The Cryogenic Thermal System Design of NASA’s James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) 2005-01-3041

The thermal design and modeling of NASA’s James Webb Space Telescope (JWST) Integrated Science Instrument Module (ISIM) is described. The ISIM utilizes a series of large radiators to passively cool its three near-infrared instruments to below 37 Kelvin. A single mid-infrared instrument is further cooled to below 7 Kelvin via stored solid Hydrogen (SH₂). These complex cooling requirements, combined with the JWST concept of a large deployed aperture optical telescope, also passively cooled to below 50 Kelvin, makes JWST one of the most unique and thermally challenging space missions flown to date. Currently in the preliminary design stage and scheduled for launch in 2010, NASA’s JWST is expected to replace the Hubble Space Telescope as the premier space based astronomical observatory. With a six meter primary mirror aperture operating at cryogenic temperatures, the scientific instruments (SIs) on board are expected to observe the early universe, one million to a few billion years old, when the first stars and galaxies began to form.

Approaching 1800 Kilograms, the ISIM system consists of four scientific instruments, a SH₂ dewar system, the structure they attach to, heat straps, heat switches, and a large radiator system. Approximately sixteen square meters of radiator is required to reject the 400–500 milliWatts of ISIM generated power and parasitic heat. Along with the challenge of providing an on-orbit passive design, the ISIM thermal system must also control temperatures during the early-orbit and cool-down phases of the mission.

After a brief overview of the JWST mission and observatory configuration the ISIM thermal system is described in detail along with the thermal analysis approach. Steady state heat load predictions and transient modeling results are also described.