Was trying to find the exact actual propulsion train arrangement for the Freedom class LCS to see why a total and ongoing loss of propulsion would be possible on the Milwaukee.
From what I have found so far it has 4 prime movers (2 Rolls Royce gas turbines and 2 Colt-Pielstick diesel engines) and 4 water jets, 2 of which (apparently the inboard ones) are steerable.
Not familiar with the speed (RPM) of a water jet impeller but this would imply a reduction gear for at least the gas turbine driven jets and most likely the diesel driven ones as well. Some kind of clutch arrangement would also seem to be required for prime mover starting.
However, one story includes the following:
[I]“The crew cleaned the combining gear filters following established procedures, but locked the port shaft as a precautionary measure to prevent possible shaft damage. Thursday evening, while conducting routine steering checks, the ship lost pressure in the starboard combining gear lube oil system. The casualty was due to similar metallic debris contamination of the filter.”[/I]
Locking a “port” shaft and talking about a “stbd” shaft doesn’t make immediate sense to me with 4 jets 4 prime movers? Do these gear boxes have two outputs?
Then I found this SNAME paper abstract:
[I]The USS Freedom propulsion system consists of a quartet of waterjets driven by a combined diesel and gas turbine (CODAG) power plant through a complex shafting system. The two propulsion shafting systems (port and starboard) each have a gas turbine and/or a diesel engine coupled to “combining reduction gearbox.” The combining reduction gearbox drives a “splitter reduction gearbox” through a “gear coupling lineshaft.” The splitter reduction gearbox, in turn, drives two waterjets via a “boost waterjet*lineshaft” and a “steerable waterjet lineshaft.” The boost waterjet lineshaft is concentric with the inboard quill shaft of the splitter reduction gearbox and is engaged via an overhanging clutch. The diesel engines, gearboxes, and gas turbines are resiliently mounted. There are four different shafting arrangements with a total of 28 support bearings, four gearboxes, two over-hanging clutches, and four axial (longitudinal) flexible couplings. The gear coupling lineshaft operates at over 1400 RPM and is supported by bearings with a relatively high bearing load influence number to load ratio. All these factors present unique challenges to the installation and alignment of the shafting system.[/I]
Hey what could go wrong with this arrangement? With metal in two of gearbox filters I would say the “unique challenges to the installation and alignment of the shafting system” were not entirely overcome.
The very concept which would seem to have been intended to provide redundancy (by allowing either prime mover to power either jet and to have a port starboard system) has injected a complexity in design and construction that resulted in having 4 working prime movers and 4 water jets an not being able to run even one set to get home.
Also found an earlier propulsion loss incident on the lead ship the Freedom which seemed to be related to the SSDG’s being lost and that affecting propulsion until they got more generators on line.
where this can be found:
[I]The initial problem was traced to turbocharger exhaust leaks on the No. 2 ship service diesel generator (SSDG), one of four generators that supply the ship with electrical power. Further examination after the power loss found similar problems on the No. 3 generator.
Problems with load shedding, or reducing electrical power to avoid overloading the generators, also were found, Doss said in the statement.
After a brief period of single generator operations, the crew brought more generators online and then brought the engines back online, restoring propulsion.[/I]
[I]The Littoral Combat Ship (LCS-1) USS Freedom’s ship service diesel generators (SSDG) continue to give the vessel fits, recently causing the LCS to lose propulsion briefly while heading out to participate in at-sea exercises off the coast of Singapore and forcing it to return to port, the U.S. Navy says.[/I]
One can’t help but notice the rather strong opposition to this program on gcaptain threads but if it is gobbling this loudly and often maybe it is a turkey.