Configurations of aerospace vehicles have historically been developed in a sequential process in which the fidelity of analyses increases as the level of definition of the configuration is increased. At the earliest stages of a program, the gross vehicle characteristics (payload weight, fuel volume, wing area, target mass fractions, etc.) necessary to meet the mission requirements are selected without a real configuration definition. Once the range of the gross parameters is defined, a configuration designer begins outlining candidate configurations within that range. At this stage of development, a configuration definition consists mainly of a three-view sketch of the vehicle, the outer mold line (OML) definition, and a layout of major systems.
Since a wide variety of possible configurations can be defined that satisfy the gross vehicle characteristics, a set of trade studies or an optimization is performed to further refine the design. At this stage, the vehicle definition is driven primarily by aerodynamic considerations (aerodynamic performance and stability & control). The aerodynamic analysis used to evaluate configurations is typically a linear aerodynamic code, but in some cases might be higher-order computational fluid dynamics (CFD). Many of the systems requirements (i.e. total fuel volume, engine thrust, actuator requirements, etc.) can be estimated based on the aerodynamic calculations. Typically at this stage of vehicle definition, the structural aspects of a configuration are estimated using highly simplified models, such as areal or volumetric weights, or perhaps parametric weight equations. Using processes such as these, a reasonably good assessment and refinement of the vehicle is usually performed, and a successful vehicle configuration is the outcome.
Once the candidate configurations have been narrowed down to a “best” configuration, a more detailed assessment of the vehicle is performed. This involves a higher level of fidelity analysis in all disciplines. In the structures area, structural FEMs are constructed, loads are calculated, and the structure is sized based on the calculated loads. Depending on the configuration and the mission, other analyses might also be required such as Thermal Protection System (TPS) sizing, aeroelastic loads, modal frequencies, aeroelastic and aeroservoelastic stability, and aerothermoelastic interactions. There are many outputs of this structural analysis. The most obvious is an estimate of the structural weight of the vehicle. This weight estimate is typically more accurate than the simplified weight estimates used earlier in configuration definition, and can be used to refine the vehicle performance and viability assessment.
A much less obvious output of the structural analysis and sizing process is the identification of structural show stoppers. In the development of conventional configurations for which a large body of experience is available (e.g. body and wing subsonic aircraft), it is rare for real show stoppers to appear that make a configuration infeasible. This is due to the fact that configuration designers have been working with these configurations for many years, and have the benefit of having seen many successful (and some unsuccessful) configurations. For the most part, the pitfalls are known. However, when unconventional configurations are being designed (and certainly all reusable launch vehicles must be considered unconventional at this time), there is a much smaller heuristic experience base upon which to draw, and it is very possible that an unforeseen structural issue will prove to make the configuration impossible. Since this process has historically taken six months to a year, many man-years of effort could easily be wasted on configurations that have show stopping structural issues.
It is clear that structural analysis and sizing earlier in the configuration definition cycle would be valuable, both by increasing the fidelity of the early performance estimates and by identifying show-stopping issues before a large amount of resources have been committed. In the past, the reason structural analysis and sizing have not been included early in the design cycle is the lengthy structural analysis and sizing cycle time (several components of which are described below). An analysis cycle of six months to a year simply cannot be included in configuration trades that may turn around several times a month. Fortunately, improvements in structural analysis processes and approaches are dramatically reducing the cycle time for structural sizing to the point where it is almost possible to insert detailed structural sizing into the early configuration definition cycle.
This paper will discuss several enabling technologies for accelerating the structural sizing process to the point that it is useful in early studies. The key items for making this happen can be classified according to whether they enhance the sizing process itself or speed up the preparation of inputs to the sizing process:
Vehicle Sizing Process
- Global Sizing
- Global/Local Interface
- Local Sizing
Rapid Input Generation for Sizing
- Parametric Modeling
- Rapid FEM Generation
- Rapid Loads and Dynamics Analysis
- Rapid FEM Mass Estimation and Distribution
Each of these technologies will be discussed, and the current state of the art will be assessed. The ongoing development of an integrated rapid structural analysis and sizing framework will also be discussed, as well as several applications of the sizing process to realistic configurations.