Design of the International Space Station Radiators to Tolerate Fluid Freezing
Expert Analysis
The customer selected liquid Ammonia as the heat transport fluid for the International Space Station External Active Thermal Control System (EATCS) early in its design because of its superior heat transport and heat transfer properties. This was an excellent choice for all aspects of the system except for one shortcoming - the Ammonia freezing temperature (-108°F) is well above the minimum temperature environment that the radiator would experience in the low earth orbit planned for the Station.
The Space Station liquid ammonia heat rejection radiator system contract was issued well after the selection of liquid ammonia for the EATCS and many of the components and subsystems were well into the design when the radiator freezing problem was uncovered. The minimum radiator environment temperature was determined to be -135°F, 27°F colder than the freezing temperature of ammonia (-108°F). The customer had two choices: (1) changing the heat transport fluid to one with a lower freezing temperature, a multimillion dollar cost and major schedule program impact, or (2) designing the heat rejection radiator to operate at below freezing temperatures. The second choice was judged to be a much smaller program impact if it could be accomplished.
The project manager assembled a multidiscipline team to evolve a radiator system design which would function in below freezing temperatures. The radiator was required to tolerate the freezing ammonia conditions and yet continue rejecting heat in the -135°F environment, which means some ammonia must continue to flow thru the radiator in these conditions. After considering several concepts, a variable radiator tube spacing approach was selected in which the tubes would freeze in selective order, outer tubes first, as the environment temperature and/or heat load decrease, shifting the flow to the middle tubes as the outer tubes freeze. The tube spacing profile was tailored so that all heat loads were met for all temperature environments. This solved the performance aspect of the design, enabling the radiator panels to reject heat even at environments well below the ammonia freezing temperature.
When ammonia is frozen solid in the radiator tubes and later thaws, a very high pressure may be generated inside the tubes because ammonia expands 10% on thawing. This could damage the tube. An innovative design was evolved that involved balancing the strength and elastic elongation properties of the radiator tube materials with the compressibility of the liquid ammonia to achieve acceptable pressures. These together with proper sizing of the tube diameter and tube thickness resulted in a design that would tolerate repeated freezing and thawing of the ammonia inside the tubes without damage.
The innovative freeze tolerant radiator design minimized the cost and schedule impacts of the freezing environment on the International Space Station Program by eliminating the need for a major redesign to the entire EATCS.
This consultant was the Project Manager for this effort, directing the multidiscipline team on a day-to-day basis, and with total responsibility for the outcome.
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