Tuesday, June 19, 2012

DDG-1000 Zumwalt - The Wooden Ship

The Navy learned its lesson concerning the use of aluminum in the construction of ships after some disastrous fires.  Of course, they then promptly forgot the lesson and built the LCS out of aluminum.  Now, along comes the DDG-1000 Zumwalt and the lesson may or may not have been relearned.  Instead of aluminum or steel, the DDG-1000 superstructure is being built of a balsa wood core composite.  Yep, wood!  And glue!

The following information was taken from an article posted at compositesworld.com. (1)

The wood composite material was selected to meet fire-retardance/fire containment requirements, reduce radar and IR signature and weight, and control construction costs.  Apparently, balsa burns more slowly than foam and better insulates the opposite sandwich skin from heat.  The 2-3 inch wood core is sandwiched between layers of carbon fiber and vinyl ester with a stainless steel mesh integrated into the external skin, providing electromagnetic interference (EMI) shielding and a lightning ground in the otherwise nonconductive panels.  The wood composite panels are being applied to the upper four levels of the superstructure (the lower three are steel) and the hangar.  Similar materials were used to build the prominent pyramidal mast enclosures on the LPD-17 class.


DDG-1000 Superstructure - Wood and Glue!

One of the noticeable features of the Zumwalt superstructure is the large expanses of flat sides.  This was apparently dictated by a desire to keep tooling and mold costs down. The wood composite panels can be as large as 120 ft long by 60 ft wide.  The performance of the composite construction was verified by the construction of a 1/3-scale model of the deckhouse, which was tested at China Lake Naval Air Weapons Station in California’s Mojave desert.

Structural support beams are made from similar composites.

This is fascinating technology.  Of course, it remains to be seen how the wood composite materials will perform in use and in combat.  Can they absorb the continual flexing of a ship at sea that causes the cracks in the aluminum superstructures of the LCS and Ticonderoga classes?  Will they provide any measure of protection from shrapnel?  Most resins produce toxic fumes when burned – will this be a manageable problem?  Will the composites provide sufficient electromagnetic transparency for the radars – there are reports that suggest that the LPD-17 class enclosed masts have such problems.  Can repairs and maintenance be performed at sea?

Whether it turns out to be a success or bust, the DDG-1000 is going to be a very interesting ship to watch over the next several years.


(1) http://www.compositesworld.com/articles/ddg-1000-zumwalt-stealth-warship, DDG-1000 Zumwalt: Stealth warship - U.S. Navy navigates radar transparency, cost and weight challenges with composite superstructure design, Michael R. LeGault, 1/18/2010, Source: Composites Technology

11 comments:

  1. Guarantee that it cant be repaired at sea. The whole concept the DDG-1k and LCS are designed around means any damage is a mission kill and it will either sink or have to return to a shipyard.

    And i doubt the composite superstructure will hold up.

    Most composite boards flex fine...for a while. Then they begin to lose strength and water gets in....its all over.

    Add to that the effect of flexing is going to be far different in a actucal 120ft long panel and a small model tested on a lake.

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  2. I don't know whether you meant that you don't think the panels can be repaired at sea due to technical reasons or if you were pointing out the manning issue. The LCS was minimally manned with the concept being that no maintenance or repair would be performed at sea, by the crew; it would all be done in port by support personnel. The DDG-1000 is manned with the same concept though reports suggest that the crew will be somewhat more appropriately sized. Nonetheless, there will be no time for the crew to do repairs, I suspect.

    Good point, either way!

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    1. Both really.

      Think about it. The pannels are massive and one peice. They are also a composite of other materials. How can they fix them?

      The navy has made it clear that its focus is no risk. The ships cant take damage they werent designed for it. Its that way in all new navy ships.

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    2. You bring up an excellent point! In WWII, ships were generally designed to withstand hits from a peer (destroyer was armored against a destroyer, BB against a BB, and so on). Now, the Navy seems to either feel that they won't take hits at all or they're willing to write off ships from one or two hits. At multi-billions of dollars per ship, that's a big write off!

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  3. I hope we have iron men, 'cuz now we have wooden ships.

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  4. The USS Constitution is no longer the only commissioned wooden warship in the U.S. Navy!

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    1. Well, outside of the 14 ships of the MCM-1 Class which have been around for over 20 years, which interestingly is a wood composite mix although it uses Douglas Fir and traditional fiberglass. Also, the MHC-51 class were completely composite (fiberglass) construction. And from these and other examples, you can find the answer to your questions.

      Can they absorb the continual flexing of a ship at sea that causes the cracks in the aluminum superstructures of the LCS and Ticonderoga classes? First, the LCS cracking was caused by manufacturing problems not fatique cracking (and they had steel cracking too for that matter). But in this application the composite superstructure should have no cracking issues especially given that they are on top of steel superstructure. There may be some join issues between the steel and composite structures, time will tell.

      Will they provide any measure of protection from shrapnel? Yes, MHC-51 class hulls are bulletproof up to 7.62mm AP rounds! In many respects, MCM-1 hulls are far more damage resistant than steel although they do splinter. They are certainly more fire resistant (hard to believe but true, they are trated not to burn, no heat transfer, and they don't lose strength as a result of heating!) than steel.

      Most resins produce toxic fumes when burned – will this be a manageable problem? Yes, no issue on numerous small fires on composite ships (or race cars for that matter). Also it is the top of the ship, which reduces the likelyhood of fire in that there is probably no DFM. lube oil, or other flammable liquid (most likely source of a fire)stored there. Outside of Belknap (special case) what ship has had a major superstructure fire?

