Countries are racing to develop hypersonic platforms and weapons having speeds between Mach 5 ( 6125 kilometers per hour) to 10 Machs. Future Spaceplanes shall enable intercontinental travel at very high speeds, travelling on a sub-orbital trajectory, journey times from the UK to Australia could be cut from the current duration of around 20 hours to as little as two hours.
The U.S. Defense Intelligence Agency puts a fine point on the value of hypersonic capabilities. “Developments in hypersonic propulsion will revolutionize warfare by providing the ability to strike targets more quickly, at greater distances, and with greater firepower,” the agency said in congressional testimony
There are two types of weapons emerging: hypersonic cruise missiles and hypersonic glide vehicles. Most long-range missiles follow a ballistic curve that takes them high above the atmosphere and then down through it, a trajectory that can be detected early and modeled accurately. Boost-glide missiles ride a ballistic launcher to attain hypersonic speed, then use momentum to glide at low altitude while taking maneuvers to elude ground defenses. Scramjets use air-breathing engines to travel far, fast, and lower still, making them that much harder to detect and shoot down.
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.
The traditional turbojet engines, which can manage supersonic speeds, uses a turbine near the inlet to compress air for combustion. The top speed of traditional jet-turbine engines maxes out at roughly Mach 2.5. The ramjet, which requires supersonic speed, instead uses the forward motion of the vehicle to “ram” air into the combustion chamber. Ramjets can then take you to around Mach 4, but then they too lose their efficiency.
Hypersonic platforms 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. The scramjet, or supersonic combustion ramjet, is optimized for hypersonic speeds. Scramjets are ‘airbreathing’ aircraft because rather than carrying both fuel and the oxygen needed to provide acceleration, they carry only hydrogen fuel and ‘pull’ the oxygen needed to burn it from the atmosphere.
Like a conventional turbojet engine, a scramjet inhales air through its inlet, compresses and mixes it with fuel, the compression causing the temperature to rise and ignition to occur.. This generates huge amounts of thrust and enables the jet to travel at speeds far in excess of the 1,350mph top speed of Concorde.
Unlike a turbojet engine, it does not have any turbine blades to compress air, but instead relies on air being forced through its inlet as it is pushed through the atmosphere at high speeds, often propelled by a rocket booster. Also like a ramjet, there are few or no moving parts, making the scramjet geometrically quite simple. The result is a jet engine with no moving parts, which cannot produce thrust on its own from a standstill.
For future, reusable space transportation systems, as well as for hypersonic flight vehicles, scramjet propulsion system is very likely to offer an economic alternative to classical, expendable and hence expensive rocket driven systems and is one of the key technologies for hypersonic flight.
The realization of Hypersonic platforms and weapons critically depend on the hypersonic propulsion. A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the distinction being that the combustion process takes place supersonically. However, combustion of air and fuel in a ramjet takes place at velocities below Mach 1, while combustion of air and fuel in a scramjet takes place at supersonic speeds. At higher speeds, it is necessary to combust supersonically to maximize the efficiency of the combustion process and to avoid the losses induced by a final normal shock.
At full scale, to get to their initial scramjet operating velocity of Mach 4, or some 3000 mph, scramjets need some other propulsion system to initially accelerate the vehicle. However, hypersonic engines such as scramjets cannot provide effective thrust at speeds much below Mach 3.5. This gap in capability means that any air-breathing hypersonic vehicles developed today would use disposable rockets for one-time boosts up to operating speed, limiting the vehicles’ usefulness. Therefore countries are experimenting with new concepts and developing next generation engine technologies for hypersonic flight.
The absolute upper limit for hypersonic flight is about Mach 15, Goyne says, but scramjet tech will be developed around a paradigm of Mach 5 or 6. What’s more, the flight corridor of a scramjet-powered hypersonic vehicle is in the stratosphere at 66,000ft to 98,000ft. Go too high and the atmosphere runs out of oxygen to burn. Go too low and the denser atmosphere causes friction leading to excessive drag and aeroheating.
These vehicles have to contend with large increase in Aerodynamic drag, which roughly scales with the square of airspeed; double the speed, and the drag goes up four times. Streamlined shapes can partly overcome this problem, but the solution then, as it is now, is more thrust which can be provided with improved engines.
