Hypersonic weapons do not zig and zag across the sky. The stresses and heat generated by a 10,000-pound glide vehicle travelling at five to 15 times the speed of sound are so great a sharp turn would rip the weapon apart. Yet even a slight adjustment can make these weapons unpredictable and nearly impossible to intercept, given their speed, and both China and Russia have developed weapons that pose significant and destabilizing threats. U.S. military leaders say it will take about a decade to counter those threats with new defenses.
Neither the Missile Defense Agency nor the Space Force have revealed the extent to which they can track hypersonic weapons or how close the United States is to being able to intercept a hypersonic weapon. Unlike ballistic missiles that travel predictable trajectories, hypersonic weapons can fly low, evading radar and can maneuver during the cruise phase. They are intended to quickly destroy high-value targets, such as aircraft carriers.
Experts at MIT and the University of Colorado say current U.S. defense can detect these new weapons, but do not yet have the ability to strike a hypersonic weapon in mid-course. Working in their favor is that maneuvering the super-fast weapons comes with a price: Any change in course generates drag, greatly reducing the weapons’ range.
The long pole of the tent is making faster and more agile interceptors.David Wright, Massachusetts Institute of Technology
Currently, Overhead Persistent Infrared missile warning satellites, like Space-Based Infrared System (SBIRS) and Defense Support Program (DSP), in a variety of orbits, detect missile and space launches the moment they begin. Ground-based radars detect and track missiles in-flight, within and above the atmosphere, Space Operations Command spokesperson Mike Pierson told Air Force Magazine.
But the threat posed by hypersonic weapons requires new capabilities.
“Hypersonics were absolutely considered as part of our threat-informed analysis,” said Pierson. Space Force worked with the National Reconnaissance Office, MDA, private industry, and others to complete an integrated force design analysis and recommendation for missile warning and tracking.
“We’re in the process of developing new force designs for future space architectures,” he added. “There are some sensitivities behind some of these questions as they get into future space architectures, which are still pre-decisional.”
In January 2021, MDA awarded contracts to develop a Hypersonic and Ballistic Tracking Space Sensor (HBTSS) to L3Harris Technologies and Northrop Grumman. These new satellites will integrate with ground radar to track hypersonic missiles anywhere on the globe. MDA plans on-orbit demonstrations of two prototype satellites by 2023. In November, MDA awarded three new Glide Phase Interceptor (GBI) contracts to develop the ability to intercept a hypersonic weapon in mid-course by the late 2020s.
“When you look at defending hypersonics, our focus has been on the regional fight,” said MDA Director Vice Adm. John Hill on Dec. 6, following the initial Fielding of the Long Range Discrimination Radar at Clear Space Force Station, Alaska.
Hill said that U.S. aircraft carrier battle groups are now equipped with a Sea-Based Terminal (SBT) missile defense capability.
“They’re going to have that destroyer operating to protect the carrier against the end game of a hypersonic,” he said.
SBT uses the Aegis Baseline 9C capability, including the SPY-1 radar and SM-6 interceptor to defend “against anti-ship ballistic missiles and some hypersonic threats,” the Missile Defense Agency told Air Force Magazine.
But both MDA and experts warn that successfully defeating hypersonics ultimately must start before the terminal phase of a missile’s trajectory.
“We have a program that we are working toward … that takes us further back into that trajectory for a layered defense against hypersonic [threats], and that would be in the glide phase,” Hill said.
“If you can defeat in the ballistic phase of a hypersonic or when it’s launched by an aircraft or launched by a cruise missile, that’s, step one, right? Then if you can kill it in the glide phase,” he said. “That’s great because really all you’ve got left is terminal capability.”
Asked what capability the military now has to track and target hypersonic weapons during the glide phase, Hill said: “We do not have that capability … today.”
Maneuverability Is ‘Oversold’
Maneuverability of a 10,000 to 20,000 pound weapon system traveling at hypersonic velocity is limited, experts say.
“You see these videos, you see where these things are just like cruising around, going around things and stuff,” said MIT physicist Dr. David Wright, a research affiliate at the Laboratory for Nuclear Security and Policy. “That just isn’t the way it goes. Because the thing is going so damn fast, that to change the direction … you have to do a heck of a lot of work, and it takes a long time.”
