Wednesday, July 19, 2017

Ship Superstructures

I’ve noted, over the years, the Navy’s trend toward larger and larger superstructures.  Much of this is due to stealth considerations but not all.  Let’s take a look at the trend and then we'll discuss the implications.

WWII early war Gato submarines had fairly large superstructures.  As the war progressed, these were steadily cut down until only the barest structure remained.  This was done to reduce silhouettes and the concomitant chance of visual detection.  Look at the Gato profiles below and note that the superstructure was reduced by around 50% which significantly lowered the profile and reduced the total visible superstructure bulk.

Gato - Early War

Gato - Late War


WWII ships in general and the Fletcher class destroyer, specifically, had narrow, small superstructures.  In the drawings below, note the relatively wide deck areas on both sides of the superstructure, stretching the length of the ship.  The superstructure was around 50% of the hull width for much of the length of the superstructure, widening out to around 80% at the forward end.  Also note that the height of the superstructure was fairly short.  Combined with a low lying hull, the overall profile was quite short.

Fletcher DD


Note the very small superstructure on the Baltimore class cruiser shown below.   On a relative basis, there is more deck space than on the Fletchers.  Note that the available horizontal deck space allows the placement of large numbers of weapons.  There is also a large section amidships that has no superstructure!



Baltimore Class CA


Now, let's take a look at a modern destroyer, the Burke class DDG.  In contrast to the WWII designs, note that the superstructure is massively large and in most areas spans the width of the ship.  Excluding the flight deck which is not usable space, the only available large deck space is the bow area or top of the hangar which is only usable for equipment if there is no deck penetration.


Burke Class DDG


Look at the LCS.  Note that the stealthy, slanted superstructure spans the entire width of the hull and covers the deck from just behind the forward gun, all the way aft to the hangar.  There is no horizontal deck space in the area of the superstructure.  Also, note the relative height of the superstructure compared to the overall height of the ship from the bottom of the hull to the top of the superstructure.  Finally, note the width of the superstruture even at the very top.  It's still quite wide at around 80% of the width of the hull.  That's a lot of weight to carry quite high on the ship.  The bulky superstructure also makes the ship quite visible.


Freedom Class LCS



Even with stealth shaping, size still equates to radar signature.  The smaller the superstructure, the smaller the radar signature for a given shape.  Just as WWII sailors understood that a smaller superstructure translated to a smaller visual signature, so too does a smaller superstructure, today, translate to a smaller radar signature.

Another issue with large superstructures is top weight and stability.  A large superstructure means a higher proportion of weight higher up which negatively impacts stability margins.(distance the vertical center of gravity can shift in response to weight growth before stability is compromised).  Typical stability margins range from 0.3 m for amphibious ships to 0.8 m for carriers.  The LCS, for example, with its overly large superstructure, has a stability margin of 0.15 m - a very low value compared to other classes (1).

Another aspect of superstructure size is its impact on deck space and deck working space.  If the superstructure gets too large, the available horizontal deck space for mounting guns, boat cranes and storage, underway replenishment equipment, etc. becomes very limited.  Similarly, limited deck space impacts the working space for the crew, be it line handling, weapons operation, resupply, boat handling, etc.  Take a close look at the Freedom LCS.  It has very little usable deck space relative to its size.

WWII ships had plenty of horizontal deck space and mounted large numbers of weapons and equipment.  With modern slanted superstructures, deck space is at a premium and negatively impacts the number of weapons a ship can carry.  Often, it is necessary to carve out platforms higher up or on top of the superstructure which, again, impacts stability.  

Take a look at the LCS, for example.  The weapon pits, 30 mm gun mounts, etc. are at the top of the superstructure.  The problem with this is that it places the weight high up which makes the ship top heavy.  People talk about adding weapons to the LCS to make it more useful but such discussions overlook the fact that adding such weapons would be difficult due to the lack of deck space.  Mounting the weapons on top of the superstructure worsens the already marginal stability.

