On the Self-starting Constraints for Busemann Intakes with Overboard Spillage N. Moradian, E.V. Timofeev, R. Tahir, and S. M¨ older 1 Introduction The air intake is an important component of hypersonic airbreathing engines. It is essentially a converging duct decelerating and compressing airflow and supplying the compressed air to the engine’s combustor. Molder and Szpiro [1] proposed to use the Busemann flow [2] as the basis for hypersonic air intake design. The Busemann flow is a conically symmetric flow that compresses and contracts isentropically, with only small final portion of compression resulting from the downstream weak conical shock, see Fig. 1a. The advantages of using Busemann-flow-based intakes have pre- viously been confirmed and discussed in theoretical and experimental studies [3, 4]. Applications to flight vehicles were considered in [5]. To operate at its maximum ef- ficiency for a design freestream Mach number the Busemann intake must be started, i.e., all incoming supersonic flow must be captured and the design steady super- sonic flow (Busemann flow) must be established throughout the intake. There are several techniques to start high-contraction intakes. Arguably, one of the simplest approaches is starting via overboard mass spillage. The intake is to be designed to include both a fully enclosed internal compression section, which is subjected to the starting constraints of the Kantrowitz theory [6], as well as an external compres- sion section in which the incoming flow is only partially encased by intake’s walls, therefore providing the possibility for overboard spillage of excessive mass during the starting process. The incoming flow first goes through the external compression section which serves as a “preconditioner” decreasing the flow Mach number in front of the internal compression section as well as the contraction of that section in such a way that the Kantrowitz self-starting condition is satisfied. The present paper is devoted to the design of Busemann intakes with overboard spillage and the sub- sequent theoretical and numerical assessment of their self-starting characteristics. N. Moradian · E.V. Timofeev McGill University, Dept. of Mechanical Engineering, Montreal, QC, H3A0C3, Canada R. Tahir RBT Consultants, Toronto, ON, M9V 1X1, Canada S. M¨ older Ryerson University, Toronto, ON, M5B 2K3, Canada c  Springer International Publishing Switzerland 201 1027 R. Bonazza and D. Ranjan (eds.), 29th International Symposium on Shock Waves 2, DOI: 10.1007/978-3-319-16838-8_ 37 5