The Royal Navy’s carrier, Prince of Wales (QE class), was
sidelined with a propulsion casualty one day into a 2022 attempted voyage to
the US and has been undergoing repairs ever since. The problem, estimated to cost $31M to fix, was
caused by a miniscule misalignment of the propeller shaft.
The US Navy encountered something similar when the Aegis
cruise, Port Royal, gently drifted aground off Hawaii and suffered unrecoverable
misalignment of its radar arrays.
Militaries should not be producing machinery that is so
sensitive that it cannot withstand typical combat shocks.
We discussed pod propulsion (see “PodPropulsion”) in a previous post and noted that it potentially offered
many benefits. I’m certainly no expert
on pods but it would seem that a pod would bypass all shaft alignment issues.
I like the idea of pod propulsion from a combat damage
resilience perspective, separate from whatever its propulsion performance
is. A pod, even if it’s rendered
unrepairable, can be simply unbolted from the outside of the stern and a
replacement put in place. There is no
need to open the ship up, as there is a for a damaged or misaligned shaft.
The investigation into the cause of the starboard [propeller] shaft fault found that there was an installation error. More specifically, Wallace added that based on “initial reports” the shaft was misaligned by as much as 0.8mm to 1mm.[1]The reason this caught my eye was the apparent sensitivity of the shaft to very, very small misalignment. One can’t help but wonder what would happen if the ship were subjected to vibrations, shock, and whipsawing from an explosion in combat. Would the shaft hold up or would it inevitably wind up misaligned by 0.8 mm or more and be rendered unfit?
 |
| Pods - No more shafts? |
Again, I’m not a propulsion pod expert, by any means, and
I’m sure that pods have their own unique problems but they seem well worth
investigating.
__________________________
https://breakingdefense.com/2023/05/millimeters-cost-millions-uk-still-to-decide-who-should-cough-up-for-31m-aircraft-carrier-repair-bill/
Since pods are electrically powered, I imagine that not only will they be easier to swap out, the engines could be tuned in a manner that would allow them to run at a given optimal RPM. Much like the way a locomotive engine runs in narrow range of RPM. This decreases the stress on the engine, making it more reliable. From what I have read recently about the pods, they are also often steerable allowing the ship to maneuver in ways not available with the normal configuration. The more I have read about the pods, the more I wonder why this hasn't already been implemented. Maybe be cause it does make sense? I can see the Navy saying they need to study it more, while international shipping is adopting it already.
ReplyDeletehttps://new.abb.com/marine/systems-and-solutions/azipod
https://www.siemens-energy.com/global/en/offerings/industrial-applications/marine/podded-propulsion.html
I think cruise ships use them extensively. They have large hotel electricity loads so it makes complete sense to have an electric propulsion system.
DeleteCargo ships still use shafts because they can direct drive from slow speed marine diesel engines. Simple and cheap.
I know the Navy is often conservative on these things, but if they plan ships with lots of extra electrical generation then there are fewer steps to implementing electric propulsion.
"the more I wonder why this hasn't already been implemented."
DeleteThe biggest obstacle (aside from institutional inertia) is that they have not, generally, been available in large enough sizes (meaning horsepower) to achieve the 30+ knot speeds a warship needs. There have been some recent pods with more HP. Alternatively, we can mount additional pods to gain the required HP.
Another nice thing about podded propulsion is that it free's up a lot of internal space. You can also locate the gen sets in different locations around the ship other than the standard midships area, giving you more options in case of battle damage. The emergency diesels on ships that I served on were always located in the bow area or fantail. Propulsion pods do require their own ducted AC cooling units to keep them cool in warm waters. The colder the motor the more efficent it is. Plus side is they would run great in arctic waters, as well as greater manuevering in restricted waters.
ReplyDeleteCynically, a somewhat slower but reliable podded warship might be seen as preferable to a much faster "Drydock Queen".
ReplyDeleteYou're thinking operationally! Excellent! In this day of supersonic missiles, a few knots of speed more or less is probably irrelevant compared to reliability and maintainability ... assuming pods have those characteristics.
DeleteIt always amazes me that the power requirement of a ship increases at the cube of velocity. And fuel consumption per mile increases with the square of velocity. So if warship speed isn't as important anymore then there is a massive opportunity to take cost and complexity out of the ships.
