Interactive design tool for turbine based combined cycle engines

A workstation-based interactive aerodynamic design tool has been developed to evaluate the performance and design constraints of a turbine based combined cycle engine concept for a hypersonic cruise vehicle. This tool uses one-dimensional aerodynamic and thermodynamic analysis techniques to model the inlet, turbine engine, ramjet burner, mixer ejector and single expansion ramp nozzle performance across the flight regime from sea level static to Mach 6.0 at altitude. The thermodynamic analysis may be performed using approximately thermally perfect or calorically perfect gasses. The design flight conditions, geometry, and parameters used in the analysis can be varied through a graphical user interface (GUI) and the change in engine performance is calculated and displayed immediately. A variety of graphical formats are used to present the results to the designer including numerical results, moving bar charts, and interactive schematic drawings. The tool provides printed output for performance plotting packages and has a restart capability. The GUI employs X based graphics widgets and the simulator runs on a single SGI workstation. The paper will detail the numerical methods used in the simulator and how it can been used in preliminary aerodynamic design. "Copyright c 1997 by the American Institute of Aeronautics and Astronautics, Inc. No copyright is asserted in the United States under Title 17, U.S. Code. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. INTRODUCTION Recent advances in computer related technologies are changing the ways that engineers perform preliminary aerodynamic design studies. In the past, preliminary design was conducted using charts, tables and graphs of the performance of earlier similar configurations and final aerodynamic design was tested and verified using wind tunnel results. With the advent of large, powerful mainframe computers, some of the preliminary design tables and graphs could be numerically generated by solving the equations of motion and some of the final design results could be verified using computational fluid dynamics (CFD). Today's workstations and personal computers have computing power equal to that of the older mainframes. When coupled with a window operating systems and a graphical user interface, (GUI), a workstation can now be used to solve preliminary aerodynamic design problems interactively. This paper will describe the development of a software tool to perform preliminary aerodynamic design and evaluation of a propulsion system for a hypersonic cruise vehicle. The tool is based on previous flow solvers developed by the author, Refs. 1-4 , to perform preliminary aerodynamic design or educational analysis using interactive computer graphics on a single workstation. In Ref. 1 an interactive inlet design tool was developed to solve for the flow through external compression inlets. Through the use of a GUI, the designer could change the geometry, and the upstream and downstream flow conditions and immediately see the effects on inlet performance and drag. As the geometry and flow Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc. Figure 1: Turbine Based Combined Cycle (TBCC) propulsion system conditions were altered, the application would recompute the important flow variables and redisplay the geometry, shock wave locations, and output flow parameters. This package has been extended to also solve for the flow through rectangular mixed compression inlets, Ref. 2. In Ref. 3 some of the coding from the the inlet design program was used to produce an educational package to study simple turbojet engines. In this package, the user sets input design conditions using the GUI and the code performs a Brayton cycle analysis as outlined in Ref. 5 to determine thrust and specific fuel consumption. This package has also recently been extended to analyze turbofan and afterburning turbojet engines, Ref. 4. The propulsion system to be modeled in this application is a turbine based combined cycle (TBCC) engine for a hypersonic cruise vehicle. The conceptual vehicle would cruise at Mach 10 using a hydrogen burning scramjet propulsion system. Scramjet powered vehicles, however, must be accelerated by some other means until sufficient ram compression exists for full supersonic combustion. While a variety of propulsion systems have been proposed to accelerate the vehicle, this study employs conventional fueled turbojets which are integrated with the ramjet/scramjet flow path. The turbojets would be used for take-off, low speed cruise, ferry and, in conjunction with the ramjet, would accelerate the vehicle to near Mach 3. At this point sufficient ram compression exists to operate the scramjet flow path in a subsonic combustion "single throat" ramjet mode, Ref. 6. As Mach number then increases to approximately Mach 6, the engine transitions to full supersonic combustion. A major concern in using a combined tubojet/ramjet/scramjet propulsion system is the integration of the turbojet with the ramjet/scramjet flow path since the turbojet represents a weight and volume penalty at high speed cruise. Details of the conceptual design, shown in Fig. 1, and its predicted performance can be found in Ref. 7. In Fig. 1, a row of turbojets are placed in a bay in the fuselage above the ramjet flow path. The turbojets and ramjets share a common inlet and nozzle while an ejector is used to control the airflow split between them. The ejector flap can be used to close off the ramjet flowpath for low speed operation as shown in Fig. 2a. A thermal choke is achieved at the ramjet spraybar which acts as a low speed afterburner for the turbojet. Soon after take-off the ejector flap is raised as shown in Fig. 2b. The ejector allows more flow to pass through the ramjet throat as the airflow through the turbojet decreases with Mach and altitude. Near Mach 3, a two position inlet flap is lowered while the ejector flap is raised to completely isolate the tubojet from the high temperature ramjet/scramjet stream as shown in Fig. 2c. The ramjet then accelerates the system to Mach 6 for scramjet transition.