ible that can each provide controlled thrust 

 in any direction (Fig. 8.8). For forward pro- 

 pulsion the pitch of the blades remains fixed; 

 for translational thrust in any other direc- 

 tion the blade pitch can be cyclically varied 

 with each revolution of the propeller. By 

 thrust coupling the blades, rotation of the 

 vehicle in any angular motion can be pro- 

 vided, thus six degrees of freedom are possi- 

 ble with the TPS. Model tests by the U.S. 



Naval Ship R&D Center in the late 1960's 

 showed the system to be capable of its pro- 

 posed potential (up to 2.2 knots) beyond 

 which the model became unstable. A station- 

 ary tail provided the model good stability at 

 3 to 5 knots, but beyond 6 knots the vehicle 

 once again became inherently unstable. 

 While 2.2 knots are more than satisfactory 

 for submersible speed, other factors contrib- 

 uted to the demise of the TPS: The system is 



Fig 8 8 One end of a cigar-shaped tandem propulsion system (U S Navy) 



376 



