When missiles fly beyond Mach 5, materials melt, airflow turns turbulent, and budgets enter the stratosphere
It's obvious why the militaries of the world want missiles that can follow erratic paths at low altitude while flying at five times the speed of sound, eluding any chance at detection or interception.
“Think of it as delivering a pizza, except it's not a pizza," says Bradley Wheaton, a specialist in hypersonics at the Johns Hopkins University Applied Physics Laboratory (APL), in Maryland. “In the United States, just 15 minutes can cover the East Coast; a really fast missile takes 20 minutes to get to the West Coast. At these speeds, you have a factor of 50 increase in the area covered per unit of time."
So the question isn't why the great powers are pursuing hypersonic arms, but why they are doing so now. Quick answer: They are once again locked in an arms race.
The wider world first heard of this type of weaponry in March 2018, when Russian president Vladimir Putin gave a speech describing his country's plans for a nuclear-powered cruise missile that could fly around the world at blinding speed, then snake around hills and dales to a target. His bold assertions have been questioned, particularly the part about nuclear power. Even so, a year later a nuclear accident killed seven people near a testing range off the northern coast of Russia, and U.S. intelligence officials speculated that it involved hypersonic experiments.
The nature of that accident is still shrouded in mystery, but it's clear there's been a huge increase in the research effort in hypersonics. Here's a roundup of what the superpowers of the 21st century are doing to pursue what is, in fact, an old concept.
The hypersonic missiles in use or in testing in China and Russia can apparently carry either conventional warheads, aimed at ships and other small military targets, or nuclear ones, aimed at cities and government centers. These ship killers could deprive the United States of its preeminence at sea, which is more than enough reason for China, for instance, to develop hypersonics. But a nuclear-armed version that leaves the defender too little time to launch a retaliatory strike would do even more to shift the balance of power, because it would dismantle the painstakingly constructed system of deterrence known as mutually assured destruction, or by the jocular acronym MAD.
“The nuclear side is very destabilizing, which is why the Russians are going after it," says Christopher Combs, a professor of mechanical engineering at the University of Texas at San Antonio. “But on the U.S. side we see no need for that, so we're going conventional."
That is indeed the official U.S. policy. But in August, some months after Combs spoke with IEEE Spectrum, an Aviation Week article pointed out that an Air Force agency charged with nuclear weapons requested that companies submit ideas for a “thermal protection system that can support [a] hypersonic glide to ICBM ranges." Soon after that, the request was hastily taken down, and the U.S. Air Force felt compelled to restate its policy not to pursue nuclear-capable hypersonic weapons.
Inhaling a Hurricane
The turbojet, which can manage supersonic speeds, uses a turbine near the inlet to compress air for combustion. The ramjet, which requires supersonic speed, instead uses the forward motion of the vehicle to “ram" air into the combustion chamber. The scramjet, or supersonic combustion ramjet, is optimized for hypersonic speeds. Its specially shaped inlets and combustion walls allow the very process of combustion to proceed supersonically—a feat that has been compared to keeping a match lit in a hurricane. Illustration: James Provost
Today's forays into hypersonic research have deep roots, reaching back to the late 1950s, in both the United States and the Soviet Union. Although this work continued for decades, in 1994, a few years after the Cold War ended with the dissolution of the Soviet Union, the United States pulled the plug on research into hypersonic flight, including its last and biggest program, the Rockwell X-30. Nicknamed the “Orient Express," the X-30 was to have been a crewed transport that would top out at 25 times the speed of sound, Mach 25—enough to take off from Washington, D.C., and land in Tokyo 2 hours later. Russia also discontinued research in this area during the 1990s, when its economy was in tatters.
Today's test vehicles just pick up where the old ones left off, explains Alexander Fedorov, a professor at Moscow Institute of Physics and Technology and an expert on hypersonic flow at the boundary layer, which is right next to the vehicle's skin. “What's flying now is just a demonstration of technology—the science is 30 years old," he says.
Fedorov has lectured in the United States; he even helps U.S. graduate students with their research. He laments how the arms race has stifled international cooperation, adding that he himself has “zero knowledge" about the military project Putin touted two years ago. “But I know that people are working on it," he adds.