The Burke has a VLS cluster mounted on top of the hangar due to the lack of main deck space and that elevated mounting negatively impacts the stability.  

The modern trend of larger superstructures also results in more functions being placed above the main deck, in less armored (to the extent that anything is armored on modern ships) and less protected spaces.  Remember, unlike torpedoes, anti-ship missiles tend to hit the superstructure.  We are going to lose combat functionality by having more of it "exposed" in the superstructure rather than buried in the hull where armor, tanks, and void spaces help provide a measure of protection.

The odd part about the entire trend towards slanted, stealthy superstructures that span the width of the ship is that I've seen no evidence that narrower superstructures with more exposed deck space are any less stealthy.  I've read that vertical sides and bulkheads generate a significant radar return but I've never read a word suggesting that horizontal deck space generates a significant return.  If the sending/receiving radar were positioned directly overhead then the deck would constitute a perpendicular surface and would generate a large return but any other angle will just scatter the radar wave away from the sending/receiving unit and if the unit is directly overhead then the ship has already been spotted!

Admittedly, my understanding of ship radar stealth is rudimentary, at best.  However, until someone can demonstrate why a horizontal deck surface is bad, I've got believe that the benefits of additional deck space far outweight any supposed gain in stealth and the additional drawbacks to a slanted superstructure, such as top heaviness and lack of weapons mounting space, further reinforce that belief.  We need to re-examine the entire ship stealth concept as it relates to equipment, weapons, and sensor mounting.




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(1) Government Accountability Office, "LITTORAL COMBAT SHIP - Additional Testing and Improved Weight Management Needed Prior to Further Investments", GAO-14-749, July 2014

32 comments:

  1. Surfaces meeting at ninety-degree angles can make nice corner reflectors for radar. Another factor to consider that many WWII designs were weight-limited (armor, ammunition) while later designs tend to be volume-limited (relatively bulky-but-light electronics and missiles).
    Bill the Shoe

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    1. Its more that an incoming radar ping from say a plane will ping off the wall, then floor then go straight back the way it came, and there are about a million ways to do that, just like in billiards.

      Stealth is about managing you RCS not making you invisible. In zumwalt there is just 2 angles you can get a return from, port and starboard, so you turn your ship to make the radar reflection NOT return off those.

      In traditional ships with no managed RSC, no matter how you turn you are always reflecting Radar pings back the way it came.

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  2. My understanding is that compared to WW2 era crew space an order of magnitude larger and that helicopter/uav deck landing and hanger was ~30% of OHP length.

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    1. To amplify the above found the following paper, though 1981 arguments still seems applicable.

      SNAME Transactions, Vol. 89, 1981, pp. 179-210
      Major Factors in Frigate Design William H. Garzke, Jr., and George Kerr

      "In FFG7, for example, the stowage, maintenance, arming, and operation of two helicopters require 133 ft of main deck length at the stern.[LOA 445 ft/ hanger & platform of LOA = 29.9%]"

      "The number of helicopters to be used in the basic ship mission will be a major factor in any modern frigate design. The stowage and operation of helicopters involves considerable area and volume on the weather deck, and, as the number of helicopters grows, so does the impact--particularly in the volume of superstructure. The larger superstructure means a larger ship both in length (to accommodate the length of hangar and the landing platform) and beam (to provide the requisite stability)."

      "When comparing the USSR Krivak to the FFG7, it was pointed out in that the Soviet frigate has a higher payload (armament/electronics) ratio than the Oliver Hazard Perry because she is armed with more gun mounts and missile launchers rather than embarking a helicopter complement."

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  3. I definitely like the concept of reducing superstructure with the aim of reducing top weight and potentially adding more weapons, etc.

    Presumably though, the hull would need to be increased in size to provide space for whatever was inside the superstructure? Not necessarily a bad thing (as it would allow even more weapons, etc) but I'm no engineer, so I don't know if there would be any side effects.

    The comment referencing the Soviet Krivaks got me thinking of something else... As I recall, at least some of the Soviet era ships had partially recessed hangars accessed via ramp or lift. Could this be considered (on ships that require aircraft) to further reduce top-weight, allowing more stuff to be mounted?

