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Seminar Report SCRAMJET

THE SCRAMJET ENGINE

INTRODUCTION

One thing has always been true about rockets: The farther and faster you want to go, the bigger you rocket needs to be.

Why? Rockets combine a liquid fuel with liquid oxygen to create thrust. Take away the need for liquid oxygen and your spacecraft can be smaller or carry more pay load.

That's the idea behind a different propulsion system called "scramjet," or Supersonic Combustion Ramjet

The oxygen needed by the engine to combust is taken from the atmospheric air passing through the vehicle, instead of from a tank onboard

Its mechanically simple as it has no moving parts.

All this makes the craft smaller, lighter, faster and have more room to carry payload.

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HISTORY OF SCRAMJET

During and after World War II, tremendous amounts of time and effort were put into researching high-speed jet- and rocket-powered aircraft. The Bell X-l attained supersonic flight in 1947, and by the early 1960s, rapid progress towards faster aircraft suggested that operational aircraft would be flying at "hypersonic" speeds within a few years. Except for specialized rocket research vehicles like the American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach 1 to Mach 2.

In the realm of civilian air transport, the primary goal has been reducing operating cost, rather than increasing flight speeds. Because supersonic flight requires significant amounts of fuel, airlines have favored subsonic jumbo jets rather than supersonic transports. The production supersonic airliners, the Concorde and Tupolev Tu-144 operated at a financial loss (with the possible exception of British Airways that never opened the accounts). Military aircraft design focused on maneuverability and stealth, features thought to be incompatible with hypersonic aerodynamics.

Hypersonic flight concepts haven't gone away, however, and low-level investigations have continued over the past few decades. Presently, the US military and NASA have formulated a "National Hypersonics Strategy" to investigate a range of options for hypersonic flight. Other nations such as Australia, France, and Russia have also progressed in hypersonic propulsion research.

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Different U.S. organizations have accepted hypersonic flight as a common goal. The U.S. Army desires hypersonic missiles that can attack mobile missile launchers quickly. NASA believes hypersonics could help develop economical, reusable launch vehicles.

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The Air Force is interested in a wide range of hypersonic systems, from air-launched cruise missiles to orbital spaceplanes, that the service believes could bring about a true "aerospace force."

The University of Queensland, Australia reported in 1995 the first development of a scramjet that achieved more thrust than drag[l] and in 2002 successfully tested the HyShot Scramjet system.

And the most recent successful tests were achieved by NASA's Hyper-X project in 2004 (around Mach 10). Currently research and development is going on for a craft that can break the Mach 10 barrier.

THEORY

What is a scramjet?

In a conventional ramjet, the incoming supersonic airflow is slowed to subsonic speeds by multiple shock waves, created by back-pressuring the engine. Fuel is added to the subsonic airflow, the mixture combusts, and exhaust gases accelerate through a narrow throat, or mechanical choke, to supersonic speeds. By contrast, the airflow in a pure scramjet remains supersonic throughout the combustion process and does not require a choking mechanism, which provides optimal performance over a wider operating range of Mach numbers. Modern scramjet engines can function as both a ramjet and scramjet and seamlessly make the transition between the two.

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About MACH Number and Speed of Sound

As an aircraft moves through the air, the air molecules near the aircraft are disturbed and move around the aircraft. Exactly how the air re-acts to the aircraft depends upon the ratio of the speed of the aircraft to the speed of sound through the air. Because of the importance of this speed ratio, aerodynamicists have designated it with a special parameter called the Mach number in honor of Ernst Mach, a late 19th century physicist who studied gas dynamics.

Basic Definitions:

speed of sound: The speed of sound is a basic property of the

atmosphere that changes with temperature. For a given set of conditions, the speed of sound defines the velocity

t which sound waves travel through a substance, such as air. Scientists have devised a standard atmosphere model that defines typical values for the speed of sound that change with altitude.

Mach number: Mach number is a quantity that defines how

quickly a vehicle travels with respect to the speed of sound. The Mach number (M) is simply the ratio of the vehicle's velocity (V) divided by the speed of sound at that altitude (a).

For example, an aircraft flying at Mach 0.8 is traveling at 80% of the speed of sound while a missile cruising at Mach 3 is traveling at three times the speed of sound.

