KILO Class SS Design Characteristics Recovered from Open Sources
By Bruce Rule - August 25, 2013
KILO Class Submarine Main Propulsion Motor
In response to tasking to determine the volume of a dc motor that develops 6,000 horsepower at 500 rpm, an electrical engineer with special knowledge of the design characteristics of large dc motors used for marine propulsion derived a probable value of 370 to 390 cubic feet. This estimate was based on scaling from the dimensions of a known marine dc propulsion motor that developed 4,700 horsepower at 1,000 rpm. (As a point of interest, two of these 4,700 horsepower motors drove a single propeller shaft of a surface ship - a cable-layer - through a two-pinion gear system with a reduction ratio of about 8:1.)
The dimensions of the PG-141 main propulsion motor known to be installed in all Project 877 KILO Class submarines indicate a volume of 380 cubic feet, a value not known to the engineer before he made his estimate of 370-390 cubic feet.
Based on the principle that the volume of a dc motor determines the torque it can produce with a given set of flux density and current density values, the engineer determined that if a dc motor develops 6,000 horsepower at 500 rpm while a second motor develops 6,000 horsepower at 250 rpm with the same flux and current densities, the volume of the 250 rpm motor would have to be approximately twice the volume of the 500 rpm motor.
Within measurement error, the volume of the PG-141M main propulsion motor installed on Project 636 KILO Class submarines (determined from published to scale shaft line schematics) is the same as the volume of the PG-141 motor installed on Project 877 KILO Class submarines: 380 cubic feet. The dimensions required if the PG-141M had twice the volume of the PG-141 (760 cu ft) significantly exceed the 6.4-foot diameter and an 11.9-foot length measured from a detailed schematic of the PG-141.
Collectively, the above information is consistent with the use of a single-stage reduction gear system - probably a planetary design for space considerations - to reduce the maximum output speed of 500 rpm of the PG-141 motor to the 250 rpm maximum output speed of the PG-141M; hence the letter “M” (for Modification) appears to be the designation of the motor which has what the Russians refer to as a “built-in reducer.”
If the main propulsion motor installed on all Project 636 KILO Class submarines had twice the volume of the PG-141, it would be essentially a different motor, probably would not carry the PG-141M designation and could not be accommodated within the apace available in Project 636 KILO units.. Note, open sources confirm a reduction gear with a ratio of about three to one was used in the Project 1710 BELUGA Class submarine.
Comments: (1) The unanswered question is why Project 877 KILOs were built without a planetary gear to reduce the acoustic vulnerability associated with a high speed propeller? (2) This approach to the Soviet/Russian acknowledged noise reduction benefits of a lower speed propeller is an example of the oft-quoted Russian axiom: "Better is the enemy of good enough."