Countries Race to overcome Technological Challenges for Hypersonic Flight
Stephen Pendergast
Systems Engineering Consulting of Complex Radar, Sonar, Navigation and Satellite Comm Systems
idstch.com | International Defense Security & Technology Inc.
edited from the Rajesh Uppal original.
38-48 minutes
The era of hypersonic flight had arrived. Hypersonic refers to aircraft, missiles, rockets, and spacecraft that can reach speeds through the atmosphere faster than Mach 5, which is near 4,000 miles per hour depending on atmospheric temperature and density. The large driving forces behind hypersonic research emerge from the need to overcome military air and ballistic missile defences, reduce cost to space, and develop faster global transportation for both military and civilian purposes. 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 antisatellite 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. China has made significantly more advances than the U.S. in the past five years.
Systems that operate at hypersonic speeds—five times the speed of sound (Mach 5) and beyond—offer the potential for military operations from longer ranges with shorter response times and enhanced effectiveness compared to current military systems. Such systems could provide significant payoff for future U.S. offensive strike operations, particularly as adversaries’ capabilities advance, says DARPA.
Hypersonic speed is defined as anything above Mach 5. There are two types of weapons emerging: hypersonic cruise missiles and hypersonic glide vehicles. Hypersonic cruise missiles are powered all the way to their targets using an advanced propulsion system called a SCRAMJET which can operate between Mach 5 and Mach 15. These are very, very fast reducing time for defences to react. However, In order to maintain sustained hypersonic flight, a vehicle must also endure the extreme temperatures of flying at such speeds and require ultra high temperature materials.
Boost-glide vehicles, one of the most common hypersonic weapon designs, are unpowered and require some sort of booster to get them to the appropriate speed and altitude, after which they glide back down to earth. Ballistic missiles, or derivatives thereof, have traditionally served as the launch platform for these systems. “It’s like a plane with no engine on it. It uses aerodynamic forces to maintain stability to fly along and to maneuver,” said Rand senior engineer George Nacouzi. What’s more, Moore notes that because it’s maneuverable “it can keep it’s target a secret up until the last few seconds of it’s flight.”
Hypersonic cruise missiles can fly at altitudes up to 100,000 feet whereas hypersonic glide vehicles can fly above 100,000 feet. An “air-breathing” hypersonic vehicle approach allows less range than boost-glide but greater maneuverability. Air-breathers can also be significantly smaller. A rocket has to carry large amounts of oxidizer to burn its fuel. A jet just sucks in oxygen from the atmosphere. But normal jets don’t have to suck in air moving at Mach 5-plus. A jet that works at hypersonic speeds will require some breakthroughs — and, again, 3D printing can help grow the exotic components.
Challenges of Hypersonic Flight
Creating hypersonic technology presents several tough, complex engineering challenges. Designing a HV often appears daunting and difficult. Numerous attempts have been made in the past with many successes and failures. Only a few programs ever became operational vehicles. Two hurdles which essentially link into a co-hurdle are the extreme hypersonic flight conditions and hypersonic mission objectives.
For example, mission objectives include space access and global transportation. Space access not only requires the attainment of very high vehicle velocities but also must traverse the varying atmospheric layers to reach an orbital path. Global access missions also benefit from hypersonic speeds but do not require the large orbital altitudes. Therefore, HV systems provide the desired speeds but demand complexity.
Hypersonic flight realm traverses a complex environment as, “ranging from high in the stratosphere to operations into and cross the demarcation of spaceflight, where the laws of aerodynamics cease to apply and the laws of ballistic, Keplerian trajectories, and Hohmann transfers take over.” Hypersonic revolution blends the twin stream of space and aeronautics research into a confluence.
NASA categorizes speeds between Mach 5 ( 6125 kilometers per hour) to 10 Machs as Hypersonic. The upper limit of hypersonic is believed to be around Mach 25. University of Queensland, Australia claims that flight beyond Mach 14 is difficult to sustain, and it becomes hard to burn fuel and the drag becomes too high. Aerodynamic drag roughly scales with the square of airspeed; double the speed, and the drag goes up four times. Streamlined shapes have partly overcome this problem, but the solution then, as it is now, is more thrust. However, hypersonic speeds are frequently limited not by available thrust or drag, but by the heat buildup caused by atmospheric friction.
