High in the sky over northwestern China, a wedge-shaped unmanned vehicle separated from a rocket. Coasting along at up to Mach 6, or six times the speed of sound, the Xingkong-2 “waverider” hypersonic cruise missile (HCM) bobbed and weaved through the stratosphere, surfing on its own shock waves. At least that’s how the weapon’s developer, the China Academy of Aerospace Aerodynamics, described the August 2018 test. (China did not release any video footage.) The HCM’s speed and maneuverability, crowed the Communist Party’s Global Times, would enable the new weapon to “break through any current generation anti-missile defense system.”
For decades, the U.S. military—and its adversaries—have coveted missiles that travel at hypersonic speed, generally defined as Mach 5 or greater. Intercontinental ballistic missiles (ICBMs) meet that definition when they re-enter the atmosphere from space. But because they arc along a predictable ballistic path, like a bullet, they lack the element of surprise. In contrast, hypersonic weapons such as China’s waverider maneuver aerodynamically, enabling them to dodge defenses and keep an adversary guessing about the target.
Since the dawn of the Cold War, the Pentagon has periodically thrown its weight behind the development of maneuverable hypersonic weapons, only to shy away when technological hurdles such as propulsion, control, and heat resistance proved daunting. “You see a flurry of activity, a lot of investment, and then we conclude it’s a bridge too far,” says aerospace engineer Mark Lewis, director of defense research and engineering for modernization at the U.S. Department of Defense (DOD). “The community was underfunded and largely forgotten for many years,” adds Daniel DeLaurentis, director of Purdue University’s Institute for Global Security and Defense Innovation.
Now, DOD is leading a new charge, pouring more than $1 billion annually into hypersonic research. Competition from ambitious programs in China and Russia is a key motivator. Although hype and secrecy muddy the picture, all three nations appear to have made substantial progress in overcoming key obstacles, such as protecting hypersonic craft from savage frictional heating. Russia recently unveiled a weapon called the Kinzhal, said to reach Mach 10 under its own power, and another that is boosted by a rocket to an astonishing Mach 27. China showed off a rocket-boosted hypersonic glide vehicle (HGV) of its own, the Dongfeng-17, in a recent military parade. The United States, meanwhile, is testing several hypersonic weapons. “It’s a race to the Moon sort of thing,” says Iain Boyd, an aerospace engineer at the University of Colorado, Boulder. “National pride is at stake.”
THE ASAHI SHIMBUN VIA GETTY IMAGES
This new arms race promises to upend strategic calculations. Russian officials have cast nuclear-armed hypersonic craft as a hedge against future U.S. prowess at shooting down ICBMs, which could undermine nuclear deterrence.
China’s military, in contrast, sees hypersonic weapons (as well as cyberwarfare and electromagnetic pulse strikes) as an “assassin’s mace”: a folklore term for a weapon that gives an advantage against a better-armed foe, says Larry Wortzel, a senior fellow at the American Foreign Policy Council who serves on the U.S.-China Economic and Security Review Commission. If tensions were to spike over Taiwan or the South China Sea, for instance, China might be tempted to launch preemptive strikes with conventional hypersonic weapons that could cripple U.S. forces in the Pacific Ocean, Wortzel says. China’s hypersonic weapons, he warns, “seem deliberately targeted at upending the tenuous strategic stability that has been in place since the end of the Cold War.”
For now, maneuverability at hypersonic speeds makes the weapons nearly impossible to shoot down—“unstoppable,” as a headline in The New York Times put it last summer. But, “Unstoppable today does not mean unstoppable tomorrow,” says Shari Feth, a materials engineer at the U.S. Missile Defense Agency (MDA). She’s at the vanguard of U.S. efforts to field countermeasures against hypersonic weapons. “There are technologies that could be developed that could be used for a more robust defense,” Feth says. “But we have more work to do to get there.”
The United States has spent decades trying to get hypersonic flight right. The first vehicle to exceed Mach 5 was a two-stage rocket, dubbed Project Bumper, launched in 1949. After four failed tests, the V-2 rocket lifted off from a missile range in New Mexico, releasing a second-stage sounding rocket that attained Mach 6.7.
