A rare SSN Sierra II in titanium for about same size as a Akula clearly more light with this metal, less detectable with MAD also, for silent Sierra are less quiet as Akula but a delicate metal for weld, maintain.
As almost all Russians sub's with double hulls with the space btw 2 for ballast so it weighs more as others sub's with much more big ballast, advantages : more resistant, dive more deep, resistant to damage, disadvantage : many vents for filling / emptying big ballasts, less hydrodynamic, quiet.
Sierra I in reserve.
Pskov, equipped on the front of a support for an acoustic indiscreet testing kit
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