Cancellation of Zumwalt took out the majority of the class, and the future cruisers in addition. No more large-number purchases planned, no money savings forthcoming (but also, nothing much invested at that point to begin with.) Not only this, but cost cuts required by the Nunn-McCurdy breach forced the Zumwalt program to drop SPY-4. Only one complete dual set had been produced and installed--on the Ford herself--at the point where it all came to a halt. A new clean slate, basically.
SPY-6 is scaleable, that is its single biggest asset. It is flexible. It can be fixed or on a rotating mount, and is made up of individual units which can be installed in any number from one onward. SPY-3 & 4 are fixed size and weight arrays, so to mount them the necessary space and weight must be made available in the design.
Neither system was initially suited to the hull of a Burke. Nowhere in their basic design is there room for a SPY-3 & 4 combo, and the numbers of SPY-6 units the Navy wanted for the required radar array performance desired, coupled with the necessary cooling, was too much top weight. Then, a breakthrough in the cooling enabled the weight to be moved lower. SPY-6 in the desired number of units could now be mounted, and that became the production array for the Flight III program. Boom! SPY-6 just "won the lottery." It is now the current "in-production" radar, so has gained the economy-of-scale cost savings from being put into production. And because it is made up of scaleable units, a smaller array of them can be back-fitted to the Flight IIAs. Standardization is big, logistically and cost-wise. Since SPY-6 is so flexible to mount (and is the now in-production radar) it is now going on the Fords as well (and I have read that Ford herself will one day have to go be refitted with SPY-6.) It was actually something of an accident...not really a planned thing at all...but SPY-6 has become the production radar.
So much for the S band. Those are the biggest arrays, and hardest to make space and weight for, and now that problem is solved. Now for an X band. SPY-3 is a big, fixed, inflexible proposition to mount, and will not fit on a Burke. And Burkes, no longer the Zumwalts, are now driving all radar design. They will be the most numerous. SPQ-9B was an "expedient." It was a ready-to-go off the shelf item that fit the requirement for the immediate short term need, and could actually be mounted within the existing space and weight. (Sort of how SPY-6 itself appeared.) As noted, we will now put our "vaunted acquisition abilities" to the task of screwing up...er...obtaining...a better(?) purpose built array which will fit the ships using it.
Has this solved the mystery for you?
On Wikipedia , it is stated:
"To cut costs, the first 12 SPY-6 sets will have an X-band component based on the existing SPQ-9B rotating radar, to be replaced by a new X-band radar in set that will be more capable against future threats".
Apparently, also the USN is convinced now that a pair of radars will be needed.
Still wondering about SPY-3/4 vs. SPY-6.
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