“The physics are enormously difficult at hypersonic speed,” aerospace engineer Steven Beresh of Sandia’s aerosciences department said. The air and gases react differently than at subsonic speed; materials are put under extreme temperatures and pressure; and there is the added challenge of guidance mechanisms also needing to withstand those pressures. “We have some information, but not enough information,” he said. “We’ve mostly been dealing with re-entry vehicles. Before, the idea was to just have the vehicle survive; now, it needs to thrive. We’re trying to fly through it.
The hypersonic regime introduces a number of flow attributes such as: extremely high turbulence, pressure, temperature, density, vorticity, and energy, thin shock layers, viscous interactions, entropy layers, changes in vehicle stability and control; and physical-chemical gas changes such as ionization, dissociation, equilibrium effects, and other molecular phenomena.
In addition, the hypersonic designer must remain aware of the other flow regimes since a hypersonic vehicle will have to transition from rest to the designed hypersonic flight Mach number and transition throughout the various characteristics of the atmosphere. The need to operate in several flight regimes can lead to unforeseen aerodynamic conditions, especially in air-breathing propulsion systems. While a certain shape or lift-to-drag (L/D) configuration may be efficient at low hypersonic Mach numbers (say 4-8), it may exhibit a severe degradation in aerodynamic performance outside this envelope. This would be the case, for example, during the takeoff and landing phases of a space plane.
When moving at such velocity the heat generated by air and gas in the atmosphere is extremely hot and can have a serious impact on an aircraft or projectile’s structural integrity. That is because the temperatures hitting the aircraft can reach anywhere from 2,000 to 3,000 °C. The structural problems are primarily caused by processes called oxidation and ablation. This is the when extremely hot air and gas remove surface layers from the metallic materials of the aircraft or object traveling at such high speeds.
The heating depends on factors such as the vehicle’s speed and contours. When a space shuttle returning from orbit hit the upper atmosphere at Mach 25, its blunt leading edges heated to 1400°C, which a skin of carbon-carbon composites helped it withstand. Newer hypersonic craft tend to have sharper edges—in part to assist with maneuverability—that can exceed 2000°C. Turbulence can make things worse. At hypersonic speeds, the boundary layer around the vehicle thickens, and a smooth, laminar flow can suddenly break up into eddies and swirls that cause temperature spikes on the vehicle’s skin.
Launched in June 2011, the 2-year project examined a phenomenon that bedevils spacecraft re-entering Earth’s atmosphere. At hypersonic speeds, laminar air flow over a surface can suddenly turn turbulent, creating intense temperature spikes on the vehicle’s surface. To study those heat fluctuations, the Transhyberian team performed wind tunnel experiments and computer simulations at the Belgian institute, the German Aerospace Center, and three Russian institutions—including the Central Research Institute of Machine Building, or TsNIIMash, a spacecraft and missile design center in Korolyov where both arrested scientists work. The research showed that locally heating or cooling the surface could help control the temperature spikes—a finding that could improve the design of hypersonic aircraft.
High air speeds also pose challenges for engines on HCMs, which unlike HGVs have their own power plants. HCMs use a supersonic combustion ramjet, or “scramjet,” to accelerate. “It’s the simplest type of jet engine you could ever imagine … just an open tube” in which air mixes with fuel, Lewis who served as chief scientist of the U.S. Air Force from 2004 to 2008. “It’s also perhaps the most complicated type you can imagine because of the extreme conditions under which it operates.” At hypersonic speeds, air molecules spend milliseconds in the engine tube—scant time for fuel and air to mix properly. And when a vehicle pitches and yaws, airflow into the engine changes, which can result in uneven combustion and thrust. Tweaks to get a better burn have ramifications for, say, how the aircraft withstands shock waves. “Everything is incredibly coupled. You are designing a fully integrated vehicle,” Lewis says. It took the United States 46 years to realize its first working scramjet: NASA’s $230 million X-43a, an uncrewed vehicle that flew in 2004.
