SCRAMJET - Engines for Planes and Rockets - Article Example

SCRAMJET Introduction Planes and rockets are a demonstration of the aspirations of humans to conquer the vast expanses of the atmosphere around earth and the space of the universe that we live in. A true beginning to these aspirations started with the small step by the Wright Brothers almost a century of ago with the demonstration of manned flight. The subsequent period in the twentieth century has seen giant leaps being taken in manned flights of airplanes commercial and passenger spanning wide distances across the globe with ever increasing frequency. By the middle of the twentieth century airspace domination saw a new entry in the term of rockets, which have enabled man to transcend into space and reach other orbital bodies like the moon and the planets of our solar system. More rocket flights occur now then ever and as science and technology have developed, so has the sophistication in propulsion technology. New sophistication in propulsion technology now allows mankind to dream of traveling around the earth at rapid speeds, and also addressing the growing concern of atmosphere pollution that occur from the burning of fuels that the many plane flights and rocket flights that happen around the globe. The new development in propulsion technology that raises the possibility of rapid travel in space without polluting the environment is the Scramjet. A scramjet is the short term used for Supersonic Combustion RAMjet (Scramjet). In simple terms a scramjet uses the oxygen in the air to burn hydrogen as a fuel to provide it the thrust, with an exhaust that consists of water. History of Jets The early airplanes from the first flight onwards were powered by engines that were gas powered reciprocating internal-combustion engines that drove a propeller, which was used to propel the aircraft on the ground and into the air. The first jet engine in the form of the turbo jet engine was developed in 1930, but the jet engine was to take several years more before it became commercially useful as a propulsion system for airplanes (History of Early Jet Engines). Figure – 1 A Jet Engine Propelled Aircraft (Bellis, 2009) Dr. Hans von Ohain and Sir Frank Whittle working separately are credited with the invention of the jet engine. By 1939 the first airplane using a turbojet engine for propulsion took to the air, while the first jet to fly using the jet engine developed by Frank Whittle took to the skies in 1941. That was the start of jet airplanes and in few decades time jet airplanes were to dominate the skies (Bellis, 2009). Different Kinds of Jets Figure – 2 How Air Flows Through a Jet Engine (NASA, 2009) Figure – 3 Parts of a Jet Engine (NASA, 2009). Over time basically four kinds of turbine jet engines have found a place in propelling airplanes. These are turbojet, turbofan, turboprop and ramjet engines. The propulsive force in the turbojet engine and the turbofan jet engine is a consequence of the reaction forces generated by the exhaust gas. Turbofan engines are normally classified on the basis of the amount of mass airflow that bypasses the main engine typically into high-bypass and low-bypass engines. Whereas in the turboprop jet engine the energy of the hot exhaust gas is used to drive a separate turbine, the power from which is employed to provide shaft power to rotate the propeller. The exhaust gas thus emanating from the nozzle is bereft of most of its energy, and the remaining small portion of energy is used to augment the thrust provided by the propeller. In the turboshaft jet engine however, all the usable energy of the gas is extracted and employed for the purpose of conversion into shaft power in a an additional free turbine, which provides the propulsive force derived from the revolutions of fan blades (Hunecke, 2003). The Ramjet has been a subsequent development to the turbine jet engines and is a very simple jet engine that has no moving parts and can be considered as a turbojet in which the rotating parts of the engine are non-existent. The speed generated in the jet rams or forces air into the engine. This means that for the ramjet to start functioning it has to be assisted to attain the required speed, which is above the speed of sound (NASA, 2009). The Turbojet Engine Figure – 4 Picture of a Turbojet Engine (The Internet Encyclopedia of Science, 2009). The turbojet engine comprises of an air intake inlet, a multi-stage compressor, a combustion chamber, a single stage or multi-stage turbine, and exhaust nozzle. During the working of the turbojet engine air gets sucked via the air intake inlet, which ensures the delivery of a continuous and uniform flow of air into the compressor. The compressor is fast rotating air pump that is responsible for raising the pressure. The resultant energy transfer to the air causes not only an increase in the pressure of air, but also the temperature and density of