Interactive Inverse Design Optimization of Fuselage Shape for Low-Boom Supersonic Concepts

There are several research papers on how to design a supersonic configuration that has desirable low-boom characteristics as determined by the Seebass-George-Darden boom minimization theory (SGD theory). The low-boom signatures predicted by the SGD theory could be realized by wing-fuselage configurations. However, for a given low-boom signature generated by the SGD theory, it is still an open question whether one could develop a feasible aircraft configuration with nacelles and tails that has a similar low-boom ground signature as that predicted by the SGD theory. Past attempts indicated that no feasible aircraft configuration with nacelles and tails would have a total equivalent area distribution matching one of the equivalent area distributions corresponding to the low-boom ground signatures determined by the SGD theory. There are essentially three alternative methods for generating a supersonic concept with a shaped boom ground signature: (i) use a direct optimization method that minimizes numerical figures of merit for low-boom characteristics, (ii) construct new “realizable” target equivalent area distributions (or near-field pressure distributions) that result in shaped boom ground signatures, and (iii) develop new tools to help designers find acceptable low-boom configurations. There were many attempts, with mixed results, using the first two methods to obtain supersonic configurations that have shaped boom ground signatures. This paper introduces a tool called BOSS (Boom Optimization using Smoothest Shape modifications). BOSS utilizes interactive inverse design optimization to develop a fuselage shape that yields a low-boom aircraft configuration. The paper also demonstrates how BOSS could be used to help design realistic aircraft concepts with low-boom ground signatures. A fundamental reason for developing BOSS is the need to generate feasible low-boom conceptual designs that are appropriate for further refinement using CFD-based preliminary design methods. BOSS was not developed to provide a numerical solution to the inverse design problem. Instead, BOSS was intended to help designers find the “right” configuration among infinitely many possible configurations that are equally good using any numerical figure of merit. BOSS uses the smoothest shape modification strategy for modifying the fuselage radius distribution at 100 or more longitudinal locations to find a smooth fuselage shape that reduces the discrepancies between the design and target equivalent area distributions over any specified range of effective distance. For any given supersonic concept (with wing, fuselage, nacelles, tails, and/or canards), a designer can examine the differences between the design and target equivalent areas, decide which part of the design equivalent area curve needs to be modified, choose a desirable rate for the reduction of the discrepancies over the specified range, and select a parameter for smoothness control of the fuselage shape. BOSS will then generate a fuselage shape based on the designer’s inputs in a matter of seconds. If the generated solution is not acceptable, the designer can work on a different part of the equivalent area curve, change the rate of reduction, or relax the smoothness control until a desirable solution is found. The new configuration will be analyzed by PBOOM (a sonic boom analysis code) to see whether it has an acceptable low-boom ground signature. If not, the designer can use BOSS to further reduce the differences between the design and target equivalent areas until the configuration has an acceptable low-boom ground signature. Using BOSS and PBOOM, the designer can generate a realistic, smooth fuselage shape that results in a supersonic configuration with a low-boom ground signature in a few hours. In addition, a designer can use BOSS to quickly eliminate any configuration that cannot achieve low-boom characteristics with fuselage shaping alone. For any given wing planform and layout of aircraft components, BOSS reduces the design time of low-boom supersonic concepts from months to hours. More importantly, BOSS allows a quick closure of the fuselage shaping process because BOSS will let the designer see how much deterioration of the fuselage shape is necessary for any further reduction of the discrepancies between the design and target equivalent area distributions. A conceptual design case study is documented to demonstrate how BOSS can be used to develop a low-boom supersonic concept from a low-drag supersonic concept. The paper also contains a study on how perturbations in the equivalent area distribution affect the ground signature shape and how new target area distributions for low-boom signatures can be constructed using superposition of equivalent area distributions determined by the SGD theory.