DeleteYou can use this to increase range or make changes to the powertrain. The gas turbines most US Navy ships use are very cheap per MW with expensive fuel. Nuclear reactors have cheap fuel but are expensive to install per MW. So the math on which powertrain type is better can change rapidly if you reduce top speed. As batteries get better there might be an opportunity to use them for "burst" speed to run away from a torpedo or to run other systems if those are important features.
Ship speed is an extremely important requirement to nail down because of the power laws involved.
No surprise at all!
ReplyDeleteUK has lost civilian ship building industry for a long time. While it constructed the two large carriers, they simply had lost most experiences already.
The issue of the post was not whether the UK had lost some shipbuilding expertise that caused it to produce an out of spec shaft by 0.8-1.0 mm but whether it is wise to build COMBAT ships that are acutely sensitive to exceedingly minor variances?
DeleteI am no expert but I read, then wrote about this eight years ago because the advantages of pods are obvious and proven.
ReplyDeleteThe US Navy built the Zumwalt class destroyer with new technologies but they were too immature and too expensive so only three will sail. It lacks important advances such as a podded drive system (right) that provides several advantages: 1) unprecedented 360 degree maneuverability so the ship can turn on a dime to evade missiles and torpedoes; 2) bow thrusters or tugs are not needed to dock or maneuver in shallow waters; 3) no vulnerable drive shaft or rudder is needed; 4) redundant systems (assuming at least two pods); 5) damaged pods can be replaced in a day and repaired ashore. The cruise ship industry has eagerly embraced this new technology to cut costs.
From what I can gather, it's all about speed and HP. Can they generate the HP for the required speed. The last time I looked at it, it was pretty iffy. Since then, I've seen some bigger units appear and it may well be doable now.
DeleteThe Constellation frigate would have been a good opportunity to try it out by building a single prototype but that chance is gone.
One of the otherwise useless Zumwalts could be retrofitted with pods as a prototype although that would be quite expensive since the ships are already built.
I also have questions about a pod's combat damage resilience but that's what prototypes are for!
The Queen Mary 2 is capable of 30 knots max & 26 knots cruise, it uses 4 pods for propulsion. So speed is not a factor.
DeleteMA
"So speed is not a factor."
DeleteIt appears that the QM2 pods generate 29 khp each for a total of 116 khp. For comparison, Wiki lists the Burke as having 4x LM2500 at 26.25 kbhp each for a total of 105 khp. However, the QM2 displacement is 79,000 t versus the Burke at 9500 t. Thus, the Burke has significantly greater hp/ton.
Still, it would seem the QM2 pods are in the vicinity of being combat useful (30+ knot speed).
CNO, have you heard recently of the Bryan McGrath appointment to the National Commission on the Future of the Navy?
ReplyDeleteHe posted his 10 points regarding what he considers his key points to believe in. Notably, Forward Presence he considers as not a mission and he intends to address readiness problem first.
Link here if you are intterested: https://conservativewahoo.substack.com/p/what-i-believe?utm_source=profile&utm_medium=reader2
I wonder if he knows the the US has more shipyard workers than Japan and nearly as many as South Korea. It is a lack of competition in the US shipbuilding industry (due to the Jones Act) that makes it uncompetitive, not a lack of government funding. The vast majority of revenues for shipyards comes from the government, so there is no lack of that and that business stays whether we open our shipyards to global competition or not.
Delete"10 points"
DeleteI've read that and find it woefully lacking and centrally incorrect. What's your analysis?
"I wonder if he knows the the US has more shipyard workers than Japan and nearly as many as South Korea."
DeleteIs that true? Cite some numbers.
If it is, is it relevant? For example, I suspect (with no data to back me up!) that half of our 'workers' are probably administrative rather than trades. Further, I know that a significant number of our workers are employed in small yards and would be irrelevant to a discussion of naval or larger commercial construction needs. Again, cite some data.
You've brought up an interesting topic. Follow through on it with some data!
The number in traditional shipyards is around 100,000. The census says 150,000, but I think they include small boat manufacturers.
