Navigation Brief
This is an e-newsletter originally published Oct. 29
ALEXANRIA – Good evening, Drifters
Somewhere in Florida, an amphibian former naval officer and commentator is opening this email with his blood pressure rising.
Yes, CDR Salamander has lots and lots of receipts on the littoral combat ship. And, to be fair, the Navy hasn’t exactly been able to prove him wrong just yet. The latest data point that the LCS may have some significant problems is the unfortunate deployment of the LCS Detroit, which ended with Detroit limping its way back to Mayport this week with a broken combining gear.
Let me explain what that is: The LCS is a combined diesel and gas propulsion train. It has two lower-power diesel engines that run the ship at lower speeds, then it has two gas turbines that COMBINE the power of the diesels with the power of two gigantic gas turbine engines in what’s called the combining gear.
It’s a complex transmission that connects power from the two large gas turbine engines and two main propulsion diesel engines to the ship’s propulsion shafts, which transmits to water jets that propel the ship through the water. That’s how the LCS goes zoom. Or is supposed to.
Read my story on that here:
Another littoral combat ship breaks down on deployment
One of the real joys of being a reporter is you get to be a kind of knowledge vampire, where you take people who have dedicated their lives to a certain subject, drain them of their knowledge and use it for your work.
This is what I’ve done here in the latest in the Drift Conversations Series. Tonight, we welcome Associate Professor Naval Architecture and Marine Engineering Matthew Collette to The Drift, to chat about what he knows best.
With that, let’s Drift!
-DBL
The Thing About CODAG
The Drift: This combined diesel and gas propulsion train, is it fundamentally unreliable? Was this just never going to work?
Matthew Collette: We've had combined diesel or gas, and diesel and gas, plants for a long time. The Germans did the design work for the first combining gear plant in the late 1950s. But it hasn’t won out because of its complexity. Most navies offer another approach. There's been a handful of ships built with this technology. The national security cutter, its goal is to split one gas turbine amongst two shaft, as opposed to combining a gas turbine and diesel on each draft.
So, it's something that should work. If a student did it, I wouldn't tell them they were wrong. But it's like everything else on LCS: It was driven by the speed requirement. Because the diesels are about 20% of the power of the gas turbine, you don't want to leave them out when you're trying to hit that sprint speed. It's such a demanding requirement.
And, yeah, 42 megawatts is big. It's certainly up there in terms of the size of combining gearboxes we've seen. And, so, my guess – and I have no inside scoop on this – is it just is right on the edge of not working. Or it works if it can be maintained perfectly but reality happens at sea and then we get a shut down or another problem.
So, it is not a technology that strikes me as inherently problematic. But like a lot of the LCS decisions, we were kind of trying to get every little last ounce of performance out of the vessel. And you stack up a whole bunch of those decisions that the ship is not as reliable as you would like.
TD: Is this a fixable problem?
MC: Fixing LCS now is a huge problem, because we generally don't design gearboxes to be replaced during the ship’s service life. Could you switch it out with an either/or gearbox instead of an combining gearbox? I mean, I think you can on the whiteboard and it would drop the top speed a little, but we're trying to prove the reliability. I'm not sure you could do that in the confines of the ship, however.
TD: Why is that? Is it because the ship was designed with a certain propulsion train in mind?
MC: I think is mainly because we don't assume the gearboxes have to be replaced during the ship’s service life. With a gas turbine, we leave room so it can be pulled out and exchanged. The diesel engines, we leave room so you can pull off the cylinder heads, take off the cylinder liners and everything else. The gearbox is normally assumed to be functional for the ship's whole service life. So, other than routine inspection and maintenance, we're not assuming you have to get in there and swap it out. If you do, now you're talking a major dry dock experience where you might have to cut into the bottom shell or the side shell to get it out of the vessel. And you'd be very limited in what you can get in its place, because there's not a lot of extra room left around it. It's a difficult problem to get around that.
TD: So, is the solution just to never engage the gas turbines if that is what breaks the combining gear?
MC: Operating in that way really kind of hurts my heart as a naval architect, because you have a high-speed vessel, that's going to be driven around 11-12 knots for the rest of its life. I could do that and probably increase the reliability. Without the details of how the gears failing, it's much harder to say how easy it is going to be to fix this.
If it’s filters, or the lube oil system is not strong enough for certain operating conditions? You could upgrade those parts of the gearbox. If there's a fundamental design flaw, then it gets much harder to figure out how you're going to fix this issue cost effectively.
TD: Okay, so we’ve got about 11 more of these to come off the lines between 2021 and 2024, between both classes. Would it be feasible to just say, hey, stop the presses? Let’s redesign the propulsion train, even if it takes two years?
MC: In theory, you would think so because the gearbox should be simpler and potentially smaller. Without seeing the plans of the ship, it's hard to know exactly if that’s possible. But that would certainly be something that if I were on the staff at NAVSEA, I bring to the CNO as an option.
If we don't really think we need 40+ knots and we're happy to live in the high 30s this is an option. The Navy has been kind of hot and cold about how critical this high-speed requirement really is for their operations. You may be, in effect, giving up nothing to get rid of it. Or if they've decided it is critical, then maybe that's not acceptable. So, it really comes down to how important they think getting every knot of speed of that platform is.
TD: If the Navy came to you and said, “Matthew, we just want pickup trucks now. We want something we can drive around that’s reliable and all you really need to worry about is changing the oil.” What would the go-to configurations be?
MC: On surface ships today, I think the two biggest ones are the diesel or gas turbine engines, where you don't try to combine them in the same gearbox. It's an either/or switch. So, you either clutch in the diesel, or you clutch the gas turbine. And that's simpler, because there's two power paths to the gearbox, but they're never on at the same time so they are both design independent of each other. Or diesel electric that we're seeing a lot of other ships.
Or. if you look at what the Danish did with their recent frigates, they've just given up on the higher speed altogether and they're accepting mid-20s on a pure diesel propulsion, which is pretty easy to pull off.
--
Our sincerest thanks to Mathew for joining The Drift tonight. I have a feeling he’ll be back on this forum if he cares to be.
On to The Hotwash!
The Hotwash
Straight to the links tonight, I reckon.
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