the air. This air is discharged into the combustion chamber, where the jet engine fuel in injected and burnt, which causes a large transfer of energy to the air, by the chemical reaction, involved in the burning of the fuel. At this stage the pressure of the air remains constant, but there is a drastic increase in the temperature of the air. The air is thus prepared to produce work efficiently in a gas turbine (Hunecke, 2003). The first place in the turbojet engine where energy from the prepared air is absorbed is the gas turbine. The gas turbine can be considered as a complimentary part of the compressor. To the gas turbine a hollow shaft or spool is connected rigidly. The function of the gas turbine is the conversion of the energy absorbed from the air into mechanical energy for driving the compressor and other engine accessories. The air that passes out of the gas turbine has not been depleted of its energy. The three components of the turbojet engine comprising the compressor, the combustion chamber, and the turbine have in combination made the air ready to for the function of providing thrust. A very large part of the heat and pressure energy is available for conversion into kinetic energy. This is the function carried out by the exhaust nozzle of the turbojet engine. The exhaust nozzle is tube like in shape and converts the heat and pressure energy into velocity. A pre-requisite for the generation of thrust by the turbojet engine is high exhaust velocity. An example of the turbojet engine is the General Electric J79, which was used to power the F-104 and F-4 combat aircraft of the USAF in the 1960s (Hunecke, 2003). Figure – 5 Image of the General Electric J79 Turbojet Engine (Absolute Astronomy, 2009) The GE J79 Turbojet Engine is a very huge turbojet engine, which was designed with the purpose of delivering high thrust in comparison to its weight. The length of the engine was seventeen feet and it was three feet in diameter. It incorporates a single shaft with a high pressure compressor stages with variable-incidence stator blades. The moving parts of the engine have been so designed that their movement can be controlled by the fuel supplied to the engine. This single shaft turbojet with a 17-stage compressor has been so designed by the unique arrangement of the variable stator blades to cause the engine to generate pressure similar to a twin-shaft turbojet engine, thereby making it a more efficient turbojet engine with a lower weight. The variable stator blades are mostly made of titanium. It was first time that titanium was used in large turbojet engines. The Figure – 6 Close View of the General Electric J79 Turbojet Engine (Cegelski, 2007) specifications of the GE J79 Turbojet Engine read as turbojet, compressor – 17-stage axial with variable stators, turbine – 3-stage, mass flow – 170lbs/sec, power ration – 13 (Cegelski, 2007). The Turboprop Engine Figure – 7 Turboprop Engine (Toffs World, 2008). In essence the design of a turboprop engine shows similarity to that of a turbojet engine, with a few differences. These differences are the presence of an additional turbine for driving the propeller, a two-spool design of the rotational machinery, and the presence of a mechanical reduction gear for altering the high rotational speed of the turbine in keeping with the moderate speed of the propeller. The core component in the turboprop engine is the gas generator, which consists of the compressor, combustion chamber, and the turbine sections. It is the addition of the inlet and exhaust nozzle that makes it similar to the turbojet. The design of the turboprop engine causes more energy to be absorbed from the hot gases than is required to for driving the compressor and the auxiliaries, so that the excess power generated in the shaft can be used to drive a propeller (Hunecke, 2003). The design of a turbojet engine is such that it accelerates a relatively low air mass flow to a high exhaust velocity, but the turboprop engine is designed conversely to accelerate a relatively high air mass flow to a low velocity. The consequence of this is that the turboprop engine is vastly superior to the turbojet engine in fuel efficiency, which however is at the cost of reduction in flight speed and enhanced cabin noise. The fuel efficiency advantage of the turboprop engine has resulted in its use for cargo aircraft and for passenger aircraft developed with fuel efficiency in mind. An example of the turboprop engine is the General Motors Allison T56 turboprop engine, which is used in the Lockheed C-130 Hercules military transport aircraft (Hunecke, 2003). Figure – 8 Image of the Allison T56 Turboprop Engine (Defense Industry Daily, 2007) The Allison T56 engine uses diesel as a fuel and consists of a gas turbine engine drives a propeller through the means of s reduction gear box. The engine possesses a 14 stage axial flow compressor that has a pressure ration of 9.5 to 1 that