JWST ISIM Primary Structure and Kinematic Mount Configuration

摘要

The JWST Integrated Science Instrument Module (ISM) includes a large metering structure (approx. 2m x 2m x 1.5 (micro)m) that houses 4 instruments and a substantial dewar. The total ISM mass is 1300-1400 kg, including an allocation of 300 kg (21-23%) for the structure, and the ISM first mode frequency requirement is 25 Hz. The structure mounts to the back of the JWST primary mirror structure, and the on-orbit nominal operating temperature is in the range of 32-37 K, with on-orbit shifts of .5 K during operation. Stringent dimensional stability and repeatability requirements combined with mass limitations led to the selection of a composite bonded frame concept comprised of biased laminate tubes with axial Young's Modulus of greater than 30 msi (207 GPa). Even with the superb material specific stiffness, achieving the required frequency for the given mass allocations in conjunction with severe spatial limitations imposed by the instrument complement and the Optical Telescope interface has proven challenging. In response to and kinematic mount configurations, and settled on a concept comprised of over 70 m of tubes, over 50 bonded joint assemblies, and a 'split bi-pod' kinematic mount configuration. In this presentation we will review the evolution of the ISIM primary structure tube topology and kinematic mount configuration to the current baseline concept. We will also show optimization procedures used and challenges resulting from complex joints under launch loads. Two additional key ISIM structure challenges of meeting thermal distortion and stability requirements and metal-composite bonded joint survivability at cryogenic temperatures are covered in other presentations.