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Discussed are some critical theoretical and experimental research issues to be investigated for candidate two-phase thermal control systems (and their components), to define what is to be done to develop reliable systems, for near and far future planetary applications envisaged. An earlier publication SAE-2007-01-3242 (“Design of planetary two-phase thermal control systems, using experimental data of terrestrial model systems, built according to thermal-gravitational modelling and scaling laws”), discussed that such advanced thermal control systems are one of the key technologies needed for future applications within the framework of the NASA Authorization Act 2005. This act specifies a programme to be established to develop sustained human presence on the Moon, including a robust pre-curser programme to promote exploration, science, commerce and US preeminence in space, also as a stepping stone to future exploration of Mars and other planetary destinations.

Advanced thermal management systems are one of the key technologies needed for future applications within the framework of the NASA Authorization Act 2005. This Act specifies that a programme shall be established to develop a sustained human presence on the Moon, including a robust pre-curser programme to promote exploration, science, commerce and US pre-eminence in space, and as a stepping stone to future exploration of Mars and other destinations. This paper will elucidate the development of two-phase thermal management systems for Moon and Mars applications, using data obtained from experiments with terrestrial scale-model systems, built according to the thermal-gravitational modelling and scaling laws (derived from dimension-analytical similarity considerations). It will include also important supporting issues, like the compilation of flow pattern maps at various gravity levels and writing down the constitutive heat and mass transfer equations for these maps.

Several publications describe research carried out at NLR on the thermal-gravitational modelling and scaling of two-phase heat transport systems for spacecraft applications. They dealt with mechanically and capillary pumped two-phase loops. The activities pertained to pure geometric, pure fluid to fluid, or hybrid scaling between a prototype system and a model at the same gravity level, and between a prototype in micro-gravity and a model on earth. Recent publications also include the scaling aspects of a prototype loop for a Moon or Mars base application and a terrestrial model. The work discussed here was carried out in the last couple of years. It concerns scaling to super-gravity levels, and was done because a promising super-gravity application for (two-phase) heat transport systems can be the cooling of high power electronics in spinning satellites and in military aircraft.

In order to demonstrate two-phase heat transport system technology in orbit, the Dutch-Belgian Two-Phase eXperiment TPX was successfully flown as Get Away Special G557, aboard STS60, February 1994. Based on TPX conclusions and lessons learned, a reflight experiment TPX II is being developed in order to usefully fill the time gap between TPX and possible future full-scale Capillary Pumped Loop flights. The characteristics of TPX II, intended to fly early 1998, are discussed in detail: configuration and component changes, updates of objectives/scenario, current status, results of pre-launch (components) testing and outlook.

Condensers are crucial components of two-phase heat transport systems envisaged for future large spacecraft. To properly design such condensers, one uses experimental data, obtained from ground testing and reduced gravity aircraft and rocket flight testing, plus results of thermal modelling and scaling calculations. A frequently reported result of such activities, is that condensation lengths required in low-gravity environment exceed the corresponding lengths on earth (in horizontal ducts) up to one order of magnitude and more, while the accompanying pressure drops are almost the same.

Two mechanically pumped two-phase test rigs were built at NLR in order to experimentally study critical issues of spacecraft two-phase thermal management systems: a 5 kW, 31 mm ID, freon loop, focusing on the critical components of the ESA Two-Phase Heat Transport System. a 300 W, 4.93 mm ID, ammonia loop, to support the development of the ESA Capillary Pumped Loop Experiment (for the in-orbit demonstration of two-phase heat transport system technology) and to experimentally support two-phase thermal modelling and scaling activities. The rigs are described in detail. Typical test results are presented.

A possible solution to handle the heat dissipated by the batteries of the Columbus Polar Platform is to equip each battery ORU (Orbital Replacement Unit) with a dedicated heat pipe radiator. Such a radiator, being stowed during launch, has to be deployed in orbit and might be chosen to be steerable to achieve maximum radiator performance, hence minimum radiator size. In such radiator systems the coupling between battery and deployable or steerable radiator has to incorporate a rotatable or flexible thermal joint. Drivers for the design of such joints are a low thermal resistance, hence a limited temperature drop across the joint to optimize radiator size, and a small deployment/retraction torque or steering torque. Different concepts for moveable thermal joints are described and quantitatively discussed.

Two-phase heat transport systems are currently considered for the thermal management of future large power spacecraft. The monitoring of the quality, being the relative vapour mass content, of the two-phase mixture at various locations in the system, is valuable - possibly indispensable - for the proper operation of such a system. This paper reviews concepts for quality monitoring. Only a few concepts turn out to be suitable for spacecraft applications. Promising concepts are based on the capacitance, sonic velocity and index of refraction. These concepts are described and quantitatively analyzed. Applicability, advantages, restrictions and some hardware aspects are discussed.