HGVs pose other challenges. The rocket that carries the glider reaches speeds far greater than those of an HCM, meaning engineers must use materials that are even more resistant to heat. Still, HGVs are easier to maneuver because they lack a scramjet, with its acute sensitivity to pitch and yaw. “It almost becomes a religious discussion—rockets versus air breathing,” Lewis says. “The ultimate answer is we probably want both.”
Studies have uncovered some of the lingering elements that continue to plague vehicle aerodynamics, viz. Limited capabilities of ground testing facilities for the simulation of hypersonic flows. Other challenges are the limited aerothermodynamic flight test database. The stringent access restrictions to existing databases and the limited verification efforts of computational fluid dynamics (CFD) aerothermodynamic codes against ground test data. The vehicle is controlled during its flight and its terminal phase using the aerodynamic surfaces. The integrated vehicle and flight control system of the hypersonicc vehicles are critical for maneuvering the vehicle.
Hypersonics represent a variety of technological challenges, particularly for electronics and electro-optics manufacturers, to protect against chock, vibration, and extremes in electronics cooling and thermal management that non-hypersonic weapons rarely face. While a bullet shot from a gun, for example, travels at about 1700 miles per hour, or Mach 2.2, the hypersonic weapons with speed more than twice are expected to give higher acceleration and shock to ruggedized electronics to deal with.
Hypersonic Technology Requirements
Some of the technology requirements are Heat: At hypersonic speeds, friction and air resistance create an incredible amount of heat, which needs to be managed through tough but lightweight heat shields and thermal protection systems. Sensors and electronics must also be hardened to withstand extreme conditions. Innovative Materials: Managing extreme heat and speed means applying advanced materials and composites that can withstand extreme environments. Maneuverability: Hypersonic systems are designed to operate in contested environments and must be capable of overcoming a wide range of defenses. Communications that become a significant challenge during hypersonic flight. A system must maintain connectivity to operators and decision-makers through a global communications and sensor systems. A system moving at a mile every second needs to operate with an incredible degree of precise maneuverability.
Today active research and development is in progress on all aspects of hypersonic flight, from materials to withstand high temperatures generated in the atmosphere, to more efficient propulsion systems, to size, weight and power (SWaP)-constrained enhanced electronics for sensors, guidance, communications, and other harsh-environment applications.
The Hypersonic International Flight Research Experimentation (HIFiRE) program is a hypersonic flight test program executed by the United States AFRL and the Australian DSTO.1,2 Its purpose is to develop and validate technologies critical to next generation hypersonic aerospace systems. Candidate technology areas include, but are not limited to, propulsion, propulsion-airframe integration, aerodynamics and aerothermodynamics, high temperature materials and structures, thermal management strategies, guidance, navigation, and control, sensors, and system components.
The Hypersonic Air-breathing Weapon Concept (HAWC) program is a joint DARPA/U.S. Air Force (USAF) effort that seeks to develop and demonstrate critical technologies to enable an effective and affordable air-launched hypersonic cruise missile. These demonstrations seek to open the door to new, responsive long-range strike capabilities against time-critical or heavily defended targets. The program intends to emphasize efficient, rapid and affordable flight tests to validate key technologies.
HAWC plans to pursue flight demonstrations to address three critical technology challenge areas or program pillars—air vehicle feasibility, effectiveness, and affordability. Technologies of interest include:
- Advanced air vehicle configurations capable of efficient hypersonic flight
- Hydrocarbon scramjet-powered propulsion to enable sustained hypersonic cruise
- Approaches to managing the thermal stresses of high-temperature cruise
- Affordable system designs and manufacturing approaches
- HAWC technologies could also extend to future reusable hypersonic air platforms for applications such as intelligence, surveillance and reconnaissance (ISR) and space access.
The Tactical Boost Glide (TBG) program is a joint DARPA/U.S. Air Force (USAF) effort that aims to develop and demonstrate technologies to enable future air-launched, tactical-range hypersonic boost glide systems. In a boost glide system, a rocket accelerates its payload to high speeds. The payload then separates from the rocket and glides unpowered to its destination.
