The implementation of cost effective debris protection in unmanned spacecraft

Abstract

The spatial density of debris in popular regions of low Earth orbit is sufficiently high that the impact risk to unmanned spacecraft can no longer be ignored. To enhance protection, designers generally consider increasing the shielding of existing materials and structures. This may be through the addition of layers of high-strength materials such as Nextel and Kevlar to MLI-covered surfaces, or by modifying the design of honeycomb panels. The benefit of this type of protection comes at the price of increased mass and cost. Another approach is to consider the vulnerability and layout of equipment on a spacecraft, and configure the spacecraft so that mission-critical items are protected from the most vulnerable regions, possibly by less critical neighbouring items. This approach provides protection enhancement without incurring significant mass penalty. However, up until now, it has generally been poorly considered. Part of the reason for this has been the lack of a methodology for evaluating and comparing the survivability of different spacecraft designs. The development of a solution to this problem forms the core of this research.

The basis of the solution is the construction of a software tool called SHIELD. This has some similarities to existing tools such as ESABASE / DEBRIS, in that it determines the impact and penetration distributions on a representative 3-D model of a given spacecraft, and the probabilities of penetration. However, in SHIELD, the computation goes a step further by calculating the probability of failure of the satellite. SHIELD does this by determining the internal damage caused by each of the penetrating particles distributed on the satellite. Any equipment that lies within the line of sight of a given particles trajectory is considered vulnerable. Multi-wall ballistic limit equations are called up to ascertain the extent of penetration inside the satellite, and therefore identify exactly which units are damaged. Knowing the criticality of these units, the consequences for the mission are then easily derived. When all penetrators are considered in this fashion, the result is the desired satellite failure probability. To derive a true measure of a satellite's survivability, SHIELD combines this failure probability with information on the various costs associated with the mission, i.e. a life cycle cost model. The result is a 'survivability metric' that enables the cost-effectiveness of radically different protection strategies to be determined and compared in a completely objective manner. This metric is also incorporated into a genetic algorithm, which is capable of searching many competing protection solutions and converging on cost-effective options.

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