Performance Assessment of the ESM Ullage Pressure Control Assembly

The European Service Module (ESM) implements a bi-propellant propulsion system that provides translational thrust and attitude
control for Orion. The propellant tank pressure is regulated by the pressure control assembly (PCA). There are two PCAs, one for each of the MON and the MMH commodities. Each PCA comprises two redundant branches which control the flow between the high pressure helium tank and the propellant tank. Each branch integrates one bi-stable valve and two mono-stable valves in series with a flow control orifice. The flight software reads the ullage pressure and commands the valves open and closed in a bang-bang fashion to target a predefined pressure set point. The flow dynamics throughout the PCA are very rapid and the resulting interactions between the valves need to be managed to limit the loads. A pneumatic model of the PCA was developed and validated against experimental results. The model was then used to assess the results observed during the acceptance tests of the flight hardware.