Different speed regions:

subsonic: A vehicle that is traveling slower than the speed

of sound (M<1) is said to be flying at subsonic speeds.

transonic: As the speed of the object approaches the

speed of sound, the flight Mach number is nearly equal to

on e, M = 1, www.seminarstopics.com

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and the flow is said to be transonic.

supersonic: A vehicle that is traveling faster than the

speed of sound (M>1) is said to be flying at supersonic speeds.

sound barrier: The term sound barrier is often associated

with supersonic flight. In particular, "breaking the sound barrier" is the process of accelerating through Mach 1 and going from subsonic to supersonic speeds.

hypersonic: For speeds greater than five times the speed

of sound, M > 5, the flow is said to be hypersonic.

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About the Engine

The scramjet provides the most integrated engine-vehicle design for aircraft and missiles. The engine occupies the entire lower surface of the vehicle body. The propulsion system consists of five major engine and two vehicle components: the internal inlet, isolator, combustor, internal nozzle, and fuel supply subsystem, and the craft's forebody, essential for air induction, and aftbody, which is a critical part of the nozzle component.

The high-speed air-induction system consists of the vehicle forebody and internal inlet, which capture and compress air for processing by the engine's other components. Unlike jet engines, vehicles flying at high supersonic or hypersonic speeds can achieve adequate compression without a mechanical compressor. The forebody provides the initial compression, and the internal inlet provides the final compression. The air undergoes a reduction in Mach number and an increase in pressure and temperature as it passes through shock waves at the forebody and internal inlet.

The isolator in a scramjet is a critical component. It allows a supersonic flow to adjust to a static back-pressure higher than the inlet static pressure. When the combustion process begins to separate the boundary layer, a precombustion shock forms in the isolator. The isolator also enables the combustor to achieve the required heat release and handle the induced rise in combustor pressure without creating a condition called inlet unstart, in which shock waves prevent airflow from entering the isolator.

The combustor accepts the airflow and provides efficient fuel-air mixing at several points along its length, which optimizes engine thrust.

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The expansion system, consisting of the internal nozzle and vehicle aftbody, controls the expansion of the highpressure, high-temperature gas mixture to produce net thrust. The expansion process converts the potential energy generated by the combustor to kinetic energy.

The important physical phenomena in the scramjet nozzle include flow chemistry, boundarylayer effects, nonuniform flow conditions, shear-layer interaction, and three-dimensional effects. The design of the nozzle has a major effect on the efficiency of the engine and the vehicle, because it influences the craft's pitch and lift.

Changing from subsonic to supersonic combustion, the kinetic energy of the freestream air entering the scramjet engine is large compared to the energy released by the reaction of the oxygen content of the air with a fuel (say hydrogen). Thus the heat released from combustion at Mach 25 is around 10% of the total enthalpy of the working fluid. Depending on the fuel, the kinetic energy of the air and the potential combustion heat release will be equal at around Mach 8. Thus the design of a scramjet engine is as much about minimising drag as maximising thrust.

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Operations

An air-breathing hypersonic vehicle requires several types of engine operations to reach scramjet speeds. The vehicle may utilize one of several propulsion systems to accelerate from takeoff to Mach 3. Two examples are a bank of gas-turbine engines in the vehicle, or the use of rockets, either internal or external to the engine. At Mach 3-4, a scramjet transitions from low-speed propulsion to a situation in which the shock system has sufficient strength to create a region(s) of subsonic flow at the entrance to the combustor. In a conventional ramjet, the inlet and diffuser decelerate the air to low subsonic speeds by increasing the diffuser area, which ensures complete combustion at subsonic speeds. A converging- diverging nozzle behind the combustor creates a physical throat and generates the desired engine thrust. The required choking in a scramjet, however, is provided within the combustor by means of a thermal throat, which needs no physical narrowing of the nozzle. This choke is created by the right combination of area distribution, fuel-air mixing, and heat release.

During the time a scramjet-powered vehicle accelerates from Mach 3 to 8, the airbreathing propulsion system undergoes a transition between Mach 5 and 7. Here, a mixture of ramjet and scramjet combustion occurs. The total rise in temperature and pressure across the combustor begins to decrease. Consequently, a weaker precombustion system is required, and the precombustion shock is pulled back from the inlet throat toward the entrance to the combustor. As speeds increase beyond Mach 5, the use of supersonic combustion can provide higher performance .

