NOVEL ORBITAL DISCONNECT SUPPORT FOR CRYOGENIC TANKS

The orbital cryogenic lifetime of Dewars can be significantly increase d if the supports which fix the cryogen tank to the Dewar vacuum vesse l are disconnected on orbit under 0 g conditions. In a study performed under an Estec contract, 16 different concepts of purely passive and of active cryogen tank supports were assessed. Based on the outcome of this assessment a 'semi-active' disconnect concept was selected which makes use of the change of length of a 'shape memory alloy' (SMA) sup port element which is located at the warm end of a conventional strap- type cryogen tank support. This SMA element is at its extended length at room and lower temperatures and its length is designed such that th e strap tank suspension has no pretension or even any contact to the t ank support strap at room and lower temperatures. Upon heating the SMA element electrically to almost-equal-to 85-degrees-C, it contracts su ch that the tank support straps come into contact with the SMA element and are pretensioned by its contraction. In a conventional strap tank support with lower and upper straps the upper straps are equipped wit h SMA elements. These elements are not in contact with the straps on t he ground. For structural tests on the ground, and for launch and land ing (of a reusable Dewar) the SMA elements are activated/heated. Durin g all other periods on the ground and when on orbit the SMA elements a re not activated. Hence on the ground heat transfer via conduction fro m the cryogen tank straps occurs via the lower straps only. On orbit t he SMA elements are in their extended length mode, such that all the s uspension straps are out of contact, resulting in negligible heat tran sfer via conduction due to unavoidable rest contacts under practically zero load.