The era of hypersonic flight had arrived. Countries are developing future hypersonic Spaceplanes , enabling intercontinental travel at very high speeds, that could cut the journey times from the UK to Australia from the current duration of around 20 hours to as little as two hours. They shall also provide revolutionary military capability like prompt global strike, launch on demand, satellite servicing and anti satellite missions. The Military of United States, Russia and other countries are developing sixth-generation fighters that may be capable of achieving hypersonic speeds. There is global race to develop Hypersonic Missiles such as US HTV-2 and X-51, Chinese WU-14, Russian Yu-71, that travel at least five times the speed of sound (Mach 5) or more. These vehicles can fly along the edge of the space and can glide and maneuver to the targets.
Hypersonic flights, re-entry, and propulsion vehicles, regardless of their design, require maneuverability of materials against high temperature erosion in excess of 2400°C. At speeds of Mach 5 and higher, aerodynamic friction can heat an aircraft’s exterior enough to melt steel. Atmospheric re-entry vehicles, rockets and scramjet powered air-breathing hypersonic cruise vehicles primarily encounter high pressures and heat flux on the leading edges due to air stagnation and shock waves. Also the respective propulsion systems experience high temperature exothermic combustion reactions to produce thrust.
One of the toughest design challenges of next-generation hypersonic munitions is developing navigation, guidance, sensors, and communications subsystems that are rugged enough to operate through the extreme heat, shock, and vibration of hypersonic flight.
To address the challenge DARPA initiated the Materials, Architectures, and Characterization for Hypersonics (MACH) programme in 2018. The programme seeks to develop and demonstrate new design and material solutions for sharp, shape-stable, high heat flux capable leading edge systems for hypersonic vehicles travelling more than five times the speed of sound. Proposed research should investigate innovative approaches that enable revolutionary advances in the materials, design and implementation of shape-stable, high heat flux capable leading edge systems.
DARPA is seeking expertise in thermal engineering and design, advanced computational materials development, architected materials design, fabrication and testing (including net shape fabrication of high temperature metals, ceramics, and their composites), hypersonic leading-edge design and performance, and advanced thermal protection systems. DARPA has specified that it does not want research “that primarily results in evolutionary improvements to the existing state of practice”.
The MACH programme will comprise two technical areas. The first area aims to develop and mature a fully integrated passive thermal management system to cool leading edges based on scalable net-shape manufacturing and advanced thermal design. The second technical area will focus on next-generation hypersonic materials research, applying modern high-fidelity computation capabilities to develop new passive and active thermal management concepts, coatings, and materials for future cooled hypersonic leading edge applications.
“The purpose is to develop new leading-edge technologies for the front of the vehicle, which meets the atmosphere first, enabling it to go faster or go deeper into the atmosphere,” DARPA’s Carter explains. “Looking at it from a thermal management perspective, the design of the vehicle is dominated by heating at hypersonic speeds. These are topics that recently the electronics and program management world has started to address. A common holy grail in electronics is one kilowatt per square centimeter of heating, which is very similar to what is required for the leading edge of hypersonics. “We’re trying to advance the technology very quickly and develop a leading edge component that future designers can use in the vehicle,” Carter continues. “The approaches you see there are things like heat pipes, which are used extensively in high-performance electronics.”
Bill Carter, Program Manager in DARPA’s Defense Sciences Office (DSO), said, “For decades people have studied cooling the hot leading edges of hypersonic vehicles but haven’t been able to demonstrate practical concepts in flight. “The key is developing scalable materials architectures that enable mass transport to spread and reject heat. In recent years we’ve seen advances in thermal engineering and manufacturing that could enable the design and fabrication of very complex architectures not possible in the past. If successful, we could see a breakthrough in mitigating aerothermal effects at the leading edge that would enhance hypersonic performance,” he added.
Moving heat from hot components like microprocessors and leading-edge aeronautic structures also is a big issue. This is how trees keep cool through leaves and was applied to some of the earliest hypersonic platforms back in the 1950s. In addition, film cooling, which is used in turbine engine blades today, enables materials to survive in environments where they ordinarily would melt. Carter says he hopes MACH will lead to development of a disruptive technology “that will get us on a new design curve that will transcend materials we use today, such as carbon/carbon composites. In a way, it’s history coming back to us. Carbon/carbon was one reason we stopped working on thermal issues.” The next five to ten years will be critical to the development of new and advanced technologies required for hypersonics, such as MACH.
