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A scramjet (supersonic combustion ramjet) is reminiscent of a ramjet. The basic components of scramjet are inlet, diffuser, fuel injector, flame holder, igniter, and combustion chamber and exhaust nozzle. The basic principle of scramjet is same as that of any type of engine, intake, compression, combustion, exhaust. NASA expects that future versions of this engine will serve as a low cost way to get payloads into orbit by lifting space cargoes to nearly atmospheric altitudes before they continue their journeys on rocket power. Scramjet advantages include simplicity of design, half an engine, carry more payloads, lower thrust to weight ratio, etc. Scramjet disadvantages include additional propulsion requirements, testing difficulties, lack of stealth, need of additional engines etc. Scramjet is used for space applications, civil applications and military applications. Recent progress in scramjet developments are Hyshot, Hyper-X, GASL project and implimentationile etc.
Space was always a dream for man. There was always a passion for human beings since the time of antiquity to fly like a bird. Here the passion takes precedence. His dream has no limits. It leads him to do lot of experiments to foray the Milky Way. Some may have failed but finally he succeeded in his attempts and that pave the way for Aeronautical Technology.
One thing has always been true about rockets: The farther and faster you want to go, the bigger your rocket needs to be. 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.
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-1 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 North American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach 1 to Mach 2.
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 atmosphere passing through the vehicle, instead of from a tank onboard. The craft becomes smaller, lighter and faster. Researchers predict scramjet speeds could reach 15 times the speed of sound. An 18-hour trip to Tokyo from New York City becomes a 2-hour flight.
The university of Queensland’s Hyshot team, Australia reported in 1995, the first development of a scramjet and in 2002 successfully tested the first ever scramjet system. It had a speed of Mach 7, or seven times the speed of sound.
A ramjet, sometimes referred to as a stovepipe jet, or an athodyd, is a form of jet engine using the engine's forward motion to compress incoming air, without a rotary compressor. Ramjets cannot produce thrust at zero airspeed and thus cannot move an aircraft from a standstill. Ramjets require considerable forward speed to operate well, and as a class work most efficiently at speeds around Mach 3. This type of jet can operate up to speeds of Mach 6.
. An object moving at high speed through air generates a high pressure region in front and a low pressure region to the rear. A ramjet uses this high pressure in front of the engine to force air through the tube, where it is heated by combusting some of it with fuel. It is then passed through a nozzle to accelerate it to supersonic speeds. This acceleration gives the ramjet forward thrust.
Figure-1: simple working of ramjet engine
FROM RAMJET TO SCRAMJET
Beyond Mach = 5 (hypersonic domain), ramjet is less and less efficient. Increasing of air stagnation temperature and pressure tends to limit the performance and to increase the thermal and mechanical loads on the combustion chamber walls
To bypass these issues, the solution is to maintain the flow supersonic from the air inlet to the engine exit and to achieve the combustion in the supersonic flow
A scramjet (supersonic combustion ramjet) is a variant of a ramjet air breathing combustion jet engine in which the combustion process takes place in supersonic airflow. As in ramjets, a scramjet relies on high vehicle speed to forcefully compress and decelerate the incoming air before combustion (hence ramjet), but whereas a ramjet decelerates the air to subsonic velocities before combustion, airflow in a scramjet is supersonic throughout the entire engine. This allows the scramjet to efficiently operate at extremely high speeds: theoretical project and implimentationions place the top speed of a scramjet between Mach 12 and Mach 24, which is near orbital velocity
Figure-2: simple working of scramjet engine
The scramjet is composed of three basic components: a converging inlet, where incoming air is compressed and decelerated; a combustor, where gaseous fuel is burned with atmospheric oxygen to produce heat; and a diverging nozzle, where the heated air is accelerated to produce thrust. Unlike a typical jet engine, such as a turbojet or turbofan engine, a scramjet does not use rotating, fan-like components to compress the air; rather, the incredible speed of the aircraft moving through the atmosphere causes the air to compress within the nozzle. As such, very few moving parts are needed in a scramjet, which greatly simplifies both the design and operation of the engine. In comparison, typical turbojet engines require inlet fans, multiple stages of rotating compressor, and multiple rotating turbine stages, all of which add weight, complexity, and a greater number of failure points to the engine. It is this simplicity that allows scramjets to operate at such high velocities, as the conditions encountered in hypersonic flight severely hamper the operation of conventional turbo machinery.
Scramjet engines are a type of jet engine, and rely on the combustion of fuel and an oxidizer to produce thrust. Similar to conventional jet engines, scramjet-powered aircraft carry the fuel on board, and obtain the oxidizer by the ingestion of atmospheric oxygen (as compared to rockets, which carry both fuel and an oxidizing agent). This requirement limits scramjets to suborbital atmospheric flight, where the oxygen content of the air is sufficient to maintain combustion.
4. SCRAMJET PROPULSION
A ramjet engine provides a simple, light propulsion system for high speed flight. Likewise, the supersonic combustion ramjet, or scramjet, provides high thrust and low weight for hypersonic flight speeds. Unlike a turbojet engine, ramjets and scramjets have no moving parts, only an inlet, and a combustor that consists of a fuel injector and a flame holder, and a nozzle. When mounted on a high speed aircraft, large amounts of surrounding air are continuously brought into the engine inlet because of the forward motion of the aircraft. The air is slowed going through the inlet, and the dynamic pressure due to velocity is converted into higher static pressure. At the exit of the inlet, the air is at a much higher pressure than free stream. While the free stream velocity may be either subsonic or supersonic, the flow exiting the inlet of a ramjet is always subsonic. The flow exiting a scramjet inlet is supersonic and has fewer shock losses than a ramjet inlet at the same vehicle velocity.
In the burner, a small amount of fuel is combined with the air and ignited. In a typical engine, 100 pounds of air/sec. is combined with only 2 pounds of fuel/sec. Most of the hot exhaust has come from the surrounding air. Flame holders in the burner localize the combustion process. Burning occurs subsonically in the ramjet and supersonically in the scramjet. Leaving the burner, the hot exhaust passes through a nozzle, which is shaped to accelerate the flow. Because the exit velocity is greater than the free stream velocity, thrust is created as described by the general thrust equation. For ramjet and scramjet engines, the exit mass flow is nearly equal to the free stream mass flow, since very little fuel is added to the stream.
5. ADVANTAGES OF SCRAMJET
Special cooling and materials: Unlike a rocket that quickly passes mostly vertically through the atmosphere or a turbojet or ramjet that flies a “depressed trajectory”, staying within the atmosphere at hypersonic speeds .Because scramjet have only mediocre thrust-to-weight ratios , acceleration would be limited. Therefore time in the atmosphere at hypersonic speeds would be considerable, possibly 15-30 minutes. Similar to a reentering space vehicle, heat insulation from atmospheric friction would be a formidable task. The time in the atmosphere would be greater than that for a typical space capsule, but less than that of the space shuttle. Often, however, the coolant is the fuel itself, much in the same that modern rockets use their own fuel and oxidizer as coolant for their engines. Both scramjets and conventional rockets are at risk in the event of a cooling failure.
Half an engine: The typical wave rider scramjet concept involves, effectively, only half an engine. The shock wave of the vehicle itself compresses the expanding gases, forming the other half .Likewise; only fuel (the light component) needs tank, pumps, etc. This greatly reduces craft mass and construction effort, but the resultant engine is still very much heavier than an equivalent rocket or convection turbojet engine of similar thrust.
