LCS Variable Depth Sonar

There have been several recent articles about the LCS ASW module being overweight and how it’s not really a problem and what amazing new capabilities the module will bring to ASW warfare.  Of course, the module consists of technology that’s been in use for years so it’s kind of hard to figure how the module will bring amazing new capabilities but that’s not the point of this post.  Further, the Navy assures us that modules diet plan to lose weight is not a problem.  Of course, the weight issues have been known for years and, presumably, the Navy has been attempting to lighten the module for quite a while now and this is the result.  This version is, one assumes, the lightest version that the Navy could assemble over the last couple of years of effort while still maintaining the requisite capability.  Thus, it’s hard to imagine where significant additional weight loss will come from without cutting into capability but, again, that’s not the point of this post, either.

Setting the preceding aside, the main component of the ASW module, such as it is, is the Thales CAPTAS 4 variable depth sonar (VDS) or a similar variant.  This, according to the Navy is an amazing piece of technology that will allow us to accomplish ASW feats never before seen.  Of course, the Thales VDS has been around and in use by foreign navies for years and the US Navy never before deemed it worthwhile.  Kind of makes you wonder, doesn’t it?

Be that as it may, if the VDS can add capability that’s a good thing.

What is it that makes the VDS useful over a conventional hull-mounted sonar, anyway?  Well, as the name implies, the VDS can be raised or lowered to any desired depth, somewhat akin to a helo’s dunking sonar.  This allows the sonar to listen under thermoclines.  Thermoclines are temperature gradients in the ocean that form layers.  The layers tend to reflect sound at the boundaries between layers.  Thus, sound at a lower layer can be trapped or channeled within that layer and not propagate to the layer above or below it.  A submarine can hide in a lower layer that a surface ship’s sonar can’t effectively penetrate.  Hence, the value of a VDS is that it can be lowered into a lower thermal layer and detect a sub that a ship with just a hull-mounted sonar can’t.

Which, again, begs the question, why hasn’t the Navy pursued VDS systems before?  I have no answer for that.  The old Soviet ships had VDS but I have no idea to what extent, if any, they were used.

Moving on …  Does the VDS bring with it any drawbacks or limitations?  As you might suspect from the question, the answer is yes. 

The first limitation is that the VDS requires around 20 minutes to deploy or retrieve.  Thus, the LCS can’t just hop around the liquid battlefield, nimbly racing to and fro collecting data.  Instead, it’s going to have to carefully pick its spots with the knowledge that once it commits to a spot, it will be there for an extended period.

Thales VDS

This leads to the second limitation and that is the vessel’s speed while the VDS is deployed.  The LCS was predicated on speed, speed, and more speed (not sure why but the Navy assures us that speed is critical).  Unfortunately, the VDS only functions at very low speed and, therefore, the LCS is limited to very low speed while the VDS is deployed.  Higher speeds will not only negate the VDS performance but may damage the unit if towed at too high a speed.  This presents a bit of a dilemma.  If the VDS is deployed and a torpedo is detected, the LCS can’t use its speed to escape.  It becomes a relatively immobile target.  On the other hand, if the LCS chooses to retain its speed capability, it can’t deploy the VDS and, thus, won’t be able to hear a sub or torpedo.  Hmm …  That’s a real Catch-22:  use the VDS and forfeit speed or use speed and forfeit the VDS.  The LCS can’t have both. 

Presumably, the Navy has already made the decision that the VDS is more important than speed or else they would not have pursued the VDS option.  Of course, that leads one to wonder what use the Navy foresaw for speed in the first place.  But, I digress …

If the Navy is committed to using the VDS extensively, that implies that the LCS is no longer removed from the ASW battle.  Sonar performance in littoral waters tends to be short range.  Recall that the original LCS ASW concept called for remote unmanned vehicles and vast underwater sensor nets that would be deployed and allow the LCS to stand off from the actual ASW battlespace.  So, with the LCS now conceptually committed to up close and personal ASW battles, we have to wonder about the LCS’ suitability for such a role.  Lacking the built-in quieting that a purpose-designed ASW vessel would have and operating noisy waterjets would seem to put the LCS at an enormous tactical disadvantage in ASW operations.

All of this leads one to wonder how carefully the Navy thought out the revised ASW module concept.  Has anyone gamed out how a loud, speed impaired LCS with no ship-mounted ASW weapons will fare in an ASW battle?  I strongly suspect that the answer is no and that a lot of sailors will pay the price to find out that the LCS is unsuited for ASW work. 

The VDS was a knee jerk reaction to the abject failure of the original ASW module and was not well thought out.  The Navy has to start thinking ahead and ensuring that they have logical and viable concepts of operation before jumping into development paths that make little sense.

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