Bradford’s Flow Control Unit (FCU) technology provides precise control of the xenon propellant flow rates to ion thrusters. The FCU makes use of proportional flow control valves to precisely control the flow rate of xenon by controlling the pressure differential over a fixed hydraulic resistance (i.e. thermally controlled flow restrictor).

In principle one of two control logics can be applied.

FCU with Closed Loop Control

Using the downstream temperature-corrected pressure measurement as a signal for closed-loop feedback, variable current is applied to the proportional flow control valve to control the effective flow restrictor inlet pressure and thereby control the mass flow rate to the thrusters.

FCU with Open Loop Control

In this architecture, the flow rate control is performed using a feedback device outside the FCU, e.g. the thruster current. Based on the thruster current, variable current is applied to the proportional valve inside the FCU to control the flow rate. This open loop control strategy allows for a simplified and more cost-effective FCU, while sharing the same component heritage and qualification status as the closed-loop FCU. The neutralizer mass flow rate is set to a fixed value, which is set by using a defined hydraulic resistance only. Solenoid isolation valves are employed in the FCU, to provide xenon isolation and branch selection.