Journal of Aeronautics & Aerospace Engineering
Open Access

Wave Propagation, Shock Waves and Expansion Fans in Scramjet Engine Design

Chiahui Yen^{* *}Correspondence: Chiahui Yen, Department of Aeronautical Engineering, Chaoyang University of Technology, Taichung, Taiwan, Email:

Author info »

About the Study

Scramjet engines are one of the most advanced propulsion technologies, capable of achieving hypersonic flight speeds. Scramjets are unique because they operate at supersonic speeds, unlike usual ramjets or turbojets that rely on subsonic combustion processes. An important aspect of scramjet design is the complex interaction of wave propagation, shock waves and expansion fans, which control how air flows through the engine at hypersonic speeds.

Wave propagation in scramjets

In fluid dynamics, wave propagation is the flow of disturbances like sound waves, pressure waves, or temperature gradients across a medium. In the context of scramjets, air is compressed and accelerated as it enters the inlet at supersonic or hypersonic speeds. This creates various types of waves, particularly shock waves and expansion waves, that influence how the air is processed for combustion.

At hypersonic speeds, air entering the scramjet is traveling faster than the speed of sound in the medium. This means that disturbances in the air cannot propagate upstream, which fundamentally changes the behavior of wave propagation compared to subsonic flows. These supersonic characteristics require special design considerations to manage the compression, combustion, and exhaust efficiently.

Shock waves in scramjet engines

These waves are abrupt discontinuities in pressure, temperature, and density that occur when air is compressed at supersonic speeds. In scramjet engines, shock waves play an important role in compressing the incoming air before it reaches the combustor, where fuel is injected and burned. There are two primary types of shock waves relevant to scramjet design:

Oblique shock waves: These waves form at an angle to the incoming flow and are generally used to compress the air in the inlet of the scramjet. By reflecting off the engine walls, a series of oblique shock waves successively compresses the airflow to the required conditions. Unlike normal shock waves, oblique shocks produce less drag and allow more efficient compression.

Normal shock waves: They form perpendicular to the flow direction and cause an instantaneous, more intense increase in pressure and temperature. In scramjets, normal shocks are less desirable because they lead to greater losses in stagnation pressure and cause more drag. However, they may still occur in certain regions of the flow, such as near the combustor inlet, depending on the design.

Expansion fans in scramjet engines

While shock waves compress air, expansion fans are regions where the air expands, reducing its pressure and temperature. Expansion fans occur when the flow turns away from itself, as in the case of air exiting the combustor or flowing around convex surfaces within the engine.

An expansion fan consists of a series of expansion waves, each gradually decreasing the pressure and temperature of the airflow. These waves are not as abrupt as shock waves, but they are still important to engine performance, particularly in managing the exit flow from the engine nozzle.

The engine's nozzle geometry plays an important role in shaping expansion fans. A well designed nozzle will use expansion fans to increase the flow velocity while minimizing losses in total pressure. This process is necessary for achieving the desired thrust at hypersonic speeds. Improper nozzle design can result in flow separation, turbulence, or unsteady expansion patterns, all of which degrade engine performance.

Interaction of shock waves and expansion fans

In scramjet engines, the interaction between shock waves and expansion fans must be carefully controlled. These two phenomena work in tandem to compress and expand air through the engine stages. A scramjet operates in a delicate balance between compression and expansion processes. If the shock waves over compress the air, it can cause excessive heating and lead to flow separation or unsteady combustion. On the other hand, if expansion fans cause too much pressure drop, the combustor may not receive enough air for efficient combustion, leading to performance losses.

Importance of managing waves in scramjet design

The behavior of wave propagation, shock waves, and expansion fans is fundamental to scramjet engine design. Shock waves compress the incoming air for efficient combustion, while expansion fans ensure that the exhaust flows smoothly at hypersonic speeds. Together, they control the efficiency, stability, and thrust generation capabilities of the engine.