In the spring of 2018, the Department of Defense (DOD) began publicly sounding alarm bells about the nation’s need to rapidly accelerate the development of technologies that could underpin a range of hypersonic weapons – strike vehicles capable of flight at Mach 5 and above.

Reacting to Russian President Vladamir Putin’s assertion in March 2018 that the Russian military had developed a hypersonic missile system it calls “Kinzhal” or Dagger, and Chinese hypersonic weapons developments including “Starry Sky-2” (a maneuverable hypersonic aircraft capable of carrying nuclear weapons), Michael Griffin, the Pentagon’s research and development head, said hypersonic weapons development is the Defense Department’s “highest technical priority” at a House Armed Services Committee hearing the following month.

 

The speed of and range of hypersonic weapons and their ability to maneuver could make them nearly impossible to counter with existing air defense systems. Griffin added that in his opinion, Chinese development of a “pretty mature system for conventional strike” at multi-thousand-kilometer ranges is the “most significant advance by our adversaries.” 

In response, the Defense Department has pushed forward coordinated research and development of several hypersonic weapons that fit into two categories. Boost-glide missiles use rocket propulsion to boost them to hypersonic speed up to the edge of outer space, at which point they glide at hypersonic speed to a target. They can be launched from mobile ground-based vehicles, surface or undersea vessels, or aircraft. Hypersonic air-breathing weapons would likely be launched from aircraft, employ a rocket booster to accelerate to Mach 5 or faster, and then use a hydrocarbon scramjet to sustain hypersonic cruise.

The U.S Air Force awarded Lockheed Martin contracts for two hypersonic weapons last year, including a contract for the Hypersonic Conventional Strike Weapon (HCSW) worth up to $928 million over its life cycle, and up to $480 million for the AGM-183 Air-Launched Rapid Response Weapon (ARRW). The boost-glide ARRW prototype flew on the wing of a B-52 in June of this year, with the first operational flight test of the weapon to occur by the end of 2020. The air-launched HCSW is scheduled to fly before 2021.

 

The Defense Advanced Research Projects Agency (DARPA) is working with the U.S. Air Force, U.S. Navy, and U.S. Army on two hypersonic weapons programs. The air-launched Hypersonic Air-Breathing Weapon Concept and the air-, land-, or sea-launched Tactical Boost Glide program are scheduled to deliver flying prototypes by the mid-2020s. Lockheed is also reported to be at work on a hypersonic fighter aircraft, while Boeing is understood to be developing a hypersonic reconnaissance aircraft. 

The efforts are at varying stages of technological maturity, but all are reliant on a mix of basic research emanating from academia, the defense industry, and the Defense Department’s research laboratories.

U.S. Combat Capabilities Development Command’s Army Research Laboratory (ARL)

ARL is engaged in basic and applied hypersonics research spanning a variety of hypersonic weapons applications associated with the Army’s No. 1 modernization priority: Long-Range Precision Fires. 

“Hypersonics is a piece of the three echelons that fall under Long Range Precision Fires,”  said Dr. Frank Fresconi, a researcher and lead for Long Range Distributed and Cooperative Engagements at ARL. “Some of those echelons will exceed Mach 5 and be in the hypersonic regime, and that’s where we have some technology gaps that we are trying to address.”

 

The science and technology research ARL is focused on will underpin future hypersonic projectile and missile development and fill in technological gaps for the three echelons: Tactical Fires for cannon artillery, Operational Fires for rocket artillery, and Strategic Fires, which includes both cannon and rocket-propelled solutions capable of being fired at strategic ranges (distances beyond the battlefield). 

The Army is looking to extend the range of its M777A2 Lightweight Towed 155mm Howitzer and the M109A7 Paladin Self-Propelled Howitzer platforms with three new projectiles that could be fired at hypersonic speed as part of its Next Generation 155 mm artillery ammunition family. The service wants the rocket-assisted XM1113 projectile to be capable of hitting targets at distances beyond 70 kilometers, and is studying hypervelocity or ramjet technology to enable the round to achieve that range.

 

Simultaneously, the Army is developing long-range missiles under the Precision Strike Missile program, the successor to its Army Tactical Missile System. Having recently completed a preliminary design review, the DeepStrike surface-to-surface missile will be capable of striking land-based targets up to 499 kilometers away and could be ready by 2025. Designed with room for continual capability upgrades after initial fielding, precision strike missiles like DeepStrike may benefit from advances in the kinds of hypersonic research ARL is currently undertaking.