The boost-glide hypersonic weapons would offer certain unique attributes to military planners. Compared to ballistic missiles, boost-glide weapons have potentially 5 to 10 times the speed of sound, nearly double the range, can generally transport a heavier payload over a given range, are capable of midcourse maneuvering, and fly at lower altitudes.
The TBG program plans to focus on three primary objectives:
- Vehicle Feasibility—Vehicle concepts possessing the required aerodynamic and aerothermal performance, controllability and robustness for a wide operational envelope
- Effectiveness—System attributes and subsystems required to be effective in relevant operational environments
- Affordability—Approaches to reducing cost and increasing value for both the demonstration system and future operational system
Lockheed is working on a number of innovative technologies to enable long-duration, maneuverable, hypersonic flight, company CEO Marillyn Hewson told reporters. These breakthroughs include new thermal protection systems, innovative aerodynamic shapes, navigation guidance and control improvements, and long-range communication capabilities, she said.
Other relevant areas that fall under this category consist of control surfaces such as fins, elevons, tailerons, flaperons, etc. The technological factors associated with these control surfaces include:
- The requirement to employ thin structures that reduce drag.
- The need to overcome the thermal protection barriers imposed by the thin surface requirement
- The need to design for longer life cycles and mitigate oxidation.
- The need to integrate both hot and cold structures (e.g., in actuators).
An interdisciplinary approach is required for realizing hyper-sonic vehicles , as the design of each of the subsystem has an effect on the other.
China has invested heavily in facilities, including sophisticated wind tunnels and shock tubes that use blast waves to study hypersonic flows. “Ten years ago, they were duplicating what others had done,” Boyd says. “Now, they’re publishing innovative ideas.” At a 2017 hypersonic conference in Xiamen, China, Chinese scientists presented more than 250 papers—about 10 times the number presented by U.S. researchers.
The hypersonic wind tunnel is used to test flight characteristics in a hypersonic region of Mach number 5 or more. For example, in March 2018, China’s state media announced construction on an 870-foot wind tunnel capable of simulating conditions from Mach 10 to Mach 25. Scheduled for completion in 2020, it will join existing wind tunnels able to simulate environments from Mach 5 to Mach 9. The U.S., by comparison, has Mach 5 to Mach 9 wind tunnels, but they are smaller than the Chinese tunnels, and capable of tests lasting only a few seconds.
Scramjet Technologies
Propulsion systems play a critical role in determining the range, payload, and speed of a missile. The major propulsion systems currently used in rockets and missiles are turbofans, turbojets, and ramjets. Supersonic systems, which travel at a speed of Mach 2-4, run on ramjet technology. However, in recent years, there has been an increasing demand for rockets with higher speeds. This has led to the development of a new propulsion system called Supersonic Combustion Ramjet, or scramjet.
Solid, liquid, and hybrid rockets, in conjunction with turbine, ramjet, and scramjet engines, embody some of the available propulsion concepts that are capable of hypersonic flight. Two branches emerge as the dominant hypersonic engine mechanisms, the rocket motor and the air-breather. Ramjets become less efficient at higher Mach numbers due to the ramjet’s subsonic combustion. Scramjets potentially hold the capability to realize the objective of a long range airliner at hypersonic speeds and, complement the traditional rocket in space launchers.
The Air Force, in collaboration with DARPA, NASA, and the Navy, is developing scramjet—supersonic combustion ramjet—technologies that may contribute to the long-range strike mission in the future. In this type of vehicle, the engine gets the oxygen it needs for combustion from the atmosphere passing through the vehicle, instead of from a tank onboard. This eliminates the need for heavy reservoir oxygen tanks, and makes the vehicle far smaller, lighter, and faster than a conventional rocket.
Ultra High Temperature Materials
Hypersonic platforms are ultimately limited by the capacities of the materials available. The Airframe of Hypersonic Vehicles like SR-72 ( Lockheed Martin reconnaissance drone with strike capability) , must include advanced materials to stay intact while subjected to high dynamic loads, and to withstand the extreme aerodynamic heating of hypersonic flight, as air friction alone would melt conventional materials, writes Dora E. Musielak, Ph.D. University of Texas at Arlington.
