Feasibility of the motorized momentum exchange tether system : an investigation of system risk

• Main Author: Draper, Christopher Hayward
• Published: University of Glasgow 2006
• Subjects: T Technology (General)
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.768666

This thesis examines the feasibility of a motorized momentum exchange tether (MMET) system being used to perform commercial space launches. The MMET system is an on-orbit launch concept that could be used to reduce the cost of access to space, thereby catalysing a broader range of space-enabled business concepts. The research presented in this thesis assumes this cost of access to space for a reasonable launch system can be presented as the adverse financial risk of its operation. Under this assumption, the research concludes that an MMET-based system would be a feasible alternative to an equivalently capable conventional system if the risk associated with the system is less than that associated with the alternative. To illustrate the concepts and approaches presented within, this thesis presents an assessment of the proposed Lunar Staged MMET (LSM) mission, an assessment that indicates the MMET is a feasible alternative for completing such a mission under specific analytical and market conditions. The expected financial risk is presented in this thesis as the product of the mission cost and the probability of mission failure. The cost of each mission is calculated from the perspective of the end customer, and the long-term price of such services is computed using publicly available data and the assumption that the commercial space industry can be modelled as an oligopoly. Support for such a model is contained in the literature and through this research, which compares the quarterly financial data published by the Boeing Company against the international commercial launch rate. The probability of system failure associated with an MMET-based unconventional launch system must account for a number of factors. For the first, conventional stage of the system, assessing the probability of stage failure is found through an examination of observed failure rates relative to conventional engineering reliability estimates for conventional launch vehicles. Through this examination, a novel approach to calculating the rate at which the probability of failure for vehicles produced within a variant class changes as a function of time is presented, an approach that offers a valid technique for applying reliability growth across a series of vehicles that are best considered to be independent vehicles. The thesis goes on to present the results of research into various component aspects that are vital to the design and analysis of a tether-based system. First, the research explores the strength of tethers modelled as braided aramid ropes, which supports claims of strain dependence regarding aramid fibre strength that can have significant strength benefits and indicates that this phenomenon should be accounted for in any operational architecture. Second, the thesis presents an empirical hypervelocity impact effects equation calibrated for use with tethers, which indicates that the currently accepted approach to oblique hypervelocity impacts may not be appropriate for tether analyses. Thirdly, research into fractured impactor dispersion after a hypervelocity impact on tether targets is presented, which indicates that the commonly accepted one-impact- one-failure assumption employed for multi-line tether analyses may not be sufficient. TetherLife, an analytical program developed to calculate the expected lifetime of an MMET system given various sub-span parameters, employs the products of these research areas to calculate the mean time to failure for a range of tether sizes and orientations. After combining the probability of failure associated with the conventional launch vehicle component of the MMET-based unconventional launch system, the probability of failure associated with the MMET system, the probability or failure associated with handing a payload between systems, and the likely cost of deploying a suitable set of MMET systems, a comparison can be made between the financial risk associated with completing a specific mission using an MMET based unconventional launch system verses completion of the same mission using conventional means. For the LSM mission examined within the research, an MMET-based system would be a reasonable option if an average of 85 missions per year are required, contingent on specific analytical assumptions. While such a number of lunar supply missions are not currently required, the conclusion that the MMET system can be an alternative to a conventional system under various circumstances offers support for continuing current research on system design and analysis.