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RAMJET AND SCRAMJET DEVELOPMENTS

Today's launch systems all have a common problem: a multiple in fuel has to be carried for every kg of payload to guarantee that the payload can reach an orbital speed. For example: 85 per cent of the almost 750t launch mass of Ariane 5 are made up of fuel for the solid fuel boosters and the main stage, which is run on liquid hydrogen and oxygen. Every kilogramm of tank capacity, which can be saved, increases the carrier's performance.

It would be ideal, if the atmosphere's available oxygen could be used on the flight through the atmosphere. Conventional turbojet engines, which are nowadays used in most commercial aircraft and military jets, cannot be used at flying speeds of about Mach 3.

This is where the so-called ramjet engines come into their own. These can be used far into the hypersonic area, i.e. at speeds of Mach 5 and over. As well as in space transportation it is also feasible to use these engines in future high-speed transport aircraft. They combine a greater range with the lowest possible weight and a more compact construction than systems currently in use.

Ramjet engines have been researched since the 50s. In 1957 the French experimental aircraft Nord Griffon II flew for the first time with a combined engine, which consisted of a conventional turbojet and a ramjet engine. By the end of this research program this aircraft completed about 200 flights and reached a new speed record with a top speed of Mach 2.19. Ramjet engines have also been used in the past in air-to-ground and ground-to-air rocket weapons. Here ramjets and rocket engines were combined. Ramjet engines are nowadays the target of intensive research again, only because of technological advances in new heat resistant materials and because of pressure to develop high performance and more cost efficient space transport systems.

A ramjet engine is an engine system which does not have any moving parts. At high flying speeds air in the intakes is compressed so much just by the forward movement that a compressor, which is needed in turbojet engines, is not necessary. This is the main difference between Ramjets and conventional turbojet engines, which are basically made up of five components: intake, compressor, combustion chamber, turbine and nozzle.

In conventional jet engines thrust is created in three stages: Intake (through flight build up) and compressor provide a building up of pressure, in the combustion chamber air is enriched with energy as fuel is burnt. In the turbine and the nozzle the air expands, while the inner energy of the gas is changed into kinetic energy and thrust.

As flight speeds increase, the quality of the engine process deteriorates. This can be demonstrated with the help of fuel specific impulses. This thermodynamic quantity describes the created pressure per fuel mass. This value decreases with increasing speed. In other words: At higher Mach numbers the fuel consumption increases much more than thrust can bei generated. Above Mach 3 the fuel specific impulse of a ramjet engine is better than that of a turbojet engine. The compressor is the main reason for this.

This turbojet component, which has several stages, rotors and stators causes losses. Furthermore turning parts wheels do not contribute to engine processes at high Mach speeds. With the flight speed the pressure, created in the air intake through flight build up rises considerably. The share the compressor contributes to the entire compression sinks accordingly: At Mach 1 the value is about 50 percent, at Mach 2 just 15 percent and at Mach 3 less than four percent.

From about three times the speed of sound the compression created by the speed is enough to keep the engine process going. The compressor is really not needed at higher speeds. Additionally the rise in temperature caused by the build up is considerable. The build up temperature at Mach 8 - depending on the altitude - is between 3,000 and 4,000 degrees Kelvin (between 2,727 and 3,727 *C), at Mach 12 about 8,000 degrees Kelvin.

Conventional compressors cannot be used at such high temperatures, because the compressor blades cannot be cooled and materials, which are able to withstand these temperatures, do not exist. The logical consequence: You omit the compressors. This will also make the turbine superfluous, which sole purpose is to drive the compressor. This is how the ramjet engine is a far simpler construction, consisting only of intake, combustion chamber and nozzle.

In conventional turbot engines kerosene is burnt at relatively low airflow speeds of about Mach 0.2. This enables a good mixing of air and injected fuel and causes a highly effective degree of combustion. One is also attempting to utilise this advantage in ramjet engines, especially a lot more experience has been gained in the past at combustions speeds below the speed of sound.

Reducing the flow speed in the engine at flying speeds of Mach 3 or 4 to a subsonic combustion can be achieved without problems. It will, however, become difficult, when the flying speed is to be increased further. The high airstream speeds have to be reduced to moderate combustion chamber speeds in the intake diffuser. This causes losses. These losses increase with higher airspeed. The result ist that the quality of the entire engine process suffers and thrust decreases.

From a flying speed of Mach 6 combustion with supersonic flow makes , since a higher specific impulse is experienced above these speeds. Even if supersonic combustion on its own is less effective, it produces fewer losses at the intake than subsonic combustion. While the engines with subsonic combustion are called Ramjet, those with supersonic combustion are Scramjets (Supersonic Combustion Ramjets). Scramjets open the speed regime up to Mach 20.

There are consequences for the construction of the engine for the different way of combustion of ram and scramjets. In the intake diffuser of the ramjet the flow speed has to be at subsonic speed. A subsonic diffuser follows - mainly in the flow canal - with an widening cross section in which pressure is built up and speed is further lowered.

