Multidisciplinary Design Techniques Applied to Conceptual Aerospace Vehicle Design

Multidisciplinary design optimization (MDO) is an emerging discipline within aerospace engineering. Its goal is to bring structure and efficiency to the complex design process associated with advanced aerospace launch vehicles. Aerospace vehicles generally require input from a variety of traditional aerospace disciplines - aerodynamics, structures, performance, etc. As such, traditional optimization methods cannot always be applied. Several multidisciplinary techniques and methods were proposed as potentially applicable to this class of design problem. Among the candidate options are calculus-based (or gradient-based) optimization schemes and parametric schemes based on design of experiments theory. A brief overview of several applicable multidisciplinary design optimization methods is included. Methods from the calculus-based class and the parametric class are reviewed, but the research application reported focuses on methods from the parametric class. A vehicle of current interest was chosen as a test application for this research. The rocket-based combined-cycle (RBCC) single-stage-to-orbit (SSTO) launch vehicle combines elements of rocket and airbreathing propulsion in an attempt to produce an attractive option for launching medium sized payloads into low earth orbit. The RBCC SSTO presents a particularly difficult problem for traditional one-variable-at-a-time optimization methods because of the lack of an adequate experience base and the highly coupled nature of the design variables. MDO, however, with it's structured approach to design, is well suited to this problem. The result of the application of Taguchi methods, central composite designs, and response surface methods to the design optimization of the RBCC SSTO are presented. Attention is given to the aspect of Taguchi methods that attempts to locate a 'robust' design - that is, a design that is least sensitive to uncontrollable influences on the design. Near-optimum minimum dry weight solutions are determined for the vehicle. A summary and evaluation of the various parametric MDO methods employed in the research are included. Recommendations for additional research are provided

Multidisciplinary Design of a RBCC SSTO using Taguchi Methods

AIAA/AHS/ASEE Aerospace Design Conference Irvine, CA, February 1993.This paper presents the results of an effort to optimize a conceptual design of a conical (winged- cone) single-stage-to-orbit launch vehicle. The vehicle employs a rocket-based combined-cycle (RBCC) engine capable of four different operating modes -- ejector, ramjet, scramjet, and rocket. The RBCC engine offers higher engine thrust-to-weight ratios than competing airbreathing engine cycles while maintaining an Isp advantage over rocket engines. The Taguchi method, a parametric multidisciplinary design method, was used to evaluate the effects of changing 8 design variables (2 of which were discrete) in an "all at the same time" approach rather than the traditional ³one variable at a time² trade study approach that is more typical of conceptual aerospace vehicle design. Design variables pertained to both the vehicle geometry (cone half-angle, engine cowl wrap around angle) and trajectory parameters (dynamic pressure limits, heating rate limits, and airbreathing mode to rocket mode transition Mach number). The vehicle payload was fixed at 10,000 lbs to 100 Nmi circular polar orbit. Vehicle dry weight and gross weight were determined for each of the 27 point designs performed. Using the method, a combination of design variables was found that produces a very attractive vehicle dry weight and gross weight. While not necessarily producing an optimum design, the Taguchi method has demonstrated considerable utility in characterizing the complex design space and identifying a vehicle with which to start a more detailed optimization process