Computerized Handbook Performs Satellite Testing Fixture Calculations in an Half Hour Compared to Four to Eight Hours Previously

One function of Goddard Space Flight Center is the responsibility of designing and building payload testing and qualification systems for military and commercial satellites. These tests are designed to measure the ability of the satellite payload to withstand the forces encountered during launch and landing in the Space Shuttle, and even during ground transportation if necessary. This may consist of designing fixturing used for a variety of static, dynamic, thermal, acoustic, vibrational and electromagnetic tests. In designing these fixtures it is frequently necessary to optimize structural members such as centrifuge fixturing to be strong enough to hold a satellite while it is being spun up to a 20 g. acceleration loading. Optimization is important in many of these tests because extra fixture weight dictates the use of larger and more expensive testing equipment. Performing parametric optimization could take between 4 and 8 hours for a given subassembly using traditional hand calculation methods. By switching to a computerized engineering handbook, The Desktop Engineer from Desktop Engineering, Woodcliff Lake, New Jersey, the time required can be reduced to about 1/2 hour for the subassembly and improve the accuracy of the calculations as well as provide much better documentation.

One example of a test on which improvements of this scale were demonstrated was the static load test of the Cosmic Background Explorer (COBE) satellite. The Desktop Engineer was used in approximately 5 different aspects of the design of the static test fixturing for this payload. A representative application was its use to design static test hardware. This consisted of a hydraulic actuator which applied static loads to the payload structure. The satellite itself had a triangular cross-section so the fixture incorporated a triangular plate reinforced by beams. A downward pulling force was then applied to the satellite structure by a hydraulic actuator attached to the fixture.

The first step was to analyze the triangular plate to make sure that it would not be overstressed during the testing process. The "Plates" module of The Desktop Engineer was selected which required only a few simple input parameters: the size and thickness of the panel, material properties, boundary conditions and loads. Since optimization was not required, a single run with DE/CAASE was performed which took less than 5 minutes. After determining that the plate could withstand testing forces, the triangular fixture was analyzed using the beams module. Again, the geometry of the structure, material properties, boundary conditions and loads was entered. The initial analysis run again took under 5 minutes and repeating the analysis after changing member size or loads the geometry of the structure took only a minute or two. In less than 30 minutes, the structure was optimized to a far greater degree than could have ever achieved with hand calculations. Even a very cursory optimization routine would have taken 4-5 hours by hand. Another important advantage of using The Desktop Engineer is that the program provides complete documentation for every calculation which was used in the report to NASA.

Another project on which The Desktop Engineer was utilized was the design of a centrifuge test clamping fixture for the Extreme Ultraviolet Explorer (EUVE). The centrifuge test was used to determine whether or not the approximately 16 foot long, 5 foot diameter satellite could withstand the dynamic forces exerted by a hard landing on the Space Shuttle. A rather complex constraint system was required in order to simulate the way that the satellite is held inside the shuttle. This type of fixture was particularly difficult to design by hand because there were several structural members which interact to absorb the load. In order to analyze this type of structure by hand it was necessary to make a number of simplifying assumptions involving how much of the load was absorbed by each member. Only the most rudimentary optimization routine could be accomplished within a 6 to 8 hour time frame, thus the design was of necessity quite conservative when hand calculations were used. On the other hand, with the "Frames" module of The Desktop Engineer, the distribution of loads among the various members was handled automatically. The fixture was modeled and material properties, loads and boundary conditions were entered. The initial analysis run took 5 minutes and was optimized in half an hour to a considerable degree of accuracy, resulting in a substantially lighter fixture than could have achieved using hand methods.

The Desktop Engineer incorporates solutions to over 5000 structural/mechanical engineering equations found in over 100 reference books. In most cases, the program will do the equation substitutions, integration, and boundary condition analysis in exactly the way an engineer would solve the problem by hand. In some of the more complicated analyses, the computerized handbook utilizes the finite element method to obtain a solution; however, the input and output format remains similar to the other modules and the user is not required to know finite element techniques. It includes 37 modules grouped into the following categories: geometric and material properties; beams and columns; rings, cables, arches and frames; plates, shells and pressure vessels; natural frequencies; user-defined modules; and miscellaneous. Modules are self-prompting to help the user find items like material properties and calculate section properties like area, moment of inertia, etc. In addition, several utility modules are included to assist the designer with complex problems. A superposition module allows the designer to look at the effects of several concurrent loads on the same structure. Another utility module allows the user to calculate stresses in addition to forces and displacements calculated by the other modules. The program also has a convenient material property database and several modules that look at complex geometries and dynamic conditions.