An investigation of the feasibility of a spacecraft multifunctional structure using commercial electrochemical cells

Abstract

Multifunctional structures offer the potential for large savings in the mass and cost of spacecraft missions. By combining the functions of one or more subsystems with the primary structure, mass is reduced and internal volume freed up for additional payload, or removed to reduce structural mass. Lithium batteries, increasingly preferred to other power storage solutions, can be employed to produce such structures by incorporating prismatic batteries into structural sandwich panels. Such “powerstructures” can reduce the mass and volume of the power storage subsystem.
After reviewing the current work in the field of multifunctional structures, this thesis describes the objective of the research, to examine the usefulness and feasibility of a multifunctional structure based on commercial lithium cells and sandwich structures. The next section presents a study that quantifies the benefits of this technology, showing maximum savings of up to 2% of total mass, and 0.5-1% for common spacecraft designs.
The next section describes experimental investigations into the mechanical suitability of commercial PLI cells for use in the multifunctional structure. Firstly, the effect of launch vibration was considered: 15 and 25 grms tests showed no measurable loss in electrical performance. Then, the structural attributes of the cells were measured using a dynamic shear test. The shear modulus of the cells was found to be rather lower than that of an aluminium honeycomb core material.
Consideration is then given to the practical implications of a multifunctional structure. The feasibility of manufacturing is assessed through the construction of a trial panel, showing that the cells lose some capacity and suffer an increase in internal resistance in a high-temperature adhesive cure and that a cold-bonding process may thus be preferable. The resultant panel was then vibrated on an electrodynamic shaker to both assess the resilience of the cells and test the reliability of finite element models. These finite element models are then used for a simple optimisation, showing that a welldesigned powerstructure can have structural performance comparable to a conventional design.
The final section weighs the benefits of using a multifunctional structure against the potential disadvantages in terms of cost, design time and flexibility, as well as assessing the validity of assumptions made in the work. The conclusion is that a multifunctional structure of this type, whilst not worthwhile for all mission types, could potentially increase the feasibility of short-term spacecraft missions using small satellites (of the order of 100 kg) with large energy storage requirements.

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