In the new race, Fedorov says, Russia has experience without much money, China has money without much experience, and the United States has both, although it revived its efforts later than did Russia or China and is now playing catch-up. For fiscal 2021, U.S. research agencies have budgeted US $3.2 billion[PDF] for all hypersonic weapons research, up from $2.6 billion in the previous year.
Other programs are under way in India and Australia; even Israel and Iran are in the game, if on the sidelines. But Fedorov suggests that the Chinese are the ones to watch: They used to talk at international meetings, he says, but now they mostly just listen, which is what you'd expect if they had started working on truly new ideas—of which, he reiterates, there are very few on display. All the competing powers have shown vehicles that are “very conservative," he says.
One good reason for the rarity of radical designs is the enormous expense of the research. Engineers can learn only so much by running tests on the ground, using computational fluid-flow models and hypersonic wind tunnels, which themselves cost a pretty penny (and simulate only some limited aspects of hypersonic flight). Engineers really need to fly their creations, and usually when they do, they use up the test vehicle. That makes design iteration very costly.
I Said Mach 7, Scotty
This rendering of the NASA X-43 experimental hypersonic vehicle, generated using computational fluid dynamics, shows shock waves produced at Mach 7—seven times the speed of sound—while the scramjet that powers its flight is operating. Such computations could not have been done in the early days of hypersonic research, when engineers were armed only with slide rules and wind tunnels. Note how the hypersonic shock waves form at the vehicle's leading edges and at the air inlets. In merely supersonic flight the shock wave typically fans out and away from the vehicle, but at hypersonic speeds it may nestle close by. This boundary layer can be aerodynamically useful—by creating lift, for instance—but it can also cause problems should turbulence develop. Image: NASA
It's no wonder hypersonic prototypes fail so often. In mere supersonic flight, passing Mach 1 is a clear-cut thing: The plane outdistances the sound waves that it imparts to the air to produce a shock wave, which forms the familiar two-beat sonic boom. But as the vehicle exceeds Mach 5, the density of the air just behind the shock wave diminishes, allowing the wave to nestle along the surface of the vehicle. That in-your-face layer poses no aerodynamic problems, and it could even be an advantage, when it's smooth. But it can become turbulent in a heartbeat.
“Predicting when it's going turbulent is hard," says Wheaton, of Johns Hopkins APL. “And it's important because when it does, heating goes up, and it affects how control surfaces can steer. Also, there's more drag."
The pioneers of hypersonic flight learned about turbulence the hard way. On one of its many flights, in 1967, the U.S. Air Force's X-15 experimental hypersonic plane went into a spin, killing the pilot, Michael J. Adams. The right stuff, indeed.
Hypersonic missiles come in two varieties. The first kind, launched into space on the tip of a ballistic missile, punches down into the atmosphere, then uses momentum to maneuver. Such “boost-glide" missiles have no jet engines and thus need no air inlets, so it's easy to make them symmetrical, typically a tube with a cone-shape tip. Every part of the skin gets equal exposure to the air, which at these speeds breaks down into a plume of plasma, like the one that puts astronauts in radio silence during reentry.
Boost-glide missiles are now operational. China appears to have deployed the first one, called the Dongfeng-17, a ballistic missile that carries glide vehicles. Some of those gliders are billed as capable of knocking out U.S. Navy supercarriers. For such a mission it need not pack a nuclear or even a conventional warhead, instead relying on its enormous kinetic energy to destroy its target. And there's nothing that any country can now do to defend against it.
“Those things are going so fast, you're not going to get it," General Mark Milley, chairman of the Joint Chiefs of Staff, said in March, in testimony[PDF] before Congress.
You might think that you give up the element of surprise by starting with a ballistic trajectory. But not completely. Once the hypersonic missile comes out of its dive to fly horizontally, it becomes invisible to sparsely spaced radars, particularly the handful based in the Pacific Ocean. And that flat flight path can swerve a lot. That's not because of any AI-managed magic—the vehicle just follows a randomized, preprogrammed set of turns. But the effect on those playing defense is the same: The pizza arrives before they can find their wallets.
