Demonstration of No-Vent Fill and Solidification of Nitrogen for the Nicmos Dewar

Abstract

In order to minimize parasitic heat loads and achieve a five-year mission lifetime, the NICMOS solid nitrogen dewar has been designed with a single fluid line serving as both fill and vent for the cryogen tank. A no-vent fill of liquid nitrogen (LN₂) is necessary to achieve a reasonable fill time with this configuration. High density of the solid nitrogen (SN₂) is also desired in order to maximize dewar efficiency. A test was conducted at Ball Electro-Optics and Cryogenics Division in December of 1992 to demonstrate the capability to perform a no-vent fill of LN₂ and to determine the attainable solid nitrogen density. The test was successful in achieving a complete no-vent fill of the 67 L test tank with LN₂. The loaded LN₂ was then solidified using cold helium gas circulated through the tank wall heat exchanger. A series of additional loading steps achieved a bulk cryogen fill density of 94% of the maximum SN₂ density at 60K. The no-vent fill procedure should be applicable to other solid cryogen systems in which the cryogen can exist in a liquid phase, and could be a candidate method for on-orbit replenishment of certain spaceborne liquid cryogen systems.