Simplicity of design: Scramjets have few to no moving parts. Most of their body consists of numerous surfaces. With simple fuel pumps, reduced total components, and the reentry system being the crank itself, scramjet development tends to be more of a materials and modeling problem than anything else.
Carry more payloads: An advantage of hypersonic air breathing (typically scramjet) vehicle is avoiding or at least reducing the need for carrying oxidizer.75% of the entire assembly weight is liquid oxygen. If carrying this could be eliminated, the vehicle could be lighter at takeoff and hopefully carry more pay loads .That could be a major advantage, but the central motivation in pursuing hypersonic air breathing vehicles would be to reduce costs.
Costs: Reducing the amount of fuel and oxidizer, as in scramjets, means that the vehicle itself becomes a much larger percentage of the costs (rocket fuels are already cheap).Indeed, the unit cost of the vehicle can be expected to end up far higher, since the aerospace hardware cost is probably about two orders of magnitude higher than liquid oxygen and tank age. Still, if scramjets enable reusable vehicles, this could theoretically be a cost benefit. Whether equipment subject to the extreme conditions of a scramjet can be reused sufficiently many times is unclear; all flown scramjet tests are only designed to survive for short periods.
It is likely that a scramjet vehicle would need to lift more load than a rocket of equal takeoff weight in order to be equally as cost efficient (if the scramjet is a non-reusable vehicle).
6. DISADVANTAGES OF SCRAMJET
Additional propulsion requirements: A scramjet cannot produce efficient thrust unless boosted to high speed, at least Mach 5. Therefore a horizontal take off aircraft could need convectional rocket engines to take off, sufficiently large to move a heavy craft. Also needed would be fuel for such engines, plus all engine associated mounting structure and control systems .So another propulsion method would be needed to reach scramjet operating speed. That could be ramjets or rockets. Those would also need their own separate fuel supply, structure and systems. Many proposals instead call for a first stage of droppable solid rocket boosters, which greatly simplifies the design.
Testing difficulties: Unlike jet or rocket propulsion systems facilities which can be tested on the ground, testing scramjet designs uses extremely expensive hypersonic test chambers or expensive launch vehicles, both of which lead to high instrumentation costs. Launched test vehicles very typically end with destruction of the test item and instrumentation.
Lack of stealth: There is no published way to make a scramjet powered vehicle stealthy, since the vehicle would be very hot due to its high speed within the atmosphere. So it should be easy to detect with infrared sensors.
Scramjet speed could reach 15 times the speed of sound. 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 minutes flight. I.e. an 18 hour trip to Tokyo from New York City or from becomes a 2 hour flight.
Scramjet can be used o propel missiles .They are found almost exclusively in missiles where they are boosted to operating speeds by a rocket engine or being attached ton another aircraft, typically a fighter. Currently used scramjet propelled missiles are
(1) British Bloodhound Surface to air missile
(2) British MBDA Meteor Air to air missile
(3) Russian Indian Brahmos Supersonic cruise missile
8. SCRAMJET AND N.A.S.A
In its maiden test flight last June, a hypersonic plane developed by NASA veered off course and was destroyed. Despite the failure, the agency in now trying to breathe new life into it tests of the craft’s novel engine, called a scramjet. NASA expects that future versions of the engine will swerve as a low cost way to get pay loads into orbit by lifting space cargoes to nearly stratospheric altitudes before they continue their journeys on rocket power.
A conventional jet engine, with its spinning blades and turbines, would tear apart at lower speeds; but the scramjet has no moving parts. That means air can safely rush through it at many times the speed of sound, combust with hydrogen fuel to boost the vehicle to hypersonic speeds (above mach 5).Of course, conventional liquid fuelled rockets fly even faster, but they must carry both fuel and oxygen needed to burn it-an expensive proposition. A future craft with both scramjet and rocket power could travel to the edge of space before firing its rockets, requiring less oxygen and leaving more room for the pay load
If NASA does get its craft off the ground, those waiting for a cheaper, more efficient way to space can begin breathe easier.
9. 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 and implimentation by an Australian team at the University of Queensland. The university's HyShot project and implimentation 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.
To coordinate hypersonic technology development, the various factions interested in hypersonic research have formed two integrated product teams (IPTs): one to consolidate Army, Air Force, and Navy hypersonic weapons research, the other to consolidate Air Force and NASA space transportation and hypersonic aircraft work. Current funding levels are relatively low, no more than US$85 million per year in total, but are expected to rise.
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 and implimentation.
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 project and implimentations are now under consideration:
X-43B: A scaled-up version of the X-43A, to be powered by the ISTAR engine. ISTAR will use a hydrocarbon-based liquid-rocket mode for initial boost, a ramjet mode for speeds above Mach 2.5, and a scramjet mode for speeds above Mach 5 to take it to maximum speeds of at least Mach 7. A version intended for space launch could then return to rocket mode for final boost into space. ISTAR is based on a proprietary Aerojet design called a "strutjet", which is currently undergoing wind-tunnel testing.
X-43C: NASA is in discussions with the Air Force on development of a variant of the X-43A that would use the HyTECH hydrocarbon-fueled scramjet engine.
While most scramjet designs to date have used hydrogen fuel, HyTech runs on conventional kerosene-type hydrocarbon fuels, which are much more practical for support of operational vehicles. A full-scale engine is now being built, which will use its own fuel for cooling. Using fuel for engine cooling is nothing new, but the cooling system will also act as a chemical reactor, breaking long-chain hydrocarbons down into short-chain hydrocarbons that burn more rapidly.
X-43D: A version of the X-43A with a hydrogen-powered scramjet engine with a maximum speed of Mach 15.
Hypersonic development efforts are also in progress in other nations. The French are now considering their own scramjet test vehicle and are in discussions with the Russians for boosters that would carry it to launch speeds. The approach is very similar to that used with the current NASA X-43A demonstrator.
Several scramjet designs are now under investigation with Russian assistance. One of these options or a combination of them will be selected by ONERA, the French aerospace research agency, with the EADS conglomerate providing technical backup. The notional immediate goal of the study is to produce a hypersonic air-to-surface missile named "Promethee", which would be about 6 meters (20 ft) long and weigh 1,700 kilograms (3,750 lb).
10. SCRAMJET PROGRAMMES
On July 30, 2002, the University of Queensland's HyShot team conducted the first ever test successful flight of a scramjet.
The team took a unique approach to the problem of accelerating the engine to the necessary speed by using an Orion-Terrier rocket to take the aircraft up on a parabolic trajectory to an altitude of 314 km. As the craft re-entered the atmosphere, it dropped to a speed of Mach 7.6. The scramjet engine then started, and it flew at about Mach 7.6 for 6 seconds. . This was achieved on a lean budget of just A$1.5 million (US $1.1 million), a tiny fraction of NASA's $US 250 million to develop the X-43A.
NASA has partially explained the tremendous difference in cost between the two project and implimentations by pointing out that the American vehicle has an engine fully incorporated into an airframe with a full complement of flight control surfaces available.