Deletehttps://www.enotrans.org/article/decline-u-s-shipbuilding-industry-cautionary-tale-foreign-subsidies-destroying-u-s-jobs/
In Japan there are 70,000-80,000 with this being report being ten years old.
https://www.oecd.org/japan/PeerReview-Shipbuilding-Japan.pdf
South Korea had almost 200,000 before COVID, but has dropped to around 100,000 now.
https://maritimefairtrade.org/south-korea-faces-shortage-of-shipbuilding-workers/
The mechanism by which the Jones Act makes our ship builders uncompetitive is that commercial shipyards tend to be extremely specialized. They make the same ship over and over. The US market is not big enough for that so you have our commercial yards building bulk ships, wind turbine installing ships, etc. all together. No foreign shipyards can come in and compete. Then of course government-funded shipyards are government-funded shipyards. So I'm sure you are correct that there are too many administrative workers. The way to fix that is open up competition so that our shipyards are forced to specialize and become more productive. A tiny fraction of our shipyard workers employed at global productivity levels would make us a top 5 ship building country.
The Brits should have studied the LPD CLass, at least the 1st 5 ships, that were eating gears due to lack of alignment bolts. All this CAD push isn't delivering what it is supposed to. I have seen companies refuse to buy the whole CAD package because of cost and so you do not get thermal, vibration, stress, or space conflict analyses that these packages can give you to at least help focus the experienced human to probable problem areas.
ReplyDeleteCynics suggest that a key part of the explanation for the building of the two RN carriers is that the Rosyth shipyard got much of the work. It is located in the parliamentary constituency that was Gordon Brown's - Chancellor of the Exchequer and later Prime Minister.
ReplyDeleteI knew Brown when we were young adults - a capable chap. It seems a career in politics amplifies weaknesses of character.
Worth noting that HMS Prince of Wales was lost primarily due to battle(Torpedo) damage resulting in shaft distortion and misalignment. Cascading power loss (leading to inability to effectively use her AA armament) and flooding were the results.
ReplyDeleteShaft damage is a common occurrence in battle. The recent sinking of the Norwegian frigate, Helge Ingstad, was due in part to flooding through the shaft assembly. Shafts are a recognized damage control weakness.
Delete.8-1mm is about .03-.04" I've done alignment on rotating equipment and I think there is more going on than misalignment. Would need to know shaft length and distance between bearing supports.
ReplyDeleteThe azipod has a shaft to connect the engine to the screw, but it is a much shorter one so it is theoretically less likely to get as far out of line, and it doesn't require the shaft alley space (where HNoMS Helge Ingstad went wrong) of conventional screws.
ReplyDeleteThere are a number of naval vessels with azipods. The Spanish/Australian Juan Carlos/Canberra and the French Mistral class come quickly to mind. I have proposed both of them for the USN as smaller and cheaper (hence lower risk) alternatives to the current LHAs/LHDs.
"The azipod has a shaft to connect the engine to the screw,"
Delete? Are you referring to the shaft within the pod? I guess that's technically a shaft but not in the sense that we're thinking of on a conventional propulsion system. Depending on the pod size and design, the pod shaft would be inches or a few feet in length.
"I guess that's technically a shaft but not in the sense that we're thinking of on a conventional propulsion system. Depending on the pod size and design, the pod shaft would be inches or a few feet in length."
DeleteThat's pretty much what I said.
Something that critical to the operation of the propeller shaft should have been checked and verified before Prince of Wales ever left port.
ReplyDeleteGood post.
ReplyDeleteI wonder what switching to pods would do to a ship's signature.
"I wonder what switching to pods would do to a ship's signature."
DeleteThat's a good question and the answer would strongly influence my support (or lack thereof) for pods. I have no information on pod acoustic signatures but I would speculate that they would be quieter than a conventional ship since the entire large, noisy, gear machinery and shaft train would be eliminated. Of course, the motor in each pod would have to be acoustically isolated but that seems doable since we do that now for ship's machinery.
I've also read studies that claim the pods produce a much more streamlined flow (less flow noise) but I've not read a direct acoustic comparison study.
I addressed this some weeks back when we were discussing armor:
ReplyDelete"Nuclear powered and all electric. Work with Mr. Bechtel. More than 86,000 SHP because you’ll need more than a Tico to drive a armored ship through the water. I don’t care how big the ship is.
