Propulsion system with differential throttling of electric thrusters

1. A propulsion system for a spacecraft, the propulsion system comprising:

at least one group of electric thrusters spatially distributed in a multi-axis system; and

a controller in communication with each of the electric thrusters, the controller configured to differentially throttle thrusts of the electric thrusters with respect to propulsion force about two axes of the multi-axis system,

wherein the at least one group of electric thrusters includes a first group of electric thrusters and a second group of electric thrusters, and the controller includes a primary mode of operation in which the controller is configured to differentially throttle the thrusts of the first group in response to a first ascension angle and differentially throttle the thrusts of the second group for a predetermined time period in response to a second, different ascension angle, and

the controller includes a redundancy mode of operation triggered in response to an inoperability of at least one of the electric thrusters in the first group, and in the redundancy mode the controller is configured to differentially throttle a complimentary pair of the electric thrusters of the first group in which the thrust of the complimentary pair of the electric thrusters in non-primary off-axis directions cancels.

2. The propulsion system as recited in claim 1 , wherein the controller is configured to differentially throttle the thrusts with respect to unloading accumulated angular momentum of momentum wheels about the two axes of the multi-axis system.

3. The propulsion system as recited in claim 1 , wherein two or more of the electric thrusters are oriented in different directions.

4. The propulsion system as recited in claim 1 , wherein, with respect to the multi-axis system, each of the electric thrusters is oriented at approximately 45°, or positive or negative multiples thereof, to each of three orthogonal axes.

5. The propulsion system as recited in claim 1 , wherein the controller is configured to differentially throttle the thrusts of the first group with respect to the two axes of the multi-axis system and differentially throttle the thrusts of the second group with respect to a third axis of the multi-axis system.

6. The propulsion system as recited in claim 1 , wherein the first group is symmetrically arranged with respect to the second group.

7. The propulsion system as recited in claim 1 , wherein the propulsion system is gimbal-less.

8. The propulsion system as recited in claim 1 , wherein the controller is configured to differentially throttle the thrusts in with simultaneous or overlapping firing.

9. A method for a propulsion system, the method comprising:

for at least one group of electric thrusters spatially distributed in a multi-axis system, differentially throttling thrusts provided by the electric thrusters with respect to propulsion force about two axes of the multi-axis system,

wherein the at least one group includes a first group of electric thrusters and a second group of electric thrusters, and, in a primary mode of operation, differentially throttling the thrusts of the first group in response to a first ascension angle and differentially throttling the thrusts of the second group for a predetermined time period in response to a second, different ascension angle, and

triggering a redundancy mode of operation in response to an inoperability of at least one of the electric thrusters in the first group, and in the redundancy mode differentially throttling a complimentary pair of the electric thrusters of the first group in which the thrust of the complimentary pair of the electric thrusters in non-primary off-axis directions cancels.

10. The method as recited in claim 9 , including differentially throttling the thrusts with respect to unloading accumulated angular momentum of momentum wheels about the two axes of the multi-axis system.

11. The method as recited in claim 9 , including differentially throttling the thrusts of the first group with respect to the two axes of the multi-axis system and differentially throttling the thrusts of the second group with respect to a third axis of the multi-axis system.

12. The propulsion system as recited in claim 1 , wherein the propulsion system is gimbal-less, and, with respect to the multi-axis system, each of the electric thrusters is oriented at approximately 45°, or positive or negative multiples thereof, to each of three orthogonal axes.

13. The propulsion system as recited in claim 12 , wherein multiple pairs of the electric thrusters are oriented in different, complimentary directions such that thrust in non-primary off-axis directions cancels.