NASA's Hyper-X program is the successor to the National Aerospace Plane (NASP) program which was cancelled in November 1994. This program involves flight testing through the construction of the X-43 vehicles. NASA first successfully flew its X-43A scramjet test vehicle on March 27, 2004 (an earlier test, on June 2, 2001 went out of control and had to be destroyed). Unlike the University of Queensland's vehicle, it took a horizontal trajectory. After it separated from its mother craft and booster, it briefly achieved a speed of 5,000 miles per hour (8,000 km/h), the equivalent of Mach 7, easily breaking the previous speed record for level flight of an air-breathing vehicle. Its engines ran for eleven seconds, and in that time it covered a distance of 15 miles (24 km). The Guinness Book of Records certified the X-43A's flight as the current Aircraft Speed Record holder on 30 August 2004. The third X-43 flight set a new speed record of 6,600 mph (10,621 km/h), nearly Mach 10 on 16 November 2004. It was boosted by a modified Pegasus rocket which was launched from a Boeing B-52 at 13,157 meters (40,000 feet). After a free flight where the scramjet operated for about ten seconds the craft made a planned crash into the Pacific ocean off the coast of southern California. The X-43A craft were designed to crash into the ocean without recovery. Duct geometry and performance of the X-43 are classified.
(a) RUSSIA AND FRANCE (AND NASA)
On November 17, 1992, Russian scientists with some additional French support successfully launched a scramjet engine in Kazakhstan. From 1994 to 1998 NASA worked with the Russian central institute of aviation motors (CIAM) to test a dual-mode scramjet engine. Four tests took place, reaching Mach numbers of 5.5, 5.35, 5.8, and 6.5. The final test took place aboard a modified SA-5 surface to air missile launched from the Sary Shagan test range in the Republic of Kazakhstan on 12 February 1998. Data regarding whether the internal combustion took place in supersonic air streams was inconclusive, according to NASA. No net thrust was achieved. The tests also included French partners.
(b) GASL PROJECTILE
At a test facility at Arnold Air Force Base in the U.S. state of Tennessee, GASL fired a project and implimentationile equipped with a hydrocarbon-powered scramjet engine from a large gun. On July 26, 2001, the four inch (100 mm) wide project and implimentationile covered a distance of 260 feet (79 m) in 30 milliseconds (roughly 5,900 mph or 9,500 km/h). The project and implimentationile is supposedly a model for a missile design. Many do not consider this to be a scramjet "flight," as the test took place near ground level. However, the test environment was described as being very realistic.
11. SCRAMJET TESTING REACHES MAJOR MILESTONE
Figure-6: Test on scramjet
A team of researchers from Air Force and industry achieved a major milestone on the development path to demonstrate a hydrocarbon fueled, supersonic combustion ramjet, or scramjet, engine. Such propulsive power will enable weapons that will dramatically increase range and decrease the reaction time when employed against high-value targets at long standoff ranges.
Built under the AFRL’s Propulsion Directorate’s Hi-Tech program, the Performance Test Engine, or PTE, successfully completed a series of free jet tests at Mach 4.5 and 6.5. The PTE is an integrated engine with inlet, combustor, and nozzle. Pratt & Whitney developed this heavyweight, heat sink demonstrator engine under contract to AFRL. The tests were conducted at the GASL facilities at Ronkonkoma, New York. The PTE met or exceeded performance goals.
The Hi-Tech program is the latest in a long series of Air Force efforts to prove the viability and utility of the supersonic combustion ramjet engine. The program is focused to establish a scramjet technology base with near term applications to hypersonic cruise missiles. This technology base can be expanded to include reusable hypersonic vehicles such as strike/reconnaissance and affordable access to space vehicles. By maturing scramjet propulsion, researchers will provide a key component to a new breed of propulsion systems known as the combined or combination cycle engines. These combine turbine, ramjet, scramjet and/or rocket engines, using each of the different cycles to the fullest advantage of their respective efficiencies to optimize overall system performance. Such propulsion systems have the potential to enable a family of vehicles, including global range, high speed aircraft, and “space plane” type vehicles for on-demand access to space.
Scramjet programme is a fast developing field in the present world. There are many applications with scramjet. It provides a cheaper and efficient access to space. Scramjet has the potential for supersonic or hypersonic transportation. Scramjet technologies are also used for military applications. But scramjet technologies still need developments. Scramjet in future will provide us cheaper and faster access to any part in this universe. Also the craft will become smaller and lighter and can carry more payloads.
1. Aircraft and Missile Propulsion M J ZUCROW, JOHN WILLEY
2. Aircraft Propulsion P J Mc MAHON, HARPET ROW
3. Hypersonic Air breathing Propulsion W H HEISER, D T PRATT
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In aerodynamics, hypersonic speeds are speeds that are highly supersonic. In the 1970s, the term generally came to refer to speeds of Mach 5 (5 times the speed of sound) and above. The hypersonic regime is a subset of the supersonic regime.
Supersonic airflow is decidedly different from subsonic flow. Nearly everything about the way an aircraft flies changes dramatically as an aircraft accelerates to supersonic speeds. Even with this strong demarcation, there is still some debate as to the definition of "supersonic". One definition is that the aircraft, as a whole, is traveling at Mach 1 or greater. More technical definitions state that you are only supersonic if the airflow over the entire aircraft is supersonic, which occurs around Mach 1.2 on typical designs. The range Mach 0.8 to 1.2 is therefore considered transonic.
Considering the problems with this simple definition, it should be no surprise that the precise Mach number at which a craft can be said to be fully hypersonic is even more elusive, especially since physical changes in the airflow (molecular dissociation, ionization) occur at quite different speeds. Generally, a combination of effects become important "as a whole" around Mach 5. The hypersonic regime is often defined as speeds where ramjets do not produce net thrust. This is a nebulous definition in itself, as there exists a proposed change to allow them to operate in the hypersonic regime
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A scramjet (supersonic combustion ramjet) is a variation of a ramjet with the key difference being that the flow in the combustor is supersonic. At higher speeds it is necessary to combust supersonically to maximize the efficiency of the combustion process. Projections for the top speed of a scramjet engine (without additional oxidiser input) vary between Mach 12 and Mach 24 (orbital velocity), but the X-30 research gave Mach 17 due to combustion rate issues. By way of contrast, the fastest conventional air-breathing, manned vehicles, such as the U.S. Air Force SR-71, achieve approximately Mach 3.4 and rockets achieved Mach 30+ during the Apollo Program.
Like a ramjet, a scramjet essentially consists of a constricted tube through which inlet air is compressed by the high speed of the vehicle, a combustion chamber where fuel is combusted, and a nozzle through which the exhaust jet leaves at higher speed than the inlet air. Also like a ramjet, there are few or no moving parts. In particular there is no high speed turbine as in a turbofan or turbojet engine that is expensive to produce and can be a major point of failure.
A scramjet requires supersonic airflow through the engine, thus, similar to a ramjet, scramjets have a minimum functional speed. This speed is uncertain due to the low number of working scramjets, relative youth of the field, and the largely classified nature of research using complete scramjet engines. However, it is likely to be at least Mach 5 for a pure scramjet, with higher Mach numbers 7-9 more likely. Thus scramjets require acceleration to hypersonic speed via other means. A hybrid ramjet/scramjet would have a lower minimum functional Mach number, and some sources indicate the NASA X-43A research vehicle is a hybrid design. Recent tests of prototypes have used a booster rocket to obtain the necessary velocity. Air breathing engines should have significantly better specific impulse while within the atmosphere than rocket engines.
However, scramjets have weight and complexity issues that must be considered. While very short suborbital scramjet test flights have been successfully performed, perhaps significantly no flown scramjet has ever been successfully designed to survive a flight test. The viability of scramjet vehicles is hotly contested in aerospace and space vehicle circles, in part because many of the parameters which would eventually define the efficiency of such a vehicle remain uncertain. This has led to grandiose claims from both sides, which have been intensified by the large amount of funding involved in any hypersonic testing. Some notable aerospace gurus such as Henry Spencer and Jim Oberg have gone so far as calling orbital scramjets 'the hardest way to reach orbit', or even 'scamjets' due to the extreme technical challenges involved. Major, well funded project and implimentations, like the X-30 were cancelled before producing any working hardware.
