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After a general introduction on two-phase thermal control system issues, the paper reviews the status of and lists commonalities and differences between the only two currently developed aerospace-related mechanically pumped two-phase thermal control systems. These are the Russian Segment Active Thermal Control System (RSATCS) hybrid two-phase ammonia thermal control system for the Russian segment of the International Space Station ISS, and the Tracker Thermal Control System (TTCS) hybrid two-phase carbon dioxide thermal control loop for the AMS-2 attached ISS payload.

As it is not possible to directly use commercial liquid flow meters in spacecraft fluid loops, a study was carried out for the European Space Agency to adapt commercial flow meter assemblies for space applications. The various activities (described in detail) eventually led to the selection of two commercial units, which were redesigned/adapted to be used in spacecraft single-phase (water) and two-phase (ammonia) thermal control loops. These flow meter assemblies were tested according to an agreed test programme, that included performance and calibration tests in a test bench (developed during the study), vibration testing and EMC/EMI testing. The results are discussed in order to assess to what extent the study objectives were met. Recommendations for future work are given also.

The Alpha Magnetic Spectrometer AMS-2 is planned for a five years mission as attached payload on ISS, the International Space Station. It is an international experiment searching for anti-matter, dark matter, and missing matter. AMS-2, an improved version of AMS-1 flown on STS 91, consists of various particle detector systems, one of these being the (Silicon) Tracker. The trade-off based choice and the experimental feasibility demonstration of a mechanically pumped two-phase CO2 cooling loop for the Tracker is discussed in detail. The current status and ongoing and planned development activities are discussed.

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.

The paper deals with heat and mass transfer research issues related to the development of spacecraft active thermal control systems, more specifically development of two-phase heat transport system technology. It focuses on design and development supporting theoretical work: the thermal/gravitational scaling of two-phase heat transport systems, including the aspects of gravity level dependent two-phase flow pattern mapping and condensation.

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.

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.

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.

Aspects of the gravitational scaling of spacecraft two-phase heat transport systems are discussed. Supporting similitude considerations, based on dimension analysis, and useful equations and correlations are given.