By Bruce Rule - August 27, 2013
If you Google “Asking the Right Question,” you will find references to Critical Thinking and to that old homily that “The question may be as or more important than the answer,” which, I think, is an overstatement but, nevertheless, is an interesting perspective.
Much of what I have posted on this site has been provided for two purposes: (1), to inform those long cut-off from the information stream that once was their professional concern about matters that may remain of interest, and which can be discussed on this site, and (2), to stress to those on active-duty that analysts should think “beyond the gram” and try to determine why they are seeing what is provided by their displays, and the operational, design, engineering, performance and acoustic detectability implications of that information.
This is where “asking the right question” applies. Perhaps the best example (that may be discussed) of what can result from asking the right question is the previously posted assessment that Project 636 KILO Class Russian diesel submarines almost certainly use a single-stage planetary gear to reduce the maximum propeller rpm from 500 rpm (older Project 877 KILOs) to 250 rpm for 636s.
The right question in this case was: “Is there any size difference between a dc motor that delivers 6,000 hp at 500 tpm and one that delivers 6,000 hp at half that speed: 250 rpm. Note that www.militarism.navy.ru/636/htm states that then-Soviet designers “reduced the speed of the main propulsion shaft line two-fold to provide a substantial reduction in the acoustic signature of Project 636 submarines.” The length of the inner (pressure) hull of 636 KILO units was increased by two frames (2 X 600mm), apparently, in part, to accommodate the reduction gear. (This Internet reference is provided to allay concerns that the maximum 250 rpm value for 636 units is classified information.)
The earlier posting on this subject provides a technical explanation of why a dc motor that delivers 6,000 hp at 250 rpm must have approximately twice the volume of a motor that delivers 6,000 rpm at 500 rpm. The writer, not having the technical background to answer that question, went, as discussed in the earlier posting, to an electrical engineer ((at the University of Kentucky (UK)) who provided an authoritative (and compelling) technical answer to that question. That answer was: twice the size. Further, and not previously discussed, is the consideration that placing the additional weight of a 250 rpm motor so far aft in the KILO would create serious stability (trim) problems, not so much of an issue with a reduction gear that is built into the engine block as previously done with 2D42 and 1D43 submarine diesels).
The writer did not – and still does not – understand the “flux density and current density value” factors the UK engineer cited in his assessment but that does not diminish the value of the assessment.
So, the bottom line: ask every question that careful consideration of acoustic data raises, and go where ever necessary to get the answer, and that may often be from beyond the Navy. (The UK engineer was not given any information on the motor other than it was a marine application.)
As has been said by someone else on this site, the only stupid question is the one not asked.
To those now on duty at CUS and elsewhere throughout the System, I say: “Do you own research; develop your own technical resources, i.e., be your own experts.”