Three Paths to the Target
Most long-range missiles follow a ballistic curve that takes them high above the atmosphere and then down through it, a trajectory that can be detected early and modeled accurately. Boost-glide missiles ride a ballistic launcher to attain hypersonic speed, then use momentum to glide at low altitude while taking maneuvers to elude ground defenses. Scramjets use air-breathing engines to travel far, fast, and lower still, making them that much harder to detect and shoot down. Illustration: James Provost
The second kind of hypersonic missile gets the bulk of its impulse from a jet engine that inhales air really fast, whirls it together with fuel, and burns the mixture in the instant that it tarries in the combustion chamber before blowing out the back as exhaust. Because these engines don't need compressors but simply use the force of forward movement to ram air inside, and because that combustion proceeds supersonically, they are called supersonic ram jets—scramjets, for short.
One advantage the scramjet has over the boost-glide missile is its ability to stay below radar and continue to maneuver over great distances, all the way to its target. And because it never enters outer space, it doesn't need to ride a rocket booster, although it does need some powerful helper to get it up to the speed at which first a ramjet, and then a scramjet, can work.
Another advantage of the scramjet is that it can, in principle, be applied for civilian purposes, moving people or packages that absolutely, positively have to be there quickly. The Europeans have such a project. So do the Chinese, and Boeing has shown a concept. Everyone talks up this possibility because, frankly, it's the only peaceable talking point there is for hypersonics. Don't forget, though, that supersonic commercial flight happened long ago, made no money, and ended—and supersonic flight is way easier.
The scramjet has one big disadvantage: It's a lot harder technically. Any hypersonic vehicle must fend off the rapidly moving air outside, which can heat the leading edges to as high as 3,000 °C. But that heat and stress is nothing like the hellfire inside a scramjet engine. There, the heat cannot radiate away, it's hard to keep the flame lit, and the insides can come apart second by second, affecting airflow and stability. Five minutes is a long time in this business.
That's why scramjets, though conceived in the 1950s, still remain a work in progress. In the early 2000s, NASA's X-43 used scramjets for about 10 seconds in flight. In 2013, Boeing's X-51 Waverider flew at hypersonic speed for 210 seconds while under scramjet power.
Tests on the ground have fared better. In May, workers at the Beijing Academy of Sciences ran a scramjet for 10 minutes, according to a report in the South China Morning Post. Two years earlier, the leader of the project, Fan Xuejun, told the same newspaper that a factory was being built to construct a variety of scramjets, some supposedly for civilian application. One engine would use a combined cycle, with a turbojet to get off the ground, a ramjet to accelerate to near-hypersonic speed, a scramjet to blow past Mach 5, and maybe even a rocket to top off the thrust. That's a lot of moving parts—and an ambition worthy of Elon Musk. But even Musk might hesitate to follow Putin's proposal to use a nuclear reactor for energy.
Like Greased Lightning: Boeing's X-51 Waverider (in white) is shown slung under the wing of a B-52 bomber, just before a test flight of the hypersonic vehicle in 2012. The B-52 mothership was needed to attain the necessary altitude; a rocket booster on the missile pushed it fast enough into the supersonic range to allow the scramjet to ignite. In 2013, the X-51 flew hypersonically on scramjet power for 210 seconds, the official record. Photo: Boeing
The cost of developing a scramjet capability is only one part of the economic challenge. The other is making the engine cheap enough to deploy and use in a routine way. To do that, you need fuel you can rely on. Early researchers worked with a class of highly energetic fuels that would react on contact with air, like triethylaluminum.
“It's a fantastic scramjet engine fuel, but very toxic, a bit like the hydrazine fuels used in rockets nowadays, and this became an inhibitor," says David Van Wie, of Johns Hopkins APL, explaining why triethylaluminum was dropped from serious consideration.
Next up was liquid hydrogen, which is also very reactive. But it needs elaborate cooling. Worse, it packs a rather low amount of energy into a given volume, and as a cryogenic fuel it is inconvenient to store and transport. It has been and still is used in experimental missiles, such as the X-43.
Today's choice for practical missiles is hydrocarbons, of the same ilk as jet fuel, but fancier. The Chinese scramjet that burned for 10 minutes—like others on the drawing board around the world—burns hydrocarbons. Here the problem lies in breaking down the hydrocarbon's long molecular chains fast so the shards can bind with oxygen in the split second when the substances meet and mate. And a split second isn't enough—you have to do it continuously, one split second after another, “like keeping a match lit in a hurricane," in the oft-quoted words of NASA spokesman Gary Creech, back in 2004.
