Component-level weight analysis for RBCC engines

Abstract

Rocket-based combined-cycle (RBCC) engines have recently received increased attention for use on advanced, reusable space launch vehicles. By combining conventional rocket and airbreathing operating modes into an integrated unit, they have given designers a middle ground between the highthrust, low-Isp characteristics of a pure rocket and the low-thrust, high-Isp of pure airbreathers. Engine weight (or thrust-to-weight ratio) is a highly sensitive parameter in the design of advanced reusable launch vehicles. While substantial experience exists with ground-test engines from the 1960's, little parametric data exists to help conceptual designers predict weight for today's advanced technology, flight-weight RBCC engines. This paper reports a new set of component-level parametric weight estimating equations for advanced RBCC engines. These equations are derived from topdown regression analysis of historical data and include variables to account for advanced technologies and materials. Component weight equations are given as functions of engine geometry, internal pressure, flight modes, etc. Taken together, the equations are used to build up an overall RBCC weight estimation model — WATES. This spreadsheet-based model is not intended to replace a more detailed weight analysis, but rather to assist conceptual vehicle designers in assessing the relative advantages of various engine concepts. Sample RBCC engine weight predictions are given. ' Assistant Professor, School of Aerospace Engineering, senior member AIAA. f Graduate Research Assistant, School of Aerospace Engineering, student member AIAA. Copyright © 1997 by John R. Olds and David J. McCormick. Published by the American Institute of Aeronautics and Astronautics, Inc. with permission. NOMENCLATURE Ac inlet or cowl frontal area (sq. inches) Cf complexity factor for variable geometry ERJ ejector ramjet ESJ ejector scramjet ISP specific impulse (sec.) L/D length-to-diameter ratio LH2 liquid hydrogen LOX liquid oxygen M,mns transition Mach number to rocket mode MDC Mixer-Diffuser-Combustor Pint internal static design pressure (psia) PR fan (total) pressure ratio RBCC Rocket-Based Combined-Cycle SERJ supercharged ejector ramjet SESJ supercharged ejector scramjet SLS sea-level static (takeoff condition) T/W thrust-to-weight ratio TAD Technology Availability Date TRF Technology Reduction Factor V internal volume of MDC (cubic inches) WATES Weight Assessment Tool for Engine Scaling WBS Weight Breakdown Structure WER Weight Estimating Rlationship Wp rocket primary weight flow rate (Ib/s) Ws secondary or inlet weight flow rate (Ib/s) Wtota! total or nozzle weight flow rate (Ib/s)