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Computer drawing of Brayton cycle with T-s plot.

To move a hypersonic cruising aircraft through the air, we have to use some kind of propulsion system to generate thrust. Because of the heat generated at stagnation points in a hypersonic flow, the gas turbine engine is not suited for this regime. A better choice for an air-breathing propulsion system would be a ramjet for Mach numbers less than 6, and a scramjet for Mach numbers greater than 6. Ramjets and scramjets rely on the forward speed of the vehicle to compress the air in the inlet instead of using the mechanical compressor of a gas turbine. The combustion section of a ramjet is similar to the gas turbine, but the ramjet needs no power turbine since there is no compressor. The thermodynamics of of a ramjet/scramjet and a turbine engine are quite similar.

To understand how a ramjet works, we must study the basic thermodynamics of gases. Gases have various properties that we can observe with our senses, including the gas pressure p, temperature T, mass, and volume V that contains the gas. Careful, scientific observation has determined that these variables are related to one another, and the values of these properties determine the state of the gas. A thermodynamic process, such as heating or compressing the gas, changes the values of the state variables in a manner which is described by the laws of thermodynamics. The work done by a gas and the heat transferred to a gas depend on the beginning and ending states of the gas and on the process used to change the state. It is possible to perform a series of processes, in which the state is changed during each process, but the gas eventually returns to its original state. Such a series of processes is called a cycle and forms the basis for understanding engine operation.

On this page we discuss the Brayton Thermodynamic Cycle which is used in ramjets and scramjets. The figure shows a T-s diagram of the Brayton cycle. Using the turbine engine station numbering system, we begin with free stream conditions at station 0. In cruising flight, the inlet slows the air stream to compress it to station 2 conditions. As the flow slows, some of the energy associated with the aircraft velocity increases the static pressure of the air and the flow is compressed. Ideally, the compression is isentropic and the static temperature is also increased as shown by the dashed lines on the plot. For an ideal, isentropic compression a vertical line on the T-s diagram describes the process. In reality, the compression is not isentropic and the compression process line leans to the right because of the increase in entropy of the flow. The non-isentropic effects are the result of shock waves in the inlet. For the ramjet, there is a terminal normal shock in the inlet that brings the flow to subsonic conditions at the burner. As speed increases, the losses through this shock eventually decrease the level of pressure that can be achieved in the burner, and this sets a limit on the use of ramjets. For supersonic combustion ramjets (scramjets) there is no normal shock and the inlet shock losses associated with the normal shock are avoided. The combustion process in the burner occurs at constant pressure from station 3 to station 5. The temperature increase depends on the type of fuel used and the fuel-air ratio. For scramjets, there may be additional entropy losses associated with the mixing of the fuel and the air. Following combustion, the hot exhaust is then passed through the nozzle. Ideally, the nozzle brings the flow isentropically back to free stream pressure from station 5 to station 8. Since ramjets and scramjets often use converging-diverging nozzle designs, there is often a mismatch between the external flow pressure and the free stream. The area under the T-s diagram is proportional to the useful work and thrust generated by the engine.


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Editor: Tom Benson
NASA Official: Tom Benson
Last Updated: May 07 2021

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