 

ARL researchers are using high fidelity computational mechanics tools that include “multi-physics like heat transfer, fluid mechanics, and structures chemistry,” Fresconi explained, attempting to understand the superheated air or “plasma” that flows around hypersonic vehicles as they fly beyond Mach 5. If they can accurately calculate the dynamic chemistry and fluid dynamics of the plasma, they can develop more effective heat shields, enabling hypersonic vehicles to better survive the intense temperatures that come with hypersonic velocity. 

 “The hypersonic regime is where we have some technology gaps that we are trying to address,” Fresconi continued. “They include things like trying to understand the aero-thermal environment and having more survivable materials under those high thermal loads. Another challenge is materials that can survive the thermal loads that have “optical access” – in other words, an aperture for a sensor. How can you make that thing survive and still perform the way it needs to?”

 

Hypersonic efforts in which the Army is partnered with DARPA include the Operational Fires program to develop a ground-launched system enabling hypersonic boost-glide weapons to penetrate modern enemy air defenses and rapidly and precisely engage critical time-sensitive targets. The Army is also tasked with managing production of the Common Hypersonic Glide Body for variants of hypersonic boost-glide missiles that all three services intend to employ. 

ARL is also working on developing materials for thermal protection systems for these efforts, but Fresconi said that the Operational Fires and Common Hypersonic Glide Body programs have achieved technological readiness levels beyond the basic and applied research the laboratory does. 

“We do support those programs in a limited sense,” Fresconi said, while noting that ARL’s “core focus” is on scientific and technical gaps. “Those programs have certain gaps that they are either living with or are trying to mitigate, but we focus on research that looks into the future, beyond these current hypersonic efforts.”

U.S. Naval Research Laboratory (NRL)

Like its Army and Air Force counterparts, NRL’s basic and applied research efforts support ongoing and future hypersonic weapons development. 

The Navy is cooperating with the Army and the Air Force on research underpinning efforts like the Alternate Re-Entry System, a maneuverable warhead also known as the Common Hypersonic Glide Body that could be employed by boost-glide hypersonic missiles fired from Air Force bombers and land-based Army launchers as well as Navy submarines or surface vessels. Capable of maneuvering at hypersonic speed, the warhead would be very difficult for enemy air defenses to counter.

But as mentioned, a vehicle moving at hypersonic speed experiences extreme heating from the friction of the air molecules through which it moves. The heat gets even higher when the vehicle maneuvers. That’s why NRL is also exploring new materials, cooling systems, and aerodynamic designs that can mitigate the damaging effects of extreme heat. 

Researchers at NRL’s Space Mechanical Systems Development Branch are working on a new aerodynamic design for hypersonic vehicles that will allow them to maneuver without generating the additional friction and heat that traditional aerodynamic control mechanisms (flaps, ailerons, elevators, rudders, etc.) would cause at speeds above Mach 5. They call their design a “morphing waverider.” 

The idea is similar to the concept of “wing warping” used by the Wright Brothers and other aviation pioneers at the dawn of powered flight. Wing warping allows for control in three dimensions by changing the shape of a wing – as birds do – rather than employing moving control surfaces. A team led by NRL mechanical engineer Jesse Maxwell is taking the idea a step further, aiming to achieve a smooth, seamless control surface – one without ailerons, flaps or hinges – to which they can introduce small deviations through morphing – changing the  aircraft’s shape, specifically the underside of the vehicle, rather than just its wing.

Making small “smooth” changes to the waverider’s geometry would allow for controllability of the craft without “the intense heating that you get with normal control surfaces for low-speed aircraft,” Maxwell said.

A waverider – a design concept for hypersonic aircraft that dates back to the 1950s – uses the shock waves it produces during flight as a lifting surface, a concept known as “compression lift.” Like most hypersonic designs, it’s highly efficient, but only within a narrow altitude, speed, and atmospheric density range. In addition to the maneuverability it could help create, morphing could allow a waverider to be efficient across a range of atmospheric environments, from near-space to lower altitudes.

 

“They [hypersonic vehicles] produce a lot of lift for that one configuration,” explained Austin Phoenix, NRL mechanical engineer and partner with Maxwell on the morphing waverider. “But if they slow down or speed up, which is what most things do, they reduce their efficiency. The ideal morphing waverider would maintain the perfect geometry across the entire flight.”

During 2017 and 2018, Maxwell and Phoenix conducted wind tunnel testing with models of their morphing waverider. Their goal is to arrive at a design that works well at hypersonic speed, whether flying like a conventional aircraft or a spacecraft. Potential space applications are where their focus is, but a successful morphing waverider could have a range of applications, from hypersonic air travel to hypersonic weapons. 

