Hypersonics weaponry—an emerging missile technology that sends warheads gliding through the atmosphere at high speeds—has garnered a great deal of attention in the press. In a recent post I showed that claims of their “revolutionary” advantages are highly exaggerated. Hypersonic weapons travel more slowly than existing ballistic missiles, can be detected by existing satellite technologies, and do not meaningfully alter the balance between missile offense and defense.
But there is one more oft-cited advantage of hypersonic missiles: accuracy. According to press reports, the ability of these weapons to strike targets with pinpoint precision ranges from “phenomenal” to “terrifying.”
While assertions of the revolutionary speed and evasiveness of these weapons fall apart under closer scrutiny, do claims of accuracy hold up? And do they justify the budding hypersonic arms race?
They do not. In fact, several factors unique to hypersonic flight conditions hinder precise warhead delivery. In this respect, hypersonic weapons will perform no better than existing ballistic missile technologies.
Why accuracy matters
Missiles are used to deliver an explosive payload over long distances. Existing ballistic missiles accomplish this by lofting warheads high into outer space. Hypersonic missiles fly at lower altitudes, through the atmosphere. In both cases, missiles must fall back to Earth near their targets, since the explosives they carry will only inflict damage within a certain radius of their detonation. The closer to its target a missile strikes, the more likely that target will be destroyed. Thus, increased accuracy can render missiles more effective.
The precise accuracy required to destroy a target depends on the type of explosive ordinance a missile carries. Modern nuclear warheads are so powerful that even poor accuracy is often acceptable. Many of the technologies developed to improve accuracy are not deployed on modern nuclear-armed missiles, since there is little benefit to enhanced precision. In contrast, weaker conventional explosives necessitate higher accuracies. Because the US hypersonic weapons program is focused on the prompt delivery of conventional explosives, it requires a highly-accurate missile.
The sources of (in)accuracy
Once a target is identified and located, a missile must know how it needs to fly to reach that target (known as guidance) and must execute the maneuvers necessary to get there (known as control).
Guidance systems track a missile’s position and motion at a given moment. This can be accomplished in several ways, and often multiple techniques are used in tandem. Inertial guidance systems use internal components (accelerometers, gyroscopes, etc.) to track every move a missile makes; an onboard computer then calculates its trajectory using simple equations of motion. These self-contained systems are highly reliable, since they require no external information to function, but can accumulate substantial errors over long flight times. Better precision can be obtained if external references are used. Advanced guidance systems track a missile’s position relative to stars, terrain, or GPS satellites.
Once a missile determines its position and motion, it can maneuver to correct for any deviations from an ideal trajectory. During the initial portions of flight, when missiles are accelerated by powerful rockets, this is achieved by controlling the direction of rocket thrust. Later, they can take advantage of aerodynamic forces to maneuver while within the atmosphere, if equipped with the necessary wings or fins.
All of these guidance and control components must compete with the many external factors that can cause a missile to veer off course. Gravity anomalies—differences between the actual local force of gravity and the predicted value—introduce errors to missile navigation. Unpredictable winds and variations in air density jostle missiles while they travel through the atmosphere. Hypersonic flight through dense air also produces immense heating of a missile’s surface, scouring away material. The resulting damage alters missile aerodynamics, degrading control.
The hypersonic (dis)advantage
Accounting for these factors, hypersonic weapons begin to look less appealing.
To be sure, the maneuverability of hypersonic missiles is far superior to that of ballistic missiles. Since hypersonic weapons fly through the atmosphere, they can take advantage of aerodynamic forces to course-correct over most of their flight paths. Ballistic missiles, in contrast, can execute corrective maneuvers only for brief periods during their initial ascent out of the atmosphere and final descent to Earth, assuming they are fitted with a special maneuvering reentry vehicle (MaRV). Hypersonic weapons might, therefore, hold an advantage in terms of control.
But this potential advantage is counterbalanced by the myriad challenges presented by hypersonic flight. Because hypersonic missiles spend most of their flight time at low altitudes—in contrast to high-flying ballistic missiles—they are subjected to unpredictable atmospheric forces for longer durations. In fact, ballistic missiles are designed to dart through the atmosphere as quickly as possible before striking their targets, since this enhances accuracy by minimizing the influence of wind and variable air density. Here, ballistic missiles have the advantage.
During their long-term atmospheric flight, hypersonic weapons also experience more extreme heating. No material can withstand the resulting temperatures for long. Thermal damage to the surface of these missiles has caused loss of control in flight tests. Ballistic missiles, since they fly primarily through the vacuum of outer space, are less vulnerable to thermal effects.
When it comes to guidance, both hypersonic and ballistic missiles can be equipped with the same navigation technologies. But long-term atmospheric flight still limits hypersonic weapon performance. The immense heating generated by their high-speed, low-altitude flight induces chemical reactions in the surrounding air, generating plasma—a sheathe of electrically-charged gas—around these missiles. This plasma can absorb electromagnetic radiation, blocking communication with satellites and other external sources of guidance.
Hypersonic weapons come up short
Considering these factors together, the question becomes this: does the enhanced maneuverability of hypersonic weapons outweigh the numerous disadvantages of high-speed atmospheric flight?
It does not. The test record shows that maneuverability is not the limiting factor for accuracy. Ballistic missiles equipped with MaRVs, like the Pershing II, could strike within about 30 meters of their targets as far back as the 1980s. By 2002, a Trident D5 ballistic missile equipped with an experimental MaRV was able to steer to within a few meters of its navigation system’s target, despite the limited maneuvering time allowed by a ballistic trajectory.
With existing ballistic missile technologies providing sufficient maneuverability for course correction, there is little to be gained from a hypersonic weapon’s increased maneuvering time. Instead, accuracy is determined by factors like guidance precision and atmospheric effects—areas in which hypersonic weapons face unique challenges.
Takeaways
In short, hypersonic weapons possess no intrinsic accuracy advantage over competing technologies, like MaRV-equipped ballistic missiles. In some respects, they are actually at a disadvantage.
With a great deal of investment into design and development, many of the challenges of hypersonic flight could probably be overcome. Current work on advanced cooling systems, for example, could mitigate surface damage from missile heating.
But ultimately, this would yield a weapon no more accurate than currently-existing missile technologies. When it comes to the purported accuracy advantages of hypersonic missiles, those lauding these systems have missed the mark.
The featured image in this blog is courtesy Wikimedia.