Beyond Mach 5, air pressure and temperature become too high in the engine for it to operate at optimal levels,” Bowcutt said. “To prevent this condition, the air is allowed to remain at supersonic speed in the engine, which by definition makes it a scramjet. These are very harsh environments that develop inside the engine. These are very high speeds, very high speed shear, very inhomogeneous distribution of temperature and pressure. That makes the prediction and the distribution of the phenomena very difficult.
Other concept is turbine based combined cycle and rocket based combined cycle for operating the vehicle from zero mach number, i.e. dual mode scramjets. The most practical concept at the moment is the turbine-based combined cycle, says Chris Goyne, Professor of Mechanical and Aerospace Engineering and Director of the Aerospace Research Laboratory at the University of Virginia in Charlottesville, So, in this case, he says, a gas turbine or a turbojet engine used to takeoff on the runway and accelerate up to the scramjet takeover speed.
Rather than using turbofan blades to compress air before combustion, scramjets use the forward speed of the aircraft. It is an air-breathing beast that has presented some fascinating engineering challenges. The size of the engine, the rate at which chemical combustion occurs, the temperature, the fuel type, the altitude and therefore air pressure and oxygen mix — they all affect design and engineering.
Its specially shaped inlets and combustion walls allow the very process of combustion to proceed supersonically—a feat that has been compared to keeping a match lit in a hurricane. “Designing ramjet and scramjet inlets and nozzles are challenging both aerodynamically, in terms of shape, and structurally,” Bowcutt said. “For a ramjet or scramjet that operates over a speed range greater than about one Mach number, inlets and nozzles must employ variable geometry to efficiently compress the ingested air and expand the engine exhaust respectively. This is particularly challenging because the inlet and nozzle structures, and the seals they employ, get very hot and must be made of special, high-temperature materials.
The scramjet propulsions are limited by several factors including lower component efficiencies, complex scramjet combustion process, energy density of the fuels, cold start of engine . “If you take the length of a typical combustor, say 20cm [8in] or 50cm and divide it by the velocity of 3,000mph [4,800km/h] to 6,000mph, you will find that the typical time that an air molecule spends within the combustor is of the order of a fraction of a millisecond,” says Javier Urzay, a senior research engineer in aerospace at the Center for Turbulence Research at Stanford University. “The main evolution has been in trying to increase that flow-through time, making flow-through times that are longer so you have longer times to burn fuel and oxidizer.”
A secondary challenge is the complexity of the flow structures in hypersonic flight, said Professor Russell Boyce, Director of UNSW Canberra Space. “It is beautiful science, but at the same time it is very complex,” Boyce said. “You can turn nice, well-behaved and well-understood flow into disaster flow in milliseconds, depending on the physics, and depending on what you do with the aircraft. “With a rocket, the aerodynamics is easy — you just put a heat shield on the front. It is long and slender and apart from the fins, there’s nothing sticking out into the flow. The engine is not air-breathing, so nothing flows in. It’s just a nice, self-contained combustion challenge. Scramjets are the opposite.”
The thermal issues are also a limiting factor when it comes to technology challenges, both the internal heating in the combustor and the external aerodynamic heating. The materials required for scramjet propulsion and hupersonic flight are faced with numerous challenges. Some of them are thermal mechanical loads on the structure and required structural integrity at sustained high temperatures, ultra high temperature materials and thermal management.
“Cooling the internal engine structure with fuel is also required. In a scramjet, designing and fabricating fuel injectors that mix most of the fuel with air is challenging. It must be able to ignite combustion and keep the flame burning. It’s also necessary to keep combustion pressure contained in the engine because it can possibly lead to what is called engine ‘unstart’.”
“It took about 50 years to develop scramjet technology to the point it could be demonstrated in flight,” said Dr Kevin Bowcutt, Boeing Senior Technical Fellow and Chief Scientist of Hypersonics. “It took so long because of the extreme thermal environment encountered at hypersonic speeds and the challenges of ground testing at hypersonic speed due to high flow energy and temperature. Hypersonic flow physics are also difficult to analyse theoretically or simulate in computers.”