The testing Maxwell and Phoenix undertook was done at the U.S. Naval Academy’s low-speed wind tunnel. But to simulate a wider range of conditions including higher speeds, the pair needed a more capable facility. After studying available and expensive alternatives, the engineers concluded that they could cost-effectively acquire a wind tunnel for use at NRL from a company with a pre-existing design. 

The new NRL-based wind tunnel, forecast to be complete by the end of 2019, will be uniquely capable, with the ability to vary pressure to simulate variations in altitude and wind speed in situ. That will be ideal for studying waverider models which will morph their shape to adapt to varying flight profiles. 

“We can accelerate and decelerate, climb, and descend,” Maxwell said. “We will be able to fly [simulations of] this vehicle anywhere from about sea level up to over 100,000 feet at speeds of Mach 1.3 up to at least Mach 5 early on – and eventually Mach 6 and a half.”

The Aerodynamics Research Center, University of Texas, Arlington (UTS) and the Office of Naval Naval Research (ONR) Wind Tunnel

The newly operational ONR/UTA arc-heated hypersonic wind tunnel located at the University of Texas’ Arlington branch is the result of joint investment by ONR and DARPA in 2014. 

ONR is aligned with NRL but separate. The organization conducts science and technology research on two tiers. At the department level, it investigates and supports technology research areas outlined in the “Naval Research and Development Framework.” Simultaneously, ONR directorates oversee investment portfolios for discovery and invention, future naval capabilities, innovative naval prototypes, and more.  

The advanced ONR/UTA wind tunnel was the vision of Dr. Luca Maddalena, director of UTA’s Aerodynamics Research Center.  “We needed a facility that was a radical change,” Maddalena said, “with the capability for a materials development component and a fundamental research component.” 

Five years ago, ONR and DARPA recognized the need for infrastructure to support hypersonic research. Maddalena agrees with other researchers in the field that the United States must invest more in facilities to support hypersonic research and said, “that’s a credit to the sponsoring agencies.”

The arc-heated tunnel is the only one of its kind at a U.S.-based university, capable of superheating air as a gas that can flow around a structure to simulate the plasma flow that forms around hypersonic vehicles traveling at 3,500 miles per hour or more. Friction causes the surfaces of hypersonic vehicles to heat up to more than 8,000 degrees Kelvin, or about 15,000 degrees Fahrenheit. 

At these temperatures, air also undergoes chemical reactions. Nitrogen and oxygen molecules start to dissociate and form a reacting mixture of atomic oxygen and nitrogen, plus regular molecular oxygen and nitrogen. This superheated air, or plasma, evolves as it flows around the vehicle, making it necessary for engineers to calculate simultaneously its chemistry and fluid dynamics to develop an effective heat shield.

 

Maddalena’s team will use two additional ONR grants totaling $1.5 million to begin research with the tunnel. The first, an $820,000 award for fundamental research, will develop diagnostic techniques to characterize the plasma flow, thereby improving the understanding of the relationship between arc-jet test and actual flight environments. The second grant, a $690,000 award, will allow the Aerodynamics Research Center to purchase a femtosecond laser system for the tunnel. 

Since the flow in the wind tunnel will be heated beyond temperatures at which any type of physical measurement device placed in it would survive, the femtosecond laser system will allow Maddalena and his team to non-intrusively measure the temperature and composition of the plasma flow. The laser system is so advanced that it has never been used in an arc-jet facility, and will take six months to build.

“The objective of the work is two-fold,” Maddalena explained. “First is to increase knowledge of complex hypersonic flows [around aero structures]. They are reacting chemically, and they have a complex model that has to be developed and then applied at a fundamental level to predict what happens with a hypersonic vehicle. We will specifically develop laser-based diagnostics to understand these complex flows.”

The second objective is to use the ONR/UTA hypersonic wind tunnel to aid in the development of materials that can be used to insulate hypersonic vehicles.

“With a facility like this, you can put thermal protection material candidates – heat shields – in it and see how they perform,” Maddalena added. “We can support other researchers and other organizations working on heat shields for hypersonics to see to what extent they are making progress with new materials.”

Maddalena said the wind tunnel is also of great interest to commercial space companies working to develop reusable space vehicles that will enter and leave earth’s atmosphere repeatedly. 

“So you can move from defense to space exploration in general,” Maddalena concluded. “This facility can also support heat shield testing for planetary entry for probes. Major players in that area have already expressed interest and we are having discussions. This is very important for hypersonic studies and for our nation.”

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This article originally appears in Defense R&D Outlook 2019.