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The International Space Station (ISS) Active Thermal Control System uses a single phase liquid ammonia system to collect and reject waste heat from the various space station systems. The expected cold environments in which the Heat Rejection Subsystem (HRS) radiators of the heat rejection system are to operate fall as low as -102.8 °C (-153 °F). Because the ammonia working fluid freezes at -77.7 °C (-108 °F) and since the environment temperatures are to remain below this level for 30 minutes per orbit, design approaches have been identified, implemented, and tested to ensure that the ISS Active Thermal Control System radiators will perform under these environments. There are several items of concern in a freeze-tolerant design. The flow tubes imbedded in the panel, from which heat is rejected, must be designed to tolerate potentially high pressure during a thaw. The supply and return manifold tubing must be designed to prevent ammonia from freezing within them.

The Photovoltaic Radiator (PVR) of the International Space Station (ISS) is a critical component of the Space Station’s thermal control system. It will cool the photovoltaic power system electronic equipment and the batteries used for power storage. The PVR will also provide environmental cooling for the service module during early phases of the Space Station. Prior to on-orbit operation, verification testing of the PVR was required to ensure the Orbital Replacement Unit (ORU) would operate as expected in the extreme on-orbit thermal environments. The on-orbit operational requirements included extreme thermal gradients, thermal cycles, and a range of coolant temperatures and flow rates, ORU pressure drop, deployment time, and electrical power consumption. The PVR ORU was tested in the world’s largest vacuum chamber at NASA Lewis Research Center’s (LeRC) Plum Brook Station Space Power Facility (SPF) located at Sandusky, Ohio.

The Heat Rejection Subsystem (HRS) Radiators Orbital Replaceable Unit (ORU) for the International Space Station has undergone thermal vacuum qualification testing at NASA Plum Brook Station, Space Power Facility in Ohio. The testing was conducted from December 1996 through January 1997 and October 4 through 18, 1997. This testing included confirmation of the heater control assembly (HCA) and heater performance that was initially tested during December 1996 through January 1997. Deployment system functional operations were tested for both hot and cold conditions using both the Integrated Motor Control Assembly (IMCA) and the Extravehicular Activity (EVA) drive.

The Photovoltaic Radiator (PVR) is designed to reject waste heat of the Early External Active Thermal Control System (EEATCS) and the Photovoltaic Thermal Control System (PV TCS) of the International Space Station (ISS). Two EEATCS PVR units and one PV TCS PVR unit will be on the Assured Early Research (AER) phase of the ISS and all four PV TCS PVR units will be on the assembly complete configuration of the ISS. Thermal environments of the AER mission and assembly complete configuration present challenging thermal designs to maintain the EEATCS and PV TCS PVR units functioning within the temperature operating limits of the structural components and the ammonia fluid.

Heat rejection requirements for the Photovoltaic Radiator (PVR) are derived from the Photovoltaic Module power generation and storage system electrical power requirements imposed by NASA. The requirement has been added to provide heat rejection for the Early Extended Active Thermal Control System (EEATCS) to support the Assured Early Research phase of the International Space Station (ISS) Mission. Early mission requirements have resulted in two of the units being coated with a silver Teflon tape. The original Thermal Control System (TCS) requirements require one of the units to be coated with Z-93. Qualification testing of the PVR includes both single panel and assembly level testing. Single panel testing is part of a qualification test program, which is designed to verify the pressure drop and thermal performance of the PVR.

Due to the extreme cold conditions in space, the International Space Station (ISS) Photovoltaic Radiators (PVR) were in danger of freezing the ammonia within the fluid loops. This would be catastrophic since the pressure buildup during thaw could potentially break the fluid flow lines. Design changes needed to be made to the radiators to insure their worthiness. Freeze tolerant flow tubes, optical coating changes to the manifold covers, multilayer insulation (MLI) around the fluid lines, and thermal isolation of manifold covers by mesh screen isolators are being combined to achieve an acceptable passive design.

The results of design and analysis of a heater system for the photovoltaic radiators (PVR)s on the International Space Station (ISS) is presented. The PVRs are designed to reject heat from batteries being charged by the solar cell arrays. Also, during the early buildup of the ISS, two of the PVRs are dedicated to rejecting waste heat from the man inhabited modules. The PVRs will at times experience environmental conditions that are severe enough to freeze the transport fluid (ammonia) which could cause failure of the coolant delivery lines. The heater system is designed to prevent this situation from occurring in the more vulnerable portions of the PVR. The activities involved in the design process and the thermal analyses used to support the design by aiding in the location of sensor, set point temperatures and failure mode analysis are presented.