To date, within the US space program, there have been only three construction EVA experiments - Experimental Assembly of Structures in EVA (EASE), Assembly Concept for Construction of Erectable Space Structures (ACCESS), both on STS-61B, and the ASM space station truss assembly test on STS-49. The purpose of all of these experiments was to demonstrate the construction of large truss structures. These experiments demonstrated the assembly of trusses both large and small, from foot restraints within the Shuttle and in free flight along side the truss, and in many other conditions. A test of the maneuverability of a truss by an astronaut was also demonstrated. In general, these experiments together demonstrated a variety of construction abilities as well as confirmed assembly times taken in the neutral buoyancy facilities (Table 9.3). Table 9.3 ACCESSAssembly Times vs. NBS Times. Unfortunately, simulated zero-g facilities only give a static understanding of the tasks and are incapable of simulating the dynamics of the apparatus' with which the astronaut must work. This problem was very noticeable on STS-61B when capture of an INTELSAT satellite was attempted. In attempts to attach the capture bar to the satellite, the primary tool used in this task, astronauts found the dynamics of the Spacecraft to be incompatible with the tools they had. The process of trying to use the capture bar to dock the astronaut to the satellite would induce motions by the satellite that the crew had not anticipated or trained for. Finally, after three attempts with the capture bar, the mission was accomplished by using astronauts to control the dynamics of the satellite by holding the satellite in place while die capture bar was attached. This lack of dynamics and control simulation in the neutral buoyancy facilities will continue to be a problem for simulating EVA unless a computer controlled simulation can be developed. In a space construction environment, the assembly of the structure part of the Spacecraft, which has been main area of emphasis of experiments in space construction, is only one part of the total assembly. Usually, members have fluid lines, electrical wire harnesses, and thermal control devices attached to them. This additional hardware complicates erectable construction because each non- structural connection requires verification and testing. Sometimes this requirement makes it necessary to assemble parts of the structure on the ground due to large concentrations of these connections, their difficulty in verification, or the crucialness of the connection. An example of a design where this necessary was in the Space Station Freedom. Here large elements were chosen to be preassembled terrestrially instead of in space constructed because of the high EVA times associated with assembly, although the original goal was full erectability of the station. Unfortunately, this process of using large pieces decreases the efficiency with which a station can be packed into a launch vehicle. A more preassembled Spacecraft leads to the volume limit of the launch vehicle being reached before the mass limit, and thus, forces the need for more launch vehicles to put the station in space. In most cases, there is a trade off between fully erectable structures and structures delivered to orbit in large piece, with launch and EVA costs as the main parameters. This trade off is usually unique for each Spacecraft and is not done in a systematic Design I way, although research is being done to automate this decision [Jolly, 1992]. Engineering Overlap Issues In addition to the choice of the size of pieces, many other engineering areas have issues that overlap with erectability of space structures. In general, the configuration of the design as well as the sizing issues with the overall Spacecraft and its individual sub-assembly pieces are concerns that are dual to engineering and construction and must have inputs from both areas before a final design is chosen. Configuration issues usually deal with what kind of designs will be chosen and how certain elements will be connected together. Questions of whether to use a box truss or a tetrahedral truss
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