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nd Mach 6, decelerating airflow to subsonic speeds for combustion results in parts of the airflow almost halting, which creates high pressures and heat-transfer rates. Somewhere between Mach 5 and 6, the combination of these factors indicates a switch to scramjet operation. When the vehicle accelerates beyond Mach 7, the combustion process can no longer separate the airflow, and the engine operates in scramjet mode without a precombustion shock. The inlet shocks propagate through the entire engine. Beyond Mach 8, physics dictates supersonic combustion because the engine cannot survive the pressure and heat buildup caused by slowing the airflow to subsonic speeds.

Scramjet operation at Mach 5-15 presents several technical problems to achieving efficiency. These challenges include fuel-air mixing, management of engine heat loads, increased heating on leading edges, and developing structures and materials that can withstand hypersonic flight. When the velocity of the injected fuel equals that of the airstream entering the scramjet combustor, which occurs at about Mach 12, mixing the air and fuel becomes difficult. And at higher Mach numbers, the high temperatures in the combustor cause dissociation and ionization. These factors— coupled with already-complex flow phenomena such as supersonic mixing, isolator- combustor interactions, and flame propagation— pose obstacles to flow-path design, fuel injection, and thermal management of the combustor.

Several sources contribute to engine heating during hypersonic flight, including heating of the vehicle skin from subsystems such as

pum ps, hydr auli cs, and elec tron ics, as well as com bust ion. The rma l-man age men t sche mes focu s on

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the engine in hypersonic vehicles because of its potential for extremely high heat loads.

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The engine represents a particularly challenging problem because the flow path is characterized by very high thermal, mechanical, and acoustic loading, as well as a corrosive mix of hot oxygen and combustion products. If the engine is left uncooled, temperatures in the combustor would exceed 5,000 °F, which is higher than the melting point of most metals. Fortunately, a combination of structural design, material selection, and active cooling can manage the high temperatures.

Hypersonic vehicles also pose an extraordinary challenge for structures and materials. The airframe and engine require lightweight, high-temperature materials and structural configurations that can withstand the extreme environment of hypersonic flight.

The challenges include: Very high temperatures Heating of the whole vehicle

Steady-state and transient localized heating from shock waves High aerodynamic loads

High fluctuating pressure loads

The potential for severe flutter, vibration, fluctuating and thermally-induced pressures

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About the Fuel used

The scramjet is an airbreather, meaning that it gets its oxygen from the surrounding air. However, the scramjet is significantly different from other kinds of jet engines, like turbojets and ramjets, in one key way. In most jets, the air pulled into the engines is slowed below Mach 1 and is combusted at subsonic speeds. The air within the scramjet combustion chamber, however, remains supersonic. The challenge of making a scramjet work is properly mixing the high-speed air with fuel while combusting and expanding that mixture before it exits the tail of the vehicle. This process typically occurs in less than 1 millisecond (0.001 seconds). Furthermore, the scramjet must burn enough fuel to generate an enormous amount of energy needed to overcome the tremendous drag forces experienced when flying at hypersonic speeds. In order to make a scramjet work, researchers must choose a fuel that can burn rapidly and generate a large amount of thrust. Hydrogen meets these criteria. One way to illustrate the differences between various fuels and their energy content is a measurement called the Lower Heating Value (LHV). The LHV describes the amount of energy released when a fuel is combusted and all of the remaining combustion products remain in gaseous form. The LHV for hydrogen is 119,600 kJ/kg. JP-8, another fuel commonly used in military aircraft, has a LHV of only 43,190 kJ/kg, less than half that of hydrogen. Simply put, hydrogen provides more "bang" per kilogram than JP-8, or any other hydrocarbon fuel for that matter.

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There are also other advantages to using hydrogen as a fuel. First of all, hydrogen is extremely flammable; it only takes a small amount of energy to ignite it and make it burn. Hydrogen also has a wide flammability range, meaning that it can burn when it occupies anywhere from 4% to 74% of the air by volume. Since hydrogen is a gas, it mixes very easily with air allowing for very efficient combustion. Another advantage over hydrocarbon-based fuels like JP-8 or gasoline is that hydrogen does not produce any harmful pollutants like carbon monoxide (CO), carbon dioxide (C02), or particulate matter during the combustion process. It is for this reason alone that many researchers have promoted hydrogen as a fuel in the public transportation industry.

