NASA and Computational Structural Analysis

David C. Aronstein

NASA research has been pivotal in its support of computational ana­lytical methods for structural analysis and design, particularly through the NASTRAN program. NASA Centers have evolved structural analy­sis programs tailored to their own needs, such as assessing high-tem­perature aerothermodynamic structural loading for high-performance aircraft. NASA-developed structural tools have been adopted through­out the aerospace industry and are available on the Agency Web site.

HE FIELD OF COMPUTER METHODS in structural analysis, and the contributions of the National Aeronautics and Space Administration (NASA) to it, is wide-ranging. Nearly every NASA Center has a struc­tural analysis group in some form. These groups conduct research and assist industry in grappling with a broad spectrum of problems. This paper is an attempt to show both aspects: the origins, evolution, and application of NASA Structural Analysis System (NASTRAN), and the variety and depth of other NASA activities and contributions to the field of computational structural methods.

In general terms, the goal of structural analysis is to establish that a product has the required strength and stiffness—structural integrity— to perform its function throughout its intended life. Its strength must exceed the loads to which the product is subjected, by some safety mar­gin, the value of which depends on the application.

With aircraft, loads derive from level flight, maneuvering flight, gusts, landings, engine thrust and torque, vibration, temperature and pressure differences, and other sources. Load cases may be specified by regula­tory agency, by the customer, and/or by the company practice and expe­rience. Many of the loads depend on the weight of the aircraft, and the weight in turn depends on the design of the structure. This makes the structural design process iterative. Because of this, and also because a large fraction of an aircraft’s weight is not actually accounted for by pri­mary structure, initial weight estimates are usually based on experience

rather than on a detailed buildup of structural material. A sizing pro­cess must be performed to reconcile the predicted empty weight and its relationship to the assumed maximum gross weight, with the required payload, fuel, and mission performance.[786]

After the sizing process has converged, the initial design is docu­mented in the form of a three-view drawing with supporting data. From there, the process is approximately as follows:

• The weights group generates an initial estimate of the weights of the major airframe components.

• The loads group analyzes the vehicle at the defined condition(s) to determine forces, bending moments, etc., in the major components and interfaces.

• The structures group defines the primary load paths and sizes the primary structural members to provide the required strength.

• Secondary load paths, etc., are defined to the required level of detail.

Process details vary between different organizations, but at some point, the structural definition reaches a level of maturity to enable a check of the initial weight estimate. Then the whole designmay be iterated, if required. Iteration may also be driven by maturing requirements or by evolution in other aspects of the design, e. g., aerodynamics, propulsion, etc.