      Will the composites provide sufficient electromagnetic transparency for the radars – there are reports that suggest that the LPD-17 class enclosed masts have such problems. Radars are mounted on the face of the enclosures, not behind them as in an enclosed mast. And it is not like we haven't had composite enclosed radomes forever. Shoot, the X-Band radar has a composite radome yet it seems to do OK.

      Can repairs and maintenance be performed at sea? Yes but why? Most surface damage is easily repaired (a little sanding, a little epoxy) until a major repair. Significant repair are a bigger problem but then please explain to me how you fix major damage on a steel ship. You cover it up! You can do that on a composite deckhouse with nails (OK screws but nails sounds better), epoxy, and plywood! No one fixes things permenantly at sea. Sure, your industrial area will have to have a more controlled environment to fix it than if you had a steel or alunimum ship and prep work is a little more difficult (you need to scarf the joints for one), but it is not real difficult. The Navy has been doing similiar repairs for years (In addition to MHCs and MCMs composite aircraft parts have been around for a longtime).

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    2. Excellent comments! Thanks for jumping in. Let's dig a bit further and learn some more!

      You say the LCS cracking is due to manf defects rather than fatigue. I've read the Nav Surf Warfare Center report on the cracks as well as the various DOT&E/DTE reports and PEO Riedel May-11 crack memo and nothing that I recall in any of them suggests the cracking is due to manf defects. In fact, the crack locations appear to be at logical stress points. What defects are you referring to? I am absolutely not making a criticism; I just want to learn more about this!

      You suggest that the composites, being mounted on top of steel, will not be subject to flex stress and yet this is the exact situation with the LCS and Ticos; their superstructures sit on steel hulls and yet they flex and crack. Am I misunderstanding what you meant?

      Your comments about shrapnel resistance are fascinating. I wonder if the balsa wood composite will provide the same level of protection as the other hulls which use a much, much denser wood? What do you think?

      I think you're right that toxic fumes from fires are manageable if for no other reason than firefighters where self-contained breathing equipment anyway, for smoke inhalation, if nothing else.

      My understanding is that the Zumwalt radars will be behind the outer panels. Admittedly, my understanding on this point is way less than 100% certain. Have you seen something definitive that states otherwise? I'd love to look further into a source that discusses that!

      You make an excellent point that repairs at sea are not normally considered permanent and, certainly, a temporary patch can be placed over a hole. However, a steel ship can often make moderately serviceable repairs with welded materials to allow a ship to stay on station and in action. I wonder if it will be the same for DDG-1000 or if repairs require an immediate trip home? This is not necessarily a criticism of the Zumwalt, just a question.

      I love your comments. Please follow up and let's all learn a bit more about this class of ship. Thanks!

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    3. Let me clarify as I was not real clear.

      The cracking that you will see with both LCS ships is largely a result of the design requirement to go 40+ knots. In it not fatigue cracking so much as overstressing of the hull and superstructure causing the welds to fail. When you build ships to go fast you except a certain amount of cracking as a result because everything is built very light. Case in point, the WESTPAC EXPRESS (and most all similiar vessels). In a former life when I worked for MSC I periodically worked with the WESTPAC EXPRESS. My Ops guys had the ABS certified welder and local ABS rep on speed dial because they inevitably needed to do voyage repairs because of cracking, especially when they hit weather. It was a basic part of doing business.

      The Ticos, Spruances, and Perry's have cracking issues because of lack of expansion joints (although they don't have leaky expansion joints!). The failure here is because the elasticity of Aluminum and Steel are different and because Aluminum does not have a fatigue or endurance limit like steel. So if you flex it, it will eventually fail. And without expansion joints, the aluminum carries a lot of the flexing load as opposed to the main deck carrying the load.

      What we have here is different. The small portion of the deckhouse will carry relatively little load since composite is a lot more elastic that steel and it is only part of the superstructure (the top of the front half). So I would expect that it won't see a lot of cracking but is more likely to see issues along the join line where the composite attaches to the steel.

      The composite panals may or may not be bulletproof. It all depends on how strong the inner and outer carbonfiber layers are. But basically, they will act like bulletproof vests and spread the force of the impact across the panel. If the outer skin is strong enough to not break immediately, it will stop the bullet. In any event, I would expect it to be more damage tolerant than Aluminum just by the nature of the materials.

      In that same vein, the potential of toxic fumes is greatly mitigated by the fact that there are MILSTDs that have to be met in regards to fumes. And it is not like the carbonfiber or balsa is going to burn as I am sure it is treated). There is a long history of balsa cored composites, I just don't see a problem.

      Everything I have seen says the radars will be mounted flush with the superstructure not in it. So while they will be behind some kind of radome like the SPY-1, it will not be the balsa cored structural panels.

      Small repairs to the skin can be easily made with fiberglass and epoxy (think Bondo). Basically you are just stopping water penetration. Larger holes can be covered with plywood or other suitable material (even aluminum or steel if you want). Again, it is high up on the superstructure. However, anything that significantly damaged the deckhouse probably damaged all the antennas that are sunk into the deckhouse. In any event, you don't have the issue of flooding like in hull repairs. Again, its only the upper four decks of seven. Not much up there but antennas. Its basically a tall deckhouse replacing the traditional mast.

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    4. "In a former life when ..."

      We've got zombies reading this blog?! I may have missed my target audience.

      Just having some fun!

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  5. Thanks for following up. Your comments all seem reasonable although my experience in industry suggests that even the upper deckhouse will experience flexing and stress. Consider a skyscraper building. Even the top floors experience flex stress and, in fact, are designed to allow for it (which leads one to wonder why the Navy can't seem to design a ship to allow for it). You make a good point about the steel/composite joint being a potential trouble area.

    As I said, it will be fascinating to watch the development of this class, not just for the reasons discussed here but because of the variety of new technologies and methodologies incorporated into the design.

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