SLCM-N: Necessary or Excessive?


The guided-missile cruiser USS Philippine Sea launches a Tomahawk cruise missile against ISIS targets as seen from the aircraft carrier USS George H.W. Bush Credit: ©US Navy, Mass Communication Specialist 1st Class Eric Garst

By Eric Gomez, Senior Fellow at the Cato Institute, USA

In October 2022, the White House released its long-awaited Nuclear Posture Review (NPR), which laid out the Biden administration’s nuclear policies. Perhaps the most controversial policy change in the 2022 NPR was the cancellation of the Sea-Launched Nuclear Cruise Missile or “SLCM-N,” which was introduced in the 2018 NPR. Supporters argue that growing nuclear threats make the SLCM-N more important than ever before. Opponents point to the opportunity costs of adding another weapon to an already stressed nuclear modernization plan.  

This article examines the potential role for the SLCM-N in the U.S. nuclear arsenal, provides an overview of the cases for and against it, and argues that the Biden administration made the correct choice.  

SLCM-N’s Origin and Role 

The SLCM-N’s raison d’être in the 2018 NPR was twofold. First, the Trump administration was worried about Russia’s large arsenal of low-yield or “tactical” nuclear weapons and the potential for “deterrence gaps” if the United States did not expand its own stock of low-yield weapons. Second, the Trump administration argued that the threat of deploying SLCM-N could pressure Moscow to return to compliance with the Intermediate-Range Nuclear Forces (INF) Treaty. 

The arms control rationale for SLCM-N was quickly discarded when the Trump administration withdrew from the INF Treaty in August 2019. Since then, debates over SLCM-N have focused on the role of the weapon in the U.S. nuclear arsenal.  

Since the SLCM-N was in the early stages of research and development prior to its cancelation there is very little unclassified information about its technical features. However, it is possible to glean information about the SLCM-N’s potential characteristics and its operational niche by examining the general features of other cruise missiles.  

Cruise missiles typically use an on-board jet engine to fly through the atmosphere slower than the speed of sound from their launch vehicle (e.g., aircraft, submarine) to their target. Their relatively slow flight speed allows cruise missiles to maneuver as they fly, which means they can take indirect routes toward their target. If an attacker knows the location of its opponents’ air defense systems, they could program a flight path that that minimizes the defender’s ability to detect the missile.  

Diagram showing notional flight paths of a ballistic missile, hypersonic glide vehicle, and cruise missile ©Government Accountability Office

Ballistic missiles, by comparison, travel much faster than cruise missiles but are easier to detect by early warning systems. Satellites with infrared sensors can detect a ballistic missile as it exits the atmosphere much more easily than a cruise missile flying within the atmosphere. Ground-based radar systems that look upward can see a large portion of a ballistic missile’s flight path, but due to the curvature of the Earth the same system may not detect a cruise missile if the missile is flying below the radar horizon.  

To sum up, cruise missiles are easier to shoot down once they are detected because of their relatively slow flying speed. However, they are harder to detect in the first place because they can fly below radar coverage and take a winding path to their target. The most likely launch platform for the SLCM-N, a nuclear-powered attack submarine, is famously difficult to locate, which further adds to the early warning challenge.   

The SLCM-N was going to be a tactical nuclear weapon. The current inventory of U.S. tactical nuclear weapons consists of air-launched cruise missiles carried by the B-52 bomber, the W76-2 warhead for the Trident submarine-launched ballistic missile, and gravity bombs carried by a mix of stealthy (B-2, F-35) and non-stealthy (F-16, F-15) aircraft.  

The operational niche for SLCM-N in the tactical nuclear arsenal has to do with its difficult to detect launcher. Air-launched cruise missiles are difficult to detect, but the bomber that carries them, the B-52, is easier to spot. The W76-2 is hard to detect before launch because it is on a submarine, but its ballistic flight path makes it easier to detect after launch. SLCM-N would be hard to detect both before and after launch.  