“Cooling is interesting because you are trying to get heat off a very hot vehicle. SWaP is important because these are very constrained platforms. Based on the aerodynamic principles involved and launch capabilities, you have a highly SWaP-constrained platform. So, advances in electronics, fuel and materials in general will be very important.”
“You cannot recreate the conditions a hypersonic vehicle will experience in flight in the lab; there’s always some kind of gap, but I believe we will close that gap. We have new tools in the toolbox, not only advances on the computational side, but in meeting the longstanding challenge of scaling. Nobody in the world can do this today, but I believe we will crack it. That will enable us to do small frames much more quickly and develop flight vehicles scaled up using the computational capabilities we’re developing. For the first couple of iterations, we’ll follow the discipline we have used for more than a century — crawl, walk, run — but we will be shortening that walking step very quickly.” At the same time, hypersonics requires greater care than other programs when it comes to making changes, both external and internal.
A greater understanding of material composition and applications in just the past five or so years has set the stage for a new century of development that could take hypersonic technology into areas never before considered. “Our ability to model materials at the atomic scale is really emerging as a way to not only understand materials but to be predictive tools. When you marry that up with AI [artificial intelligence], we have a truly new way to approach materials development,” DARPA’s Carter says. “These new capabilities are very inspiring and I’m anticipating the next century will be just as exciting as the last in materials science as we integrate all that into multidisciplinary design, looking at how the mission may drive fundamental development. We’ve tried to model MACH on that new future, not only making new materials, but what is driving that development so sensible engineers will want to use them.
One area is new materials that involve compositionally complex alloys (CCAs). “For the past two centuries, we have looked at the periodic table and added small amounts of other materials,” Carter says. “CCAs bring together at least five or more elements in an attempt to confuse nature that actually works. They have some interesting properties — high temp, anti-corrosion, fatigue, and toughness you don’t see in traditional alloys. The story of composites really has yet to be written. Some of those have properties that could be very useful in building other aspects of the vehicle.”
Other thermal-management design approaches, such as insulating and highly conductive materials to manage heat pathways inside of hypersonic vehicles also are of considerable concern. “In the world of thermal management, we are still making substantial inroads in ultra-high heat,” Carter says. “The headroom to go to even higher heat flux and temperatures still has a long way to go.
“If we are successful, we can see dramatic improvements in the capability of these platforms in velocity, range, the atmospheric conditions we can fly in,” Carter says. “We’re also thinking about manufacturability, so I expect to see an industrial base to produce these structures. And we’ll see American ingenuity come to the fore in other areas of hypersonics. One is how we model those, which is a cornerstone of how we develop systems. I expect to see dramatic advances not only in modeling materials but in modeling vehicles; model-driven design is being done today, but it’s not as connected and powerful as we would like.”
“Hypersonics is a very interconnected design process. Every change you make has to be connected to every other component, unlike building an airplane. With MACH, we’re talking about a leading-edge technology that will improve the capability of the vehicle with very little redesign required,” he says. “It’s one thing to have an aeroshell on the leading edge, but you also have to have all the communications and other stuff on the inside protected from the heat of hypersonic flight. Just swapping out one component could leave you vulnerable to a thermal shift.”
DARPA’s HEAT developing rugged materials for hypersonic radomes and infrared windows
High speed aerospace systems, particularly hypersonic systems, require the use of an RF radome or IR window (both hereafter collectively referred to as “apertures”) to protect sensitive electronics from the aggressive aerothermal environment of high speed flight while providing
transparency to the RF or IR electromagnetic energy used for guidance, communication, and sensing. These aperture materials are subjected to extreme thermal, mechanical, and chemical environments during hypersonic flight that can limit the performance of the aperture and/or the platform. For example, shock waves and high heat loads produced during flight impart wavefront distortions and boresight errors upon guidance electronics, while aerothermalmechanical loads challenge aperture integration into flight vehicle aerostructures. As such,
current hypersonic aperture implementations are constrained by their interdependent materials response to the high heat loads and dynamic stress states of hypersonic flight.
In the HEAT program, DARPA will develop and demonstrate RF and IR apertures that are capable of enabling operation in high enthalpy environments not achievable today using state-ofthe-art monolithic and fiber-reinforced solutions. Successful solutions will advance multiple performance objectives simultaneously in order to realize RF and IR apertures for future hypersonic platforms, including controlling thermo-optical and elastic-optical effects; maintaining desired transmission amplitude and bandwidth; reduced thermal deformation, mismatch, and radiation; rate of manufacture, cost and scalability competitive with current state of the art apertures; weather erosion resistance and vehicle integration.