Hypersonic weapons can have control rudders and fins, which can be adjusted to change altitude or direction, but at the cost of both speed and range. Hypersonic missiles can maneuver in the mid-part of their flight, dropping to about 50 km altitude and traveling a good distance of their trajectory using lift forces to maneuver. Material strength under the overwhelming heat and pressure created at that speed is a primary limiting factor.
Dr. Iain Boyd, director of the Center for National Security Initiatives at the University of Colorado Boulder, said even small adjustments can make a big difference. “You’re not maneuvering so that you’re turning very large angles,” he said. “You’re just doing a little bit. And that’s more than enough to make it difficult to track.”
Wright explained the challenge. In lower altitudes, both lift and drag are greater due to greater atmospheric density. “
“The point is that if the vehicle drops to lower altitude, the atmospheric density is larger,” Wright said. “That allows it to create more lift force so it can turn more quickly, but it also increases the amount of drag it experiences.
Wright calculated that for a hypersonic glide vehicle traveling at Mach 15, it would take seven minutes to turn 30 degrees, and that if it were flying at an altitude of 40 km, it would have to drop 2.5 km to achieve that objective.
“I have a radius of curvature of like 4,000 kilometers,” he said. “It’s a very slow turn. And at the same time, I’m losing like 15 percent of my range.” Dropping instead by 5 km, the turn can be made in half the time—but the price of that maneuver would be 25 percent of vehicle range.
“People have really gotten this idea that it’s like this speed boat, you’re just cruising around,” he said. “The timing is much slower than that and it’s much more costly. So, I think that’s been oversold.”
MIT Hypersonics Research Laboratory Director Dr. Wesley L. Harris is currently working with a team of seven Ph.D. students to understand the loads, pressure distributions, and heat transfer on hypersonic vehicles.
“Those are enormous loads,” he said. “These are usually 20,000 [pounds]no less than 10,000-pound vehicles flying at Mach Five. So, a slight deflection can generate a big, heavy load leading to shockwaves, even leading to separation of fins, control surfaces, on these vehicles, enormous pressure, therefore force loads and shockwave boundary interaction, on these vehicles.”
Wright believes the enormous restrictions on vehicles moving that fast has been left out of the debate on hypersonics.
“I think there are all these myths about [hypersonics being] undetectable and maneuvering and all that kind of stuff,” he said.
MDA is nonetheless preparing to defend against hypersonic weapons at all phases of its trajectory.
“Hypersonic weapons can travel at exceptional speeds with unpredictable flight paths that challenge existing defensive systems,” the Missile Defense Agency said in a statement. “MDA is developing a systems-level defensive architecture to provide a layered defense to address a growing threat posed by hypersonic weapon systems.”
The “confounding” aspect of tracking a hypersonic glide vehicle, Boyd explains, is how it differs from a ballistic missile, cruising at an altitude that missile defense systems have never before needed to look.
“These new systems, really, they’re flying in a different part of the sky, and they can maneuver all along the trajectory,” he said. “They are a much more difficult challenge on the defensive side.”
At an altitude between 20 and 60 km, hypersonic weapons operate both far above airplanes and far below satellites in low-Earth orbit.
“One of the things about these hypersonic systems is that they kind of lie at the seams in between things that we have in place,” Boyd said. Neither space-based sensors designed to detect ballistic missiles nor ground-based radars are designed to address this threat.
“There’s actually a lot of real estate out there, but we’re not used to looking,” he said. “They’re not following a predictable trajectory, and they don’t really have to maneuver very much to be lost to us.”
The SBIRS can detect rocket launches by identifying their heat signature. Theoretically, they could also track hypersonic weapons by watching heat they generate in flight, Wright said.
“If you’re flying something at Mach 10 through the thick atmosphere at 50 kilometers, the thing is so hot that you’re creating a red glow that can be seen by these satellites,” he said. So the issue is not that an incoming missile is invisible, he added.
“This idea that it’s just going to be a surprise ignores the fact that one of the physics properties of these things is they create so much heat [that] they can be seen by early warning satellites.”
But Boyd notes that a satellite would have to continuously track such a missile and that as it slows, the heat signature could become too faint to pick up.