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-1 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 North American X-15 and other rocket-powered spacecraft, aircraft top speeds have remained level, generally in the range of Mach 1 to Mach 3.
In the realm of civilian air transport, the primary goal has been reducing operating cost, rather than increasing flight speeds. Because supersonic flight, using conventional jet engines, requires significant amounts of fuel, airlines have favored subsonic jumbo jets rather than supersonic transports. The production supersonic airliners, Concorde and the Tupolev Tu-144, operated with little profit for the French and Russian airlines but British Airways flew Concorde at a 60% profit margin over its commercial life.Military combat aircraft design has focused on maneuverability, more recently combined with stealth. These features are thought to be incompatible with hypersonic aerodynamics because of the very high speeds and temperatures of hypersonic flight.
In the United States, from 1986-1993, a reasonably serious attempt to develop a single stage to orbit reusable spaceplane using scramjet engines was made, but the Rockwell X-30 (NASP) program failed.
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, Russia, and India have also progressed in hypersonic propulsion research.
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. 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."
There are several claims as to which group were the first to demonstrate a "working" scramjet, where "working" in this case can refer to:
• Demonstration of supersonic combustion in a ground test
• Demonstration of net thrust in a ground test
• Demonstration of supersonic combustion or net thrust in a ground test with realistic fuels and/or realistic wind tunnel flow conditions.
• Demonstration of supersonic combustion in a flight test
• Demonstration of net thrust in a flight test.
The problem is complicated by the release of previously classified material and by partial publication, where claims are made, but specific parts of an experiment are kept secret. Additionally experimental difficulties in verifying that supersonic combustion actually occurred, or that actual net thrust was produced mean that at least four consortia have legitimate claims to "firsts", with several nations and institutions involved in each consortium (For a further listing see Scramjet Programs). On June 15, 2007, the US Defense Advanced Research Project Agency (DARPA) and the Australian Defence Science and Technology Organization (DSTO), announced a successful scramjet flight at Mach 10 using rocket engines to boost the test vehicle to hypersonic speeds, at the Woomera Rocket Range in Central Australia.
A scramjet is a type of jet engine designed to operate at the high speeds typically associated with rockets. Its main difference from a rocket is that it collects air from the atmosphere to burn its fuel, rather than carrying an oxidizing substance on board. More conventional jets (turbojets, turbofans and ramjets) share this characteristic but are unsuitable for the high speeds at which scramjets can operate.
FIGURE 1.X-43A with scramjet attached to the underside
Turbine-based engines, while efficient for flight at subsonic and supersonic speeds, quickly lose their efficiency at higher Mach numbers. As air enters the compressor, its pressure and temperature increases, with high Mach numbers resulting in high temperatures. High temperatures are undesirable because they can cause melting or structural failure of the engine, and because the energy released from combustion reduces as the temperature of the fuel-air mixture increases. As the available energy decreases, the drag increases with Mach number squared. The maximum operating speed of a turbine-based engine can be increased by cooling the air in the inlet, and by combining the turbine with other thrust-producing technologies like afterburners or ramjets .
Ramjets are easier to build for higher operating temperatures than turbojets, and produce less drag. They are thus capable of flight at higher speeds than turbojets (but with the drawback that they cannot usefully operate below about 400mph). However, ramjets must slow intake air down to subsonic speed for fuel mixing and combustion by compressing it at the inlet. At conventional supersonic speeds with subsonic combustion this is more efficient than using a bladed compressor, but at higher speeds a problem develops. The shock wave which forms during the compression process causes a high drag on the engine. The drag on the engine is eventually more than can even theoretically be compensated for by the thrust produced. Similarly to the turbojet, the compression at high speeds causes high temperatures which reduce the combustion efficiency.
For an engine to be efficient, it must have low drag and good combustion efficiency. The theoretical upper operating limit for engines with subsonic combustion is not a hard line, but lies somewhere between Mach 4 and Mach 8 depending on the fuel used.
3.1 Diagram illustrating the principle of scramjet operation
The scramjet is intended to avoid the high drag and low combustion efficiency of other types of engine at high Mach number by maintaining supersonic airflow through the whole engine. The lack of a strong shock, as in a ramjet, significantly reduces the drag of the engine. Because intake air is decelerated less than with a ramjet, it is also heated less and fuel can be burned more efficiently. The difficulty is that at these higher airflow velocities, the fuel must be mixed and burned in a very short time, and that any error in the geometry of the engine will result in a high drag.
A very simple scramjet would look like two kitchen funnels attached by their small ends. The first funnel is the intake, into which air is forced, compressing and heating in the process. At the narrow section where the funnels join and compression is greatest, fuel is added and combusted which heats the gas further. The gas expands and exits through the second funnel, like the nozzle of a rocket, and thrust is produced.
Note that most artists' impressions of scramjet-powered vehicles depict waveriders, on which the underside of the vehicle forms the intake and nozzle of the engine; the two are asymmetric and contribute directly to the lift of the aircraft. A waverider is the required form for a hypersonic lifting body.
3.2 Scramjets integrate air and space
The world's first scramjet engine to demonstrate operability at Mach 4.5-6.5 using conventional fuel.
As the 21st century unfolds, a revolutionary engine technology is aiming to fly craft at high Mach speeds and seamlessly integrate air-to-space operations. The supersonic combustion ramjet, or scramjet, uses no rotating parts, will power vehicles hundreds of miles in minutes, and will make rapid global travel and affordable access to space a reality.
These goals drew closer to achievement this spring when the first scramjet-powered aircraft flew on its own. On the afternoon of March 27, an unpiloted X-43A, a National Aeronautics and Space Administration (NASA) craft mounted on a Pegasus booster rocket, dropped from a B-52 flying at 40,000 ft off the coast of California. The rocket sent the experimental aircraft soaring to its test altitude of 95,000 ft, where the X-43A separated from its booster, and its scramjet engine fired for a planned 10-s test, achieving an incredible Mach 7, or 5,000 mph.
Data from that flight helped validate the concept of a hypersonic craft with an airbreathing engine. More flights during the next several years will expand on the engine and aerodynamics data obtained in March, and could put some scramjet vehicles in service in less than a decade. Scramjets will enable three categories of hypersonic craft: weapons, such as cruise missiles; aircraft, such as those designed for global strike and reconnaissance missions; and space-access vehicles that will take off and land like airliners.
Scramjets have a long and active development history in the United States. On the basis of theoretical studies started in the 1940s, the U.S. Air Force, Navy, and NASA began developing scramjet engines in the late 1950s. Since then, many hydrogenand hydrocarbon-fueled engine programs have helped scramjet technology evolve to its current state. The most influential of these efforts was NASA’s National Aerospace Plane (NASP) program, established in 1986 to develop a vehicle with speed greater than Mach 15 and horizontal takeoff and landing capabilities. The program ended in 1993, but the original NASP engine design, significantly modified by NASA, provided the foundation for the power plant used during the X-43A’s recent flight.
3.3 Scramjet propulsion
Thrust is the force which moves any aircraft through the air. Thrust is generated by the propulsion system of the aircraft. Different propulsion systems develop thrust in different ways, but all thrust is generated through some application of Newton's third law of motion. For every action there is an equal and opposite reaction. In any propulsion system, a working fluid is accelerated by the system and the reaction to this acceleration produces a force on the system. A general derivation of the thrust equation shows that the amount of thrust generated depends on the mass flow through the engine and the exit velocity of the gas. Engineers use a thermodynamic analysis of the scramjet to predict thrust and fuel flow.