Nevertheless, there are some disadvantages to using hydrogen as a fuel in aerospace vehicles. Hydrogen is not a dense fuel. At standard pressure and temperature, it has a density of only 0.09 kg/m3. Compare that to the density of gasoline at 750 kg/m3 or JP-8 at 800 kg/m3. While this low density is an advantage in terms of saving weight, hydrogen requires a large volume in order to store an adequate amount of chemical energy for practical use. Hydrogen gas is typically stored under pressure to increase its density, but even at 10,000 psi (68,950 kPa) it will contain only a quarter of the chemical energy stored in an equivalent volume of JP-8.

The density of hydrogen can be further increased by cooling and pressurizing the substance to the point that it becomes a liquid, but even in this form it will need a tank approximately twice the size of that required by JP-8. In addition, the cost and safety issues involved in manufacturing and storing cryogenically-cooled fuel is another major drawback. Despite the clear advantages of hydrogen described earlier, more energy can often be stored in

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smaller volumes using denser fuels. As a result, vehicles burning denser hydrocarbon fuels can usually fly longer distances than those using hydrogen.

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APPLICATIONS

Seeing its clear potential, organizations around the world are researching scramjet technology. Scramjets will likely propel missiles first, since that application requires only cruise operation instead of net thrust production. Much of the money for the current research comes from governmental defence research contracts.

Space launch vehicles may benefit from having a scramjet stage. A scramjet stage of a launch vehicle theoretically provides a specific impulse with 1000 to 4000 s whereas a rocket provides less than 600 s whilst in the atmosphere [1], potentially permitting much cheaper access to space.

One issue is that scramjets are predicted to have exceptionally poor thrust to weight ratio- around 2 . This compares unfavourably with a typical rocket engine that is usually 50-100. This is compensated for in scramjets partly because the weight of the vehicle would be carried by aerodynamic lift rather than pure rocket power (giving reduced 'gravity losses'), but scramjets would take much longer to get to orbit which offsets the advantage.

Whether this vehicle would be reusable or not is still a subject of debate and research.

An aircraft using this type of jet engine could dramatically reduce the time it takes to travel from one place to another, potentially putting any place on Earth within a 90 minute flight. However, there are questions about whether such a vehicle could carry enough fuel to make useful length trips, and there are obvious issues with sonic booms and acceptable g-loads on passengers.

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Hypersonic SCRAMJET vehicle applications

National Aerospace Plane (NASP) and X-30: During the 1980s, NASA began

considering a hypersonic single-stage-to-orbit (SSTO) vehicle to replace the Space Shuttle. The proposed National Aerospace Plane (NASP) would take off from a standard runway using some kind of low speed jet engine. Once the aircraft had reached sufficient speed, air-breathing ramjet or scramjet engines would power the aircraft to hypersonic velocities (Mach 20 or more) and to the edge of the atmosphere. A small rocket system would provide the final push into orbit, but the attractiveness of the concept was using the atmosphere to provide most of the fuel needed to get into space. NASP eventually matured into the X-30 research vehicle, which used an integrated scramjet propulsion system.

The X-30 was intended to replace the Space Shuttle but was cancelled in the early 1990s due to escalating costs and lack of military support.

X-43 Hyper-X: NASA's Hyper-X project, now known as the X-43 will be

the first vehicle using an air-breathing engine ever flight tested at hypersonic speeds.Looking much like a scaled-down X-30, the Hyper-X is a small, unpiloted vehicle intended to test an integrated scramjet engine from Mach 7 to 10. To become airborne, the X-43 will be mounted on the nose of a Pegasus rocket carried aloft and released by a B-52. The Pegasus will power the test craft to about 100,000 ft and the desired test speed before the X-43 separates and its scramjet engine engages. The Hyper-X will only fly for a few seconds before falling into the ocean, but data collected from these test flights will be used to develop practical hypersonic scramjet engines for future vehicles.

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The first X-43 test flight, conducted in June 2001, ended in failure after the Pegasus booster rocket became unstable and went out of control. In addition, three follow-on models are also being considered. First of these is the X-43C which will test a hydrocarbon-fueled dual mode scramjet being developed by Pratt & Whitney under the Air Force's HyTech program. The HyTech engine is expected to accelerate the enlarged X-43C from Mach 5 to Mach 7.

