How one optimal AUKUS submarine pathway became two

Australia will now get three in-service submarines from the United States. (Supplied: Royal Australian Navy, ABIS Jayden Fahy)

For an arm of the government tasked with a fairly straightforward mission — that is, fighting — Defence is famous for wrapping itself in impenetrable language.

Ships and planes are "platforms", weapons are "capabilities", soldiers are "personnel" and Australia's road to running a fleet of nuclear-powered submarines is a "constrained optimal pathway".

That optimal pathway has come in for plenty of scrutiny in the past few days.

Australia will only buy second-hand Virginia nuclear-powered submarines from the US during the AUKUS stopgap phase, with some analysts criticising the move and others believing it is a positive change of plan.

The plan, as of last week, was for Australia to buy two "in-service", that is, second-hand, submarines in 2032 and 2035, and a third brand-new submarine in 2038.

As of this week the plan is for Australia to buy three used submarines which the government is now arguing has been its preferred option all along.

Defence officials fronting up to estimates hearings this week have been copping questions on the surprise shift.

Was the optimal pathway of last week actually not "optimal"? And if so, why was it called the "optimal pathway"?

"You can absolutely have two constrained optimal pathways," new Defence Secretary Meghan Quinn told Greens Senator David Shoebridge.

The exchange led Senator Shoebridge to accuse AUKUS of "not only damaging the public purse, but destroying the English language".

Any economist will tell you constrained optimisation is taught in first year uni. It's basically finding the best option with the cards you've been dealt.

But the exchange highlights the trouble the government is having explaining the changes it's made and the risk to public confidence in Australia's biggest ever defence project.

Senator David Shoebridge has accused AUKUS of "damaging the public purse". (ABC News: Matt Roberts)

Back when the "optimal pathway" was first announced in 2023, questions about the complexity of the plans were already being asked.

As Richard Marles and others have pointed out over the past few days, that plan would have left Australia operating four types of submarines.

Australia is spending billions stretching out the life of its aging fleet of Collins Class submarines, so the last will still be in the water deep into the 2040s.

There are the used Virginia class submarines, which are being built in the US this decade and will be delivered in 2032 and 2035.

Richard Marles sa