There is growing interest in developing ultrahigh temperature materials (UHTM). These are materials with temperature capability greater than 1650 °C and able to withstand extreme erosive / corrosive environments. UHTM should possess high strength at high temperatures, oxidation resistance, ablation resistance, thermal shock resistance.
These materials will be required for hypersonic air breathing vehicles, hyper speed cruise missile, hypersonic aircrafts, re-usable launch vehicles to protect leading edges and nose cones that experience very high temperatures (> 2000 °C). Refractory diboride composites like ZrB2, HfB2 etc and multilayer coatings of HfC, SiC on C-C composites are most promising UHTM candidates.
Rugged Electronics
“Matching and overcoming hypersonic threats will demand technology advances across three broad areas: propulsion systems; airframe materials; embedded electronics,” wrote Lorne Graves, chief technology officer of rugged computer specialist Abaco Systems in Huntsville, Ala., in a March 2019 white paper. “While most research spending is targeting the first two areas, it is clear that innovative improvements in embedded electronics are key to creating fully functional hypersonic systems.”
He identified four primary functions embedded electronics that will need to perform for deployable hypersonic platforms:
- Mission computing, focused on responding to commands, adjusting to changing conditions and ensuring that all subsystems work in concert to accomplish a platform’s mission;
- Flight computing, controlling the path of the platform, monitoring the outputs of sensors and controlling the operational employment of sensors;
- Real-time signal processing for radar, electro-optical sensors, and electronic warfare (EW); and
- flawless and secure communications with command and control networks.
“The electronics to perform these functions may exist in a single computing enclosure or be spread across several, Abaco’s Graves wrote. “The system may even be designed so that a subsystem of electronic components can perform multiple functions, switching between them based on the mission situation. But, regardless of the configuration, hypersonic flight will present the electronics with an intimidating set of environmental challenges, taking conditions already faced by airborne electronics and raising the bar to an entirely new level of difficulty.”
“Traveling at those speeds, the vibration profiles will evolve beyond what typical standards currently call for, so you have a lot of work to do on printed circuit boards, all the separable connector failure points and mitigating the effects of those so you don’t have failures, either right away or fatigue failures, which are worse,” says Ivan Straznicky, chief technology officer for advanced packaging at Curtiss-Wright Defense Solutions in Ashburn, Va.
While external heat is a problem for external structures, such as sensors, it also significantly raises the aircraft’s internal temperature, threatening to damage sensitive electronics. “Heat is another area,” Straznicky continues. “If a missile goes back into the atmosphere at those speeds, it generates a lot of heat, so we need materials to shield against that. But you can’t get rid of that heat entirely, so it will be hot inside that missile. You need electronics that can survive above typical temperature ranges, which may require unique solutions, such as vapor chambers or perhaps some type of liquid cooling.”
Experts say they will have to design components to withstand environmental conditions as severe as 10 times higher than the maximum allowed under VITA 47 — an American National Standard that defines rugged environmental, design, and construction requirements for rugged commercial-off-the-shelf (COTS) plug-in embedded computing boards. Still, there is no ruggedization “silver bullet” to protect hypersonic electronics and sensors.
“Cooling is interesting because you are trying to get heat off a very hot vehicle. SWaP [Size, Weight and Power] 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.”
“There are potentially flight versus mission computers on the platforms, which may have very different requirements,” Straznicky points out. “The flight computer may need be certified to DO-25 4 DAL A [the most stringent design assurance level for airborne electronics], which is a high bar. Mission computers still need to be ruggedized, to the same level, but the design assurance levels impose significant additional restrictions on the hardware, such as recognizing failure modes.”
Manufacturing also can be a challenge. “Components are no longer made specifically for military applications, but predominantly for commercial,” Straznicky explains. “A lot of component manufacturers do have industrial, sometimes military, temperature characteristics, but those are few. When you design those in and assemble them onto a printed circuit board, then put cooling systems on top of that, everything needs to work reliably, such as solders. That’s a fairly esoteric specialty for ruggedization.”