In the early 1900's some of the original ideas concerning ramjet propulsion were first developed in Europe. Thrust is produced by passing the hot exhaust from the combustion of a fuel through a nozzle. The nozzle accelerates the flow, and the reaction to this acceleration produces thrust. To maintain the flow through the nozzle, the combustion must occur at a pressure that is higher than the pressure at the nozzle exit. In a ramjet, the high pressure is produced by "ramming" external air into the combustor using the forward speed of the vehicle. The external air that is brought into the propulsion system becomes the working fluid, much like a turbojet engine. The combustion process in a ramjet occurs at subsonic speeds in the combustor. For a vehicle traveling supersonically the air entering the engine must be slowed to subsonic speeds by shock waves generated in the aircraft inlet. Much above Mach 5, the performance losses from the shock waves become so great that the engine can no longer produce net thrust.
In the 1960's an improved ramjet was proposed in which the combustion in the burner would occur supersonically. In the supersonic combustion ramjet, or scramjet, the losses associated with slowing the flow would be minimized and the engine could produce net thrust for a hypersonic vehicle. Tests were begun to design the supersonic burner and to better integrate the inlet and nozzle with the airframe. Because the scramjet uses external air for combustion, it is a more efficient propulsion system for flight within the atmosphere than a rocket, which must carry all of its oxygen. Scramjets are ideally suited for hypersonic flight within the atmosphere.
CHAPTER – IV
All scramjet engines have fuel injectors, a combustion chamber, a thrust nozzle and an intake, which compresses the incoming air. Sometimes engines also include a region which acts as a flame holder, although the high stagnation temperatures mean that an area of focused waves may be used, rather than a discrete engine part as seen in turbine engines. Other engines use pyrophoric fuel additives, such as silane, to avoid such issues. An isolator between the inlet and combustion chamber is often included to improve the homogeneity of the flow in the combustor and to extend the operating range of the engine.
Computational fluid dynamic (CFD) image of the X-43A with scramjet attached to the underside at Mach 7
A scramjet is reminiscent of a ramjet. In a typical ramjet, the supersonic inflow of the engine is decelerated at the inlet to subsonic speeds and then reaccelerated through a nozzle to supersonic speeds to produce thrust. This deceleration, which is produced by a normal shock, creates a total pressure loss which limits the upper operating point of a ramjet engine.
For a scramjet, 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 minimizing drag as maximizing thrust.
This high speed makes the control of the flow within the combustion chamber more difficult. Since the flow is supersonic, no upstream influence propagates within the freestream of the combustion chamber. Thus throttling of the entrance to the thrust nozzle is not a usable control technique. In effect, a block of gas entering the combustion chamber must mix with fuel and have sufficient time for initiation and reaction, all the while travelling supersonically through the combustion chamber, before the burned gas is expanded through the thrust nozzle. This places stringent requirements on the pressure and temperature of the flow, and requires that the fuel injection and mixing be extremely efficient. Usable dynamic pressures lie in the range 20 to 200 kPa (0.2-2 bar), where
Where q =.5 ρ v^2
q Is the dynamic pressure of the gas
ρ (rho) is the density of the gas
v is the velocity of the gas
To keep the combustion rate of the fuel constant, the pressure and temperature in the engine must also be constant. This is problematic because the airflow control systems that would facilitate this are not physically possible in a scramjet launch vehicle due to the large speed and altitude range involved, meaning that it must travel at an altitude specific to its speed. Because air density reduces at higher altitudes, a scramjet must climb at a specific rate as it accelerates to maintain a constant air pressure at the intake. This optimal climb/descent profile is called a "constant dynamic pressure path".
Fuel injection and management is also potentially complex. One possibility would be that the fuel is pressurized to 100 bar by a turbo pump, heated by the fuselage, sent through the turbine and accelerated to higher speeds than the air by a nozzle. The air and fuel stream are crossed in a comb like structure, which generates a large interface. Turbulence due to the higher speed of the fuel lead to additional mixing. Complex fuels like kerosene need a long engine to complete combustion.
The minimum Mach number at which a scramjet can operate is limited by the fact that the compressed flow must be hot enough to burn the fuel, and of high enough pressure that the reaction is finished before the air moves out the back of the engine. Additionally, in order to be called a scramjet, the compressed flow must still be supersonic after combustion. Here two limits must be observed: Firstly, since when a supersonic flow is compressed it slows down, the level of compression must be low enough (or the initial speed high enough) not to slow the gas below Mach 1. If the gas within a scramjet goes below Mach 1 the engine will "choke", transitioning to subsonic flow in the combustion chamber. This effect is well known amongst experimenters on scramjets since the waves caused by choking are easily observable. Additionally, the sudden increase in pressure and temperature in the engine can lead to an acceleration of the combustion, leading to the combustion chamber exploding.
Secondly, the heating of the gas by combustion causes the speed of sound in the gas to increase (and the Mach number to decrease) even though the gas is still travelling at the same speed. Forcing the speed of air flow in the combustion chamber under Mach 1 in this way is called "thermal choking". It is clear that a pure scramjet can operate at Mach numbers of 6-8, but in the lower limit, it depends on the definition of a scramjet. Certainly there are designs where a ramjet transforms into a scramjet over the Mach 3-6 range (Dual-mode scramjets). In this range however, the engine is still receiving significant thrust from subsonic combustion of "ramjet" type.
The high cost of flight testing and the unavailability of ground facilities have hindered scramjet development. A large amount of the experimental work on scramjets has been undertaken in cryogenic facilities, direct-connect tests, or burners, each of which simulates one aspect of the engine operation. Further, vitiated facilities, storage heated facilities, arc facilities and the various types of shock tunnels each have limitations which have prevented perfect simulation of scramjet operation. The HyShot flight test showed the relevance of the 1:1 simulation of conditions in the T4 and HEG shock tunnels, despite having cold models and a short test time. The NASA-CIAM tests provided similar verification for CIAM's C-16 V/K facility and the Hyper-X project and implimentation is expected to provide similar verification for the Langley AHSTF, CHSTF and 8 ft (2.4 m) HTT.
Computational fluid dynamics has only recently reached a position to make reasonable computations in solving scramjet operation problems. Boundary layer modeling, turbulent mixing, two-phase flow, flow separation, and real-gas aerothermodynamics continue to be problems on the cutting edge of CFD. Additionally, the modeling of kinetic-limited combustion with very fast-reacting species such as hydrogen makes severe demands on computing resources. Reaction schemes are numerically stiff, having typical times as low as 10-19 seconds, requiring reduced reaction schemes.
Much of scramjet experimentation remains classified. Several groups including the US Navy with the SCRAM engine between 1968-1974, and the Hyper-X program with the X-43A have claimed successful demonstrations of scramjet technology. Since these results have not been published openly, they remain unverified and a final design method of scramjet engines still does not exist.
The final application of a scramjet engine is likely to be in conjunction with engines which can operate outside the scramjet's operating range. Dual-mode scramjets combine subsonic combustion with supersonic combustion for operation at lower speeds, and rocket-based combined cycle (RBCC) engines supplement a traditional rocket's propulsion with a scramjet, allowing for additional oxidizer to be added to the scramjet flow. RBCCs offer a possibility to extend a scramjet's operating range to higher speeds or lower intake dynamic pressures than would otherwise be possible.