That’s why scramjets, though conceived in the 1950s, still remain a work in progress. In the early 2000s, NASA’s X-43 used scramjets for about 10 seconds in flight. Since 2013, the United States Air Force X-51A Waverider has held the duration record with a 210-second burn that pushed the plane to Mach 5.
In 2016, an Indian test vehicle reached Mach 6 with the engine running for just five seconds. In Sep 2020, the Defence Research and Development Organisation (DRDO) performed a major technological feat on September 7 when it flew a cruise vehicle at a hypersonic speed of Mach six for 20 seconds. The DRDO called the cruise vehicle Hypersonic Technology Demonstrator Vehicle (HSTDV). The centrepiece of the HSTDV was the indigenously developed air-breathing scramjet engine, which formed the HSTDV’s propulsion system. If the mission’s aim was to prove this air-breathing scramjet engine in flight, it was achieved.
Moreover, developers need to understand how a scramjet would react to that length of hypersonic flight. “If we are talking military applications, we want to do at least 10min of sustained flight,” says Musielak.
Scramjet propulsion technologies
The critical technologies developed for the HSTDV mission were the scramjet engine and its ignition, sustaining the ignition, ethylene fuel, generation of maximum energy from the engine, development of materials to take care of the high temperatures that occurred due to air friction on the leading edges of the cruiser’s wings, tail surface and nose tip, and controlling the HSTDV with minimum drag and maximum thrust.
Dr. Avinash Chander, former Director General, DRDO, said: “The fuel should be ignited in milliseconds. Not many countries were able to do it at the first instance… Energy generation should be maximum and drag should be minimum.” The ignition should be sustained for the duration set for the flight. The entire HSTDV should be controlled but with maximum thrust.
The cost of developing a scramjet capability is only one part of the economic challenge. The other is making the engine cheap enough to deploy and use in a routine way. To do that, you need fuel you can rely on. Early researchers worked with a class of highly energetic fuels that would react on contact with air, like triethylaluminum.
“It’s a fantastic scramjet engine fuel, but very toxic, a bit like the hydrazine fuels used in rockets nowadays, and this became an inhibitor,” says David Van Wie, of Johns Hopkins APL, explaining why triethylaluminum was dropped from serious consideration. Next up was liquid hydrogen, which is also very reactive. But it needs elaborate cooling. Worse, it packs a rather low amount of energy into a given volume, and as a cryogenic fuel it is inconvenient to store and transport. It has been and still is used in experimental missiles, such as the X-43.
Today’s choice for practical missiles is hydrocarbons, of the same ilk as jet fuel, but fancier. The Chinese scramjet that burned for 10 minutes—like others on the drawing board around the world—burns hydrocarbons. In May 2020, workers at the Beijing Academy of Sciences ran a scramjet for 10 minutes, according to a report in the South China Morning Post. The Chinese breakthrough was based on the “world’s first systematic investigation into the effect of hydrocarbon fuel state change on the performance and stability of supersonic combustion”, the article said.
Here the problem lies in breaking down the hydrocarbon’s long molecular chains fast so the shards can bind with oxygen in the split second when the substances meet and mate. And a split second isn’t enough—you have to do it continuously, one split second after another, “like keeping a match lit in a hurricane,” in the oft-quoted words of NASA spokesman Gary Creech, back in 2004.
Supersonic air stream could raise the temperature of the engine to over 4,000 degrees Celsius – twice that of ordinary jet engines – and if the heat built up, the scramjet could explode. Fan and his colleagues dealt with this problem by directing fuel to the surface of the most heated components, such as the combustion chamber, the article said. With precise control, the fuel could absorb and dissipate the heat. The heat, in turn, would turn the fuel into a gas of carbon and hydrogen molecules eager to meet the oxygen in the compressed air, and burn. The Chinese Academy of Sciences has nominated Fan – a Princeton physics PhD who has worked on the scramjet programme since 2004 – for a national “innovator of the year” prize.
Scramjet designs try to protect the flame by shaping the inflow geometry to create an eddy, forming a calm zone. Flameouts are particularly worrisome when the missile starts jinking about, thus disrupting the airflow. “It’s the ‘unstart’ phenomenon, where the shock wave at the air inlets stops the engine, and the vehicle will be lost,” says John D. Schmisseur, a researcher at the University of Tennessee Space Institute, in Tullahoma. And you really only get to meet such gremlins in actual flight, he adds.