“In the world of flight test, products have to be much more rugged than the design products for the operational stage of an aircraft. So we push the envelope much further than we would on an operational platform,” Curtiss-Wright’s Daghigh says. “For an aircraft that typically flies at Mach 2, we push the tests to Mach 3,” Daghigh says. “For hypersonics, you have to push that even further; if it is supposed to go to Mach 8, we test it to Mach 9 or 10. In flight test, you’re in experimental mode and anything goes, which you have to be ready for because we are measuring thousands of sensors down to the nanosecond to see what happened across them all if something goes wrong at any one point. In the operational environment, they won’t take the aircraft beyond its design characteristics.”
“Uncontrolled energies at resonant frequencies can rapidly destroy components and connections throughout an embedded computing system. Some hypersonic platform designs further complicate the mechanical vibration challenge by using an engine that varies its combustion behavior based on altitude,” says Abaco’s Graves. “This innovative propulsion solution also means that the engine vibration characteristics will vary, further complicating embedded electronics designs,” Graves says. “Other environment considerations are not related to hypersonic speeds, but must still be accounted for, based on a given platform’s mission profile.”
“Vibration effects, as well as acceleration G forces and pyrotechnic shock, can all be mitigated by using damping materials at physical connection points, both within a system chassis and where the chassis is attached to the airframe,” according to Graves. But the extremes to which hypersonic flight pushes all environmental requirements — especially at speeds in the Mach 20 to Mach 30 range envisioned for hypersonic missiles — means there must be significant advances in every component and every method used to defeat those challenges. Simulations are used to identify potential weaknesses in COTS components used in the design, then those are strengthened and secured to ensure they can withstand the hypersonic environment before they ever go on a flight test vehicle.
Efforts to overcome those difficulties continue to be the focus of numerous research programs in the U.S., Russia, China, France, India, Australia, and Japan. From military labs to academia to private companies, new technologies are being developed, old technologies modified, and revolutionary designs tested to address the issues facing hypersonic missile engineers.
Conformal coatings
Experts say that environmental ruggedization to protect electronics from heat at hypersonic speeds — exterior temperatures rising higher than 4000 degrees Fahrenheit — almost certainly will rely on conformal coatings. “We have three different types of coating: acrylic, urethane and parylene, a vacuum vapor deposited film up to half a millimeter of thickness that wraps itself around everything on the board. That is the ultimate in protection. That protects against humidity and condensation, which you would have to protect against in cases of very rapid changes in temperature,” Straznicky says. “Whatever the actual shock and vibration levels and the actual environment, once released, we can work to those types of extremely harsh environments, even beyond current ruggedization requirements.”
The specific technologies used to ruggedize against heat and vibration also will vary throughout the vehicle, depending on where the electronics are installed. The nose, for example, will experience temperatures than the tail. Where sensors are installed also will impact the level of ruggedization required, such as the types of glues and fasteners used to attach them to the skin of the vehicle. The problem is two-fold — finding materials that can withstand both extreme temperatures, including a rapid transition from the cold of altitudes exceeding 100,000 feet to the intense heat of reentry and flight through the lower atmosphere, and intense shock and vibration.
Thus the true difficulty with developing a hypersonic missile is not pushing it to such high speeds, but ensuring the embedded computing systems can endure the extreme range of temperatures, G forces, vibration, humidity, and pyrotechnic shock — and do so without adding weight or bulk to the missile.
Artificial intelligence (AI)
AI is likely to make an impact on missile defence. Hypersonic missiles deployed in the field will reduce the reaction time available for missile defence systems to mitigate the threat. The human element in the chain will have to be removed or reduced. Potential systems like the space sensor array will produce large amounts of data, meaning that AI will be needed to sort through the data and look for potential threats.
Processing and Computing
A third component supporting rapid deployment is a software framework for developing high-performance, real-time embedded applications,” Graves wrote. “The AXIS Software Tool Suite from Abaco includes modules supporting the accelerated development of algorithm implementation, data movement, inter-process communications, image processing, event analysis and more capabilities. At the platform level, programs need powerful embedded electronics that can support all aspects of a mission, making software development a critical function.”