Project Bumper and subsequent efforts laid bare the daunting challenges. “This is a very unforgiving realm,” says Lewis, who served as chief scientist of the U.S. Air Force from 2004 to 2008. “You’re flying under extraordinary conditions”—extreme velocities, forces, and temperatures. The hypersonic threshold of Mach 5 is arbitrary, but at those speeds, he says, “temperatures start to get high enough to worry about.”
The heating depends on factors such as the vehicle’s speed and contours. When a space shuttle returning from orbit hit the upper atmosphere at Mach 25, its blunt leading edges heated to 1400°C, which a skin of carbon-carbon composites helped it withstand. Newer hypersonic craft tend to have sharper edges—in part to assist with maneuverability—that can exceed 2000°C. Turbulence can make things worse. At hypersonic speeds, the boundary layer around the vehicle thickens, and a smooth, laminar flow can suddenly break up into eddies and swirls that cause temperature spikes on the vehicle’s skin. “We’ve devoted a lot of fundamental research to figure out when that occurs,” Lewis says. A vehicle’s survival, he says, requires resilient superalloys and ultra–high-temperature ceramics. And perhaps novel coolants. For example, a team at the U.S. Naval Research Laboratory has devised a liquid sodium system that drains heat from a leading edge through continuous evaporation and condensation.
High air speeds also pose challenges for engines on HCMs, which unlike HGVs have their own power plants. HCMs use a supersonic combustion ramjet, or “scramjet,” to accelerate. “It’s the simplest type of jet engine you could ever imagine … just an open tube” in which air mixes with fuel, Lewis says. “It’s also perhaps the most complicated type you can imagine because of the extreme conditions under which it operates.”
At hypersonic speeds, air molecules spend milliseconds in the engine tube—scant time for fuel and air to mix properly. And when a vehicle pitches and yaws, airflow into the engine changes, which can result in uneven combustion and thrust. Tweaks to get a better burn have ramifications for, say, how the aircraft withstands shock waves. “Everything is incredibly coupled. You are designing a fully integrated vehicle,” Lewis says. It took the United States 46 years to realize its first working scramjet: NASA’s $230 million X-43a, an uncrewed vehicle that flew in 2004.
HGVs pose other challenges. The rocket that carries the glider reaches speeds far greater than those of an HCM, meaning engineers must use materials that are even more resistant to heat. Still, HGVs are easier to maneuver because they lack a scramjet, with its acute sensitivity to pitch and yaw. “It almost becomes a religious discussion—rockets versus air breathing,” Lewis says. “The ultimate answer is we probably want both.”
The United States has not yet fielded either. After decades of fits and starts, any advantage that U.S. hypersonic R&D once held has largely eroded away. Its wind tunnels and other testing infrastructure are aging. And challenges such as tweaking designs to ensure engine walls don’t melt have slowed progress on scramjets, Lewis says. “Today we are further away from routine scramjet flight than we were 10 years ago.”
From a base in the Ural Mountains on 26 December 2018, Russia’s armed forces launched a ballistic missile carrying an HGV called Avangard. After separating from its carrier in the stratosphere, the HGV zigzagged 6000 kilometers across Siberia at a searing Mach 27, Russian officials claimed, then smashed into a target on the Kamchatka Peninsula. Afterward, a beaming Russian President Vladimir Putin called Avangard “the perfect New Year’s gift for the country.” Russia’s defense ministry announced last month that it has put the nuclear-armed HGV into combat duty—allowing Putin to claim that Russia is the first country armed with hypersonic weapons.
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Russian boasts along with Chinese advances have sounded the alarm in the United States. Congress will pour more than $1 billion into military hypersonic research this year and has created a new university consortium to do basic studies. “Our work on hypersonics has really ramped up,” says Jonathan Poggie, an aerospace engineer at Purdue. His team models low-frequency shock waves “that pound on a vehicle like a hammer.”
The rising military stakes have prompted the Pentagon to consider classifying some basic hypersonic research. DOD “is very concerned about educating our enemies,” Poggie says. “They are in the middle of trying to draw these red lines,” Boyd adds. But, “If we overclassify,” he warns, “there are a number of domino effects. You’d be stifling innovation. Inevitably, that means fewer new ideas.”