USS Florida launches a Tomahawk cruise missile in a 2003 test fire ©US Navy 

Arguments For and Against SLCM-N 

The Biden administration’s decision to cancel the SLCM-N reignited debates between the missile’s proponents and opponents. 

Arguments in favor of SLCM-N tend to emphasize the value of a flexible nuclear arsenal in a time of growing international danger. This perspective contends that the world has become a more dangerous place since 2018, especially where nuclear weapons are concerned. The rapid expansion of China’s nuclear arsenal, North Korea’s plans to field tactical nuclear weapons, and Russian nuclear threats during its invasion of Ukraine are three prominent examples of recent nuclear challenges. Keeping pace with these threats requires greater flexibility. As the Wall Street Journal editorial board put it, “Multiple options cannot  hurt in a crisis.”  

Another common argument in favor of SLCM-N is the need to fill an operational niche. Although the United States already possesses low-yield cruise missiles, the ability to launch from a difficult to observe submarine provides a capability that does not currently exist. With this niche filled, U.S. military commanders can offer more options to the president in a conflict.  

Arguments against SLCM-N turn the operational niche perspective on its head. Instead of being essential, the SLCM-N is excessive because the United States already possesses similar options for limited nuclear use. The recently unveiled B-21 stealth bomber will be capable of carrying a new air-launched cruise missile currently in development. Arming the B-21 with a next-generation missile will fill the SLCM-N’s operational niche of difficult to detect missile fired from difficult to detect launcher. Additionally, the costs of the B-21 and next-generation air-launched cruise missile were already incorporated into the U.S. nuclear modernization plan before the Trump administration introduced SLCM-N.  

The opportunity costs of pursuing the SLCM-N are another mark against the weapon. SLCM-N was a late addition to a massive U.S. nuclear weapons modernization plan. A 2021 report from the Congressional Budget Office estimated that the United States will spend $634 billion on nuclear forces between 2021 and 2030. Moreover, the annual price tag for nuclear modernization is climbing as multiple major weapons platforms move from research and development to procurement. The SLCM-N would also require manufacturing new nuclear warheads, but the Department of Energy is struggling to produce new plutonium pits.   

As nuclear modernization costs increase, military services will face difficult questions about how to prioritize conventional and nuclear capabilities. Congress could delay these tradeoffs by continuing to increase defense spending, but as the defense budget approaches $1 trillion annually and economic problems like inflation persist, more money for defense is not guaranteed. The SLCM-N is a relatively small part of this problem because of the nascent state of the program, and these questions and tradeoffs would exist without it. However, canceling the SLCM-N could make these looming challenges that much easier to address.  

Nuclear capable AGM-86B air-launched cruise missile trainer loaded onto B-52H Stratofortress at Minot Air Force Base, ND ©US Air Force, Tech. Sgt. Aaron D. Allmon II 

Conclusion: Biden Got It Right 

Although the SLCM-N would have increased the flexibility of the U.S. nuclear arsenal and filled an operational niche, it was an excessive solution. A B-21 bomber armed with new air-launched cruise missiles provides a “close enough” fit with the SLCM-N’s operational niche without adding new expenses to an already expensive nuclear modernization plan.  

Improvements in nuclear flexibility must be weighed against the opportunity costs and tradeoffs of adding a new capability. Pursuing SLCM-N might create some marginal deterrent benefit, but U.S. money and time is better spent elsewhere, and deterrence will not be meaningfully degraded by cancelling the SLCM-N. Moreover, stopping the program now, when it is still in its very early stages, involves lower sunk costs.   

The SLCM-N program was excessive to need. The Biden administration did the right thing by cancelling it in the 2022 NPR.  

Tomahawk cruise missile detonates above a test target, 1986 ©US Navy/Wikimedia 

About the Author:

Eric Gomez is a senior fellow at the Cato Institute, where his research focuses on nuclear strategy, arms control, and deterrence issues in East Asia. With Caroline Dorminey, he is the co-editor of America’s Nuclear Crossroads: A Forward-Looking Anthology (Cato Institute, 2019).  

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