“Today’s processing silicon is segmented into technology types, including multi-core general purpose processors (GPPs), general purpose graphics processing units (GPGPUs), field programmable gate arrays (FPGAs), digital signal processors (DSPs) and communications processors (CPs),” Graves wrote. “As the name suggests, a GPP is not specialized — and yet, for the purposes of embedded computing, it is. A GPP is the best processor type when decision-making and context-switching is important; for example, when responding to commands.”
Each advance in modern processors, however, increases one of the most pernicious problems with hypersonic flight — heat. Even as each new generation of processor provides geometric increases in performance, increases vital to the complex requirements of a hypersonic vehicle, the internal heat generated increases, as well. In recent years, however, there has been little change in the maximum operating temperatures of most commercial silicon, typically around 100 degrees Fahrenheit.
Cooling technologies
One of the most common methods of cooling such systems is blowing forced air over them, which obviously is not possible in hypersonic flight, where external heat also must be extirpated. That leaves two other primary techniques: conduction cooling and heat pipes, sometimes combined within an advanced system design. Conduction cooling is implemented with a heat frame that encloses a circuit board where the processors are connected, and wedge locks that firmly attach the heat frame/circuit board assembly to a system chassis wall, Graves explains. The wedge locks also help address another problem — vibration — by increasing system rigidity.
Heat pipes are used to move heat directly from a surface with significant heat load, such as a multi-core GPP, to a cooled surface, often inside the finned outer surface of a chassis enclosure. A heat pipe’s effectiveness is dependent on a number of variables, but they are often an efficient way to deal with difficult hot spots in a system.
Chinese Technology Breakthroughs for Hypersonic Applications
China has been making steady progress in its own space and military operations, including advances in 3D printing, energy storage, scramjet test platforms, and intercontinental ballistic missiles (ICBMs).
TSC Beijing, a Chinese titanium manufacturer, successfully 3D printed a titanium fuselage central box for its high-speed aircraft, which cuts production time from two years down to just six months. TSC used the 3D laser printer TSC-S4510 (one of the world’s largest 3D printers) to print the fuselage, within an error tolerance of less than 0.5mm. The intended aircraft was identified only as a “high-speed” aircraft. Given its narrow wing roots, and 23-foot total length, it is likely a hypersonic UAV, write Jeffrey Lin and P.W. Singer in popular mechanics.
In May, the National Defense University of Technology (NUDT) showed off the Ling Yun, a Mach 6+, two stage scramjet testbed. NUDT hopes that the Ling Yun’s relative simplicity and reliability will make it a mass-produced platform for refining new hypersonic technologies such as thermal resistant components for communications systems, or for collecting atmospheric data in the near space. The Ling Yun’s ease of production could provide the basis for scramjet cruise missiles used to swarm enemy ships and air defenses.
Other possibilities open up if Ling Yun’s scramjet engine can scale down to a 6- to 8-inch diameter. This would open up the potential of hypersonic shells for China’s cannons that could fire hundreds of miles (the U.S. Army is also at work on such a system, targeting completion in 2023). A scramjet cannon would be cheaper and more mobile than a railgun, since it wouldn’t need to lug around massive systems for power generation and storage. Scramjet cannons would be cheaper than ballistic missiles, not to mention being harder to defend against due to smaller sensor profiles and higher rates of fire.
The DF-41 intercontinental ballistic missile (ICBM) conducted its tenth test flight on May 27, 2018. The DF-41 is a mobile, 13,000-15,000km range ICBM with a multiple warhead payload of 1.5-2 tons. The DF-41’s massive payload and Mach 25 top speed gives it enough performance to launch other systems, like a hypersonic glide vehicle with global reach (a Chinese response to the infamous Russian ICBM, Avangard) or more exotically, a multistage booster for long-range scramjet cruise missiles.
US Breakthroughs
Among the essential Pentagon efforts to regain the technological lead, the Naval Surface Warfare Center, Crane Division (NSWC Crane) in Indiana recently unveiled an innovative program that rapidly develops technologies and systems into prototypes for testing and fielding. It draws from a network of leading-edge companies and universities from which the U.S. government can mine the best ideas. The program has the mandate to assess myriad systems that might be applicable to hypersonics, from sensors and tracking technologies to propulsion and glide vehicles.
