How Fast Is Fast Enough? A Role for Supersonic Munitions in Standoff Strike

Evangelism, speculation, and anxiety attend America’s campaign to add hypersonic weapons to its standoff strike portfolio. These weapons, some of which may fly as fast as 20 times the speed of sound, have been cause for varying reactions in the national security establishment. America’s hypersonic weapon development is considered simultaneously a deterrence-restoring solution to a yawning capability gap, a threat to strategic stability with Russia and China, and an unjustified money pit in a constrained budget environment. Meanwhile, as hypersonic weapons grab headlines, the United States also remains deeply committed to acquiring far slower subsonic weapons, like the Tomahawk, that fly at speeds below Mach 1. Supersonic strike (above Mach 1, but below Mach 5) receives far less investment overall, lagging both the upper and lower velocity regimes. The Department of Defense is developing a “high-low” mix of exquisite, expensive hypersonic weapons and slower subsonic munitions despite the likelihood that future combat operations against high-technology adversaries will be munitions-intensive and, therefore, costly. An essential question — one that could deliver more options for decision-makers, enhance strategic stability, and even reduce overall costs — has been neglected: How fast is fast enough?

How did the Department of Defense arrive at its high-low mix? The common refrain is that hypersonic weapons will allow the United States to threaten “time-critical” targets like mobile missile launchers. Subsonic weapons would otherwise cover any targets that do not merit such urgency. For some missions, however, the expense of a hypersonic weapon may not be justified and a subsonic weapon will lack the necessary swiftness.

The balance across the hypersonic, supersonic, and subsonic flight regimes should be governed by some simple, informal guidelines that are fundamental to both aerodynamics and budgeting. First, the faster you go, the more it costs. Second, the farther you fly, the more it costs. The supersonic gap in the U.S. strike portfolio suggests the Pentagon has yet to decide upon the right balance among speed, range, and cost. A “high-medium-low” portfolio of hypersonic, supersonic, and subsonic munitions could provide the United States with options to spread fiscal, strategic, and technical risk across its strike portfolio, while creating new and more complex dilemmas for defending adversaries.

The Future of Warfare Will Not Be Cheap

A comprehensive look at the strike portfolio is necessary because, despite advances in precision targeting, future conflicts will require expending more munitions to destroy a given target. For much of the last century, the number of munitions required to destroy a given target declined as offensive technologies improved. U.S. airpower, for example, had progressed from the massive streaming bomber raids of World War II with hundreds of bombers and thousands of weapons, to the smaller strike “packages” of Vietnam involving scores of aircraft with hundreds of weapons, to solitary, stealthy B-2s over Belgrade and Baghdad destroying multiple targets with a few precision weapons.

Today, however, the munitions-per-target curve is set to slope upwards again because defensive systems have become more effective and more widespread. Advanced air defenses could require future strike plans to expend more munitions to stimulate, deplete, and overwhelm defenses before a sufficient number of weapons can “survive” until they hit their targets. This “salvo competition” will place fiscal pressure on the Pentagon, potentially compelling future planners to reserve the most expensive survivable weapons for only the most high-value targets while accepting the risk of taking more time to destroy other targets with less expensive, less survivable weapons.

The Case for Hypersonic Weapons

Hypersonic weapons have captured investment and attention because their speed and maneuverability might enable the United States to threaten heavily defended, time-critical, or fleeting targets like command-and-control nodes or mobile missiles. Defeating mobile missiles, in particular, continues to be one of the most challenging problems facing U.S. forces.

In the opening minutes of a potential conflict over, for example, the South China Sea or Taiwan, Chinese mobile missile launchers could unleash salvos of precision medium- and intermediate-range conventional missiles against airbases, ports, and depots in U.S. and allied territory potentially as far away as Guam. China’s anti-satellite missiles and long-range anti-ship ballistic missiles also deploy from mobile launchers.

In defending against these threats, it would be far more effective for the United States to attack the archer rather than the arrow. The above scenario therefore presents U.S. forces with an urgent and consequential game of whack-a-mole: Not all mobile missile launchers will be successfully located let alone successfully interdicted.

America’s strike arsenal, which is overwhelmingly comprised of subsonic weapons, would struggle to strike mobile missiles in time to make a difference. Although subsonic weapons could be deployed on shorter timelines from penetrating “stand-in” aircraft like the B-2 or upcoming B-21, such an approach would entail risk in loitering inside adversary defenses and would depend upon a costly, vulnerable tail of supporting assets like in-flight refueling tankers and airbases.

The United States is pursuing hypersonic weapons with the above concerns in mind. As of 2020, the Army, Navy, Air Force, and the Defense Advanced Research Projects Agency (DARPA) are developing six different hypersonic prototypes. The services have requested about $9 billion over the next five years, while DARPA was estimated to spend about $150 million in FY2020. These research programs and advanced prototypes are not fully funded “programs of record” with plans and timelines to acquire combat capabilities. Rather, they are part of the 2018 National Defense Strategy’s commitment to “prototyping and experimentation … prior to defining requirements.”

