Don't Believe the Hype About Hypersonic Missiles

This is a guest post. The views expressed here are solely those of the author and do not represent positions of IEEE Spectrum or the IEEE.

Hypersonic weapons are one of the hottest trends in military hardware. These missiles, which fly through the atmosphere at more than five times the speed of sound, are commonly depicted as a revolutionary new tool of war: ostensibly faster, less detectable, and harder to intercept than currently-deployed long-range ballistic missiles. The major military powers-China, Russia, and the United States-have bought into this belief, with each investing vast sums in a hypersonic arms race.

But there is a glaring problem with this narrative: Many claims regarding the purported advantages of hypersonic weapons are false. These missiles are, in reality, an old technology with a massive price tag and few meaningful advantages over existing ballistic missiles.

An old technology

While commonly touted as an emerging technology," these weapons were first designed nearly a century ago, in late 1930s Germany. The Treaty of Versailles, signed at the end of World War I, prohibited most weapons development by the Nazi government. But a loophole allowed work on rockets, prompting an intense German interest in missiles. Out of this grew the Amerikabomber project, aimed at developing a weapon that could strike the United States from Germany. To this end, German engineers designed the Silbervogel (Silver Bird), the first conception of a hypersonic boost-glide missile.

Yet Germany declined to pursue this design, in part because it judged the costs of hypersonic weapons to be disproportionate with their meagre benefits. In subsequent decades, ballistic missiles achieved the same goal of swift warhead delivery to distant targets. Flying primarily through the vacuum of outer space, rather than the comparatively dense atmosphere, ballistic missiles were seen as superior to the hypersonic concept in many ways.

While research on hypersonic weapons continued throughout the 20^th century, no nation deployed them, opting instead for ballistic and cruise missile technologies. Even after spending more than US $5 billion (in current year dollars) developing the Dyna-Soar hypersonic glider, the United States abandoned the design in 1963 when it could not identify any clear objective for the weapon.

Interest in hypersonic weapons was rekindled in the early 2000s, but not on the basis of a performance advantage over existing missiles. Rather, the United States sought to use conventionally-armed missiles for rapid strikes, including against non-state targets such as terrorist groups. But it was concerned that nearby nations could mistake launches for nuclear-armed ballistic missile attacks. Hypersonic gliders fly low-altitude trajectories that could be easily discriminated from those of ballistic missiles.

As this history shows, nations have consistently judged hypersonic weapons to be an unnecessary addition to existing arsenals, offering at best niche capabilities. Why today, after decades of disinterest, has the idea that hypersonic weapons outperform existing missiles taken hold? Was some unique performance advantage overlooked?

While the strategic environment has evolved, the physics of hypersonic flight-and the limitations it places on performance-remain unchanged. In a recently-published article, we report the results of computational modeling of hypersonic missile flight, showing that many common claims regarding their capabilities range from overblown to simply false.

Technical reassessment

First, consider the claim that hypersonic weapons can reach their targets faster than existing ballistic missiles. The fastest hypersonic vehicles are launched on rocket boosters, like those that launch intercontinental-range ballistic missiles. Thus, both types of missile reach the same initial speeds. But hypersonic weapons fly through the atmosphere where they are subjected to substantial drag forces. Ballistic missiles, on the other hand, fly high into outer space where they are free from these drag effects. Thus, while hypersonic weapons fly a more direct path to their targets, they lose much of their speed throughout flight, ultimately taking longer to reach their targets than comparable ballistic missiles.

Gliding through the dense atmosphere at hypersonic speeds subjects these gliders to more than just drag forces. As they slow down, hypersonic vehicles deposit large quantities of energy to the surrounding air, a portion of which is transferred back to the vehicle as thermal energy. Their surfaces commonly reach temperatures of thousands of degrees Celsius.

This extreme heating limits performance in two ways. First, it constrains glider geometry, as features like sharp noses and wings may be unable to withstand aerothermal heating. Because sharp leading edges decrease drag, these constraints degrade glider aerodynamics.

Second, this heating renders hypersonic missiles vulnerable to detection by the satellite-mounted sensors that the United States and Russia currently possess, and that China is reportedly developing. Hot objects emit light in proportion to their temperature. These satellites watch for light in the infrared band to warn of missile strikes. Ballistic missiles are visible to them during launch, when fiery rocket plumes emit a great deal of infrared light, but become harder to see after rocket burn-out, when the warhead arcs through outer space. Hypersonic missiles, on the other hand, stay hot throughout most of their glide. Our calculations indicate that incoming gliders would remain visible to existing space-based sensors not only during launch, but for nearly their entire flight.

Finally, it is often claimed that hypersonic weapons will upend the strategic balance between adversaries because they can bypass missile defenses. The reality is more complex. As discussed, the effects of atmospheric drag and heating mean that hypersonic weapons will have few, if any, advantages over existing missiles when it comes to outpacing interceptors or evading detection. Still, their low-altitude flight would allow them to fly under the reach of defenses designed to intercept ballistic missiles in outer space. Their maneuverability could also allow them to dodge interceptors in the atmosphere.

Yet the performance of hypersonic weapons against missile defenses is strategically meaningful only if it offers a new capability (i.e., if these weapons do something existing missiles cannot). This is not the case. Existing long-range missiles could easily overcome defensive systems by fairly simple means, such as firing more missiles than the adversary has interceptors, or by using countermeasures, like decoys.

Hypersonic hype

In short, technical analysis of the performance of hypersonic weapons shows that many claims as to their supposedly revolutionary" performance are misinformed. So why has an arms race coalesced around a weapon that does not work as advertised?

Development and acquisition of strategic missiles are complex, bureaucratic processes involving a broad range of actors, from the military organizations that use these weapons to the legislators who fund them. In this context, decisions commonly serve the diverse interests (financial, professional, etc.) of broad coalitions, rather than narrow technical criteria. Even if a missile does not perform to the specifications by which it is marketed, proponents are often keen to convince others of its revolutionary" nature-thus securing funding, prestige, and other benefits that accompany the successful development of a new weapon.

But now is not the time to let spending on these weapons go unscrutinized. Federal budgets are under enormous strain, and every dollar spent on hypersonic weapon development is one that does not go to other pressing needs. Beyond these financial concerns, arms racing could increase tensions and the likelihood of conflict between nations.

To be sure, research on hypersonic flight is a worthwhile endeavor, with broad applications from commercial transportation to space exploration. But when it comes to costly, ineffective hypersonic missiles, the technical basis fails to justify the price.

Cameron L. Tracy is a Kendall Fellow in the Global Security Program at the Union of Concerned Scientists. He has a PhD in materials science.

David Wright 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. He has a PhD in physics.