Design of Working Fluid Venting System for Mechanical Pumped Fluid Loop Heat Rejection System for Mars Missions
In several Mars missions flown by JPL, a mechanically pumped loop utilizing Freon-11 as the working fluid is utilized for thermal control of the spacecraft components during cruise. The fluid loop controls temperatures of components within the cruise stage as well the entry vehicle's lander or rover. During entry into the Mars atmosphere, the cruise stage needs to be separated from the entry vehicle, which requires the tubing connecting the two stages to be severed. Just cutting the tubing, without venting the freon in a controlled manner, would lead to a random direction and speed of the effluent fluid. The impulse from this would create a significant and non-deterministic nutation of the entry vehicle, which could jeopardize its entry. To avoid that impact, the Freon is vented in a very carefully controlled direction and speed to ensure that the direction of the vent vector passes along the center of gravity of the entry vehicle or is canceled by opposing nozzles. An innovative scheme of using the pressurized gas to "piston out" the Freon was designed and implemented in these missions. Additional concerns with the vent nozzle not freezing up due to the sublimation of the liquid Freon while escaping into the vacuum of space were investigated and mitigated in this design. A detailed fluid-thermal-impulse model was created to predict the resultant thrust profile and total impulse imparted on the spacecraft. This paper will describe the basic design, the corresponding design & analysis to assess and predict its performance, the trade -off of design concepts, and its implementation for flight usage.