The rocket based combined cycle can be an alternative engine to power a vehicle into space. The engine has 4 stages and one of the more challenging stages to in crease performance is the ﬁrst stage, the ejector stage. One proposed method to increase performance is by achieving higher entrainment performance and mixing at low free-stream velocities. The Exchange Inlet is used as an alternative nozzle to achieve this performance. The Exchange Inlet is compared to a conical nozzle within a RBCC engine with computational ﬂuid dynamics at various pressures. The mixing and entrainment properties are compared between the two engine conﬁgurations and additional cases are investigated with shorter mixing sections to further investigate these properties. The Exchange Inlet is found to have better entrainment than the circular nozzle in all cases.