On one hand, this prototype-first approach could support the rapid operational deployment of hypersonic capabilities. On the other, the United States is taking risk by developing these systems without comprehensive requirements (a defined set of attributes, functions, and quantities) that could illuminate how these weapons fit comparatively and complementarily into the strike portfolio. For example, developmental efforts are pursuing two different flight approaches — hypersonic glide vehicles and air-breathing hypersonic cruise missiles — that will have different velocity, range, and cost characteristics. How will these differences in speed and distance affect target sets or constrain launch platforms? Can the Pentagon afford these weapons in the numbers needed for combat? Those without security clearances are left to speculate as to how the department is accounting for these concerns.

Future operational deployment of hypersonic weapons will also have to account for their impact on strategic stability between the United States and nuclear-armed competitors like China and Russia. Both nations’ mobile launchers are capable of deploying either conventional or nuclear weapons. A hypersonic threat to a conventional mobile missile launcher is a threat to a nuclear mobile missile launcher, in this case. Furthermore, hypersonic glide vehicles have trajectories similar to boosting ballistic missiles in the early stage of flight. From the standpoint of a targeted nation attempting to peer through the fog of war, does an incoming hypersonic weapon indicate a purely conventional threat or the opening move of a nuclear first strike? This strategic-level uncertainty should give pause to future operational planners as they assign weapons to targets.

The Role of Subsonic Weapons

For decades, the United States has favored long-range strike capabilities that operate in the subsonic regime of flight. While certain systems (e.g., air-to-air missiles and dashing tactical fighter jets) operate in the supersonic regime, most strike weapons (i.e., cruise missiles and glide bombs) and strike aircraft (i.e., long-range bombers and bomb-laden tactical fighters) fly under Mach 1. This bias toward subsonic flight is not without its merits. From a fuel-consumption perspective, turbofan and turbojet engines are efficient means to cover long distances. This is a primary reason why airliners, with the exception of the Concorde, have flown at the same subsonic speeds since the dawn of the jet age.

From bombers to standoff decoys, and from strike fighters to long-range weapons, the United States is currently recapitalizing a suite of capabilities that will need hours rather than minutes to cross hundreds or thousands of miles. Billions of dollars are being dedicated to a new penetrating bomber, a family of swarming decoys, F-35 acquisition, and various stealthy cruise missiles. In future combat, where superior intelligence is best exploited with superior speed, these systems might arrive too late.

Still, America’s stealthiest and longest-range munitions are subsonic. These munitions offer the ability to sneak into contested environments and, in many cases, allow launch platforms to remain outside the range of adversary defenses. Any future joint strike portfolio should include subsonic weapons on these merits, but flight time should remain a key consideration in planning.

Current U.S. Supersonic Investments

Hypersonic weapons, however swift, maneuverable, and difficult to intercept, will be expensive to acquire and it remains to be seen whether their introduction into the U.S. arsenal or use in combat would threaten strategic stability with Russia and China. Long-range subsonic weapons, some of which are designed for stealthy penetration of complex adversary defenses, lack the speed to catch some time-critical targets. Could supersonic weapons occupy a valuable middle ground? A supersonic munition could penetrate defenses at perhaps three times or more the velocity of a typical U.S. cruise missile while providing utility against some of the targets of the upper and lower missile velocity regimes.

Unlike the hypersonic flight regime, which is currently the subject of an intensive research and development campaign, supersonic flight has been thoroughly exploited from the lab bench to the battlefield. Supersonic flight has been a part of U.S. operations for decades, with a variety of missiles and aircraft exploiting the regime. The United States has developed supersonic standoff weapons since the 1956 flight of the SSM-N-9 Regulus II but, today, the ship-launched SM-6, an adaptation of a surface-to-air missile and the ground-launched MIM-168 Army Tactical Missile System are the only such strike weapons in the U.S. arsenal.

Critically, there are no supersonic standoff strike weapons deployable from penetrating platforms like the F-35, B-2, and B-21, or “arsenal” platforms like the B-52 and B-1. These aircraft instead rely upon jet-powered subsonic munitions like the Long-Range Anti-Ship Missile or glide bombs like the Joint Standoff Weapon. While it is true that hypersonic weapons will eventually be deployable from tactical fighters, bombers, and perhaps even cargo aircraft, there are no plans for these platforms to utilize supersonic standoff strike weapons.

As a strike weapon, the SM-6 has shortcomings in range, deployability, and explosive power that can be attributed to its original mission as an air defense weapon. Launching a roughly $5 million SM-6 requires a multibillion-dollar U.S. destroyer or cruiser to approach within 230 miles of its land or sea target. Its warhead weighs 140 pounds, a far cry from the 1,000-pound warheads of the Long-Range Anti-Ship Missile and Tomahawk (warhead mass is a vital factor when penetrating ship hulls or hardened, deeply buried targets). The Block 3 version of the Army Tactical Missile System, meanwhile, offers a 500-pound unitary penetrating warhead and flight speeds over Mach 3, but is limited to a range of 155 miles.

