Jeff S. Noall and Dr. Russell L. Daines, Mechanical Engineering
A scramjet (supersonic-combustion ramjet) is a theoretical engine which, like a ramjet, converts the velocity of the inlet air into pressure. This “ram” pressure compresses the fluid stream entering the engine prior to fuel injection and combustion. In a ramjet, the incoming air is slowed to subsonic speeds (speeds less than the speed of sound) before combustion occurs. In a scramjet, the air stream remains supersonic throughout the process. This feature gives the scramjet potential in high-speed applications such as hypersonic aircraft and space-transport systems. One of the main problems facing scramjet development is controlling the behavior of the shock waves that occur in supersonic flow. For these engines to work, the geometry of the engine inlet must be designed in such a way as to place the shock waves in optimum position. Unfortunately, a given inlet configuration will correctly place the shock waves for only one specific flight Mach number. Thus, most current scramjet models call for a variable-geometry inlet. Such an inlet is complex and adds significantly to vehicle weight.
This research project focused on a proposed method to allow the scramjet engine to adapt to different inlet conditions (such as flight speed, temperature and altitude) without the need for variable-geometry mechanisms. The method uses the concept of fluidic compression where a secondary fluid jet is used to compress the upper jet (see Figure 1). The fluid in the compression jet can be supplied either by burning hydrogen in a separate air stream, or from the exhaust of a small rocket motor. The positions of the shock waves in the main air stream were able to be controlled by varying the pressure and speed of the compression jet as needed. The fluid properties of the lower stream were controlled by the amount of fuel that was burned in the compression jet. Once the intake air stream is compressed, a splitter plate separates the compression jet from the main air jet before combustion occurs. By changing the fluid properties of the compression jet in a calculated manner, the scramjet was found to be able to adapt to varying flight conditions, thereby eliminating the need for a variable-geometry configuration.
In the scramjet, the main air stream is compressed as it travels through the shock waves. If the shock waves are not in the correct position, the direction of the wave varience may cause either over or under compression. Over compression will lead to temperatures in excess of material limits, and the engine will burn up. Under compression (loss of air capture) results in a loss of thrust. Figure 2 shows that the fluidic compression model gives nearly identical compression to the fixed geometry model. This is manifest by comparing the pressure contour label values at equivalent stations. As the streamline in Figure 1 indicates, some of the fluid from the compression jet “leaks” into the main air stream. This results in a small loss of air capture in the fluidic compression model. This “leakage” will lead to a slight drop in the thrust produced by the main stream.
This study showed that fluidic compression can be used as an efficient means of compression in scramjet engines. The implementation of this method could lead to a fixed-geometry scramjet engine capable of adapting to varying flight conditions. The fluidic compression method does, however, have the significant drawback of having a need to supply fuel to the compression jet. This means that the flight vehicle would need to carry more fuel, thereby leading to a greater gross takeoff weight. The fluidic compression model also was found to capture slightly less mass flow than in the variable geometry model. These problems detract from the overall attractiveness of the fluidic compression method. Further work is required to determine the practicality of implementing fluidic compression into scramjet design.