2-4 steerable electric drive pods; no extended 20+ yard propellor shafts strung under the hull; because battle damage susceptibility."
And-
"Mr. Bechtel, I want the battleship powered by 2 S1B reactors; just like you have planned for the Columbia. Why not A-series like we use in the carriers? They’re too damned big and complex. Mostly too big in terms of space required. Remember, this is an electric ship; you’re spinning electric generators for the propulsion pods not driving screws. If the math for a couple of S1Bs doesn’t work come back and tell me so. By the way, EACH reactor has to be able to power the entire ship 100% independently. We’ll have two for redundancy."
My point IS that coupling reactors with electrically-driven pods seems likely more durable in terms of potential battle-damage.
Disney and Princess Cruises are able to make pods work. The USN has very clearly, over the years, ruled out same. My personal guess is that it's because the Surface Navy has an institutional bias AGAINST nuclear power. Likely based on 1970s views about nukes unfiltered by industry engineering improvements over the last 50+ years.
Let me add a little more- And also start off by saying I myself see horsepower as being a function of power generation.
DeleteNRC has just approved a NuScale reactor design which is rated for 50 megawatts ( https://www.scientificamerican.com/article/first-u-s-small-nuclear-reactor-design-is-approved/ )
I don't see, yet, what dimensions for that plant are. But IF a SMR won't fit in a Tico (or Idaho!) hull just use a couple of S1-Bs a la Columbia
The AZIPOD ( https://new.abb.com/marine/systems-and-solutions/azipod#benefits ) is rated at 22 megawatts. And I can't tell from the marketing literature IF 22MW is what's required to drive the AZIPOD. Kinda' like software marketing is written so you can believe whatever you want to believe.
But it's all I got to work with.
More problems with my thinking:
1. After a limited look I can't tell how much power a S1-B generates. Enough to power a AzimuthPod? Think so but don't know for sure.
2. I don't know for sure how much electrical power a AZIPOD (industry term) requires.
But this is what I DO know:
- I'll stick 3 junior officers, each with a EE degree, in a room over a weekend. And then I'll know if I can power a Tico up to 33-knots with either or both SMR or S1-B using electric azimuth pod drives.
BUSHIPS, as an organization, was succeeded by NAVSEA which has a budget of $36B approx. Part of NAVSEAs mission is 'NAVSEA and its predecessor organizations have been responsible for the design, construction, delivery, maintenance, and disposal of our Navy’s ships and ship systems.' Keyword being 'design.'
Current leader of NAVSEA is VADM Galinis who is a EE from The Academy. Galinis has been in the job since June 2020 and so is on his way out. Let's hope his successor is more than a placeholder that will consider more innovation than we've seen to-date.
"My point IS that coupling reactors with electrically-driven pods seems likely more durable in terms of potential battle-damage."
DeleteWithout thoroughly understanding ALL the pros and cons of pod propulsion, it would seem that pods offer space and weight savings along with enhanced ease of repair. However, linking that to a nuclear power requirement does not seem to be a given nor even necessarily desirable. It might be a good combination but it does not jump out as such, on balance. In particular, the issue of combat damage is a major risk factor. A single, physically minor hit/damage to, say, a circulating radioactive fluid could render the entire ship unusable. I don't intimately know the specifics of any particular nuclear reactor design so perhaps this isn't even a possible scenario?
We can get/train lots of turbine operator/techs but it's very difficult to get nuke operator/techs.
Sunken nuclear ships are an issue as is ultimate retirement and disposal.
I've done posts comparing nuclear and convention power and there is no overwhelming case for one over the other.
"can't tell from the marketing literature IF 22MW is what's required to drive the AZIPOD."
DeleteI agree that marketing brochures are not clear. For example, the QM2's four pods are described as
"Queen Mary 2's power plant comprises four sixteen-cylinder Wärtsilä 16V46CR EnviroEngine marine diesel engines, generating a combined 67,200 kW (90,100 hp) at 514 rpm, and two General Electric LM2500+ gas turbines, provide a further 50,000 kW (67,000 hp); these drive electric generators, which in turn provide the power to drive four 21,500 kW (28,800 hp) Alstom electrical motors located inside the podded propulsors ... "
It's unclear whether that means that each pod DRAWS 21.5 MW or delivers 21.5 MW to the pod props.