Figure 4. Propulsion efficiency decreases with speed as we progress through turbojets to ramjets and scramjets to rockets; hydrogen is more efficient than jet fuel.
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. 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.
Figure 5. As the vehicle speed increases from Mach 3 to Mach 8, the isolator pressure ratio passes through a peak at Mach 6. As the shock train and boundary layer retreat, the modes change from dual-mode ramjet to dual-mode scramjet to pure scramjet mode.
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 (Figure 3). Engine efficiency dictates using the ramjet until Mach 5–6. At around 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 pumps, hydraulics, and electronics, as well as combustion. Thermal-management schemes focus on the engine in hypersonic vehicles because of its potential for extremely high heat loads. 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. These 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
• erosion from airflow over the vehicle and through the engine
With the completion of the successful X-43A flight and the ground-testing of several full-sized demonstration engines, confidence in the viability of the hydrogen- and hydrocarbon-fueled scramjet engines has increased significantly. NASA plans to launch another X-43A this fall and fly it at Mach 10, or 6,750 mph.
ADVANTAGES AND DISADVANTAGES
5.1Advantages and disadvantages of scramjets
5.1.1 Special cooling and materials
Unlike a rocket that quickly passes mostly vertically through the atmosphere or a turbojet or ramjet that flies at much lower speeds, a hypersonic airbreathing vehicle optimally flies a "depressed trajectory", staying within the atmosphere at hypersonic speeds. Because scramjets have only mediocre thrust-to-weight ratios, acceleration would be limited. Therefore time in the atmosphere at hypersonic speed would be considerable, possibly 15-30 minutes. Similar to a reentering space vehicle, heat insulation would be a formidable task. The time in the atmosphere would be greater than that for a typical space capsule, but less than that of the space shuttle.
New materials offer good insulation at high temperature, but they often sacrifice themselves in the process. Therefore studies often plan on "active cooling", where coolant circulating throughout the vehicle skin prevents it from disintegrating. Often the coolant is the fuel itself, much in the same way that modern rockets use their own fuel and oxidizer as coolant for their engines. All cooling systems add weight and complexity to a launch system and reduce its efficiency. The increased cooling requirements of scramjet engines result in lower efficiency.
The efficiency of a launch vehicle depends greatly on its weight. Calculating the efficiency of an engine system is mathematically complex, and involves tradeoffs between the efficiency of the engine (takeoff fuel weight) and the complexity of the engine (takeoff dry weight), which can be expressed by the following:
• is the empty mass fraction
• is the fuel+oxidiser mass fraction
• is initial mass ratio, and is the inverse of the payload mass fraction
A scramjet increases the mass of the engine Πe over a rocket, and decreases the mass of the fuel Πfe. The logic behind efforts driving a scramjet is (for example) that the reduction in fuel decreases the total mass by 30%, while the increased engine weight adds 10% to the vehicle total mass. Unfortunately the uncertainty in the calculation of any mass or efficiency changes in a vehicle is so great that slightly different assumptions for engine efficiency or mass can provide equally good arguments for or against scramjet powered vehicles.
5.1.3 Simplicity of design
Scramjets have few to no moving parts. Most of their body consists of continuous surfaces. With simple fuel pumps, reduced total components, and the reentry system being the craft itself, scramjet development tends to be more of a materials and modelling problem than anything else.
5.1.4 Additional propulsion requirements
A scramjet cannot produce efficient thrust unless boosted to high speed, around Mach 5, depending on design, although, as mentioned earlier, it could act as a ramjet at low speeds. A horizontal take-off aircraft would need conventional turbofan or rocket engines to take off, sufficiently large to move a heavy craft. Also needed would be fuel for those engines, plus all engine associated mounting structure and control systems. Turbofan engines are heavy and cannot easily exceed about Mach 2-3, so another propulsion method would be needed to reach scramjet operating speed. That could be ramjets or rockets. Those would also need their own separate fuel supply, structure, and systems. Many proposals instead call for a first stage of droppable solid rocket boosters, which greatly simplifies the design.
5.1.5 Testing difficulties
Unlike jet or rocket propulsion systems facilities which can be tested on the ground, testing scramjet designs use extremely expensive hypersonic test chambers or expensive launch vehicles, both of which lead to high instrumentation costs. Launched test vehicles very typically end with destruction of the test item and instrumentation.
5.1.6 Lack of stealth
There is no published way to make a scramjet powered vehicle (or any other hypersonic vehicle) stealthy- since the vehicle would be very hot due to its high speed within the atmosphere it should be easy to detect with infrared sensors. However, any aggressive act against a scramjet vehicle would be difficult because of its high speed.
5.2Advantages and disadvantages for orbital vehicles
An advantage of a hypersonic airbreathing (typically scramjet) vehicle like the X-30 is avoiding or at least reducing the need for carrying oxidizer. For example the space shuttle external tank holds 616,432 kg of liquid oxygen (LOX) and 103,000 kg of liquid hydrogen (LH2). The shuttle orbiter itself weighs about 104,000 kg (max landing weight). Therefore 75% of the entire assembly weight is liquid oxygen. If carrying this could be eliminated, the vehicle could be lighter at takeoff and hopefully carry more payload. That would be a major advantage, but the central motivation in pursuing hypersonic airbreathing vehicles would be to reduce costs. Unfortunately there are several disadvantages:
5.2.1Lower thrust-weight ratio
A rocket has the advantage that its engines have very high thrust-weight ratios (~100:1), while the tank to hold the liquid oxygen approaches a tankage ratio of ~100:1 also.
Thus a rocket can achieve a very high mass fraction (Takeoff rocket mass:unfuelled rocket mass=fuel+oxidiser+structure+engines+payloadtructure+engines), which improves performance. By way of contrast the project and implimentationed thrust/weight ratio of scramjet engines of about 2 mean a very much larger percentage of the takeoff mass is engine (ignoring that this fraction increases anyway by a factor of about four due to the lack of onboard oxidiser). In addition the vehicle's lower thrust does not necessarily avoid the need for the expensive, bulky, and failure prone high performance turbopumps found in conventional liquid-fuelled rocket engines, since most scramjet designs seem to be incapable of orbital speeds in airbreathing mode, and hence extra rocket engines are needed.
5.2.2 Need additional engine(s) to reach orbit
Scramjets might be able to accelerate from approximately Mach 5-7 to around somewhere between half of orbital velocity and orbital velocity (X-30 research suggested that Mach 17 might be the limit compared to an orbital speed of mach 25, and other studies put the upper speed limit for a pure scramjet engine between Mach 10 and 25, depending on the assumptions made). Generally, another propulsion system (very typically rocket is proposed) is expected to be needed for the final acceleration into orbit. Since the delta-V is moderate and the payload fraction of scramjets high, lower performance rockets such as solids, hypergolics, or simple liquid fueled boosters might be acceptable. Opponents of scramjet research claim that most of the theoretical advantages for scramjets only accrue if a single stage to orbit (SSTO) vehicle can be successfully produced. Proponents of scramjet research claim that this is a straw man, and that SSTO vehicles are exactly as difficult to produce and bring the same benefits to rocket-powered and scramjet-powered launch vehicles.
The scramjet's heat-resistant underside potentially doubles as its reentry system, if a single-stage-to-orbit vehicle using non-ablative, non-active cooling is visualised. If an ablative shielding is used on the engine, it will probably not be usable after ascent to orbit. If active cooling is used, the loss of all fuel during the burn to orbit will also mean the loss of all cooling for the thermal protection system.
