A recent article in The New York Times strongly implied that hypersonic weapons under development at the U.S. Department of Defense are being overhyped. In particular, several points made in a recent Science & Global Security publication by Cameron L. Tracy and David Wright, “Modeling the Performance of Hypersonic Boost-Glide Missiles,” are highlighted that question the value of hypersonic weapons. Here’s what those authors got wrong in their analysis of hypersonics:
First, it is important to note that the article focused on strategic (long-range) hypersonic systems. The weapons are boosted by a rocket to high altitude and then glide and maneuver to their target, hence the terminology “boost-glide.” These long range systems form a subset of all DoD hypersonic activities. Within the simplifying assumptions made for strategic systems in the published analysis, there is certainly some merit to the data presented and conclusions drawn. Hypersonic systems may take longer to fly a particular range than existing ballistic systems. It may be possible to detect hypersonic systems from space using existing sensors.
However, the evaluation of actual hypersonic systems requires significantly more detailed analysis than that presented by the authors. In general, the flight performance and defensive challenges of military systems depend strongly on the specific technical details, and for that very reason some of these details cannot be discussed fully in the public domain.
In a general sense, though, what can be said is that maneuvering hypersonic vehicles are significantly more difficult to track than those on a purely ballistic trajectory. Once a ballistic missile is launched, it largely follows simple Newtonian mechanics to reach its target destination. So once a determination of its initial trajectory is made, it is relatively straightforward to track a ballistic missile, and there is sufficient time to respond defensively. Use of a depressed ballistic trajectory adds one extra level of complexity, but such systems still follow essentially a straight line when viewed from space.
A new and second level of complexity is provided by hypersonic systems on a maneuvering boost-glide trajectory that now adds cross-range (side-to-side) capability. This feature requires tracking of these systems throughout their trajectory, as it is no longer predictable, which greatly escalates the defensive effort.
It is also important to consider the value of strategic hypersonic systems not being fired one at a time in isolation, but rather simultaneously as part of a salvo — in numbers and in conjunction with other weapons. While the initial rocket launch signatures from space will look very similar for ballistic and hypersonic systems, it will be necessary to track all missiles in a salvo in detail to be certain of each of their trajectories. Multiple, disparate weapons fired simultaneously would therefore represent a further significant defensive challenge.
Finally, in addition to developing strategic hypersonic capabilities, the DoD is also expending a significant effort on smaller tactical (short-range) systems. These weapons could be launched from air platforms, rocket-boosted to altitude, then maneuver to their target.
The value provided by these tactical hypersonic systems is perhaps much easier to understand. For example, a Mach 8 system would cover a distance of 500 kilometers in just over 3 minutes. The short flight time provides a unique opportunity to address fleeting, high-value targets and dramatically compresses the timeline available for a defensive response. Launched from the air and boosted by a relatively small rocket also makes the initial launch and subsequent flight trajectory more difficult to detect. And again, maneuvering during flight creates additional tracking challenges.
Both China and Russia have already fielded hypersonic weapons with initial capabilities; and with vigorous development programs continuing, a broader range of such systems can be anticipated in the future. It is therefore essential that the U.S. responds through development of effective defensive measures.
Moreover, the fact that these nations have found significant value in hypersonic weapons should make it no surprise that the Pentagon has also identified effective use cases. Both strategic and tactical hypersonic systems offer new and unique military capabilities, and the U.S. must continue to pursue their development.
Iain Boyd is the H.T. Sears Memorial professor of aerospace engineering sciences at the University of Colorado. He has led studies on hypersonic technology and other topics for the U.S. government.
