Experiments In Hyper-Speed

(Source: US Army ALT Magazine; posted Aug 16, 2018)



by Ms. Mary Kate Aylward

DARPA’s Falcon Hypersonic Test Vehicle (HTV) emerges from its nose cone and prepares to re-enter the atmosphere. DARPA has conducted two test flights of the HTV; in the second, in 2011, the HTV reached a speed of Mach 20 before losing control. (Darpa image)

What are hypersonic weapons, why does the Army want them, and are they as revolutionary as they sound?



1998: U.S. intelligence locates Osama bin Laden at an al Qaeda camp in Afghanistan. Navy ships in the Arabian Sea launch cruise missiles, which take two hours to reach the target 1,100 miles away. The camp is destroyed but bin Laden survives: He had left less than an hour earlier.



2003: DOD requests funding for the Conventional Prompt Global Strike program, citing the need to be able to hit “fleeting targets.”



2011: After several failures, DOD’s first successful test of a hypersonic weapon occurs: The Army launches a missile from Hawaii that lands 30 minutes later in the Marshall Islands, approximately 2,000 nautical miles (or 2,300 standard miles) away.



2013: The Chinese military’s “Science of Military Strategy” (an authoritative study of China’s strategic position) notes: “The United States is in the process of implementing a conventional ‘Prompt Global Strike’ plan. Once it has functional capabilities, it will be used to implement conventional strikes against our nuclear missile forces and will force us into a disadvantaged, passive position.”



2014: China conducts the first of at least seven tests of a hypersonic weapon.



March 2018: Russian President Vladimir Putin claims to have finished testing an “invincible” Mach 10 hypersonic cruise missile that “can also maneuver at all phases of its flight trajectory, which also allows it to overcome all existing and, I think, prospective anti-aircraft and anti-missile defense systems, delivering nuclear and conventional warheads,” according to translations provided by the Russian government.





“Hypersonic” describes any speed faster than five times the speed of sound, which is roughly 760 miles per hour at sea level. Multiply that by five and you have a weapon that travels at least 3,800 miles per hour or more. But is speed enough to change the game? Does a missile flying at Mach 7 outperform one at Mach 3 on metrics other than speed? Apart from flying very fast, what does DOD—and what do our adversaries—think hypersonic weapons can accomplish?



“It’s really meant to kick the door open,” said Bob Strider, hypersonics chief at the U.S. Army Space and Missile Defense Command, “and then allow other assets to come in.” (The door, in this analogy, is closed by the anti-access and area denial measures a country could deploy to prevent others from entering or passing through a given area of land, air or sea.) Strider oversees the Army’s contributions to the Conventional Prompt Global Strike technology demonstration program, to support the building of a ground-launched hypersonic weapon. The Army conducted two technology demonstration flights of the Advanced Hypersonic Weapon—one successful, in 2011, and one aborted in 2014 after testers detected an anomaly with the booster seconds into the flight.



The Army, specifically, is after a long-range missile that redefines long range—Chief of Staff Gen. Mark A. Milley has stressed that he wants to see “10x” improvements. “We, the Army, have as our number one priority for modernization long-range precision fires; a subset of that is the hypersonic piece to it,” Milley said March 15 in testimony before the House Appropriations Subcommittee on Defense. What’s publicly known about DOD hypersonic progress suggests that hypersonics offer that kind of range. Less has been said about their precision, though Strider said the Army’s Advanced Hypersonic Weapon “is showing a lot of capability to be able to get where it’s supposed to get and hit with a lot of energy. … In our upcoming tests we’ll be testing those bounds more and looking at what the vehicle really is capable of as far as maneuverability.”



Some defense analysts are unconvinced that the United States needs a hypersonic strike and are skeptical of some technical claims made about hypersonic weapons, pointing out that there are other ways to hit fleeting targets, get into denied areas or strike a rogue nuclear facility—ways that cost less, and risk less.



WHY GO HYPERSONIC?



