between the thrust that is produced and a plane perpendicular to the axis a may be acceptable providing no resultant thrust of significant amplitude is generated in one direction or the other along the axis a under drive from the combined thrust of the various open valves in the first set 24 the valves 24 a and 24 c are oriented to produce side thrust in mutually opposite directions parallel to an axis b1 whereas the valves 24 b and 24 d are oriented to produce side thrust in mutually opposite directions parallel to an axis c1 the axes b1 and c1 in this example are orthogonal or substantially orthogonal and they intersect each other on the axis a it should be observed that the thrust axes of the valves 24 a and 24 c are parallel but not in alignment since they are situated on opposite sides of the axis b1 likewise the thrust axes of the valves 24 b and 24 d are parallel but not in alignment since they are preferably situated on either side of the axis c1 thus the thrusts from the valves in the set 24 give rise to rotational torque in a direction f1 about the axis a in the second set 26 the valves 26 a and 26 c are oriented to.