BY RICHARD STONE
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.”
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.” (Science.org)