There are other problems besides flameout that arise when you’re inhaling a tornado, writes Philip E. Ross. One expert, who requested anonymity, puts it this way: “If you’re ingesting air, it’s no longer air; it’s a complex mix of ionized atmosphere,” he says. “There’s no water anymore; it’s all hydrogen and oxygen, and the nitrogen is to some fraction elemental, not molecular. So combustion isn’t air and fuel—it’s whatever you’re taking in, whatever junk—which means chemistry at the inlet matters.”
Simulating the chemistry is what makes hypersonic wind-tunnel tests problematic. It’s fairly simple to see how an airfoil responds aerodynamically to Mach 5—just cool the air so that the speed of sound drops, giving a higher Mach number for a given airspeed. But blowing cold air tells you only a small part of the story because it heads off all the chemistry you want to study. True, you can instead run your wind tunnel fast, hot, and dense—at “high enthalpy,” to use the term of art—but it’s hard to keep that maelstrom going for more than a few milliseconds.
“Get the airspeed high enough to start up the chemistry and the reactions sap the energy,” says Mark Gragston, an aerospace expert who’s also at the UT Space Institute. Getting access to such monster machines isn’t easy, either. “At Arnold Air Force Base, across the street from me, the Air Force does high-enthalpy wind-tunnel experiments,” he says. “They’re booked up three years in advance.”
Other countries have more of the necessary wind tunnels; even India has about a dozen[PDF]. Right now, the United States is spending loads of money building these machines in an effort to catch up with Russia and China. You could say there is a wind-tunnel gap—one more reason U.S. researchers are keen for test flights.
Another thing about cooling the air: It does wonders for any combustion engine, even the kind that pushes pistons. Reaction Engines, in Abingdon, England, appears to be the first to try to apply this phenomenon in flight, with a special precooling unit. In its less-ambitious scheme, the precooler sits in front of the air inlet of a standard turbojet, adding power and efficiency. In its more-ambitious concept, called SABRE (Synergetic Air Breathing Rocket Engine), the engine operates in combined mode: It takes off as a turbojet assisted by the precooler and accelerates until a ramjet can switch on, adding enough thrust to reach (but not exceed) Mach 5. Then, as the vehicle climbs and the atmosphere thins out, the engine switches to pure rocket mode, finally launching a payload into orbit.
One of the promising technology is Rotating Detonation Engines that might someday offer both high velocity and decent fuel economy. The engine’s detonation chamber is essentially a thin, hollow cylinder (actually, it’s the thin, hollow space between two concentric cylinders, if you want to get specific). The engine sets off a detonation using the usual means—fuel, oxygen, pressure, heat—which sends a shockwave chasing itself through the cylindrical loop.
Here, the shockwave slams into oxygen molecules and fuel molecules with so much force that they compress, excite, and detonate. Each subsequent detonation keeps the shockwave going, and the engine keeps those detonations coming by feeding the chamber carefully timed injections of fuel and oxygen.
“What this allows the engine to do is burn fuel at a much higher rate compared to conventional combustion engines,” says Narendra Joshi, the chief engineer of propulsion technologies at GE Research. This higher burn rate creates more thrust, which is how these engines will (theoretically, one day) push aircraft into hypersonic speeds.
The combustion chamber, the thin space between the two metal cylinders in rotating detonation engine is about 10 times smaller than the chamber in conventional turbine engines which results in burning fuel at a much higher pressure. The higher the pressure, the more work the engine gets out of the molecules once they explode. “We estimate a 5 to 10 percent improvement in gas mileage,” says Stephen Heister, a propulsion engineer at Purdue University whose research includes rotating detonation engines.
Scramjets have required materials that could withstand the extreme heat created at such speed. The scramjet — from ‘supersonic combustion ramjet’ — only becomes operational at around Mach 5, or five times the speed of sound (more than 5000 km/h). “Up until about 10 years ago, people were trying to use metallic materials for scramjet construction, like titanium and nickel alloys — high-temperature alloys from the aerospace industry. But those materials just didn’t have the capability.” The materials that do have the capability to withstand the extreme operating temperatures, are carbon composites said Professor Michael Smart, mechanical engineer, director of Hypersonix and head of UQ’s HyShot Group. “These are materials like fibreglass, but instead of having polymer fibres, they have carbon fibres,” he explains.