    Lofty.

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  4. Completely uninformed opinion, but is a driver for the increased size of superstructures both the radar (SPY-1 panels and accessories) and crew habitability? While older ships had larger crews, my understanding is that the individual has far more "space" in today's ships than previously, which takes up more space within the hull. I believe DDG 51 officer berthing is in the superstructure...I would suspect that there was limited berthing in the superstructures of older ships.

    Again, could be completely wrong, and would be interested in being corrected.

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  5. Served on Gearing, Forrest Sherman, Adams and Knox class cans. Officers quarters and wardroom were on the
    01 and 02 level forward if I remember correctly. Crew berthing was tight, especially on the Gearings, we still used the old canvas racks which I thought were more comfortable than the later mattress coffin boxes.
    Point being modern warship designs are more cruise ship friendly than warship capable. Check out some of the u tube video on euro ships (FREMM). Who wouldn't want to book a cruise.

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    1. Back in 1998, I got to take a brief tour of a French destroyer; it was nice. During that period, the navy didn't have a big looming enemy so why not make our ships more European style? Comforts we can use everyday versus weapons that we will rarely use. Once an institution gets into a mentality, it's hard to get out. Last year I took a brief tour of the DDG-1000. Crew size 140; enlisted people 4 to a room. My navy experience was a 200 man berthing compartment (carrier). The modern accommodations and sparse manning are great until you take battle damage.

      MM-13B

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    2. Years ago when the Missouri was just new to Pearl, my wife and I toured her. She had a picture of her old canvas racks next to the 'new' racks that looked like shelving, but housed 30% less.

      I can't say which was better, I don't have enough experience. I will say on the canvas racks that I felt my face would be uncomfortably close to my buddies butt.

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    3. Forrest Shermans (BLANDY and DAVIS) had officer berthing forward of the wardroom on the main and second decks, with CO and commodore cabins above WR. Most officer berthing was on main deck aft. Believe the idea was to split up officer berthing to prevent a single hit from getting them all.
      Bill the Shoe

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  6. WW2 destroyers were considered top heavy but that was in comparison to other ships of the time. The extra topside weight was due to the upgrades in anti aircraft weapons. The threat from the air in the Pacific was greater than anticipated. They were fast, shallow draft, and often performed operations close to shore (littoral combat). I bet they had a better stability margin than the 0.15 for the LCS. On the AB destroyers, a lot of above deck space is taken up by the intake and exhaust for the gas turbines which are larger than required for boilers.

    MM-13B

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    1. I've often wondered why we can't design a ship with the turbine air intake and exhaust openings located closer to the engine. Every foot closer is that much more superstructure space not needed.

      Side openings through the hull? That would be an armor issue except that we don't armor our ships. Hmmm ... Just idle speculation.

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    2. There's actually an exhaust design for the the Meko's that that channels the diesel and gas turbine exhaust out the stern of the ship. The piping is internally cooled by water (helps reduce IR signature as well). I think there's a diagram of it if you go to naval technology/projects and look up the Mekos. As an old engineer I find the idea exciting, but seems like the ducting would take up a lot of internal space.

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    3. The name of the propulsion system on the Meko is CODAG-WARP. (combined diesel-gas water jet and refined propeller) propulsion system. German's always come up with neat stuff.

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    4. Steve, I had not seen that propulsion system. It's fascinating and seems to offer many benefits. Thanks for the heads up on it!

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    5. "German's always come up with neat stuff."

      True.

      But in my experience its also complex and difficult to maintain. We want simple.

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    6. I understand the turbine intakes large volume of air they take in means they have to have the flow straightened out and the other factor is salt spray, high up to keep it dry.

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    7. Gas turbines are so much lighter and smaller then the furnace/boiler/steam turbine systems that you could put them just about any place on the ship.

      The Queen Mary 2 has turbines above the desk in the funnel. No ducting required at all.