The hypersonics focus at NSWC Crane is part of a larger program there aimed at prototyping systems to address a range of critical Department of Defense priorities, from machine learning and hypersonics, to radiation-hardened microelectronics and various warfare technologies.
“There have been significant advances in computational fluid dynamics, air-breathing propulsion, and high-temperature structures and materials. Current efforts are using advanced design and manufacturing techniques. Cost is an important factor that has received significant emphasis in the current generation of programs. And, finally, the need for hypersonic systems is emerging and maturing,” AFRL’s Miller says.
The Materials Architectures and Characterization for Hypersonics (MACH) program at the U.S. Defense Advanced Research Projects Agency (DARPA) in Arlington, Va., comprises two technical areas. According to the agency, “the first seeks to develop and mature fully integrated passive thermal management systems to cool leading edges based on scalable net-shape manufacturing and advanced thermal design. The second 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.”
3D Printing Key to Hypersonic Weapons: Raytheon
“But when it comes to making hypersonic systems, which require exotic materials and strangely shaped components that conventional methods can’t handle, 3D printing may be essential,” says Raytheon’s head of advanced missile systems, Tom Bussing. “Growing” parts in a 3D printer allows you to make much more complex shapes than the traditional process — used since before the Bronze Age — of casting the basic shape in a mold and then cutting it to the final desired form.
He gave example of design of cooling system for hypersonic jet moving through the air, at Mach 5-plus, that generate extreme friction and heating of hypersonic air vehicle. “But if you want cooling vents in a traditionally manufactured component, you have to drill a bunch of holes in it (and hope you didn’t weaken it too much). If you want cooling vents in a 3D-printed component, you just program the printer to make a shape that has openings in it from the start. What’s more, if you drill out your cooling channels, they’re going to be pretty much straight; but if you grow the channels in a 3D printer, they can be helixes or other elaborate shapes that vent heat much more efficiently.“If it’s more efficient, it means you can make it smaller, [with] less cooling,” said Bussing. “[It] lasts longer, flies farther.”
Tactical Boost-Glide is the approach already tested by both Russia and China: a rocket motor boosts the missile up to hypersonic speed, after which it glides to the target. The goal is to “skip” off the atmosphere like a skipping stone over water, allowing it to go vast distances at extreme speeds. Getting this to work requires progress in aerodynamics, stability, and controls, as well as materials, Bussing said. Getting this to work requires progress in aerodynamics, stability, and controls, as well as materials, Bussing said. 3D printing can help in all these areas
An “air-breathing” hypersonic vehicle, by contrast, flies under its own jet power the whole way. This approach allows less range than boost-glide but greater maneuverability. Air-breathers can also be significantly smaller. A rocket has to carry large amounts of oxidizer to burn its fuel. A jet just sucks in oxygen from the atmosphere. But normal jets don’t have to suck in air moving at Mach 5-plus. A jet that works at hypersonic speeds will require some breakthroughs — and, again, 3D printing can help grow the exotic components.
Orbital and Nasa 3D printed scramjet engine part survives critical wind tunnel tests
Orbital ATK has successfully tested a 3D printed hypersonic engine combustor at Nasa Langley Research Centre in Virginia. The breakthrough could lead to planes that can travel 3,425mph (5,500km/h) – 4.5 times the speed of sound. The combustor was created through a manufacturing process known as powder bed fusion (PBF). In this, a layer of metal alloy powder is printed and a laser fuses areas of together based on the pattern fed into the machine by a software program.
As each layer is fused, a second is printed until the final product is complete. Any additional powder is removed and the product is polished. The combustor was successfully put through a range of hypersonic flight conditions over the course of 20 days, including one of the longest duration propulsion wind tunnel tests ever recorded. Orbital says one of the most challenging parts of the propulsion system, scramjet combustion. This houses and maintains stable combustion within an extremely volatile environment.
References and Resources also include:
https://trace.tennessee.edu/cgi/viewcontent.cgi?article=2315&context=utk_gradthes
https://www.popsci.com/chinas-hypersonic-work-speeds-up#page-5