enables it to deliver air to a can-annular combustion system. Subsequent to combustion in the combustion chamber the gases pass through a 4 stage axial flow turbine. The engine has been so designed as to operate at an unchanging speed of 13,829 RPM that can be controlled through the use of the variable pitch propeller. As a result the airflow and pressure through the engine remains constant for any altitude and speed. The combustion system consists of inner and outer casings, which are meant to provide the gas path connection between the compressor and turbine, as well as the mechanical connection, six separate combustion liners that act as controllers for the airflow distribution and aircraft pattern essential to stable combustion requirements, and six dual orifice atomizers. Fuel enters the combustion chamber via the six dual orifice atomizers axially to the liner via the inlet. The fuel and air get mixed and combustion occurs at different degrees at each of the three combustion zones. Subsequent to the combustion in the combustor the gases pass into the four stage axial flow turbine through the outlet section of the combustor, causing the turbine to rotate at high speeds, which is transferred to the propeller, through a set of reduction gears (Gas Turbine Engines). Turbofan Engines Figure – 9 Image of a Turbofan Engine (Types of Air-breathing Engines) The turbojet engine operates with propulsive efficiency at the speed of sound or above it, while the turboprop engines demonstrate propulsive efficiency below the speed of sound, leaving a gap in efficient propulsive jet engines at high sub-sonic cruise speeds of 0.8 Mach or 1000 km/h at altitudes of around 11,000 ft. The turbofan jet engine was developed to as a propulsive efficient jet engine to fill this gap between the turbojet engine and the turboprop jet engine. It displays similarity to the turboprop engine in that the design of the turbine section, where more energy is absorbed from the hot gas than is required for driving the compressor. This excess power in the shaft is used for driving a fan, a low-pressure compressor that is positioned upstream of the main compressor, A part of the air that enters the engine intake after processing by the fan section is made to bypass the inner or core part of the engine and expands in a separate outer nozzle, which provides cold thrust in addition to the normal thrust. In some kinds of turbofan jet engines the cold air and hot exhaust gas from the core engine is mixed, which further increases the propulsive efficiency of the engine (Hunecke, 2003). By employing bypass in the design, turbofan jet engines are very fuel efficient for the thrust that they deliver, and in modern times, when there is a stress on fuel efficiency, the turbofan jet engine is more commonly used in aircraft. In a turbofan jet engine the quantum of air that bypasses the core engine in relation to the quantum of air that passes through the core engine is called the bypass ration. This has led to the differentiation and development of two kinds of turbofan jet engines, namely low by-pass ratio engines and high-bypass ratio engines. A by pass ratio ranging from 0.2:1 to 1:1 makes the turbofan engine a low by-pass jet engine, while a bypass ratio of about 5:1 makes the turbofan engine a high bypass jet engine. Low by-pass turbofan jet engines are used mainly for supersonic military combat aircraft, while high-bypass turbofan jet engines are used form high-subsonic military aircraft and commercial transport aircraft. An example of low by-pass turbofan jet engine is the Pratt & Whitney TF33 (Hunecke, 2003). Figure – 10 Image of a TF33 mounted on a Boeing VC-137B Commercial Aircraft (Pratt & Whitney JT3D) Ramjet Engine The principles under which a ramjet engine functions are quite similar to those used in a liquid-rocket engine. There is one basic difference in that the ramjet uses oxygen from the air it breathes in, and hence it falls under the category of air-breathing engines. The most important component in a ramjet is the combustion chamber or the burner. In the combustion chamber the air and the combustible fuel are mixed and ignited. On ignition the mixture of gases forms an incandescent gas that becomes a jet stream through the constant replenishment of air and the combustible fuel and generates exhaust velocity. In its simple design a ramjet engine is made up of a streamlined body that has an exterior opening, through which air is forced into the body of the engine. For the engine to function a pressure higher than atmospheric pressure is necessary. This status is achieved in the design, whereby the cross sectional area of the air intake exterior opening is much smaller than the outlet of the nozzle to the exterior. The velocity of the air entering the body reduces due the expansion, which in turn leads to an increase in pressure of air in the combustion chamber. Figure – 11 Image of a Ramjet