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  7. No energy conversion process is 100% efficient. Many processes are irreversible. There will be heat rejected. In a conventional boiler propulsion plant, the two main heat rejections are the exhaust stack to the atmosphere and through the condenser to the ocean. In a gas turbine, the working fluid is air instead of the denser water/steam and the only major heat rejection is through the exhaust stack. There is a larger volume of much hotter exhaust gas as compared to a boiler/steam plant of an equivalent output. You could easily duct the gas out the sides, but I would not want to be standing on the deck in the slightest cross breeze, hot. The gasses could be sent below the waterline but that would likely cause a lot of cavitation (hot gas/steam bubbles rapidly forming and collapsing). Aircraft turboprop engines are a very similar concept to marine gas turbines; a gas turbine turns a propeller shaft. On turboprop engines, the exhaust does provide a small amount of thrust. Could we find a way to direct the exhaust gas under the ship in the aft direction and provide additional forward thrust without causing excessive cavitation? Maybe someone has already done this? This would slightly increase propulsion efficiency and greatly open up topside space. IR signature and topside mass would be reduced. Of course that’s where the deluxe suite goes.

    MM-13B

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    1. Reply to my own comment. Steve, the Meko you referenced is similar to what I was thinking; so I don't get the prize today. I think that would be a complicated engineering plant for a small ship.

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    2. I agree to complicated. I've always liked DE drive(diesel/electric)with electric drive pods.
      Saves space internally, you can split the gen packs forward and aft for survivability and you have the option of back up battery banks for silent operations while sub hunting.

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  8. In this day an age of all seeing sensors and all-knowing networking, the exposed bridge with it obsolete, analog viewing stations (glass windows) looks a little long in tooth.

    Sarcasm aside, why not berth the crew in the super structure, move the C-n-C into the bowels of the ship, route everything that was topside, to below deck? Of course, keep a token pilot house for peacetime or harbor maneuvering.

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    1. So the USS Monitor?

      I tend to agree really, other than intakes, radar tower/elec aerials/arrays, deck gun and helo hangar, what actually needs to 'stick up'?

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  9. I've always disliked the big superstructures of big LPDs, which are supposed to operate near shore. They have big, flat radar-reflecting sides. Would a low-rider like supertankers be much better?

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  10. Stealth against RADAR for a surface ship made of steel/aluminum/titanium is like perfume on a pig, methinks. Steel is gonna reflect something. Not an RF scientist just simple observations.

    Since the late 80's radar contacts can still be blips but using ISAR/SAR modes they show detail of selected blips for target discrimination. Big superstructure or smaller superstructure- it doesn't matter. Any right angles made with steel are gonna be observable.

    That is why that DDG-1000 makes me laugh (cynically...) every time I see a pic. All that mechanical-electrical technology to open doors and load missiles and guns below decks really adds cost/time better put to investing in making the ship more survivable and perform as well as what it replaces.

    Of course you can make a structure/ship less observable by cutting down those angles but it comes at a cost of more $$. Almost like the additional knots gained above certain speeds for off shore racing boats. What is good enough? Sensors are always going to improve incrementally to match.

    B2

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  11. B2, raises a good point. I always wonder what happens when all those electrical "openings" stop working if you lose power or damage due to combat? Is there a reverse,manual mode? Or does USN just assume that if you have been hit, you stop fighting anyways?

    Another thought about these fully enclosed ships because of "LO" requirements: ships are bigger and have more "openings" BUT anyone remember a few years ago when that F22 burned? It looked fine from the outside but supposedly was a complete loss inside, firefighters couldn't "open" it up enough to extinguish the fire, USAF had to issue better tools to firefighters to permit ingress when a F22 is on fire (im guessing F35 will do the same)....anyone wonder what's going to happen when you have a massive fire inside all these new "LO" ships that have far fewer openings than previous generation ships? Just my 2 cents...

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  12. OK, interesting piece. And really I could go on all day. But I won’t.

    Super structure wise, the slanty sides, are just the same metal that was in your central structure but running down the outside of the ship instead of making a little hut at the centre.

