At the Naval Postgraduate School in San Diego, CA, a master’s degree candidate conducted research to better understand the cold-flow performance of a miniature ramjet operating at speeds up to Mach 4.1 A ramjet is a type of airbreathing jet engine that uses the forward motion of an aircraft to compress air flowing into the engine. With no moving parts, it is simpler than a turbine engine, which relies on a turbine-driven compressor to compress incoming airflow.
Ramjets operate most efficiently at supersonic speeds up to roughly Mach 5. The inlet at the front of the ramjet is designed to drastically slow the incoming air, which is then mixed with fuel and combusted. Combustion exhaust is expelled through a nozzle to accelerate the ramjet to supersonic speeds, providing the thrust that propels the aircraft. Ramjets are used to propel military reconnaissance planes and missile systems, their high speeds enabling these aircraft to evade most threats and interceptors. The performance characteristics of miniaturized ramjets, which have potential for mini/micro unmanned aerial vehicle (UAV) and other defense applications, are not as well understood.
The goal of the thesis project was to use a computational model to predict the drag force on a miniature ramjet in a Mach 4 airflow, and to validate the predictions by measuring the actual drag force on a miniature ramjet subjected to Mach 4 airflows in a supersonic wind tunnel (SSWT).
A scale model of a ramjet was machined, and specially designed support struts were built for mounting the ramjet model within the SSWT to prevent the model from moving up or down during testing. In the center of each strut were two flexure beams instrumented with strain gauges designed to measure drag on the ramjet during testing. Because the temperature inside the SSWT was 68K, cryogenic strain gauges were used. Careful selection of the adhesive used to bond the strain gauges to the flexure beams was critical, since the adhesive is effectively part of the strain gauge system and may affect its performance. Master Bond EP29LPSP adhesive was selected due to its ability to retain sufficient flexibility at cryogenic temperatures. Experimental results showed that the computer model under predicted the drag force on the miniature ramjet model.