NASA Glenn Research Center is responsible for managing development of the Service Module for the Orion Crew Exploration Vehicle (CEV), which includes part of the heat rejection system for the CM (Crew Module) and SM (Service Module). The astronauts and electrical subsystems in the CM and SM generate heat that will cause overheating if not rejected to space. Heat rejection is accomplished by using heat exchangers to transfer excess heat to a heat transfer fluid which is pumped in a loop through radiators exposed to the cold of space on the outside of the Service Module. The photo below depicts the Orion service module and highlights the radiator panels heat rejection system as well as the umbilical housing, thrusters, lunar science payload, main engine, solar array and high-gain antenna.
A fluid candidate for CEV that was earlier used on Apollo is a mixture of propylene-glycol (PG) and water. This mixture is desirable due to its low toxicity and good heat transfer properties. However, under certain combinations of fluid temperature, flow rate, and space thermal environment, a radiator using a PG and water mixture can experience a condition called “stall” or “stagnation.” During stall, viscosity changes in the fluid cause a subset of radiator tubes to experience drastically reduced flow while other tubes see a corresponding increase in flow. The net effect is seen as a temperature increase at the radiator outlet. Stall has been analytically predicted using a variety of techniques, but the ability of analysis to predict radiator performance over the full range of operating conditions was previously unknown.
A “Radiator Stall Physics Experiment” was designed to simulate and control the fluid conditions leading to stall and provide data to validate the analytic methods characterizing stall performance. The Orion Stall Physics Experiment utilizes two fluid loops: one carried a PG and water mixture, the other a cold fluid (Dow Syltherm XLT) to simulate the space environment. Separate commercial laboratory chiller/heaters feed both loops and pass the fluid through six stainless steel tubes. The cold Syltherm flows in jackets over six eight-foot long tubes carrying the PG and water mixture. Both loops were instrumented for temperature, flow, and pressure.
A variety of tests were conducted on the Stall Physics Experiment to verify test rig performance while initiating stall conditions.