the axially elongated first primary oxidizer intake orifices 10 on the outer surface of the combustion chamber control plates 13 are mounted in a sliding manner so as to slidably move in a direction generally parallel to the central axis 3 on external side 1a of the outer wall 1 the control plates 13 are guided at one of their ends by studs 14 and collar 15 and may be guided at their other ends by pins 16 which extend from the outer wall 1 stub shaft 17 is rigidly attached to and extends from an end of each of the control plates 13 such that it slidably engages an oblique slot 18 defined by the actuating ring 19 which rotates about the central axis 3 the engagement of the stub shafts 17 with the oblique slots 18 are such that the control plates 13 are moved axially generally parallel to the central axis 3 as the actuating ring 19 is pivoted about the central axis 3 each control plate 13 defines a second elongated primary oxidizer intake orifice 20 having a shape and dimensions substantially identical to those of the first primary oxidizer intake orifices 10 as illustrated the second orifices 20 lie in an annular array extending about the central axis 3 such that each is circumferentially aligned with a first orifice.