A statement by the pod manufacturer says,
"The system comprises two fixed and two azimuthing pod units delivering a combined output of more than 85 MW"
This states that the OUTPUT of the pod motor is 85 MW (114,000 hp). It's also unclear whether that 85 MW is bhp or shp or something else.
Assuming 85 MW is the output from the pod motor, the input power must be something greater than 85 MW since there are always losses. If one assumes, say, a 10% loss, the input power feed (from turbines, a nuclear reactor, or whatever) must be around 94 MW.
Unless, of course, I'm completely misunderstanding all this!
Finally, what's important is delivered hp relative to ship weight (displacement as the common 'weight'). That's what determines speed (along with hydrodynamics of the hull, L/W, etc.)
I'm inclined towards reactors because I'm looking for the greatest power-generation density in the smallest form factor. Betcha' this is one of the reasons, if not THE reason, gas-turbines have replace naval steam plants. Power-density.
DeleteHowever- Despite conducting additional research I can't find the physical dimensions of a SMR anywhere; so let's take that one off the board.
Are reactors more delicate wrt battle damage? Dunno but there goes our sub force if so. So, for the moment, let's take that one off the board.
And why, exactly, do we need reactor operators? No, no- Hear me out please.
B-17s in 1943 had cables strung throughout to operate control surfaces. 40 years later, in 1983, F-16s are fly-by-wire and don't use mechanical cables. 40 more years have given us auto-pilot and self-driving cars. Fuel injection and electronic ignition in our cars; neither of which can you or I fix. Yep, you can argue we shouldn't build systems into warships which can't be fixed but seems to me the trend is clearly going towards black-box systems which either work or don't work with little or no in-between. Consider, please, your TV, your car and your watch. Form factors for all would be recognizable in 1943 but all are extraordinarily more complex, more reliable and more capable today.
So why not a black-box reactor run entirely by software? Answering engine bells. Designed as a (ARMORED!) cartridge which can be unplugged, replaced and then disposed of modularly. And all with fewer moving parts than a gas-turbine which, at minimum, means lower maintenance costs are going to drive power plants in this direction. COMNAVOPS makes the point that pods can simply be replaced vs. being repaired.
And yes, I fully acknowledge that Naval Reactors is entirely predicated on 1980s practices learned from and installed on the 688-class. But- USN used to station sailors in 100-degree engine rooms, clipboard in hand, so they could log the readout on a mechanical temperature gauge. Those days are (almost-) gone and rightfully so. The boys in NR are going to see their world change. 2063, 40 years from now, will bear little resemblance to 2023 much less 1983.
OK. All of that is pie-in-the-sky wishful thinking contingent on manufacturing and design which don't currently exist. I submit the software technology to eliminate manual operation while preserving safety margins pretty much exists now.
I've gone a bit astray on COMNAVOPS post regarding feasibility/desirability of pods. But he caused me to think about what future trends look like given the history of all product development, in almost every industry you can name, that we've seen since WW2. The trends are pretty clear.
If only the USN had an organization dedicated to thinking about those trends and how same might guide DESIGN. Oh, wait! That would squarely be in NAVSEAs wheel house. And maybe, just maybe, NAVSEA is thinking about this and has a tiger-team already dedicated.
And if they don't it'll be just one more thing I have to fix when I become SECNAV. After the next election.
"I'm inclined towards reactors because I'm looking for the greatest power-generation density"
DeleteWell, sure, if we're talking about a future technology wish list and someone actually creates a box size reactor that requires no personnel then I'm with you.
For the present, I'm focused not on power density but on combat effectiveness.
If you want to consider your hypothetical reactor then you can equally consider Star Wars type lasers, rail guns, and a host of other non-existent technologies.
Hey, I have nothing against speculation and, in fact, it serves a fantastic purpose by forcing/allowing us to peek into the possible future and start preparing for it. So, as long as you're speculating ... carry on!
"recognizable in 1943 but all are extraordinarily more complex, more reliable and more capable today."
