SHAPE: Shape Memory for a High Turn-Down Ratio

Spacecraft designed for missions beyond low earth orbit (LEO) face a difficult thermal control challenge: they are required to reject a high heat load to warm orbital environments and a low heat load to cold transit environments, necessitating a quite high turn-down ratio. This difficult challenge can be transformed into a tractable design problem for arbitrarily high turn-down ratios through the use of shape memory alloys, materials that exhibit a temperature dependent phase change such that they can be easily deformed below the transition and recover a predefined shape above the transition. In fact, shape memory alloys can be trained to behave in a two-way manner so that it takes one shape above and another below the transition temperature. Such materials make possible a passively deployed heat rejection device which adjusts vehicle and environment loads based on its operating temperature alone. This project seeks to train a shape memory material and evaluate its behavior. This project trained a shape memory material to behave in a two-way manner and evaluated the capability of the two-way shape memory effect. A literature review identified an optimum training method involving alternating constrained and unconstrained thermal cycles. The effect of two bending strain rates were investigated through training rigs of differing radii, both trained in parallel. It was identified that in both cases cold working of the material results in some deformation to the austinite shape. This cold work deformation can be addressed through careful design of the hot set shape of the material.The results indicate that the greatest delta between austinite and martensite shapes is obtained through the lesser training strain (large radius rig). Projection of the results of two-way training of the material indicate that sufficient deformation can be trained to successfully employ in a full sized heat rejection device.

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