As detailed in the classified addendum, for a given hypersonic trajectory, DARPA expects substantial improvements in key metrics over the capability of current materials that result in substantial improvement in functionality at the platform level. Although it may be possible to advance these complex combined performance metrics by improving traditional, mature monolithic or fiberreinforced materials, DARPA believes that by leveraging recent advances in multi-objective optimization, computational materials design, net-shape manufacturing, and integrated system modeling, fundamentally new approaches can be developed with dramatically improved responses to high heat loads and dynamic stress states and enable high performance sensing during critical phases of hypersonic flight.
The HEAT program will develop a new class of materials that are capable of operating at high heat fluxes and dynamic pressures to enhance hypersonic operational capability. Proposed research should investigate innovative approaches that enable revolutionary advances in the
design and implementation of aperture materials. The HEAT program will focus on (1) RF radome materials, (2) IR window materials, and (3) enabling aperture material technologies for next-generation hypersonic platforms.
Specifically excluded is research that primarily results in evolutionary improvements to the existing state of practice without significant added capability such as incremental enhancements to existing ceramics and ceramic composites currently utilized as hypersonic aperture materials.
The HEAT program is a four year, two-phase effort. Proposals should be structured as a 24-month base Phase I with a 24-month Phase II option. The program is divided into three Technical Areas (TAs). TA1 will focus on developing and testing integrated RF aperture materials. TA2 will focus on IR aperture materials. TA3 will focus on next generation aperture materials that lead to capability well beyond TA1 and/or TA2 performance metrics.
TA1 and TA2: RF and IR materials.
The TA1 and TA2 goals are to develop fully integrated RF (TA1) and IR (TA2) aperture materials solutions that are applicable for integration into typical hypersonic aeroshell materials (e.g. ceramic composites such as C-C, or C-SiC, or refractory metal alloys such as Inconel). All proposed solutions should utilize an integrated computational materials engineering (ICME) framework both to enhance materials optimization as well as to enable system-level performance projection.
Proposed solutions must be able to survive sustained operation in a high temperature oxidative environment (i.e. as described in the classified addendum). Proposals must detail the manufacturing method and vehicle integration strategy. Preliminary parametric performance models are required, validated through relevant hardware testing, that describe performance as a function of design and aerothermal operating conditions.
In Phase I, TA1 and TA2 performers will develop materials solutions that must survive the ground test campaign outlined in the classified addendum. All TA1 and TA2 proposals must include detailed estimates of effective performance and specifically address how the proposed solution will mitigate the extreme aerothermal environments detailed in the classified addendum. A parametric performance model for the RF or IR aperture material should be developed that takes into account variations in aperture geometry, aerothermal environment, and aerodynamic considerations. Phase I must include a task to estimate the manufacturability and cost of the proposed technology.
In Phase II, scaled-up TA1 and TA2 solutions selected to move forward must survive increasingly aggressive conditions as outlined in the classified addendum. Solutions must also demonstrate manufacturability such that application scale apertures can be reasonably integrated onto flight vehicles.
TA3 Next-generation materials technologies: The TA3 goal is to identify, develop, and mature materials that enable future hypersonic platforms with performance that greatly transcends TA1 and TA2 requirements. Successful TA3 approaches will explore new materials and integration approaches, novel aperture implementations, and modeling capabilities to accelerate development of next-generation apertures capable of meeting the detailed TA3 metrics given in the classified addendum.
. All proposed materials development should utilize an integrated computational materials engineering (ICME) framework both to enhance materials optimization as well as to enable system-level performance projection.
Hypersonics experts at the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., have chosen three U.S. technology companies to develop rugged RF radomes and infrared windows able to withstand the environmental extremes that future hypersonic missiles and aircraft must endure.
So far three companies are developing materials to shield sensors from heat and vibration as part of the HEAT program: the General Electric GE Global Research Division in Niskayuna, N.Y.; the Lockheed Martin Corp. Missiles and Fire Control segment in Orlando, Fla.; and the Georgia Tech Research Corp. in Atlanta.
Lockheed Martin won a $2.5 million HEAT contract , Georgia Tech won an $8.3 million HEAT contract in Feb 2021, and GE Global Research won a $7.5 million contract on 4 March. The three companies are considering solutions that may involve affordable and manufacturable means of controlling thermo-optical and elastic-optical effects; maintaining desired transmission amplitude and bandwidth; and reducing thermal deformation, mismatch, and radiation.
Lockheed Martin, Georgia Tech, and GE Global Research experts are looking into new materials that combine metals, ceramics, and coatings for high-performance structures, as well as new computational capabilities necessary to develop these materials.