DeleteWell, the reliable portion of that statement is highly debatable. Take, for example, combat aircraft. In WWII, aircraft, if they weren't physically shot up, had a very high reliability and readiness rate because they could be repaired with duct tape and a monkey wrench. In contrast, today's aircraft struggle to achieve a 50% MINIMAL readiness rate and a 25% fully mission capable rate as documented by DOT&E reports. Similarly, our ships, from WWII through the Cold War, had high readiness rates. Today's Navy is almost unable to deploy other than sporadically. I can list the nearly endless examples of ships unable to deploy or deploying with known, degraded systems but I'm sure you know the list as well as I do.
In fact, the evidence demonstrates that our modern complexity and black boxes have brought us a military with staggering readiness problems.
On a personal note, as a teenager, I could fix almost any problem on a car. Today, I can't even reach the spark plugs to change them and you need a computer to have a chance to even figure out what the problem might be and little hope of fixing it.
I'm not sure your premise holds!
I'm very conscious about arguing with you, COMNAVOPS, on your platform. I don't ever want to argue for sport, but I do want to lend whatever minor insights and different perspectives (2 complete sentences separated by a comma is an automatic FAIL (Mrs. Woods; Senior Comp)). OTOH, maybe the 'but' makes it a subjective clause?
DeleteSo. Very Respectfully, Sir...
You make my point(s) exactly.
WWII aircraft had higher readiness rates. Because they could do so much less. Mag compasses in B-17s were sturdier/stupider than the inertial guidance system in A-6 Intruders which could carry greater payloads. Which was better wrt CEP (even given back in the NOW Stone Age 70s/80s)? Losing inertial nav would degrade, if not down, a modern aircraft. Readiness stats in WWII were stoneage also. Readiness, nowadays, is a very granular thing; no aircraft or ship is 100% so readiness is not a simple GO/NO-GO thing. "We'll bomb by hand", my ass. Not in this day and age.
Your mention of auto repairability is actually a concept that was very much in my mind when I wrote my 2 prior posts. Because you pulled up that example you're likely approximately my same age. I went through all that. Re-built multiple Corvair engines with my Dad. Other engines too. Watched him re-build a Holley Quadra-Jet (Idiot; what was he thinking? Diff generation). And I myself note too, with fascinating interest, that I can do little more than oil and brakes on my 2006 Acura. It's mostly a black box.
So yeah. You could work on the 'old cars.' But. The Old Cars wouldn't last 200K miles. My 1999 Yukon is just over 200K and doing well because it's been taken care of. Doesn't matter how well you took care of a 1970 Camaro; it wouldn't run 200K without one or more engine/tranny rebuilds.
Modern cars are horribly expensive, vastly more capable and last longer than earlier models. And so it goes with weapon systems and their platforms. This is the world we live in and I don't think I'd trade it for what existed 40 years earlier (80 if you want to go back to WWII). 'Weapons of Choice' by John Birmingham postulates how a modern battlegroup would do against WWII counterparts. Fun read.
So forgive me for thinking, still, my premise holds exactly. As the great man, Thomas Sowell, said, "There are no solutions. Only tradeoffs."
In conclusion, I submit trends are clear:
- Greater cost.
- More complexity.
- Greater capability
- Greater reliability over time.
Let's use your car example wrt greater reliability. The 1908 Model T couldn't be driven across the county without breaking down. Certainly, not cross-state.
By the early 60s, you could drive a splendidly maintained auto cross-country. Nowadays you can drive cross-country without checking the oil even once.
My point is that reliability progresses with incremental improvements over years. We're at the suckage point of the reliability curve now but it will get better, over time, and will be worth it. cf Ford class.
IMHO.
"conscious about arguing with you"
DeleteDon't hesitate to argue - although I much prefer discussion rather than argument. All you need is facts and logic.
"Because they could do so much less"
Ask yourself whether they were doing less or doing exactly what they were intended to do AND NOTHING MORE. One of my overarching themes is that today's designers try to make every platform a do-everything, win-the-war-single-handed, miracle machine instead of a single purpose, exquisitely focused asset. Consider the F-35. It's supposed to fill multiple roles (AF, Navy, Marines), perform strike, A2A, ISR, EW, drone control, remote strike weapons guidance, and probably a few others. In contrast, the F6F Hellcat had one purpose and one purpose only - A2A (yes, it dabbled in bombing but that was not it's primary purpose). So, I would suggest that previous aircraft (and ships) were not less capable, they were MORE capable at their ONE designed purpose.