Reducing the amount of fuel and oxidizer, as in scramjets, means that the vehicle itself becomes a much larger percentage of the costs (rocket fuels are already cheap). Indeed, the unit cost of the vehicle can be expected to end up far higher, since aerospace hardware cost is probably about two orders of magnitude higher than liquid oxygen and tankage. Still, if scramjets enable reusable vehicles, this could theoretically be a cost benefit. Whether equipment subject to the extreme conditions of a scramjet can be reused sufficiently many times is unclear; all flown scramjet tests are only designed to survive for short periods.
The eventual cost of such a vehicle is the subject of intense debate since even the best estimates disagree whether a scramjet vehicle would be advantageous. It is likely that a scramjet vehicle would need to lift more load than a rocket of equal takeoff weight in order to be equally as cost efficient (if the scramjet is a non-reusable vehicle).
Seeing its 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 defense research contracts.
• Access to Space Study: Summary Report; Office of Space Systems Development, NASA Headquarters, Washington, DC, 1994.
• Curran, E. T.; Murthy, S. N. B.; Eds., Scramjet Propulsion; Progress in Astronautics and Aeronautics, vol. 189; AIAA: Washington, DC, 2000.
• Faulkner, R. F. The Evolution of the HySET Hydrocarbon Fueled Scramjet Engine; AIAA Paper 2003-7005; AIAA: Washington, DC, 2003.
• Heiser, W. H.; Pratt, D. T. Hypersonic Airbreathing Propulsion; AIAA: Washington, DC, 1994.
• Kandebo, S. W. New Powerplant Key to Missile Demonstrator. Aviation Week Space Technol., Sept. 2, 2002; p. 56.
• McClinton, C. R.; Andrews, E. H.; Hunt, J. L. Engine Development for Space Access: Past, Present, and Future. Int. Symp. Air Breathing Engines, Jan. 2001; ISABE Paper 2001-1074.
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SCRAMJET COMBUSTOR DEVELOPMENT
An airframe integrated scramjet propelled vehicle has advantages forapplication to several missions. In its simplest form, such a vehicle will combinethe features of quick reaction, low vulnerability to counter attack and betterpropulsion efficiency.The Supersonic Combustion Ramjet (SCRAMJET) engine has beenrecognized as the most promising air breathing propulsion system for thehypersonic flight (Mach number above 5). In recent years, the research anddevelopment of scramjet engine has promoted the study of combustion insupersonic flows. Extensive research is being carried out over the world forrealizing the scramjet technology with hydrogen fuel with significant attentionfocused on new generations of space launchers and global fast-reactionreconnaissance missions. However, application for the scramjet concept usinghigh heat sink and hydrogen fuels offers significantly enhanced mission potentialfor future military tactical missiles. Scramjet being an air-breathing engine, theperformance of the missile system based on the scramjet propulsion isenvisaged to e nhance the payload weight and missile range.Supersonic combustion ramjet engine for an air-breathing propulsionsystem has been realized and demonstrated by USA on ground and in flight. X-43 vehicle used hydrogen fuel. Hydrocarbon fuel scramjet engine is still understudy and research. Mixing, ignition and flame holding in combustor, ground testfacilities and numerical simulation of Scramjet engine are the critical challengesin the development of scramjet engine.
1.1 Scramjet engine - Technological challenges
a)Mixing, Ignition and flame holding in a scramjet combustor
Among the three critical components of the scramjet engine, the combustorpresents the most formidable problems. The complex phenomenon ofsupersonic combustion involves turbulent mixing, shock interaction and heatrelease in supersonic flow. The flow field within the combustor of scramjetengine is very complex and poses a considerable challenge in design anddevelopment of a supersonic combustor with an optimized geometry. Suchcombustor shall promote sufficient mixing of the fuel and air so that the desiredchemical reaction and thus heat release can occur within the residence time ofthe fuel-air mixture. In order to accomplish this task, it requires a clearunderstanding of fuel injection processes and thorough knowledge of theprocesses governing supersonic mixing and combustion as well as the factors,which affects the losses within the combustor. The designer shall keep in mindthe following goals namely,i) Good and rapid fuel air mixingii) Minimization of total pressure lossiii) High combustion efficiency.
b) Ground test facilities for testing of Scramjet engine.In order to carry out the experiments essentially required for the development ofthe scramjet engine and to clearly understand various complex areas associatedwith it, there is a need of scramjet test facility. Among the devices generally usedto produce the test gas to simulate air entering the scramjet combustor are archeater, ceramic storage heater and combustion burners. The scramjet groundtest facilities are available in the mid Mach number range of 5 to 8. There are nosteady flow test facilities in higher Mach number range since achievement of totaltemperatures, pressure and low pressures at exit present enormous engineeringchallenges. Free piston shock tunnels enable test with duration of onlymilliseconds at higher Mach numbers. Conventional scramjet facilities operate inthe blow down mode since continuous operations implies very large powerrequirement for heating the air.
c) Numerical simulation of Scramjet Flow fieldGround tests and classical methods alone cannot give data with sufficientaccuracy for design of hypersonic systems. Due to the closely integrated nature,component level testing will not be able to simulate accurately the complex flowfield. It is difficult to simulate Reynolds number, boundary layer transition inground test facilities. Also, the quality of air is difficult to simulate in the testfacilities. Therefore there is a need to estimate the performance in the flightbased on the results of ground tests. This can be accomplished only through theuse of mathematical modeling of the flow, which is to be solved to first reproducethe result of the ground test and then used for predicting the flight conditions.The primary unknown on a physical plane consists of modeling turbulence and itsinteraction with chemistry. The issues on the numerical front consist of evolvingalgorithms to solve the N – S equations or their variants such that sharp gradientregions near the shocks are captured with numerical diffusion or overshoot. Theprediction of wall heat transfer rate is another task to be handled both on themodeling plane and numerical experiments. One of the advantages of themathematical model is that once it stands validated it can be used to conductseveral numerical experiments on exotic ideas like with respect to enhancedmixing components with much less expense as compared to experiments. Theexperimental effort is not eliminated but reduced and better focused. This is infact the current day approach to the solution to the problems of high-speed flight.Development and realization of scramjet engine has been undertaken inUSA, Russia, Japan, France, Germany and India individually as well as throughjoint cooperation. The urgency of realizing a hypersonic air-breathing engine hasbeen felt by many agencies for civilian and military applications. Thedevelopment of the scramjet engines poses considerable challenges and itdemands multidisciplinary design, analysis, modeling, simulation and systemoptimization. The hardware realization and testing becomes equally complexand multidisciplinary.DRDL is working on a program called “Hypersonic TechnologyDemonstrator Vehicle” (HSTDV). Technological challenge for this vehicle is todemonstrate the scramjet engine at a flight mach number of 6.5. Numberof ground-based experiments have been carried out to develop the scramjetcombustor and associated test facilities also have been established in DRDL.The details of test facility and tests carried out on the development of strut-basedcombustor, Ramp-Cavity combustor and barbotage injection of kerosene withhydrogen fuel as pilot are highlighted in the subsequent sections.
