Design of a welded joint for robotic, on-orbit assembly of space trusses

In the future, some spacecraft will be so large that they must be assembled on-orbit. These spacecraft will be used for such tasks as manned missions to Mars or used as orbiting platforms for monitoring the Earth or observing the universe. Some large spacecraft will probably consist of planar truss structures to which will be attached special purpose, self-contained modules. The modules will most likely be taken to orbit fully outfitted and ready for use in heavy-lift launch vehicles. The truss members will also similarly be taken to orbit, but most unassembled. The truss structures will need to be assembled robotically because of the high costs and risks of extra-vehicular activities. Some missions will involve very large loads. To date, very few structures of any kind have been constructed in space. Two relatively simple trusses were assembled in the Space Shuttle bay in late 1985. Here the development of a design of a welded joint for on-orbit, robotic truss assembly is described. Mechanical joints for this application have been considered previously. Welded joints have the advantage of allowing the truss members to carry fluids for active cooling or other purposes. In addition, welded joints can be made more efficient structurally than mechanical joints. Also, welded joints require little maintenance (will not shake loose), and have no slop which would cause the structure to shudder under load reversal. The disadvantages of welded joints are that a more sophisticated assembly robot is required, weld flaws may be difficult to detect on-orbit, the welding process is hazardous, and welding introduces contamination to the environment. In addition, welded joints provide less structural damping than do mechanical joints. Welding on-orbit was first investigated aboard a Soyuz-6 mission in 1969 and then during a Skylab electron beam welding experiment in 1973. A hand held electron beam welding apparatus is currently being prepared for use on the MIR space station. Presently, Marshall Space Flight Center is evaluating processes appropriate for on-orbit welding. A low gravity environment has been found to have very minor effects on the welding processes appropriate for this application. This is based on tests run on-orbit as well as low gravity environments achieved by flying aircraft in parabolic trajectories. It appears that a modified form of gas tungsten arc welding (GTAW) will be most appropriate for welding together structures on-oribt. The process has been modified to work in a vacuum by providing gas to the arc zone by means of a hollow tungsten electrode with special shielding. A commercial tube welding head has been successfully modified for use on-orbit with a gas leakage rate of approximately 2.5 liters/min. To develop as realistic a joint as possible, a specific truss structure was selected on which to base the design. The structure considered was based on the 120 foot diameter aerobrake tetrahedral truss structure. The truss members were assumed to consist of graphite/epoxy tubes. Also, it was assumed that the nodes were constructed of 2219-T87 aluminum alloy. The magnitude of the member load assumed for design purposes was 100 kips

An efficient frequency response solution for nonproportionally damped systems

A method is presented to accurately and economically calculate steady state frequency responses based on the analysis of large finite element models with nonproportional damping effects. The new method is a hybrid of the traditional nonproportional and proportional damping solution methods. It captures the advantages of each computational approach without the burden of their respective shortcomings, as demonstrated with comparative analysis performed on a large finite element model

Seventy Years of Changing Great Books at St. John\u27s College

This dissertation examines a curricular approach at an institution that claims to maintain a liberal arts focus – that of the canon of Great Books as implemented as a formal curriculum at St. John’s College. My research question is: what enabled the Great Books program at St. John’s College to survive for over seventy years? The significance of this question can be seen by noticing that St. John’s College is the only college in the United States to have exclusively adopted reading the Great Books as its four-year curriculum. Other institutions that have experimented with a Great Books program prior to and since its introduction at St. John’s College have continued their existing programs as well, but many have limited their Great Books efforts to an honors course or general core requirement, if their Great Books effort survives at all. My dissertation is historical starting with the influencing factors leading to this curriculum’s introduction at St. John’s College in 1937. I then outline the implementation and document the changes to the list of Great Books comprising the program as it was updated over the subsequent seventy years as documented in St. John’s College’s academic catalogs from 1937 through 2008. I show that the list of Great Books required to be read by every student over the years has contained a consistent core while making slight adjustments

Characterization of space station multilayer insulation damage due to hypervelocity space debris impact

Four main tasks were accomplished. The first three tasks were related to the goal of measuring the degradation of the insulating capabilities of Space Station multilayer insulation (MLI) due to simulated space debris impacts at hypervelocities. The last task was associated with critically reviewing a Boeing document on the fracture characteristics of the Space Station pressure wall when subjected to a simulated hypervelocity space debris impact. In Task 1, a thermal test procedure for impact damaged MLI specimens was written. In Task 2, damaged MLI specimens were prepared. In Task 3, a computer program was written to simulate MLI thermal tests. In Task 4, the author reviewed a Boeing document describing hypervelocity impact testing on biaxially stressed plates

MLIBlast: A program to empirically predict hypervelocity impact damage to the Space Station

MLIBlast is described, which consists of a number of DOC PC based MIcrosoft BASIC program modules written to provide spacecraft designers with empirical predictions of space debris damage to orbiting spacecraft. The Spacecraft wall configuration is assumed to consist of multilayer insulation (MLI) placed between a Whipple style bumper and a pressure wall. Predictions are based on data sets of experimental results obtained from simulating debris impact on spacecraft. One module of MLIBlast facilitates creation of the data base of experimental results that is used by the damage prediction modules of the code. The user has a choice of three different prediction modules to predict damage to the bumper, the MLI, and the pressure wall

BALLIST: A computer program to empirically predict the bumper thickness required to prevent perforation of the Space Station by orbital debris

A computer program called BALLIST that is intended to be a design tool for engineers is described. BALLlST empirically predicts the bumper thickness required to prevent perforation of the Space Station pressure wall by a projectile (such as orbital debris) as a function of the projectile's velocity. 'Ballistic' limit curves (bumper thickness vs. projectile velocity) are calculated and are displayed on the screen as well as being stored in an ASCII file. A Whipple style of spacecraft wall configuration is assumed. The predictions are based on a database of impact test results. NASA/Marshall Space Flight Center currently has the capability to generate such test results. Numerical simulation results of impact conditions that can not be tested (high velocities or large particles) can also be used for predictions

After the Split - The Recent Workload of the Court of Appeals for the Fifth Judicial Circuit

Symposium on Federal Judicial Administration: Stewardship in a Changing Environmen

A comparison of the South African procurement environment with global best practice

The aim of this research was to investigate the ways in which South African organisations are embracing the potential to improve their performance through the use of effective strategic sourcing practices. Four specific indicators for effective strategic sourcing were identified, around which four research questions were formulated. This research was qualitative in nature. An exploratory, applied research process based on experience survey methodology was followed, using two survey instruments. Questionnaires were issued through a professional body to all its members, and key informant interviews were conducted with a number of subject matter experts. Content analysis was applied to the data collected. The results of this research support the idea that the procurement function in South African organisations is involved in the overall strategic planning process, is appropriately positioned and structured for effective strategic sourcing, is attempting to use appropriate tools and techniques where possible, and is applying appropriate performance measurement methods. However, improvement in a number of areas is still required if these organisations wish to achieve the full benefits that strategic sourcing can offer. The results of this research could be used by procurement managers in South Africa for guidance in terms of how to structure and manage their own functions in order to derive the greatest possible performance improvements.Dissertation (MBA)--University of Pretoria, 2010.Gordon Institute of Business Science (GIBS)unrestricte