Thermal Challenges in the Development of the Hydrazine Propelled Roll and Attitude Control Subystem for the VEGA Launcher

Abstract

The thermal design of the Roll and Attitude Control Subsystem (RACS) of the VEGA launch vehicle has been developed by Astrium GmbH to ensure that there is no possibility of a hydrazine explosion resulting from deflagration, detonation or thermal runaway between the lift off of the launch vehicle and the final depletion of the system at the end of the mission. Hydrazine vapour and residual priming gas bubbles can occur in the propellant upstream of the flow control valve seat where combinations of high temperatures and water hammer pressure peaks are generated during system operations. The thruster mounting bracket design limits the soak back heat reaching the thruster flow control valves and optimizes the conductance paths to phase change material heat sinks located on the brackets. The temperature limits applicable to the hydrazine wetted surfaces in the thruster and feed lines had to be raised beyond the heritage limits applicable to the Ariane 5 attitude control subsystem on account of the analytical temperature predictions and uncertainties taking into account the flight environmental conditions and the demanding mission requirements. These consider both roll control in the early boost phases of VEGA as well as requirements derived from the European Code of Conduct for Space Debris Mitigation resulting in the incorporation of a propellant depletion system activated by pyro valve after separation of the payload. A routine qualification of the system, its thrusters and its associated flow control valves had to give way to a more sophisticated dedicated qualification philosophy with analytical prediction of all critical mission phases as well as final verification by hot firing tests.