Optimized support systems for spaceborne dewars

In a long-lifetime dewar, heat loads to the cryogen fall into two distinct categories: parasitic heat leak (i.e. conduction and radiation from the vacuum shell) and internal heat loads (i.e. electrical power dissipation and telescope aperture heating in the case of cooled telescope systems). To accommodate launch loads, the cryogen tank support system of a spaceborne dewar must be stout, and heat conducted from the vacuum shell through the supports can have a large influence on cryogen loss rate. Sophisticated support systems using low conductivity composite materials for thermal optimization have consequently been developed. The most thermally efficient support system used to date in flight hardware is fibreglass/epoxy tension straps. To provide even better thermal efficiency than straps, various attempts have been made over the last two decades to develop dual support concepts in which a primary support reacts launch loads and a smaller, more thermally efficient support then holds the cold assembly in place after launch. This Paper compares predicted dewar performance for several cases using both tension straps and the best developed dual support approach, passive orbital disconnect struts (PODS). Results show that cryogen loss rates are very similar for both systems. Straps provide a slight advantage for storage of lighter cryogens such as helium and hydrogen, and PODS are slightly better for heavier cryogens such as nitrogen and oxygen. The PODS system is more complex than straps, and a flightworthy design does not yet exist. The performance of a flight system using straps is at this time more predictable than one using PODS.