The nozzle of a ramjet has to be designed as a so-called laval nozzle, which makes it possible to accelerate the flow to supersonic speed when leaving the combustor. Laval nozzles consist of a converging part, in which the subsonic flow is accelerated to sonic speed at the exit of the combustion chamber (Mach 1). In the nozzle's diverging section the sonic flow can then expand and accelerate further. By comparison the nozzle of a scramjet, in which the flow is already at supersonic speed, when it exits the combustion chamber, is simply diverging, its cross section increases.

Since the flow speed sinks and the pressure rises during combustion at supersonic speeds - this phenomenon is reversed at subsonic speeds - a scramjet is fitted with an additional element, the isolator, between the diffuser and the combustion chamber. The isolator's function is to isolate the combustion chamber before injection. It is supposed to prevent the pressure force, which rises during supersonic combustion, from effecting the diffuser flow via the bordering walls. This could lead to blocking the inlet.

A phenomenon called "shock train" forms within the isolator, consisting of a changing succession of compression shock waves and expansion. It is caused by the interaction of shock waves and the bordering layer of the isolator's wall and causes a (beneficial) further rise in flow pressure.

As in rockets the use of hydrogen is intended instead of kerosene as fuel for ramjet engines. The reason is the much higher energy density of hydrogen. Per kg fuel the threefold amount of energy can be added to the process. The disadvantage of hydrogen, however, is its low density and accordingly bigger tanks.

With the scramjet the mixing of air and fuel is considerably bad because of high speeds at the entrance to the combustion chamber between Mach 2 and Mach 3, at the exist between Mach 1.2 and Mach 1.6. This makes combustion not very effective. For this reason the combustion chamber has to be elongated to guarantee satisfactory mixing. There are several concepts as to how the fuel - gasous hydrogen - can be injected into a supersonic combustion chamber most effectively. One can basically differentiate between (vertical) injection via drillings in the wall and injection systems, which are placed into the flow. With the latter injection basically takes place parallel to the airflow. If the flow speeds of air and fuel are different, as a rule a turbulent shearing layer forms, which accelerates the mixture. One attempts to increase the turbulence and the resulting mixing with the help of additional vortex. However, turbulence always results in loss of flow, which means that a compromise has to be found.

This is why vertical injection, which causes considerable vortex, has proved to be no more favourable than the parallel one. Mixing close to the injection nozzle is good, although the fuel's depth of penetration into the combustion chamber is low. Furthermore there are shock waves and flow detachments near the injection area, which result in a loss of pressure. The design of the combustion chamber geometry and of fuel injection elements as well as the combustion process are some of the major challenges when developing ramjet/scramjet engines and their applications.

In order to increase the application scale of airbreathing hypersonic engines engineers have developed "dual-mode" concepts for these engines. This means that they can be used in ramjet as well as in the scramjet mode. This way the engine can be adjusted optimally to the respective flying speed. This can either be achieved with the help of a combustion chamber with variable geometry or by injecting fuel via different injection nozzles depending on the flow speed.

However, neither ramjet nor scramjet can operate at speeds below Mach 2 or 3. If the vehicle is to start of its own a combination of the ramjet engine with other engine systems becomes necessary. Since the compressor is lacking in a ramjet engine there is pressure built-up and airflow in the engine when the vehicle is not moving. This means: There is no airbreathing engine that can cover the full operational range from take-off to hypersonic speeds.

But, a hypersonic aircraft cannot just take off at Mach 3. It has to take off and cover the whole speed range. This makes it necessary to accelerate the vehicle by other means until the ramjet/scramjet engine can kick in at around Mach 3.

There are basically two concepts to chose from: With a two-stage design as suggested in the Sänger- Program, the ramjet-powered aircraft is transported by another vehicle to the required altitude and speed. With single-stage vehicles turbojet engine and ramjet are combined in one aircraft. At around Mach 3 one engine is switched off and the other takes over.

For space launch applications developers have high hopes for the so-called Rocket-based Combined Cycle Engines (RBCC). In these an airbreathing ramjet/scramjet engine is combined with a rocket engine. For the launch up to the speed in which a ramjet can be used, the vehicle is propelled by the rocket engine. Up to speeds of Mach 10 to 12 the scramjet engine provides the necessary thrust. Above these speeds the rocket engine sets in to supports the airbreathing engine and provides from Mach 20 all the required thrust necessary for orbit injection.

Before this concept can be applied, some work to research the basics of the scramjet technology has to be carried out. No vehicle with scramjet engine has flown up to now. The first practical use of this application is imminent. The first X-43A of NASA is to take to the air for a test flight. As part of NASA's Hyper-X-Program three of these almost four metre long experimental aircraft with integrated scramjet are being built. They are to reach speeds of up to Mach 10 in flight tests over the next three years.

From page 86 of FLUG REVUE 5/2000