An apparatus in accordance 6 wherein said supply means comprises an annular chamber surrounding said evacuation chamber, means for supplying {HEAD ~ said drive medium} to said annular chamber under pressure and a ring nozzle means for feeding the drive medium from {TAIL ~ said annular chamber} to said evacuation chamber at a supersonic speed 
An apparatus in accordance 6 wherein said supply means comprises an annular chamber surrounding said evacuation chamber , means for supplying { HEAD ~ said drive medium } to said annular chamber under pressure and a ring nozzle means for feeding the drive medium from { TAIL ~ said annular chamber } to said evacuation chamber at a supersonic speed
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