SM-6 and the Army Tactical Missile System are not the state of the art. Indeed, the best weapons of this nature are found abroad.

The State of the Supersonic Art

Russia and China have long included supersonic weapons in their respective strike portfolios. For decades, both nations watched the trajectory of U.S. strike munitions and platforms and developed their own measures and countermeasures accordingly. While their investments in hypersonic strike have left the United States playing catch-up, their arsenals have long included supersonic options.

The BrahMos, a joint development of the Russian and Indian defense industries, is arguably the world’s most advanced supersonic cruise missile. Capable of carrying a 660-pound warhead over 300 miles at velocities up to Mach 3, the BrahMos can be launched from ground, air, sea, and undersea platforms. The BrahMos poses a complex, stealthy, maneuvering threat for air defenses while flying as high as 49,000 feet to as low as ten feet. Each weapon costs $2.5 million. Russia and India are developing a new version, the BrahMos-ER, which can strike at Mach 4 from 500 miles away.

A notional U.S. missile inspired by the BrahMos and launchable from multiple domains is within easy reach. The United States already possesses key enabling technologies like ramjet propulsion, active radar, and satellite navigation. An acquisition program could leverage progress in the Air Force’s and Navy’s recent pushes to develop very long-range air-to-air missiles. Furthermore, some of the technologies already developed for advanced hypersonic and subsonic programs could be exploited, like compact booster rockets, automated target recognition, and jammers.

But First, Requirements

Adding a BrahMos-inspired missile (a jointly developed, 500-mile range weapon) to the U.S. strike portfolio is an attractive proposition. A supersonic standoff strike missile that’s deployable from air, land, sea, and undersea, with a range of roughly 500 miles, a flight speed of Mach 4, and a penetrating warhead of more than 500 pounds could provide heretofore unavailable options to the U.S. military. Furthermore, this weapon would fly flatter, supersonic trajectories entirely different from ballistic systems, thus relieving the potential strategic risks of hypersonic weapons being confused for nuclear weapons. For the sake of fiscal restraint, strategic stability, and operational efficacy, America’s first step toward such a weapon should be the establishment of well-defined, comprehensive requirements for its joint strike portfolio.

Requirements are the lifeblood of the Department of Defense’s acquisition process. When properly defined, they can reduce risks in both the development and use of new systems. Defining strike portfolio requirements with hard questions of efficacy and strategic stability would allow the Pentagon to assess whether there could be targets for which the cost of a given hypersonic weapon outweighs the urgency of its destruction. Conversely, there may be plans that could benefit from certain targets being destroyed on shorter, albeit not necessarily hypersonic, timelines.

Consider a notional strike utilizing a BrahMos-inspired missile against a time-critical target located 500 miles away. Planners could choose from hypersonic, supersonic, or subsonic munitions capable of flying at Mach 15, Mach 4, and just under Mach 1. These munitions would require about three, nine, or 40 minutes to reach the target. What missions can be met with which of these weapons? Given a defined set of missions, what are the political and military costs and benefits of striking about four times faster than a subsonic weapon but four times slower than a hypersonic weapon? If supersonic investments make sense, what elements of hypersonic and subsonic programs can be traded or trimmed?

Ideally, a mixed portfolio of subsonic, supersonic, and hypersonic weapons — a “high-medium-low” mix — would impose costs on the adversary, forcing them to re-evaluate the strategic value of its targets. Operationally, this adversary would have to prepare to interdict attacks coming in at different velocities and profiles that present multiple, resource-depleting dilemmas.

A mixed portfolio would also allow the Pentagon to balance its desire for speed against the constraints of its budget. While the Department of Defense’s budget has risen to historic levels in the last few years, this rate of growth had already begun to plateau before the COVID-19 pandemic began. Despite its substantial defense spending advantage over its competitors, the United States will have to make trades as the effects of COVID-19 cascade through the economy, tax base, and federal budget.

The Pentagon needs to recognize that a future where hypersonic weapons make up a small, sparingly-used portion of the arsenal is within the realm of possibility. It can prepare for this future by investing in supersonic weapons that could fill some of the gap by adding a velocity edge over subsonic munitions at less cost. Ultimately, the Pentagon will be unable to address these concerns and reap the potential benefits of supersonic weapons without well-defined, comprehensive requirements. Otherwise, it will remain impossible to determine with confidence just how fast is fast enough.

David N. Zikusoka is a research fellow at the Center for Strategic and Budgetary Assessments. He previously served in positions at the White House, Department of Defense, and U.S. Cyberspace Solarium Commission.

Image: Master Sgt. Michael Ammons