“The epoxy matrix is not like typical fibreglass. Instead it uses materials like silicon carbide as the matrix. Those carbon composites can withstand temperatures up to 1600 degrees Celsius and not be damaged. They have been around for a long time, but what has really changed in the last decade is our ability to fabricate them in all sorts of aerodynamic shapes.”
Other technology is 3D printing. Raytheon-Northrop Grumman team and a Lockheed Martin-Aerojet Rocketdyne team – say they are leaning on 3D printing to bring their scramjet-powered hypersonic cruise missiles into test flights in 2020. 3D printer could help create scramjet engine geometries not possible before. Changing the contours of the inside of a scramjet could help to better control the flow of air and fuel so as to improve performance, though it has to be durable in high-temperature environments, says Urzay. They also believe 3D printing will help save on production costs and lead times.
Northrop Grumman is 3D printing all of its engine using advanced materials, including the critical combustor. “The use of additive manufacturing techniques also enables better performance and faster production,” says Wilcox. “Advancements in this area of manufacturing have made a big difference in printing precision parts that used to be too complex and time consuming to do by other techniques. Our successful tests have confirmed that additively manufactured materials perform as intended in simulated conditions.”
Mastering the air-breathing scramjet technology will lead to the development of hypersonic missiles, faster civilian air transportation and facilities for putting satellites into orbit at a low cost.
According to reports, NASA is working on creating a new hypersonic missile which can optimise missiles for maximum range and destruction. The technology allows the AI component to use the result of computational fluid dynamic (CFD) to work around a design of a scramjet missile.
As a result, the hypersonic missile would be faster with a longer range than any missile produced by other nations.
A hypersonic propulsion company backed by Rolls-Royce, Boeing and BAE Systems has taken a step closer to developing an engine capable of powering combat jets and other aircraft at speeds of up to Mach 5 following tests of two subsystems vital to the success of the design. British-based Reaction Engines said the recently completed tests of full-scale heat exchanger and hydrogen pre-burner subsystems validated the design of what are key components required to supply heat energy and air to the core of the air-breathing engine.
The success of the trials on the heat exchanger, known as the HX3, and the pre-burner is another step in the right direction to maturing Reaction Engines’ technology. The latest tests follow trials undertaken in 2019 in Denver, where the company undertook high-temperature airflow testing for the Defense Advanced Research Projects Agency’s HTX program.
The company reported at the time that its proprietary ultra-lightweight heat exchanger used in the test was exposed to hypersonic conditions approaching 1,000 degrees Celsius, or roughly 1,800 degrees Fahrenheit. The heat exchanger performed its pre-cooler function by quenching about 1,800-degree Fahrenheit temperatures in less than one-twentieth of a second, according to the company.
Dissel said that together the three tests successfully demonstrate key subsystems not previously used in an aerospace environment. “The company is very focused on maturing the subsystems that are fundamentally new to aerospace. Pre-cooler was the big one, and now with the innovative HX3 heat exchanger and pre-burner tests, these are three key components very specific to Sabre,” he told Defense News on March 5. “We are well past the hump in terms of validating the fundamental pieces. Putting it together as an integrated device able to go five times the speed of sound is still a big challenge, so from an overall integration standpoint we are at the beginning.
New IU Center to Leverage High-Performance Computing to Advance Hypersonic Propulsion
The U.S. Department of Energy’s National Nuclear Security Administration Advanced Simulation and Computing announced in Oct 2020 it will fund a new Center for Exascale-enabled Scramjet Design at the University of Illinois at Urbana-Champaign. U of I will receive $17 million over a five-year period.
Willett Professor and Head of the Department of Aerospace Engineering Jonathan Freund is the application co-director and principal investigator of CEESD. He said air-breathing hypersonic propulsion is the key to expanding access to space, enhancing defense, and accelerating global transport. CEESD will be housed in the National Center for Supercomputing Applications at U of I with NCSA’s Director William Gropp serving as the computer science lead and co-director of CEESD. Gropp described the uniqueness of this center due to its university setting.
