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 projectile etc.
1. INTRODUCTION
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.
2. RAMJET
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.
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
3. SCRAMJET
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 projections place the top speed of a scramjet between Mach 12 and Mach 24, which is near orbital velocity
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 hypersonicflight 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 torockets, 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.
7. APPLICATIONS
CIVIL APPLICATIONS
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.
MILITARY APPLICATIONS
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 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.
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.
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:
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
HYSHOT
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. [1]. 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 projects by pointing out that the American vehicle has an engine fully incorporated into an airframe with a full complement of flight control surfaces available.
HYPER-X
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 projectile equipped with a hydrocarbon-powered scramjet engine from a large gun. On July 26, 2001, the four inch (100 mm) wide projectile covered a distance of 260 feet (79 m) in 30 milliseconds (roughly 5,900 mph or 9,500 km/h). The projectile 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
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.
12. CONCLUSION
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.
13. BIBLIOGRAPHY
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
4. http://www.wikipedia.org
5. http://www.scramjet.com
6. http://www.nasa.com
7. http://www.aerospaceweb.org