“There’s almost certainly a need to have higher sensitivity sensors to pick them up,” he said.
That’s the focus of the Missile Defense Agency’s Hypersonic and Ballistic Tracking Space Sensor contract. Phase IIb of the project will build on industry designs and risk-reduction efforts.
MDA will continue to develop HBTSS as a unique Overhead Persistent Infrared (OPIR) sensor providing fire-control quality tracking data on hypersonic threats and ballistic missile defense threats. MDA plans to work with the Space Force to integrate the future system into a unified defense.
“HBTSS is needed, since we cannot populate the earth and the oceans with terrestrial radars to meet this need,” the agency read when it announced the contract award.
“If I have a radar on the ground, and the ballistic missile is up at 1,500 kilometers, I can see it,” Wright said. But the curvature of the Earth masks weapons at lower altitude until they are within 500 km, or so, providing only about two minutes to respond.
“It’s kind of already too late,” Boyd said.
Glide Phase Interceptors
The most advanced missile defense system now in operation is the Patriot Advanced Capability (PAC)-3 system, which can reach hypersonic speeds to hit its target. Hypersonics experts believe current missile defenses could possibly protect high-value assets against hypersonic threats, provided they were placed in the correct location.
MIT’s David Wright is currently studying the susceptibility of hypersonic weapons to current generation missile defenses.“If you slow a hypersonic weapon down below about Mach 6 …you could probably have a good chance of intercepting with something like the state-of-the-art Patriot,” he said.
But the advantage will always go to the faster weapon. If the interceptor is faster than the incoming missile, it can defeat it. But “if the hypersonic weapon is going about the same speed, then it will always have the advantage,” he said.
“If you’re worried about hypersonic weapons … it appears to me that the long pole of the tent is making faster and more agile interceptors,” he said. “There’s a rule of thumb that the interceptor actually needs two to three times as much lateral acceleration as the target in order to make up for that time lag.”
Boyd disagreed. He said MDA must first overcome the tracking problems. “Because we don’t have the sensors in place, because they’re [hypersonic weapons] flying in a different part of the sky, because of the geometry of the earth, the radar would pick it up, [but] it would likely be too late for an interceptor to get up there and engage with it.”
MDA’s multi-layered tracking approach would begin with space sensors tracking the launch and cruise phase and carry through until ground-based radars could pick up the target. Tracking data would be fed to a new Glide Phase Interceptor missile defense system deployed aboard Aegis cruisers, which would seek to intercept the hypersonic glide vehicle during the cruise phase; as a backup, in case the hypersonic threat got through, the Aegis SM-6 missile defense system could still engage the target in the terminal phase.
Harris said current interceptors could be effective against a hypersonic weapon as long as they have enough notice and enough knowledge of the weapon’s size, weight, and maneuverability.
“All of those things are critically important,” he said. The interceptor need not strike the weapon directly; it only needs to be able to explode in the vicinity in order to destroy the incoming weapon. “You have to build faster, more maneuverable and more potent warheads in our defensive interceptors,” he said.
Wright is not convinced that the SM-6 would be fast enough to take out an incoming hypersonic threat in the terminal phase, as MDA has suggested.
“My sense is the SM-6 is just too slow,” he said. “It’s not a matter of sensors that you have to look at, it’s a matter of having a faster and more maneuverable interceptor.”
Indeed, even if it could strike the incoming weapon in the terminal phase, that might not prove effective. “Stuff is still going to get through,” Boyd said. “So, even if you were able to engage it right at the end, that may be too late.”
Boyd argues the U.S. needs to ramp up investment—and soon. “There should be a sense of urgency,” he said. “I think the current investment is not going to provide a comprehensive solutio. … The current investments are going to take a long time to get there.”
Meanwhile, Russia and China are racing to develop scramjet and ramjet hypersonic engines that could yield smaller, more maneuverable, and harder-to-detect weapons. The threat will not stay static, but like all weaponry, continue to evolve.
“The next generation of interceptors will have to be responsive to certain parameters driven by what we know about the offensive weapons that the bad guys have: How maneuverable are those?” Harris said. “We have to know, we must. We have no choice.”