• An Air Force Academy cadet peers into a wind tunnel used for measuring effects on hypersonic research vehicles. Development of new hypersonic craft is speeding up, but considerable research remains to overcome some technological challenges.  U.S. Air Force photo
     An Air Force Academy cadet peers into a wind tunnel used for measuring effects on hypersonic research vehicles. Development of new hypersonic craft is speeding up, but considerable research remains to overcome some technological challenges. U.S. Air Force photo

Hypersonic Applications Range Far Afield

June 1, 2019
By Robert K. Ackerman
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U.S., Russian and Chinese technologies race for supremacy.

The use of hypersonic weapons and vehicles for offensive and defensive military operations is accelerating as advanced research picks up in technologically sophisticated countries. However, speed increases are accompanied by growth in the number of technological challenges that must be overcome to build successful systems.

Russia and the United States have been pursuing hypersonic technologies for decades, notes David Van Wie, Johns Hopkins Applied Physics Laboratory sector head for air and missile defense. Recently, however, China has moved into the arena with significant advances. Van Wie relates that many Chinese technical papers and capabilities have won acclaim, placing the Middle Kingdom among the top players in the field.

The United States has investigated “a vast amount” of hypersonic technologies broadly across the discipline, he says. The country has demonstrated creativity and the ability to organize complex systems in this field, and Russia has shown similar capabilities with its experimental facilities. China, as a new entrant, nonetheless has made strong progress with its academic communities and research facilities.

“Hypersonics depress the battlespace of defensive systems,” Van Wie says. “It shortens the timelines associated with it and stresses defensive systems in terms of response times and engagements. This is independent of whether you’re operating offensive systems or working to come up with the defensive systems associated with that.” He continues that all three countries have spent enormous resources examining defensive hypersonic capabilities over the years.

“These weapons will be different from other classes of weapons and therefore likely will be operated in different ways also,” Van Wie declares, adding that the United States is highly active in both offensive and defensive hypersonic technologies.

For example, many potential hypersonic devices are coordinate-seeking weapons, Van Wie explains. They are fired at a point on the Earth’s surface, toward which the vehicle then navigates to the target. Yet others under development will need a more complex form of targeting that requires an exotic command and control. Van Wie points out that these weapons have a very short time in flight, so they do not present much opportunity for decision making after launch. In-flight targeting updates probably are the only major decision aid that can be incorporated.

New applications may be emerging in strike weapons. Van Wie believes that coming hypersonic strike systems will be rocket-powered using solid fuel. Not far beyond those weapons are scramjet systems, he suggests, noting that several countries have the technical capabilities for it.

Integrated engineering design is necessary for hypersonic scramjet vehicle development. This design must meet the severe demands presented by the interior of a scramjet engine, Van Wie notes. When high temperatures surround a vehicle’s exterior, its skin can radiate much of that heat away from the vehicle. But the interior of an engine offers no such radiation capability for heat relief, so that must be overcome. Here, active cooling—such as piping fuel through hot parts to absorb heat—can help.

The most advantageous applications of hypersonic weaponry are found in long-range strike domains. Noting they are not as useful for short ranges, Van Wie points out that long-range applications stress defenses and compress the battlespace.

On the defensive side, many interceptors already operate in the hypersonic realm, and these are likely to see improvements and advances in the near term. These advances could be applied to offensive weapons as well.

And the advances probably will come in the form of higher speeds, Van Wie offers. Describing it as “a natural evolution of military capabilities,” he observes that greater speed and standoff distance always are advantageous for combat.

But it only begins to have a game-changing effect when these complex systems are integrated together to operate in larger numbers, he says. A small number of hypersonic systems will not have a game-changing impact. “They’re not silver bullets,” he emphasizes. Militaries will be able to overcome them. “It takes acceptance, development of a complete end-to-end capability and deployment at some level of scale before you really get the game-changing impact.”

The general acceptance of the hypersonic speed threshold is Mach 5, or five times the speed of sound. But Van Wie notes this is not internationally accepted within the hypersonics community, although a consensus is forming around that. The physics of hypersonics are not in dispute: the faster an object travels in the atmosphere, the more its energy content affects the atmosphere surrounding the craft. As that energy becomes larger, it begins to drive different physical phenomena, including high-temperature gas dynamic phenomena, in the airflow around the vehicle. This mandates the use of high-temperature materials.

In designing power systems, the propulsion system of choice changes with speed increases. Turbine-based technologies give way to scramjets, Van Wie notes. And simulating these phenomena in ground-test facilities is exceedingly difficult, he says. Ground tests present imperfect simulations of what actually happens in flight, which puts the onus on actual flight testing.

“As you think about hypersonic systems, there are new and different kinds of challenges that you have to pay attention to,” Van Wie declares. “It’s not a natural evolution of vehicle design in the way we work on much lower-speed vehicles.”

Even with decades of history in hypersonic research, many challenges loom as researchers endeavor to advance the technology. Foremost among these is the aerothermal environment generated around the vehicles, Van Wie offers. Moving through the atmosphere at speeds greater than Mach 5 generates kinetic energy that is dissipated through aerodynamic drag, and a fraction of it becomes heat in the air and on the vehicle itself. “Creating vehicles that can sustain operation inside a very-high-temperature aerothermodynamic environment will continue to be a challenge as we develop new classes of vehicles,” he says. “Working through the thermal protection system and the accurate prediction of aerodynamic heating, and how to mitigate those heating effects, is the hardest part about hypersonic systems.”

Meeting this challenge may require complex engineering, he continues. New materials can help solve some of the problems, but system engineering—including an active cooling system—may complement those efforts. He offers that an overall solution likely will involve materials and a passive cooling system that might blend an ablative concept.

The vehicle’s interior will require insulation that controls heat conduction and prevents it from leaching into the vehicle with damaging effects, Van Wie points out. Sensitive electronics could be knocked out easily without blanket insulation such as silica or alumina.

Van Wie offers that most of the necessary scientific knowledge is already in hand. Near-term advances do not require any great breakthroughs. Over time, system performance will be improved as advanced materials are incorporated. Van Wie expects broader use of hypersonic weapons, emerging from technology advances, within a decade.

What must be done now is to perform the systems engineering associated with building practical hypersonic vehicles. Risk tolerance is vital as development proceeds through flight test, he declares.

“We’re standing at the threshold of first-generation systems with a lot of performance benefits that are still to be gained downstream,” he states.

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