Research on hypersonic flight goes back to the 1960s, but it has been technically challenging to achieve. At hypersonic speeds, the air molecules around the flight vehicle start to change, breaking apart or gaining a charge in a process called ionization. This subjects the hypersonic vehicle to tremendous stresses. Spacecraft, and ballistic missiles, spend most of their flight out of the atmosphere, free of the heat, pressure and friction, while hypersonic vehicles have to push through the atmosphere. “The thermal protection system for the hypersonic weapon is one of the key, very key, technologies that have to be in place because the hypersonic weapon is pretty much in the atmosphere through its flight; it gets temperatures in excess of 2,000 degrees for quite a few minutes,” said Strider.



Hypersonic flight has several applications. A reusable hypersonic airplane (of the “two hours from Beijing to London” variety) is the most distant, though NASA and the Defense Advanced Research Projects Agency have both explored preliminary steps; it’s weapons that are capable of hypersonic speeds that DOD is actively pursuing. These come in several varieties, including hypersonic cruise missiles and boost-glide vehicles. The former are powered during their flight by an attached engine; the latter are unpowered after launch and, as the name suggests, glide to their destination.



The U.S. military began pursuing hypersonic weapons in earnest under the Conventional Prompt Global Strike program in 2007. The program sought to achieve a non-nuclear strike anywhere around the globe within an hour. Now, a prompt global strike also appears useful as part of a package of options to counter anti-access and area denial measures. As concern grows about China’s efforts to close off what it considers its part of the Pacific, a weapon that could fly undetected into the denied area while the launch platform stays well outside becomes more attractive to U.S. military planners.



The Army’s Advanced Hypersonic Weapon demonstrator, tested in 2011 and 2014, relied on boost-glide technology. Rockets launch—boost—the glide vehicle to a high altitude, giving it enough speed and energy to reach its target. The glide vehicle then curves back toward the Earth’s surface, and glides or skips along the atmosphere without power for the remainder of its trajectory. (Though “glide” might suggest gentle motion, the vehicle is tearing through the atmosphere at Mach 5 or faster.)



The U.S. Army Aviation and Missile Research, Development and Engineering Center, in Huntsville, Alabama, developed the thermal protection system. The Army team collaborated with a number of national laboratories on the launcher and glide vehicle design, and refined it in wind tunnels where vehicle forces were measured at hypersonic speeds.



CHANGING THE GAME?



Hypersonics have been spoken of as game-changers (whether because of their speed or their radar-evading low flight profile), though opinions vary across the defense community as to whether current hypersonic technology is advanced enough to be revolutionary. In the “yes” column is Strider. “I see it as a game changer. I’d say there’s very few mechanisms today that could stop a hypersonic weapon.”



Whether they change the game or are an incremental shift is, to some extent, a moot point by now: China is testing hypersonics, so is Russia, and therefore, so is the United States. “I do think for better or for worse hypersonic weapons are likely to become a significant feature of the international landscape and could have quite significant strategic implications. I think we’re kind of feeling our way through what those implications could be,” said James Acton, a theoretical nuclear physicist with the Carnegie Endowment for International Peace, in a September 2017 interview with Army AL&T.



WHAT’S ON THAT WARHEAD?



Blundering into a nuclear exchange is one such possible implication that concerns defense analysts, and it has occasionally concerned Congress since the advent of the Prompt Global Strike program in the early 2000s. Congress has generally supported the program, but it has withheld funds for some requests from DOD, citing concerns about the possibility of accidental nuclear war. “Radars would provide much less warning time of a boost-glide weapon attack than a ballistic missile attack,” Acton wrote in his 2013 study “Silver Bullet?” If a targeted country does not know whether the weapon due to arrive in minutes is carrying a conventional or a nuclear warhead, would it take the risk of leaving what could be a nuclear strike unchallenged?



Acton is not convinced that DOD has made a serious case “that the strategic benefits [of having hypersonic weapons] outweigh the strategic risks of escalation with Russia and China.” Others worry about the wider risk of arms-racing and missile proliferation. Air Force Lt. Col. Jeff Schreiner wrote in a 2014 Stars and Stripes op-ed calling for a hypersonic test ban: “The tactical planner in me sees countless uses for hypersonic delivery platforms against a range of target sets. The strategic planner sees the ability to help offset other nations’ strategic assets with a conventional versus nuclear strike. The pessimist in me sees a technology that has the potential to spiral out of control in many nations into deadly new nuclear delivery platforms.”