Scramjet designs try to protect the flame by shaping the inflow geometry to create an eddy, forming a calm zone not unlike the eye of a hurricane. Flameouts are particularly worrisome when the missile starts jinking about, thus disrupting the airflow. “It's the 'unstart' phenomenon, where the shock wave at the air inlets stops the engine, and the vehicle will be lost," says John D. Schmisseur, a researcher at the University of Tennessee Space Institute, in Tullahoma. And you really only get to meet such gremlins in actual flight, he adds.
There are other problems besides flameout that arise when you're inhaling a tornado. One expert, who requested anonymity, puts it this way: “If you're ingesting air, it's no longer air; it's a complex mix of ionized atmosphere," he says. “There's no water anymore; it's all hydrogen and oxygen, and the nitrogen is to some fraction elemental, not molecular. So combustion isn't air and fuel—it's whatever you're taking in, whatever junk—which means chemistry at the inlet matters."
Simulating the chemistry is what makes hypersonic wind-tunnel tests problematic. It's fairly simple to see how an airfoil responds aerodynamically to Mach 5—just cool the air so that the speed of sound drops, giving a higher Mach number for a given airspeed. But blowing cold air tells you only a small part of the story because it heads off all the chemistry you want to study. True, you can instead run your wind tunnel fast, hot, and dense—at “high enthalpy," to use the term of art—but it's hard to keep that maelstrom going for more than a few milliseconds.
“Get the airspeed high enough to start up the chemistry and the reactions sap the energy," says Mark Gragston, an aerospace expert who's also at the UT Space Institute. Getting access to such monster machines isn't easy, either. “At Arnold Air Force Base, across the street from me, the Air Force does high-enthalpy wind-tunnel experiments," he says. “They're booked up three years in advance."
Other countries have more of the necessary wind tunnels; even India has about a dozen[PDF]. Right now, the United States is spending loads of money building these machines in an effort to catch up with Russia and China. You could say there is a wind-tunnel gap—one more reason U.S. researchers are keen for test flights.
Another thing about cooling the air: It does wonders for any combustion engine, even the kind that pushes pistons. Reaction Engines, in Abingdon, England, appears to be the first to try to apply this phenomenon in flight, with a special precooling unit. In its less-ambitious scheme, the precooler sits in front of the air inlet of a standard turbojet, adding power and efficiency. In its more-ambitious concept, called SABRE (Synergetic Air Breathing Rocket Engine), the engine operates in combined mode: It takes off as a turbojet assisted by the precooler and accelerates until a ramjet can switch on, adding enough thrust to reach (but not exceed) Mach 5. Then, as the vehicle climbs and the atmosphere thins out, the engine switches to pure rocket mode, finally launching a payload into orbit.
In principle, a precooler could work in a scramjet. But if anyone's trying that, they're not talking about it.
Fast forward five years and boost-glide missiles will no doubt be deployed in the service of multiple countries. Jump ahead another 15 or 20 years, and the world's superpowers will have scramjet missiles.
So what? Won't these things always play second fiddle to ballistic missiles? And won't the defense also have its say, by unveiling superfast antimissiles and Buck Rogers–style directed-energy weapons?
Perhaps not. The defense always has the harder job. As President John F. Kennedy noted in an interview way back in 1962, when talking about antiballistic missile defense, what you are trying to do is shoot a bullet with a bullet. And, he added, you have to shoot down not just one but many, including a bodyguard of decoys.
Today there are indeed antimissile defenses that can protect particular targets against ballistic missiles, at least when they're not being fired in massive salvos. But you can't defend everything, which is why the great powers still count on deterrence through mutually assured destruction. By that logic, if you can detect cruise missiles soon enough, you can at least make those who launched them wish they hadn't.
For that to work, we'll need better eyes in the skies. In the United States, the military wants around $100 million for research on low-orbit space sensors to detect low-flying hypersonic missiles, Aviation Week reported in 2019.
Hardly any of the recent advances in hypersonic flight result from new scientific discoveries; almost all of it stems from changes in political will. Powers that challenge the international status quo—China and Russia—have found the resources and the will to shape the arms race to their benefit. Powers that benefit from the status quo—the United States, above all—are responding in kind. Politicians fired the starting pistol, and the technologists are gamely leaping forward.
And the point? There is no point. It's an arms race.
This article appears in the December 2020 print issue as “Going Hypersonic."
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