Engine (Types of Air-breathing Engines) The more the ramjet engine is accelerated through the air the greater is the pressure build up in the combustion chamber and the increased efficiency of the engine to generate through exhaust velocity with the ignition of the mixture of air and combustible fuel in the combustion chamber. The optimum speed at which a ramjet engine becomes efficient is 2000 km/h. The deficiency of the ramjet engine lies in this aspect of its functioning, whereby they can operate only in dense layers of atmosphere and do not develop enough thrust on their own, when they are static or during low flight speeds. This makes it necessary for an initial take-off velocity of 700 km/h, before a ramjet engine can generate sufficient thrust on its own (Twigge, 1993). Figure -12 Image of the Pratt & Whitney Rocketdyne’s PWR-9221FJ dual-mode ramjet engine. (Unbuilt Projects & Aviation Technology) The ramjet engine has yet to find active use in commercial aircraft, but the engine has been tested for military purposes. One such engine is the Pratt & Whitney Rocketdyne’s PWR-9221FJ dual-mode ramjet engine. The Pratt & Whitney Rocketdyne’s PWR-9221FJ dual-mode ramjet engine has been successfully tested on the ground to delver a flight speed of up to Mach 4, though it is yet to be tested in the air. However, the successful ground test of this dual mode ramjet engine that is integrated with a variable geometry inlet and exhaust nozzle raises the possibility of using turbine-based combined-cycle propulsion to deliver high flight speeds. This dual-mode ramjet engine has been designed to function as a ramjet to attain moderate supersonic flight speeds of up to Mach 5 and as a scramjet to attain higher flight speeds of up to Mach 6. This broad range in functionality in the design of the engine reflects an attempt to employ it on a vehicle that will be capable of taking off and landing from a conventional runway (Colagouri, 2009). Scramjet Engine Figure – 13 Image of a Scramjet Engine (Scramjet Engine) A major problem that has plagued the attempts of greater air and space flight domination by humans has been the limitations in jet engines and rocket propelled ships. With jet engines it is possible to achieve flight speeds of up to Mach 3. At speeds beyond Mach 3 the heat generated in the jet engine is so high that the blades of the turbine start to melt. Rocket ships do not have this problem and can achieve high flight speeds of up to Mach 25, but have to carry vast amounts of fuel and liquid oxygen for this purpose and so carry more fuel and liquid oxygen than a payload making demonstrating work inefficiency. The air breathing ramjets and scramjets without moving parts and the need for high large quantum of fuel and oxygen do not suffer from these deficiencies and is the reason for their considered importance in the effort to reach high flight speeds efficiently (Belfiore, 2009). Figure – 14 Simple Diagram of a Scramjet Operation (Working of a Scramjet Engine) A scramjet derives its name from it being a Supersonic Combustion Ramjet. In simple terms it a scramjet may be considered as a high speed scoop that picks up air and forces it into the combustion chamber, where the oxygen from the air is used to burn the fuel, which is usually water to generate thrust through high exhaust velocities. The exhaust gas is essentially water. The balancing act here is in the design of a scoop that has a low drag in comparison to the thrust generated by the engine to enable high flight velocities (Macinnis, 2002). This part in the design of a scramjet is no different from the ramjet engine. The difference in the design of the scramjet engine is that it overcomes the speed limitation experienced with a ramjet engine. The air entering the ramjet engine has to be slowed down to subsonic speeds for the engine to run efficiently. In the slowing down of air, heat is generated and irrespective of the measures employed to cool the air a ramjet cannot go beyond a flight speed of Mach 5, as the cooling measures will be overcome and the engine will overheat and disintegrate. The scramjet engine is able to overcome this difficulty by doing away with the diffuser present in the ramjet engine to slow down the air flowing into the engine. The design however calls for an engineering feat in maintaining the ignition in the combustion system for the airspeed will be so high that it resembles keeping a match lit in a hurricane (Macinnis, 2002). There are three key features of the scramjet engine. The first is that it mixes fuel with high compresses air taken from the atmosphere to achieve near rocket-like power in a much lighter craft than a rocket. The second key feature is that for a scramjet engine to start functioning it has to be propelled to a speed of Mach 5 by a rocket or another ramjet, which is the disadvantage with a scramjet engine. The scram jet engine is capable of providing the