Fractionated satellites: a systems engineering analysis

Abstract

The current method of operating space-based assets involves the design and launch of large, monolithic spacecraft. These spacecraft are not responsive to failures or changes in mission requirements, as both require the launch of a completely new spacecraft. The concept of fractionated spacecraft was introduced in 2006 by the US Defence Advanced Research Projects Agency (DARPA) as a way of designing and operating space systems which is more responsive than current methods. The fractionated concept involves the decomposition of the traditional monolithic satellite into a number of free-flying spacecraft connected wirelessly. The free-flying satellites each carry a subset of the required subsystems and share their fractionated resources to achieve the mission objectives. Since 2006, studies of this concept have focused on analyses of the value and utility provided by fractionated spacecraft or on design studies of specific systems.

This thesis addresses two key questions: Firstly, is fractionation a mission enabling technology? Secondly, can design rules and guidelines be developed for fractionated satellites to allow continuity of measurements to be maintained and launched mass minimised? The first question is addressed by studying a payload and system for measuring coastal salinity from space. This provides an initial opportunity to assess the fractionated concept from a systems engineering point of view. The second question is addressed by undertaking a more general analysis of fractionated architectures, building on the knowledge gained in development of the fractionated coastal salinity measurement system. A computer model was developed to simulate the lifetime of different fractionated architectures when subjected to subsystem failures. A local search algorithm was used to find fractionated architectures which gave the best compromise between mass launched versus the operational time over a 50 year lifetime. The results from the application of this model showed that architectures that are highly fractionated, containing several homogeneous satellites, best achieved this compromise. These findings provide a contrast to the heterogeneously fractionated architectures proposed by DARPA. When a fractionated architecture is first implemented, the technology required to fractionate all the spacecraft subsystems may not be available. Consequently, the key to the implementation of these first fractionated architectures will be to ensure that there is redundancy in the fractionated subsystems spread across the architecture.

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