    The mass of metal is virtually the same, but you get a lot more volume, the old way, was frankly idiotic. And only built that way because we just couldn’t shake the fact that initially warships were seen as water bases castles ( see "fo'c's'le")

    Let go ! they aren’t houses on the sea, they are ships !

    Secondly, why on earth do you want large exposed deck space doing nothing ? So the little guys can walk around ? the weather gets on it, its just asking to be bombed, etc etc.

    Any part of a ship that isn’t actually doing something is pointless and should be eliminated. Having a huge amount of space under cover actually allow us to get much much more in our ships in a smaller lighter package. i.e. More Volume enclosed for less surface area.

    Having all the heavy stuff on the outside actually limits the pendulum movement, and hence superstructure CAN be higher without being AS detrimental to stability as it would first appear.

    Lets not get into weight of water sloshing across your weather deck, ice build up and the myriad different issues with the old design.

    Engineering Axiom : if you can build a thing with a less number of component this is a good thing, look at all those bits on that cruiser, walls floor gun turrets, masts, separate structures, one piled on top of another, bad, bad , bad, focuses stress, hundreds of points of failure. Why are there 2 entirely separate structures, with a third thing right in-between ????? it’s just mental.

    Guns are now set higher on a ship, because that’s a good place to shoot from, great arcs, great visibility, better range. Simple as. You design you whole ship stability at once. Not make a hull and throw weight on after ???? That would be moronic.

    Yet.

    Burkes hanger VLS are just a really bad idea added after the fact, because the design is way way out of date, As I think I’ve said before, there is only so much you can do to an old hull before it just doesn’t work any more.

    Also you have only focused on the superstructure. That has changed yes, but the changes you note are in concert with some fairly dramatic changes in hull form, if you look at a Burkes Beam in comparison with length and draft, then look at your cruiser, you will see what I mean.

    Beno

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    1. Plenty of vessels have VLS up there with the hangar, even in the days of launchers some had that too on the hangar roof level. Great firing arcs

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    2. Oh please don't get me wrong, there is nothing wrong with that arrangement, in fact some very right things, distributed silos, not taking up the precious centre of the ship, stability of launch being more amidships, efflux not obscuring forward superstructuse, all great.

      The only point I was trying to make was that they wern't part of the original design, and they are heavy. It will have effected stability.

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    3. Hi Benjamin,

      Could you clarify in more detail how the Arleigh Burke class design is "way out of date"? Do your criticisms extend to the upcoming Flight III design?

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    4. Yer, no problem.

      My comment is specific to the main hull design only. That's the sea frame and the super structure.

      Burkes were first designed and built in the 1980's, and this is still essencially the basic hull design. The evaluation work for that design looked at sizing, stealth and upgradeability. And was actually MASSIVLY ahead of its time.

      Look At the AESA aspects, and their effect on the superstructure. And you can see Burke was 20 years ahead at least.

      But the Navy knew it was dating when the commissioned DDG1000. Weight tollerances, stealth and AESA were acknowledged then to require a new ground up hull. Zumwalt was born, and then canned for cost reasons.

      The navy knows its loosing its supremacy by trying to upgrade a 35 year old Hull design. But for political and cost reasons its doing the best it can with the available resources.

      Flight 3 will contain the very best system that will fit in the hull, and these are superb. no doubt.

      Its just that the hull is beginning to be very limiting.

      Beno

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  13. The balance between superstructure and hull is a complex matter. When you're talking about WW2 ship design you're talking about displacement, and when you talking about volume. The reason the heavy cruiser designs in WW2 had smaller superstructures is because of anti-aircraft sky arcs. The treaty cruisers had poor sky arcs and poor AA fire control. After AA became mostly a matter of guided missiles, the sky arc wasn't that critical, but internal volume for electronics, armament and control was. The post-treaty battleships also show a marked reduction in superstructure because of topweight and top hamper. Instabilty became such a problem with the addition of 40mm mounts that in the NC and SD classes the metacentric height was only a couple of feet.

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