thrust to generate high flight speeds. Tests conducted by NASA in 2004 found the scramjet engine achieving flight speeds of Mach 9.6 in 10 seconds after boosted by a rocket (Macinnis, 2002). The history of the scramjet engine moving out from the laboratories to actual flight begins in 2002, when researchers involved in the HyShot program at the University of Queensland‘s Center for Hypersonics created history by actually using a scramjet engine to power a craft in flight. A small scramjet engine was attached to a rocket, which carried it to height of 200 miles and dropped it. At a height of 20 miles the scram jet engine fired and reached a flight speed of 7.6 Mach before reaching the ground. Subsequently, research into the scramjet has been actively participated in by NASA, the American Air Force and Navy, and DARPA (Macinnis, 2002). In early 2010 an experimental flight craft X51 Wave rider powered by the Pratt & Whitney Rocketdyne’s scramjet engine SJY61-2 is to be tested, and prove the possibility of extended duration hydrocarbon-fueled hypersonic flight (Colagouri, 2009). Successful testing of scram jet engines will see scram jet engines becoming more commonly used, just as jet engines replaced gas powered reciprocating internal-combustion engines that drove a propeller. The scramjet engines would dominate the field of hypersonic flight. Jet engines would be used to power these craft to speeds between Mach 2 and Mach 3, from whence the scramjet engines would kick in and provide the thrust to achieve flight speeds of Mach 6 and over. It will not be long before the flight across the Pacific Ocean to Tokyo would take about two hours (Macinnis, 2002). Table – 1 Comparison of Jet Engines Type of Jet Engine Advantages Disadvantages Turbojet Engine Simple in design and capable and efficient at supersonic flight speeds The basic design is deficient for efficiency and power for subsonic flights, and is relatively noisy. Large diameter of the engine and the requirement of heavy blades. Limitation in top speed, as beyond Mach 3 there is the potential risk for damage to the engine from heat and shock waves. Turboprop Engine High fuel efficiency at lower subsonic speeds and high shaft power in relation to weight of the engine. Limitation in achieving high flight speed. It is also noisy and complex in design of transmission. Turbofan Engine The most commonly used jet engine in modern times, with features of thrust/weight ratio over 100, simple air inlet design, high compression ratio, and good cost to thrust ratio. Operates more quietly than other jet engines due to the larger mass flow and lower total exhaust speed. For this reason it is more efficient than other jet engines in a range of subsonic airspeeds, and has a cooler exhaust temperature. Limitation in achieving supersonic flight speeds. Ramjet Engine Lightest of the jet engines with very few moving parts. Capable of hypersonic flight speeds of up to Mach 5. It is efficient at these high speeds and has a thrust to weight ratio that goes up to 30 at optimum flight speed levels. Requires high initial flight speed to start functioning, and therefore requires to be boosted to the required flight speeds by external support. It is inefficient at slow flight speeds, because of poor compression ratio, difficulty in arranging shaft power for accessories, air intake flow must be slowed down to subsonic speeds to prevent overheating of the engine. It is difficult to maintain and test. Scramjet Engine Offers the same benefits as the ramjet, with the added benefits of no reduction in speed of air intake flow in the engine, no over heating, and efficient at hypersonic speeds beyond Mach 5 Have the same disadvantages as the ramjet engine, with the exception of the need to slow down air intake flow. (Cambridge Encyclopedia, 2009). Orbital Vehicles Orbital vehicles are in essence the true space vehicles that travel away from the earth into space to reach altitudes of 161 to 321 kilometers and orbit the earth. Orbital vehicles like the Space Shuttle have been designed to remain in space for about two weeks, while International Space Station is designed to remain in space for years. The Space Shuttle system can be reused and has three engines winged spacecraft that is attached to two solid rocket boosters, with a large external tank. The two solid rocket boosters provide the the main thrust for liftung the Space Shuttle into orbit and are shed once their boosting power is no longer required. The external tank supplies the fuel and osidiser to the main engines of the Space Shuttle during launch and the external tank is also shed. The Space Shuttle attains a flight speed of around 17,000 mph. On completion of the space mission the Space Shuttle enters earth’s atmosphere on its own power. It lands on a normal conventional runway, deploying parchutes to aid in the deceleration (Anderson & Piven, 2005). Figure – 15 Image of the Space