Two propulsion concepts are currently being considered for an X-43B model. First of these is a rocket-based combined cycle (RBCC) engine under development by Aerojet, Boeing, Pratt & Whitney, and Rocketdyne. The RBCC engine is a new technology using a rocket engine fed by oxygen from the atmosphere rather than carried aboard the vehicle. The effort is being funded by NASA Marshall under the Integrated Systems Test of an Airbreathing Rocket (ISTAR) program. Meanwhile, an alternative propulsion arrangement is being developed at NASA Glenn as part of the Revolutionary Turbine Accelerator (RTA) program. The RTA engine uses a turbine-based combined cycle (TBCC) to push turbojet technology to much higher speeds than is possible with current jet engines. Regardless of the engine eventually selected, plans call for the vehicle to be air-launched at Mach 0.8 and accelerate to Mach 7 or 8 over 10 minutes. The RTA engine would accerate to about Mach 5 where a HyTech engine like that used on the X-43C would take over. Both the RBCC and TBCC vehicles would be able to glide down for landing and reuse permitting up to 25 flights.

A final proposal is for an X-43D, an evolved version of the original X-43 A. While the X-43 A is powered by an uncooled hydrogen-fueled scramjet engine, the X-43D would use a cooled, liquid-hydrogen-fueled scramjet. The upgraded engine would provide 10 seconds of power and be capable of

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Commercial Transports: Hypersonic vehicles in general and waveriders in

particular have long been touted as potential high-speed commercial transports to replace the Concorde. Some aerospace companies, airlines, and government officials have proposed vehicles cruising at Mach 7 to 12 capable of carrying passengers from New York to Tokyo in under two hours.

Military Applications: Probably the greatest proponent of hypersonic travel

over the years has been the United States military. Trends of the 1950s and 1960s indicated that military aircraft had to fly faster and higher to survive, so concepts for high-altitude fighters and bombers cruising at Mach 4 or more were not uncommon. Although the trend soon fizzled and military planners looked to maneuverability and stealth for survival, the military has recently shown renewed interest in hypersonic flight. For example, many have conjectured about the existence of a Mach 5 spy plane, the Aurora, that may be under development or perhaps already flying. If so, the Aurora may be a scramjet-powered design similar to the X-30 and X-43 research vehicles.More recently, Northrop Grumman has unveiled a concept for a hypersonic bomber designed using waverider principles.

Cruise Missiles: Though developing a man-rated hypersonic vehicle like

those described above will likely require decades of work and enormous cost, militaries around the world will likely have hypersonic cruise missiles entering service by 2015. Most current concepts for high-speed missiles are simple cylinders with no relation to waveriders.

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RECENT PROGRESS

In recent years, significant progress has been made in the development of hypersonic technology, particularly in the field of scramjet engines. While American efforts are probably the best funded, the first to demonstrate a scramjet working in an atmospheric test was a shoestring project by an Australian team at the University of Queensland. The university's HyShot project demonstrated scramjet combustion in 2002. This demonstration was somewhat limited, however; while the scramjet engine worked effectively and demonstrated supersonic combustion in action, the engine was not designed to provide thrust to propel a craft.

The US Air Force and Pratt and Whitney have cooperated on the Hypersonic Technology (HyTECH) scramjet engine, which has now been demonstrated in a wind-tunnel environment. NASA's Marshall Space Propulsion Center has introduced an Integrated Systems Test of an Air-Breathing Rocket (ISTAR) program, prompting Pratt & Whitney, Aerojet, and Rocketdyne to join forces for development.

The most advanced US hypersonics program is the US$250 million NASA Langley Hyper-X X-43A effort, which flew small test vehicles to demonstrate hydrogen-fueled scramjet engines. NASA is worked with contractors Boeing, Microcraft, and the General Applied Science Laboratory (GASL) on the project.

The NASA Langley, Marshall, and Glenn Centers are now all heavily engaged in hypersonic propulsion studies. The Glenn Center is taking leadership on a Mach 4 turbine engine of interest to the USAF. As for the X-43A Hyper-X, three follow-on projects are now under consideration.

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CONCLUSION

Imagine a jet engine that doesn't pollute the atmosphere, flies more than five times the speed of sound and carry more pay load.

This can be made into reality using Scramjet Engines that is powered by oxygen it scoops out of the air as it flies.

It will take years of work before scramjets are available for practical uses, but they could eventually revolutionize space launches and commercial flights.

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CONTENTS:

THE SCRAMJET ENGINE