Gas bearing system

1. A bearing assembly for stabilizing rotation of a rotatable element in vacuum conditions, the bearing assembly comprising:

a static gas bearing acting as a bearing for the rotatable element;

an ultrasonic vibration pump coupled to the static gas bearing, for providing pressurized gas to the static gas bearing; and

a sealed housing enclosing the gas bearing and the ultrasonic vibration pump,

wherein the ultrasonic vibration pump comprises a pump inlet through which unpressurized gas enclosed in the sealed housing may enter the ultrasonic vibration pump.

2. The bearing assembly according to claim 1 , further comprising a control unit for controlling operation of the ultrasonic vibration pump,

wherein the control unit is adapted to control a power that is supplied to the ultrasonic vibration pump depending on a rotation speed of the rotatable element.

3. The bearing assembly according to claim 2 , wherein the control unit is adapted to control the power that is supplied to the ultrasonic vibration pump in such manner that a higher pressurization is applied for rotating speeds below a predefined threshold and a lower pressurization is applied for rotating speeds above the predefined threshold.

4. The bearing assembly according to claim 2 , wherein the power to be supplied to the ultrasonic vibration pump is determined based on the rotation speed of the rotatable element, using a lookup table.

5. The bearing assembly according to claim 1 , wherein the ultrasonic vibration pump is an ultrasonic membrane pump.

6. The bearing assembly according to claim 1 ,

wherein the ultrasonic vibration pump comprises a solid state valve; and/or

wherein an inlet of the ultrasonic vibration pump is coupled to a particle filter.

7. The bearing assembly according to claim 1 , wherein the static gas bearing acts as a radial bearing for the rotatable element.

8. The bearing assembly according to claim 1 , further comprising one or more second gas bearings acting as axial bearings for the rotatable element,

wherein the one or more second gas bearings are enclosed by the sealed housing.

9. The bearing assembly according to claim 1 , wherein the static gas bearing is a spherical gas bearing.

10. The bearing assembly according to claim 1 , wherein the ultrasonic vibration pump is a first ultrasonic vibration pump, the bearing assembly further comprising at least one second ultrasonic vibration pump that is coupled to the static gas bearing,

wherein the at least one second ultrasonic vibration pump is enclosed by the sealed housing and provides redundancy to the first ultrasonic vibration pump.

11. The bearing assembly according to claim 10 , further comprising at least one third ultrasonic vibration pump that is coupled to the static gas bearing,

wherein the at least one third ultrasonic vibration pump is enclosed by the sealed housing and coupled in series with the first ultrasonic vibration pump.

12. The bearing assembly according to claim 1 , wherein a gas pressure in the sealed housing is in the range from 10 kPA to 80 kPA.

13. The bearing assembly according to claim 1 ,

wherein the sealed housing is a flexible casing; and/or

wherein the sealed housing comprises a sealed joint.

14. The bearing assembly according to claim 1 , wherein the rotatable element is one of:

a reaction wheel for a spacecraft;

a control moment gyroscope for a spacecraft; or

a reaction sphere for a spacecraft.

15. The bearing assembly according to claim 1 , wherein the ultrasonic vibration pump is a first ultrasonic vibration pump, the bearing assembly further comprising another ultrasonic vibration pump that is enclosed by the sealed housing and coupled to the static gas bearing in series with the first ultrasonic vibration pump.

16. A reaction wheel assembly for a spacecraft, the reaction wheel assembly comprising:

a reaction wheel; and

the bearing assembly according to claim 1 , for stabilizing rotation of the reaction wheel.