Shuttle (NASA, 2009) The history of orbital vehicles starts with the Sputnik 1, which was launched into orbit on October 4, 1957. All orbital vehicles need to be carried outside of earhs gravitational pull. This means overcoming the force of gravity, which is accomplished with the help of rockets used as launch vehicles, which are connected to the orbital vehicle and are shed after their boosting function is complete. A fundamental requirement in the design of these launch vehicles is that they must be cpable of acceleration from 0 mph at the launch pad to the required orbital velocity of approximately 17,000 mph within the space of a few hundred miles to overcome earth’s gravitational force. In addition the launch vehicle will need to carry all its fuel requirements for the generation of the energy required for this sustained thrust. This is a disadvantage, as it adds considerable weight to the launch vehicle, which makes for additional energy requirements or added fuel weight. This advantage can be reduced, if the oxidiser weight can be reduced through the use of an engine that cab breathe in air and use the oxygen present in the air. The current launch vehicles are rocket engines, which do not have this capability. The scramjet engine has this advantage and is considered to be capable of providing the required boost for orbital vehicles and reducing the weight of the fuel to be carried. The scramjet offers potentially twin benefits as a launch vehicle. The first is reduced costs, and the second is no damage to the environment as the exhaust gas is essentially water (Miller, 2004). Figure – 16 Image of Scramjet being Developed at NASA (Scramjets) There are disadvantages involved in the possible use of scramjet engines in launch vehicles for orbital vehicles. The scramjet engine has a lower thrust in comparison to the rocket engine. This means that expensive, bulky and failure prone high performance present in the conventional liquid-fueled rockets will be necessary boosting cost and weight. Projections of scram jet engine acceleration to orbital velocity suggest a ceiling of 17 Mach, which is insufficient compared to the required orbital velocity of 25 Mach. Yet, these deficiencies do not rukle out the scramjet becoming a launch vehiclwe for it has the potential of being developed into reusable launch vehicle in the same manner as the orbital vehicle and there providing large cost benefits. Other areas of furture use that the ramjet has potential for are military aircraft, passenger aircraft and missiles including cruise missiles (Cambridge Encyclopedia, 2009). In spite of the limitation that the upper limits of a scramjet engine still remain undetermined, the possibility of its future use as the booster for orbital vehicles cannot be ruled out, as current advances have led to the belief that it is capable of reaching speeds between 20 Mach and 25 Mach, which is adequate for launching an orbital vehicle. Another advantage of the scramjet engine is the high operational ceiling of 75 kilometers without fear of stalling, whereas the advanced turbojets have a ceiling of 40 kilometers and ramjets a ceiling of 55 kilometers. Figure – 17 Image of a Possible Scramjet Space Plane (Scramjets) There is more likelihood that a hybrid scramjet/rocket will be the driving force behind space planes. Scramjets can be integrated into the housing of a rocket, or it may become possible to integrate a rocket into the scramjet combustor, for the purpose of this combined cycle engine. In this configuration the rocket will remove the disadvantage of the scram jet and provide the thrust at takeoff, subsonic and low hypersonic flight. The scramjet takes over at high hypersonic flight to boost the vehicle to orbital velocity, propelling it into space and for maneuvering the vehicle in space and on re-entry (Scramjets). Literary References Absolute Astronomy. (2009). General Electric J79. Retrieved December 7, 2009, Web Site: http://www.absoluteastronomy.com/topics/General_Electric_J79 Anderson, E. & Piven, J. (2005). The Space Tourist’s Handbook. Philadelphia: Quirk Books. Belfiore, M. (2009). The Hypersonic Age is Near. Popular Science, 272(1), 36-41. Bellis, M. (2009). Jet Engines – Hans von Ohain and Sir Frank Whittle. Retrieved December 7, 2009, from, About.com Web Site: http://inventors.about.com/library/inventors/bljetengine.htm Cambridge Encyclopedia. (2009). Jet engine - History, Types, Type comparison, Turbojet engines, Turbofan engines, Major components, Advanced designs, Trivia. Cambridge Encyclopedia, Vol. 39. Retrieved December 7, 2009, from, State University.com Web Site: http://encyclopedia.stateuniversity.com/pages/11537/jet-engine.html Cambridge Encyclopedia. (2009). scramjet - History, Simple description, Theory, Advantages and disadvantages of scramjets, Advantages and disadvantages for orbital vehicles, Applications. 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