2. DEVELOPMENT OF THE KEROSENE FUELED STRUT BASEDSCRAMJET COMBUSTOR
2.1 TEST FACILITY:The setup consists of a Hydrogen burner as an on-line gas generator, an axisymmetricconvergent-divergent nozzle for accelerating the test gas tosupersonic speed, a circular to rectangular transition duct. The supersoniccombustor has two parts; one constant area section with backward facing stepwith fuel injection strut and the second one is diverging area combustor. Thevitiated air is allowed to expand through an axsymmetric supersonic nozzle with2.4 exit Mach number. The accelerated vitiated air flows through a transitionduct, to provide a uniform flow at the entry of the constant area combustor, withminimum losses. The total temperature and total pressure of the vitiated airfloware measured by means of temperature sensor and pressure transducerrespectively.
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07-05-2011, 12:17 PM
Deepak S V Gowda
Scramjet Technology.doc (Size: 1.19 MB / Downloads: 217)
Hypersonic aircraft having a lateral arrangement of turbojet and Scramjet engines are disclosed. The Scramjet engines may be positioned laterally outboard of the turbojet engines. In one embodiment, the turbojet inlet and outlet openings may be covered during use of the Scramjet in order to provide compression and expansion ramps for the laterally adjacent Scramjet engines. The side-by-side arrangement of the turbojet and Scramjet engines reduces the vertical thickness of the aircraft, thereby reducing drag and potentially increasing performance.
A scramjet (supersonic combustion ramjet) is a variation of a ramjet. A pure scramjet remains supersonic throughout the combustion process and does not require a choking mechanism. Modern scramjet engines are able to seamlessly make the transition between ramjet and scramjet operation. A scramjet propulsion system is a hypersonic air breathing engine in which heat addition, due to combustion of fuel and air, occurs in the flow that is supersonic relative to the engine. In a conventional ramjet, engine the incoming supersonic airflow is decelerated to subsonic speeds by means of a multi-shock intake system and diffusion process. Fuel is added to the subsonic airflow, the mixture combusts and then re-accelerates through a 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. Modern scramjet engines are able to seamlessly make the transition between ramjet and scramjet operation.
Scramjets have a long development history in the United States. In the 1940s, fundamental theoretical studies provided an understanding of high-velocity flow in ducts with heat addition. In the late 1950s, the first efforts to develop and demonstrate scramjet engines took place with Air Force, Navy and NASA laboratory experiments, which provided a foundation for the many development programs that followed. From the 1960s through today, many programs have had the objective of developing and demonstrating hydrogen and hydrocarbon-fueled scramjet engines. McClinton1 explored these developments and examined each generation along with its unique contributions to the understanding of supersonic combustion. Fry R. S2 provided a comprehensive look into advances in ramjet propulsion technology from subsonic to hypersonic flight speeds since the early 1900s. The following references provide insight into some of the key programs that have helped to evolve scramjet technology to its current state.
The most influential program in modern scramjet development was National Aero-Space Plane (NASP) program, which was established in 1986 to develop and fly a synergistically integrated low speed accelerator, ramjet and scramjet propulsion system. Designed to operate on hydrogen fuel, the X-30 (shown in Figure 1) was developed intensively over the years of the NASP program.
The original engine design from the NASP program, while significantly modified by NASA, was used as the foundation for power plant of the successful X-43A vehicle that flew at Mach 7 (5,000 miles/hour) in March 2004 3 as part of the Hyper-X program. The data collected during the flight of X-43A (Figure 2) is an important step in the validation of hypersonic air-breathing vehicle and engine design methods.
The United States Government has been furthering the development of hydrogen and hydrocarbon scramjets. The U.S. Air Force/NASA and Pratt & Whitney ground tested the first uncooled hydrocarbon-fueled scramjet engine at simulated flight Mach numbers of 4.5 and 6.5, as reported in Aviation Week & Space Technology/March 20014. Further development of this engine led to the ground demonstration of liquid JP7 hydrocarbon-fueled scramjet constructed from flight-weight (nickel-based alloys) fuel-cooled structures with the potential for satisfying requirements of future operational engines capable of powering missiles, aircraft, and access to space vehicles at sustained hypersonic speeds, as reported in Aviation Week & Space Technology/June 20035. This program was marked by the first successful test of a flight-weight scramjet operating on storable JP-7 fuel. The Defense Advanced Research Projects (DARPA)/U.S. Navy and Boeing/Aerojet/JHU have also ground demonstrated a JP10 hydrocarbon-fueled dual combustion ramjet, which was constructed from non-flight weight materials (primarily nickel alloys) and intended exclusively for hypersonic missiles, as reported in Aviation Week & Space Technology/September 20036.
What is a SCRAMJET?
A scramjet propulsion system is a hypersonic air breathing engine in which heat addition, due to combustion of fuel and air, occurs in the flow that is supersonic relative to the engine. In a conventional ramjet, engine the incoming supersonic airflow is decelerated to subsonic speeds by means of a multi-shock intake system and diffusion process. Fuel is added to the subsonic airflow, the mixture combusts and then re-accelerates through a 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. Modern scramjet engines are able to seamlessly make the transition between ramjet and scramjet operation.
Why supersonic combustion?
As flight Mach numbers increase beyond Mach 5, the use of supersonic combustion can provides higher performance (i.e. specific impulse) due to inlet efficiency offset by higher Rayleigh losses associated with combustion (Figure 3). Crossover points between ramjet and scramjet operation indicate the benefits of operating in ramjet until Mach 5-6. The process of decelerating airflow at flight Mach 6 to subsonic speeds for combustion results in near-stagnation conditions, with attendant high pressures and heat transfer rates. The engine structural integrity dictates supersonic combustion past Mach 6. Somewhere between Mach 5 and 6, the combination of these factors indicates a switch to scramjet operation. The physics beyond Mach 8 dictates supersonic combustion.
The description of geometrical configuration and design consideration are the most important requirements for understanding the aerophysics of hypersonic air-breathing engines. The most closely integrated engine/vehicle integration is observed in the case of a propulsion system with a scramjet engine. The scramjet engine occupies the entire lower surface of the vehicle body. Scramjet propulsion system consists of five major engine and two vehicle components: internal inlet, isolator, combustor, internal nozzle and the fuel supply subsystem. The vehicle forebody is an essential part of the air induction system while the vehicle aftbody is a critical part of the nozzle component. These are described schematically in Figure 4.
The primary purpose of the high-speed air induction system, comprised of the vehicle forebody and internal inlet, is to capture and compress air for processing by the remaining components of the engine. In a conventional jet engine, the inlet works in combination with the mechanical compressor to provide the necessary high pressure for the entire engine. For vehicles flying at high supersonic or hypersonic speeds, adequate compression can be achieved without a mechanical compressor. The forebody provides the initial external compression and contributes to the drag and moments of the vehicle. The internal inlet compression provides the final compression of the propulsion cycle. The forebody along with the internal inlet is designed to provide the required mass capture and aerodynamic contraction ratio at maximum inlet efficiency. The air in the captured stream tube undergoes a reduction in Mach number with an attendant increase in pressure and temperature as it passes through the system of shock waves in the forebody and internal inlet. It typically contains non-uniformities, due to oblique reflecting shock waves, which can influence the combustion process. Scramjet air induction phenomena include vehicle bow shock and isentropic turning Mach waves, shock-boundary layer interaction, non-uniform flow conditions, and three-dimensional effects.
Fuel choice, between hydrocarbon and hydrogen, is typically driven by heat-sink requirements and vehicle system-level requirements (Figure 5). Missiles and short-range aircraft may use hydrocarbon fuels for their storability and volumetric energy density. Long cruise range aircraft or space access systems tend toward hydrogen because it has superior energy release per pound of fuel, and heat absorption capability, critical to actively cooled structures exposed to scramjet environment.
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11-02-2012, 10:56 AM
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