MACH 10 NOT ENOUGH



One benefit that could counterbalance the risks of inadvertent escalation would be the ability to get around an adversary’s missile defense systems. Right now missile defenses are designed with a ballistic missile’s flight path, altitude and speed in mind, but whether that means hypersonic weapons will be able to easily break through them is unknown.



“Systems like THAAD, PAC-3, Aegis, are actually pretty good at intercepting ballistic missiles now of medium range, they’ve now been tested against intermediate-range ballistic missiles,” Acton said, referring to the Terminal High Altitude Area Defense system and the Patriot Advanced Capability 3 missile. “Those things are moving faster than many hypersonic weapons.” Apart from speed, what should set hypersonic weapons apart from ballistic missiles is the ability to maneuver, rather than fly in a straight line, as the weapon approaches the target.



“The real issue is the extent to which these things can execute very rapid terminal maneuvering, in terms of their ability to penetrate missile defenses, and we haven’t seen that demonstrated yet,” Acton said. “… These very breathless technical claims about hypersonic weapons being these silver bullets, without the question mark, that can do everything—at the very least the jury is still out.” Little data has been released after DOD’s hypersonic tests—and verifiable data about the accuracy of Russian and Chinese missiles is also scarce—so not much information is publicly available about how well current prototypes maneuver or how accurate they are.



MAKING MOVES: KEY, VERY DIFFICULT



The dynamics of hypersonic flight make it hard for a speeding missile to make rapid evasive maneuvers shortly before target impact. Think of trying to make a quick, precise turn while driving: It’s easier at 35 miles an hour, harder at 70, and much harder at 7,000 miles an hour. Strider said the Advanced Hypersonic Weapon has overcome some of the challenges. “Maneuverability is a key aspect to its military utility. … Once it’s gliding, it’s able to fly cross-range, left or right in its flight path.”



Researchers have a math problem: how much energy to put into the weapon at launch—essentially, how much of a boost to give it—to make sure it hits its target with enough force. Engineers add up the energy required to lift the vehicle above the atmosphere, the distance it needs to cover, how much time it will spend fighting the atmosphere’s drag when it re-enters, and how many deviations from a straight trajectory it needs to make.



“It’s just like you throw a paper airplane: The harder you throw it, the farther it’s going to go,” Strider explained. “Same thing here. We’ve got to put enough energy into something that weighs quite a few pounds to throw it several hundred miles. Any maneuvers you make bleed off energy because you’re not powering it [at that point in its trajectory], so you have to be careful how many maneuvers you make so you can manage the energy that’s in it to make sure you can get to the target you want to.”



The U.S. Air Force is set to demonstrate the hypersonic X-51A Waverider, which is designed to ride on its own shock wave and accelerate to about Mach 6. Hypersonics will “revolutionize military affairs in the same fashion that stealth did a generation ago, and the turbojet engine did a generation before,” according to an Air Force study. (U.S. Air Force image)



WHAT’S NEXT



Whether the Advanced Hypersonic Weapon or any land-based hypersonic vehicle will be fielded is still an open question, though Strider and others believe the United States needs to field something quickly to counter Russia’s and China’s progress. “Currently they’re due to have some operational capability in the near future, and the U.S. needs a similar capability to be able to show them we’ve got one, too,” Strider said. “And so that’s what we’re shooting to do.” Funds from the Conventional Prompt Global Strike program will shift to the Navy, which is working on a sub- or ship-launched hypersonic glide vehicle, in 2020.



At this stage, Strider’s office does not plan further tests of the Advanced Hypersonic Weapon—he is now coordinating the planned flight tests of the Navy’s developing hypersonic capability, which is similar to the Army’s in design and build—until Army leadership makes policy and budgeting decisions. The ball is in the court of the cross-functional team dedicated to long-range precision fires, headed by Brig. Gen. Stephen J. Maranian. “Gen. Maranian at Fort Sill [Oklahoma] has the responsibility to bring the best concept forward to Gen. Milley” after examining all the options to improve the Army’s long-range precision strike ability, Strider said.



“And because the hypersonic weapon that we have developed through OSD sponsorship is the only flight-proven hypersonic weapon, we think we’ve got a front seat in being the best concept to take forward.”



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