Seems fairly vague to pin it down until they actually make the speech. A "genuinely trilateral" plan could cover any combination of submarine, logistics and training options.
I'd like to think SSNX and the Aussie sub could merge, perhaps using SSN(X) sections - we already know SSNX is going to have a VLS for the first time in the RN and that system is going to be US for certain which raises the possibility of sharing hull sections.
But the SSNX likely:
- Will be significantly larger and more expensive than a Virgina. Including likely an even larger crew which may be a particular pinch point for the Australian navy.
- Won’t be available to Australia for an even longer timescale than Virgina class subs (the 1st SSNX is approx. a decade away from 1st being in the water, much greater scope for that to creep even later, greater risk of development problems and/ or technical growing-pains, plus when would any be available to Australia given that timescale? A probably unrealistically optimistic 20 years plus for the 1st?).
Given all the other challenges and risk associated with program (and the ticking clock of the Collins class continued availability) a Virgina variant appears likely to be the only horse in town.
Now if a radically different approach was/ is taken (building a number of new interim non-nuclear subs) then that may have opened up an option Australia choosing to time their joining the nuclear sub club with the SSNX. In the absence of such there would likely be a significant (near total?) run-down of the Collins sub fleet before any Australian SSNX subs were available, an approach that would call the whole rationale/ justification for nuclear subs (to deter China) into question.