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Why hypersonic weapons cannot live up to their hype

Why hypersonic weapons cannot live up to their hype
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The United States, Russia and China are developing hypersonic weapons — missiles that carry warheads through the atmosphere at more than five times the speed of sound. The fevered interest in these weapons is driving competition among these countries that costs billions of dollars per year, heightens tensions, and may undermine broader arms control efforts. 

Ironically, the strong desire for these weapons results in part from widespread, exaggerated claims about unique capabilities they reportedly offer in terms of short delivery times, maneuverability and stealth. Pentagon officials continue to spread these claims, despite the fact that technical analysis shows they are simply false.

Such claims have been repeated broadly, leading to assertions by officials that “developments in hypersonic propulsion will revolutionize warfare.” In March 2018, Russian President Vladimir PutinVladimir Vladimirovich PutinNavalny doctor says he could 'die at any moment' Sunday shows preview: Russia, US exchange sanctions; tensions over policing rise; vaccination campaign continues Russia blocks key Biden Cabinet officials from entering in retaliation for sanctions MORE described Russia’s acquisition of this technology as “the most important stage in the development of modern weapons systems.” Russia declared with much fanfare in December 2019 that it had fielded the long-range, nuclear-armed Avangard, making it the first nation to acquire hypersonic capability and upping the ante for the United States.

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To better understand these claims, we conducted detailed computer modeling of long-range hypersonic weapons using publicly available information about a U.S. hypersonic test vehicle, the HTV-2. Our analysis shows that fundamental physical limitations imposed by high-speed flight through the atmosphere render hypersonic missiles, at best, an evolutionary — not revolutionary — advancement over existing ballistic missile technologies. 

Specifically, our calculations show that hypersonic weapons travel intercontinental distances more slowly than comparable ballistic missiles flying on low-altitude trajectories, called depressed trajectories. Moreover, we find that these weapons become so hot from atmospheric drag at these high speeds that they emit an intense glow that can be seen for most of their flight by space-based sensors. As a result, claims that they are “nearly invisible” to early-warning systems are not true — instead, they are easily seen and tracked by existing early-warning satellites.

In addition, while hypersonic gliders can maneuver, the extent of that maneuvering is more limited than is frequently suggested, since maneuvering comes at a significant cost to the range the missile can reach.

Since these limitations result from the fundamental physics of an object at high speed interacting with the atmosphere, they are not specific to the HTV-2 vehicle but are general features of a broad range of hypersonic weapons — both U.S. weapons and those developed abroad. 

The Pentagon has questioned whether our analysis is relevant to current hypersonic weapons being developed since we rely on available data from a test system that is several years old. The most important design elements we assume in our analysis, however, likely have not changed much since that time, and the prospects of their changing to significantly improve hypersonic performance in the future seem dim.

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In particular, the value of the lift-to-drag ratio (L/D) of a hypersonic weapon is central to its performance. To stay aloft, these gliders must generate aerodynamic lift. But generating lift also creates aerodynamic drag that slows the glider, and L/D tells how much drag necessarily accompanies the required amount of lift. Generating high drag reduces the speed of the glider, which limits its range and maneuverability, and results in heating of the glider and the surrounding air, which produces the infrared light signal trackable by satellites. 

While L/D of commercial aircraft may be 15 or larger, achievable values at hypersonic velocities are much lower. Following decades of research and development of hypersonic technologies, the HTV-2 is estimated to have an L/D of 2.6. There is no indication that significantly higher values have become possible without sacrifices in payload volume or thermal resilience in the last decade, or will any time soon. Without this, the fundamental physical issue — that high drag reduces the speed and range of hypersonic gliders and makes them visible from space — will not change. 

Why does it matter if these weapons ultimately cannot live up to the hype around them? On the one hand, these efforts are costing billions of dollars that could be used more profitably elsewhere. 

But the most consequential effects of these weapons on global security may arise from perceptions of their performance, rather than actual capabilities. Fears that a country might fall behind its adversaries in the hypersonic arena are exacerbating tensions between the United States, Russia and China and spawning a hypersonic arms race. This dynamic could add new weapons to global arsenals and prevent progress on serious nuclear arms control. Moreover, unsubstantiated beliefs about the military utility of hypersonic weapons are stifling arms control efforts to limit these systems before they proliferate. 

Pentagon scientists and engineers who work on hypersonic weapons certainly know that many of the public claims are exaggerations. It’s time for Congress to look into this issue in detail and ensure that science and reality overcome the hype.

David Wright, Ph.D., is a research associate in the Laboratory for Nuclear Security and Policy in the Department of Nuclear Science and Engineering at the Massachusetts Institute of Technology (MIT).

Cameron Tracy, Ph.D., is a Kendall Science Fellow in the Global Security Program at the Union of Concerned Scientists, a nonprofit organization founded more than 50 years ago by scientists and students at MIT.