"Model T"
To be fair, you're comparing the very first car with later models. No one even knew what problems existed in cars!
"IMHO"
I disagree with aspects of your premise (not all!) but you present it well and I encourage that kind of contribution. Readers can decide for themselves what they agree/disagree with. My total agreement is not a requirement for a good comment!
I do see greater reliability in certain aspects of technology. That's indisputable. I'll offer an amplifying thought to yours. It may be that we've achieved improved reliability in individual components but are now going too far with our do-everything designs and REDUCING reliability overall because of that. To repeat, our modern aircraft struggle to meet 50% bare minimum flyable rates (set aside full mission capable) and that's during peacetime with, presumably, perfect maintenance conditions. Every function we try to add to a platform increases the overall risk of failure - that's indisputable, also - and that may be where we're at, now. How else do you explain our supposedly greater reliability and yet our inability to keep aircraft flying? Just something to consider.
Interesting post and trains of thought!! Heres a couple of random and at times uneducated additions:
ReplyDeleteFirst, Im not sure nuclear power is needed, and could be overkill for a pod propulsion system, and itd likely negate all cost and space savings. I understand that small reactor technology is advancing, but its hard to envision it scaling down to shipping container size like curtent turbines. On the other hand, using a nuclear plant to power the pods would have the added benefits of an even further reduced acoustic signature, and no/minimal thermal signature as well. Now how important that truly is is debateable, but at least worth mentioning.
"added benefits of an even further reduced acoustic signature"
DeleteI'm not sure how true that is. Reactors have lots of circulating fluids, pumps, valves, heat exchangers, etc., all of which make noise. An SSK is, supposedly, far quieter than a nuke sub. Lacking actual acoustic data, I can't make any definitive statement but I'd be cautious about making the assumption that a reactor is quieter than a turbine plant. Might be, might not.
Similarly, reactors have large thermal signatures (they're exothermic in the extreme!), discharge large quantities of heated water that was used for cooling, etc. Again, caution!
" An SSK is, supposedly, far quieter than a nuke sub. Lacking actual acoustic data, I can't make any definitive statement but I'd be cautious about making the assumption that a reactor is quieter than a turbine plant. Might be, might not."
DeleteThe linked article has a dramatic title, but you might find interesting some text a little over halfway down the page.
https://nationalinterest.org/blog/the-buzz/did-sweden-make-americas-nuclear-submarines-obsolete-18908
Quote:
"Nuclear powered submarines have become very quiet—at least an order of magnitude quieter than a diesel submarine with its engine running. In fact, nuclear-powered submarines may be unable to detect each other using passive sonar, as evidenced by the 2009 collision of a British and French nuclear ballistic missile submarines, both oblivious to the presence of the other."
This article is not about podded drive systems, but it says that electric motors can be made to be very quiet.
https://www.thedrive.com/the-war-zone/42900/the-truth-about-the-growing-diesel-submarine-threat-from-a-veteran-sub-hunter
About three-quarters to four-fifths down the page:
"The ASW migraine is only enhanced by a new generation of electric motors. 'Advances in solid-state power conditioning equipment and rare-earth magnets are creating an electric motor half the size and weight—for the same output—as conventional units,'"
I don't know if this is relevant to podded drive systems, but if it is, I hope this helps.
And though this drifts further away from the post's subject, you might like the recommendation a third of the way down the page that the US Navy adopt some type of modern hedgehog--someone agrees with you.
"Similarly, reactors have large thermal signatures..."
DeleteTrue... To be honest as i typed that, all I could picture were the shimmering heat waves rippling out of a Tico doing an emergency breakaway (a recollection from a Med cruise decades ago), and my mind was focused on exhaust. Now as far as inbound missiles, id think the exhaust plume would be of more concern than some warm water near the keel.
But nonthless, good points that are taken!! Itd sure be nice if we had more data to look at and compare...!!!
"as far as inbound missiles, id think the exhaust plume would be of more concern"
DeleteThe Navy has redesigned exhausts to some degree to reduce the heat signature (introducing more cool air into the exhaust). However, that's a pretty small improvement. I would like to think that a room of engineers, if turned loose, could come up with substantial improvements in exhaust signature (discharge below the water, discharge at the stern, water spray cooling of the exhaust, etc.).