“The needed supersonic combustion ram jets (scramjets) have been demonstrated but are insufficiently engineered for many applications,” Freund said. “Their promise is revolutionary but their challenge is profound—to maintain combustion, with its modest flame speeds, in supersonic air flow.
“Advanced lightweight composite materials provide a new design paradigm that can facilitate thermal management through temperature resistance and/or strategic ablation,” Freund said. “Predictive simulations, realized by the integration of multiple physical models and performance-enabled with advanced computer science methods, will constitute a fundamental advance that circumvents testing costs that currently hinder design.”
“High-performance computing is enabling for our design goals, and the center will, at the same time, provide a unique educational experience,” Gropp said. “The computer science students will be trained to work effectively with computational scientists, who are facing challenging prediction goals. Likewise, computational scientists will learn computer science approaches and opportunities within the team structure.”
The experimental work for the center will be orchestrated by AE’s Greg Elliott. Freund said, Elliott leverages experience in high-speed flow experiments, diagnostics, and their integration with predictive modeling. In all, 20 researchers are associated with the center across NCSA and six departments in The Grainger College of Engineering: Computer Science, Aerospace Engineering, Mechanical Science and Engineering, Industrial and Enterprise Systems Engineering, and Electrical and Computer Engineering.
UCF Lands Selective New DOD Award to Advance Hypersonic Propulsion Research, reported in Nov 2021
The three-year award to UCF is one of 18 research projects the Department of Defense recently announced it was funding in order to advance the state of applied hypersonics research and build a hypersonics-focused scientific and industrial workforce.
The award is through the University Consortium for Applied Hypersonics, which operates under the Department of Defense’s Office of the Under Secretary of Defense for Research and Engineering’s Joint Hypersonics Transition Office and is administered by the Texas A&M Engineering Experiment Station.
The UCF project will explore high flammability and high energy-density solid fuels for ramjets and scramjets, both of which are engines designed for hypersonic propulsion. It will be led by Kareem Ahmed, an associate professor in UCF’s Department of Mechanical Engineering and an expert in hypersonic propulsion engineering. The project has also been boosted with additional funding from UCF’s recently announced Jump Start awards which invests in the UCF Ultra-High-Speed Flow Facility for Hypersonics and Space Propulsion. The work is important because solid-fuel rocket systems are four to five times more fuel efficient than liquid-propellant systems, meaning they can go farther for less cost, and are safer to store.
The problem has been that solid fuel systems face significant challenges in low pressure conditions that occur at high altitudes, such as unstable flames, blowouts and adverse reaction flows, which reduce engine stability, Ahmed says.
To overcome this, Ahmed’s team will perform experiments to create new, advanced solid fuels that use outside air to help drive supersonic combustion in a scramjet, known as airbreathing propulsion, rather than carrying its own oxygen supply that’s mixed into the system like what’s used in rocket propellants. “The novel solid fuels will provide wider flammability limits and longer range while constraining volume and improving thermal and mechanical properties,” Ahmed says.
Solid fuel compositions, such as aluminum-lithium based fuels, and advanced manufacturing techniques like field assisted sintering technology that can achieve new compositions not previously obtained, will be explored to identify blend optimization for high altitude, hypersonic propulsion.
The high-speed, low pressure, solid-fuel reactions will be tested in UCF’s unique hypersonic high-enthalpy reaction, or HyperREACT, facility to allow the detailed exploration of the reacting flame-flow dynamics in extreme, previously inaccessible regimes using high-speed, high-resolution advanced laser diagnostics.
“This investigation will significantly accelerate the research and development efforts in hypersonics while advancing the scientific knowledge and training graduate students to maintain technological superiority,” Ahmed says. “The project leverages strong technical support and resources that have been provided by leading hypersonic industries that are developing high-speed air-breathing scramjet propulsion systems.”
Collaborators on the project include Steven Son, a professor of mechanical engineering with Purdue University, and Douglas Wolfe, a professor in the Department of Materials Science and Engineering at Pennsylvania State University. The research project will potentially include specific expertise from UCF Department of Mechanical and Aerospace Engineering faculty members Seetha Raghavan, Jayanta Kapat and Michael Kinzel.