Propellant gas supply for an ionic propulsion unit
A field of feeding propellant gas to ion thrusters, and in particular a method of feeding an ion thruster with propellant gas coming from a pressurized tank via a feed circuit including an on/off valve and, in succession downstream from the on/off valve, a high pressure restrictor, a buffer tank, and at least one low pressure restrictor. The method includes the steps of calculating a pressure setpoint for the buffer tank as a function of a flow rate setpoint, calculating the difference between the pressure setpoint for the buffer tank and a pressure measured in the buffer tank, calculating a setpoint for the opening time of the on/off valve as a function of the difference and of a pressure inside the pressurized tank, and opening the on/off valve in compliance with the opening time setpoint.
1. A method of feeding an ion thruster with propellant gas coming from a pressurized tank via a feed circuit comprising an on/off valve and, in succession downstream from said on/off valve, a high pressure restrictor, a buffer tank, and at least one low pressure restrictor, the method comprising:
calculating a pressure setpoint for the buffer tank as a first function of a flow rate setpoint;
calculating a difference between the pressure setpoint for the buffer tank and a buffer tank pressure measured in the buffer tank;
calculating an opening time setpoint for the on/off valve as a second function of said difference and of a pressurized tank pressure inside said pressurized tank; and
opening the on/off valve in compliance with said opening time setpoint,
wherein the on/off valve is a single on/off valve controlled to occupy only either an open position or a closed position and not to occupy positions between the open position and the closed position, in order to regulate, according to the flow rate setpoint, a flow rate of the propellant gas from the buffer tank to the ion thruster, and the opening of the on/off valve in compliance with the opening time setpoint is triggered only when the pressure setpoint for the buffer tank is at least 5% greater than the buffer tank pressure measured in the buffer tank.
2. The method according to claim 1 , wherein said opening time setpoint is calculated on the basis of an inverse fluid flow model for said feed circuit.
3. The method according to claim 1 , wherein the pressure setpoint for the buffer tank is calculated on the basis of an inverse fluid flow model of the at least one low pressure restrictor downstream from the buffer tank.
4. The method according to claim 1 , wherein the opening of the on/off valve in compliance with the opening time setpoint is triggered only if the buffer tank pressure measured in the buffer tank is decreasing or zero.
5. The method according to claim 1 , wherein the opening of the on/off valve in compliance with the opening time setpoint is triggered only if the buffer tank is decreasing or zero.
6. The method according to claim 1 , wherein said feed circuit includes a bifurcation downstream from the at least one low pressure restrictor, having a first branch for feeding an anode section of the ion thruster and a second branch for feeding a cathode section of the ion thruster, each of said first and second branches having a respective additional restrictor.
7. The method according to claim 1 , wherein the pressurized tank pressure inside said pressurized tank is measured indirectly on the basis of an initial pressure and of an overall flow of propellant gas that has passed through the feed circuit.
8. The method according to claim 1 , wherein said feed circuit includes a safety valve upstream from the on/off valve.
9. The method according to claim 1 , wherein the pressure setpoint for the buffer tank is calculated as Q c ,
where Q c , is the flow rate setpoint, PdC avai is an equivalent head loss of the feed circuit downstream from the buffer tank measured in liquid ohms (Lohms), k g is a coefficient of the propellant gas, and f(T) is a correction factor that is a function of a temperature T of the propellant gas.