Carbon-Fibre Reinforced Polymer Hexapod
The hexapod connects MIRI to the ISIM (Integrated Science Instrument Module) structure of the telescope. The structure is made by the Danish Space Research Institute (DSRI) and designed with 6 struts, 3 ISIM brackets and 3 MIRI brackets. The function of the hexapod is to hold MIRI mounted to JWST whilst keeping it thermally isolated the ISIM structure. The hexapod struts are made from Carbon-Fibre Reinforced Polymer (CFRP) and are a good thermal design because they long and the loads are transferred purely in tension or compression. The tubes have an inner diameter of 33 mm and a wall thickness of 1.3 mm As a result, the stresses across the section are uniform and all the material that is heat conductive is also doing its share of carrying the load. Since the length of the struts is already fixed, the cross section and the materials can be optimised. To avoid moment loads on the struts, the brackets have flexures in two directions to avoid transferring the moments.
Although the design of the struts is limited by stiffness and buckling considerations, using a high-modulus fibre would not help since there would be no advantage thermally and the high sensitivity of high-modulus fibre to microcracking is a real disadvantage.
| Weight of MIRI | 103 Kg |
| Minimum Eigenfrequency | 60 Hz |
| Max. g-load in any direction | 20 g |
| Operational temperature | 7K - 35K |
| Maximum heat flow through all 6 struts | 6 mW |
| Max. Weight of hexapod | 5 Kg |
To reduce the formation of the microcracks during the cool down period (occurs after departing the space craft launch vehicle), it is important that the fibre layers have the same thickness. Microcracks occur in the resin parallel with the fibres. Cracks are formed because the layer in the direction of the fibres have a high stiffness, high strength and a low CTE value; whilst the layer in the direction normal to the fibres have a high CTE, low strength and a low. During cool - down the layer contracts most in the direction normal to the fibres adding strain to both directions. In a laminate with same layer thickness, stresses will build up to the same level in both directions, and thereby dividing the stresses in an optimal way. The strength of the fibre layer is low in the normal direction and if the critical strength is exceeded then microcracks will formed.
The CFRP hexapod is provided by DSRI