Design issues for turbine-based and rocket-based combined cycle propulsion system inlets

This paper presents the elements of an analytical and experimental inlet program prepared by the NASA Lewis Research Center (LeRC) in support of Turbine Based Combined Cycle (TBCC) and Rocket Based Combined Cycle (RBCC) propulsion systems. The primary focus is on inlet technology for TBCC systems although areas of common interest to RBCC systems will also be discussed. The preliminary problem definition, analysis and design work are presented along with a proposed program plan for an aggressive program to meet the aerospace community's future needs for TBCC and RBCC inlet technology. The TBCC is a propulsion system that combines a turbine engine with ramjet and scramjet systems while RBCC combines the ramjet and scramjet systems with a rocket. TBCC systems combine the low speed thrust capability of a turbojet with the high speed efficiency of a ramjet and scramjet, while RBCC systems combine the high thrust-toweight (F/W) ratio of rockets with the high speed efficiency (i.e. specific impulse, Isp) of ramjets. Both systems provide a single, integrated propulsion system that is capable of generating net thrust from sea-level-static to hypersonic speeds. The subject inlet plan is a roadmap to investigate problem areas unique to the TBCC and RBCC systems. These include forebody boundary layer ingestion, offset diffusers, rectangular-to-round transition ducts, and single flow path versus dual flow path flow splitting issues. The plan outlines both computational and experimental studies designed to investigate the feasibility of various strategies to control or minimize the adverse effects of the abovementioned problem areas on inlet flow field qualities (e.g., total pressure distortion and recovery). Preliminary conceptual designs are presented for vision vehicles (both government and industry concepts) and propulsion systems. Design and analysis for these concepts were performed with method-of-characteristics (MOC), parabolized Navier-Stokes (PNS), and full NavierStokes (FNS) codes and the results are presented. An experimental program is laid out to address the aforementioned problem areas and those uncovered during the analyses. This comprehensive experimental program involves tests in the LeRC 1x1 Supersonic Wind Tunnel (SWT), Diffuser Test Rig (DTR), and 10x10 SWT. NOMENCLATURE D2 Fn Isp Ldif M0 Ps Pso PtO Pt2 q Ram/m Spil/m Z A2 inlet capture area turbojet subsonic diffuser entrance flow area turbojet subsonic diffuser exit flow area freestream captured stream tube area turbojet subsonic diffuser exit diameter thrust specific impulse turbojet subsonic diffuser length freestream Mach model static pressure freestream static pressure freestream total pressure engine face total pressure recovery freestream dynamic pressure ram drag divided by massflow spill drag divided by massflow turbojet subsonic diffuser centroid offset turbojet subsonic diffuser effective average wall angle gamma, ratio of specific heats, 1.4