants, for example. With so many targets in the area—many exceptionally vulnerable to a submarine attack—this self-imposed restraint limits the fleet commander’s ability to turn the SSNs’ combat potential into combat power.
With a large CRAW arsenal, however, the SSN needs only to slip inside the ASW screen. Once under the enemy’s logistics seatrain, it could hide in the noise of the dozens or even hundreds of craft passing on the surface and conduct a large-scale attack.
The hardest part would not be sneaking itself in but fighting its way out. The SSN would need to hold some weapons in reserve to evade a counterattack and escape the target zone—hard to do when an attack submarine is out of the fight for up to an hour after the first four Mk 48 shots—a dangerous time if escape is contested. But a substantial CRAW loadout would allow sufficient numbers to be held back for antitorpedo and antiship defense. That would leave enough munitions for several such engagements before needing to reload at a depot.
Vulnerability to ASW
The CRAW is not a perfect solution, however. Its comparatively short range might put the SSN at risk of detection or counterfire. This would be true whether the SSN were attacking or merely in the vicinity of the PLAN’s better ASW warships. Outside of a few large, high-end combatants, though, the tactical risk of detection and attack by PLAN surface forces and submarines is manageable. To engage those few ASW-capable platforms, the Mk 48 would still be the preferred weapon. For threats with limited ASW capability, U.S. attack submarines could safely engage with CRAW or simply bypass them. Once behind the enemy screen, the SSNs could do what submarines have always done: destroy enemy logistics. The Mk 48, after all, is not a perfect solution either. Assessing capacity against capability requires judgments that vary from situation to situation.
The U.S. submarine force’s torpedo inventory will be its most significant constraint in a war in the western Pacific. Given the policy and industrial limitations on Mk 48 torpedo production and the physical limits of SSN torpedo loadouts, the Navy needs to consider alternative weapon systems. Designing equipment to load dozens or even hundreds of CRAWs on each SSN, and developing the tactics to use them in mass engagements and escape maneuvers would significantly multiply the combat effectiveness of each SSN, the U.S. Navy’s most critical asset. Without weapons such as CRAW and tactics to employ them, the U.S. Navy submarine force will quickly culminate, and there might remain no other means to fight off an invasion.
1. Mark F. Cancian, Matthew Cancian, and Eric Heginbotham, The First Battle of the Next War: Wargaming a Chinese Invasion of Taiwan (Washington, DC: Center for Strategic and International Studies, 2023), 64–69; and Lonnie Henley, China Maritime Report No. 21: Civilian Shipping and Maritime Militia—The Logistics Backbone of a Taiwan Invasion (Newport, RI: China Maritime Studies Institute, U.S. Naval War College), 1–2.
2. Cancian et al., The First Battle of the Next War, 64–69.
3. Clifford Funnell, Jane’s Underwater Warfare Systems 2010–2011 (Alexandria, VA: Jane’s Information Group, 2010), 389–91.
4. See Jane’s Naval Forces, “Background Information: Mk 48 AdCap,” archived at www.janes.com/defence/naval_forces/news/juws/juws010202_1_n.shtml.
5. Funnell, Jane’s Underwater Warfare Systems, 389–91.
6. “Lockheed Martin to Build Components for MK 48 Submarine Launched Torpedoes With On-Board Sonar Guidance,” Military Aerospace, 20 December 2022, 3.
7. Cancian et al., The First Battle of the Next War, 111–16.
8. CAPT William Toti, USN (Ret.), “You Can’t Win without (More) Submarines,” U.S. Naval Institute Proceedings 149, no. 12 (December 2023).
9. Henley, Civilian Shipping and Maritime Militia, 1–3.
10. Nick Brown, Alex Pape, Michael Fabey, and Ridzwan Rahmat, Jane’s Fighting Ships (Alexandria, VA: Jane’s Information Group Inc., 2019), 942.
11. Cancian et al., The First Battle of the Next War, 68.
12. Cancian et al., 111–16.
13. Hunter Keeter, “Senior Submariner Cites Shortfall In Torpedo Production Base,” Defense Daily, 19 June 2001, 4.
14. Henley, Civilian Shipping and Maritime Militia, 17.
15. Tony Kittell, Penn State University Applied Research Laboratory, interview with author, 6 October 2023.
16. Kittell, interview.
17. Brown et al., Jane’s Fighting Ships.
18. Kittell, interview.
19. Henley, Civilian Shipping and Maritime Militia, 17.
20. Kittell, interview.
21. Funnell, Jane’s Underwater Warfare Systems.
22. Kittell, interview.
23. Christopher D. Yung, “In D-Day’s Wake,” Naval History 23, no. 3 (June 2009).