One reason pods aren't used widely in military ships is that "whipping" (not whipsawing) is likely to have a pronounced effect on a concentrated mass located at the extremity of the ship. Shock transmission to the integral electric motor is also on a more direct path than for a conventional shaft to internally mounted motor.
ReplyDeleteWhile "unbolting" sounds attractive, you're still opening up the ship, because the power transmission and cooling methods (usually electrickery and HVAC) still have to pass through the shell of the ship. That they do so through the horizontal seating for the pod, as opposed to the vertical shaft seal makes little difference. Aligning the seating from which the pods are hung is also a non-trivial exercise.
You also have to find a method of offering up a unit weighing several hundred tons to the ship where a crane can't plumb - as opposed to using chain blocks and some staging to position a prop and shaft components weighing at most a few tens of tons.
Some use pods (UK Bay and Enterprise classes) and indeed SP and AUS navy LHD, but they're not the panacea made out.
""whipping" (not whipsawing) is likely to have a pronounced effect on a concentrated mass located at the extremity of the ship."
DeleteThat's interesting. I don't know what you mean by 'whipping' in this context. Maybe you could elaborate?
Do you know this phenomenon to be a demonstrated fact or is this speculation on your part?
I note that pods eliminate the need for rudders which removes a significant weight from the stern.
I also note that as regards weight, rudders should be susceptible to the phenomenon you're describing but are apparently not. Do you have an explanation for this seeming contradiction?
Whatever works to mount a prop or rudder ought to work to mount a pod unless you know some reason why not?
It's all to do with weight - or more specifically concentration thereof. Whipping is the resonant flexing of the hull under undex loading, which means that point masses at the extremities will be subject to some significant loadings.
DeleteIn terms of your questions regarding pods vs props/rudders, I refer you to the statement on their relative weights. A pod of sufficient power to replace a single prop / shaft weighs north of 200tons. Compare that with a prop of 30 tons. One you can position and offer up using chain blocks slung from the ship. The other, you can't.
"point masses at the extremities will be subject to some significant loadings."
DeleteThe phenomenon you describe is well known and understood. The question is whether, in this specific case, it's significant enough to be a disqualifier. As similar data points, a Nimitz class rudder weighs 50 tons and has a longer moment than a pod. Likewise, each convention prop/shaft/support on a ship has significant weight and very long moments.
So, there seems to be sufficient evidence that pod can be safely mounted although I'd hope engineers would carefully calculate the moment stress before committing to a pod.
"The other, you can't."
What data/example do you have to support this statement? Pods are mounted, dismounted, and replaced on commercial ships on a routine (if not frequent) basis. I don't know the mechanics of how the execute the dismounting and replacement but they certainly are able to do it.
Given that a fairly detailed whipping analysis was carried out in the design stages for QEC, I can tell you it was. You also need to understand the difference between a large point mass concentrated at the extremity of a ship and a collection of much smaller masses distributed along a significant length. When the hull whips, that point mass has a large acceleration applied to it, which has a major effect on the point loading in way of the seating.
DeleteOn use by commercial ships - of course they do during build - and occasionally for defect rectification. But they need access to specialist supporting MHE to do it. Which costs a lot more than a set of chain blocks and some staging in the dock bottom, which is all you need to shift a prop and/or shaft section. Lets try starting with how you get a 200 ton plus pod unit into or out of the dock bottom and then how you get it under the ship? Not too many cranes in maintenance shipyards with that sort of capacity.
"Not too many cranes in maintenance shipyards with that sort of capacity."
DeleteI'm not sure why you're so focused in on this mechanical act of mounting/removing a pod. It's simply not much of an issue. We have repeated proof from the commercial sector that large pods can be mounted and removed without too much problem. The Navy's shipyards have the equipment to handle very large ships and equipment. In short, if pods are deemed desirable, their mounting/removal is not an issue.
The Navy routinely mounts and removes rudders, props, and sonars and yet you seem to think a pod is not possible? Despite commercial yards doing it routinely? Do you just not like pods and are looking for some reason why they can't be used - again, despite their common use on commercial ships?