Computational analysis of an independent ramjet stream in a combined cycle engine

A new concept for the low-speed propulsion mode in rocket-based combined cycle engines has been developed as part of the NASA GTX program. This concept, called the independent ramjet stream (IRS) cycle, is a variation of the traditional ejector ramjet (ER) design and involves the injection of hydrogen fuel directly into the airstream, where it is ignited by the rocket plume. The advantage of the IRS design is that it allows for a single large rocket instead of several smaller rockets, and its required combustor length is smaller than that of a traditional ER design. Both of these features make the IRS design lighter. Experiments and computational fluid dynamics are currently being used to evaluate the feasibility of the new design. In this work, a Navier-Stokes code valid for general reactive flows is applied to the model engine under cold-flow, ER, and IRS cycle operation. Pressure distributions corresponding to cold-flow and ER operation are compared with experimental data. The engine response under IRS cycle operation is examined for different reaction models and grid sizes. The solutions exhibit a high sensitivity to both grid resolution and reaction mechanism but do indicate that thermal throat ramjet operation is possible through the injection and burning of additional fuel into the airstream.