Follow-on Studies Using the Voyager Spacecraft Thermal Model

The 42-year-old Voyager Interstellar Mission (VIM) is operating long beyond its design life. In 2012, Voyager 1 crossed into interstellar space and Voyager 2 is believed to be very close to making the same transit. Due to declining power output from the Radioisotope Thermoelectric Generators (RTGs) the Science and Flight Operations teams continue to make difficult choices, managing both the power and thermal margins to preserve critical science observations and maintain the health of the two spacecraft. A previous paper, “Creating a Voyager Thermal Model 39 Years Into the Flight Mission, Along With Model Correlation and Application” described how a thermal model of these spacecraft was developed and correlated without many design artifacts and with limited telemetry. This paper describes how the thermal model has been used to establish an Allowable Flight Temperature (AFT) limit for hydrazine propellant in the propulsion subsystem to minimize the risk of freezing. Voyager 2 temperatures have already descended to this limit near the Roll thruster propellant lines. The Flight Operations team has investigated several ways of detecting propellant freezing based on analysis and trending of thruster performance telemetry. In addition, the Voyager thermal model is being used to predict the spacecraft response to possible changes in power state. These changes could involve turning off outboard science instruments and/or their heaters to increase power margin and hence power dissipation inside the spacecraft bus (i.e. in Bay 7, where the power regulation electronics are located). Changes might also be made to turn on or off other loads inside the bus to more effectively heat the coldest propellant lines. Many of these changes have been or will be tested first on Voyager 1 which has more power margin and does not have the power matrix commanding issues experienced on Voyager 2.