A veil of secrecy is also descending in Russia, which has produced “a rich body of hypersonic literature,” Lewis says. Security officials there recently charged two scientists with treason for sharing findings with European collaborators; the data had been approved for release but then declared secret 5 years later (see sidebar).
China, in contrast, has been surprisingly open about its research. “The Chinese are trying to establish prestige in the field,” Lewis says. The nation has invested heavily in facilities, including sophisticated wind tunnels and shock tubes that use blast waves to study hypersonic flows. “Ten years ago, they were duplicating what others had done,” Boyd says. “Now, they’re publishing innovative ideas.” At a 2017 hypersonic conference in Xiamen, China, Chinese scientists presented more than 250 papers—about 10 times the number presented by U.S. researchers.
“You see papers you’d think they wouldn’t publish in the open literature,” Poggie says. One is a recent analysis from the China Aerodynamics Research and Development Center showing that the plume of ionized gas, or plasma, left by a hypersonic vehicle is more visible on radar than the vehicle itself. That implies radar could give early warning of an incoming weapon.
Other nations are chasing the trio of leaders—or teaming up with them. Australia is collaborating with the United States on a Mach 8 HGV, and India with Russia on a Mach 7 HCM. France intends to field an HCM by 2022, and Japan is aiming for an HGV in 2026, the U.S. Congressional Research Service noted in a July 2019 report.
The United States is largely defenseless against such weapons, at least for now, in part because it can’t track them. U.S. military satellites are vigilant for flashes that reveal launches of ICBMs and cruise missiles. But they would probably lose track of even a rocket-boosted hypersonic weapon soon after it detaches from its booster, analysts say. To avoid “shooting blindly … you need to continue to track it when it starts doing these maneuvers in the atmosphere,” says Thomas Karako, director of the Missile Defense Project at the Center for Strategic & International Studies.
To remedy that shortcoming, the Pentagon plans to launch hundreds of small satellites with sensors capable of tracking heat sources an order of magnitude cooler than rocket boosters. “By proliferating them, you make it impossible to take them all out,” Karako says. The full Hypersonic and Ballistic Tracking Space Sensor network could be up and running by 2030, he adds. (The satellites would also be used to help guide U.S. hypersonic weapons.)
Once you have such sensors, “we can find a way to build the interceptors,” Karako says. Current missile defense interceptors aim to destroy ICBMs near their apex in the upper atmosphere, much higher than a hypersonic weapon flies, and they aren’t maneuverable enough to hit a swerving target. “You’ll need interceptors with more divert capability than we have,” Karako says.
U.S. Air Force video by Chris Okula
MDA is exploring various approaches that would enable interceptors to “overmatch” incoming weapons, Feth says. One possibility, she says, is to fly faster—a tall order that would demand new lightweight, heat-resistant composites and alloys.
Interceptors could destroy a hypersonic vehicle either by colliding with it or by detonating a warhead nearby. But MDA is also exploring using directed energy: lasers, neutral particle beams, and microwaves or radio waves. Directed-energy countermeasures were floated in the 1980s as elements of the United States’s “Star Wars” missile defense shield—then abandoned. Four decades later, “They are more plausible,” Karako says. Still, MDA recently scrapped plans to test a prototype 500-kilowatt airborne laser by 2025 and to develop a space-based neutral particle beam.
Even as defense scientists search for ways to thwart a hypersonic attack, diplomats and nonproliferation experts are discussing how to limit—or even outlaw—the disruptive technology. “Hypersonic weapons are primed for arms control,” argues Ankit Panda, senior fellow on the Defense Posture Project at the Federation of American Scientists, a think tank. The United Nations Office for Disarmament Affairs weighed in last year with a report exploring arms control scenarios, blasting what it called a “blinkered pursuit of a novel technology with as-yet-unproven military utility,”
Arms control treaties, however, are hardly in vogue these days. And with China, Russia, and the United States egging each other on with one high-profile test after another, the hypersonic arms race seems likely to accelerate.
Source: Science Mag