Why? Just look at the project's trajectory, starting with money. The Australian Strategic Policy Institute always insisted the submarine was going to cost far more than those early, wistful figures, conjured from the shallows. The submarine is a very hungry caterpillar, eating everything in sight. First billed as a $20b illion project, it rapidly exploded through $50 billion to $80 billion and is now closing on $100 billion, well before any proper drawings for the first boat. We're still dealing in "concepts". But there's a good reason for this, the second problem - technical advances. The design team is working in an area where scientific knowledge is progressing rapidly, but not in a good way.

The chosen propulsion system, for example, uses already outdated lead-acid batteries. Brilliant analysis from Derek Woolner (who's literally written the book on the earlier problems with our troubled Collins-class subs) helpfully points out in lucid, overwhelming technical detail four factors making it certain this submarine will be "obsolescent on delivery". The submarine's primary risk-reduction strategy is to ensure that it incorporates nothing that hasn't already been to sea: the equivalent of ordering the best bi-plane in the sky in an age of jet aircraft.

The crucial issues are, however, military. More specifically, what can a submarine can do other capabilities can't? What's the opportunity cost of this enormously expensive boat?

In the past, answering these questions has been easy: we need subs to deter invasion. Until now, anyone attempting to land on this continent would have been forced to build a hugely capable navy simply to defeat our submarines. These days, however, a D-Day style landing on Bondi is neither plausible or likely.

The other attraction was the possibility of using submarines to surreptitiously gain intelligence. That's how one of our Collinses came to be tangled up in fishing nets off Shanghai. Imagine just how outraged we'd be today if a Chinese sub was spotted in Sydney Harbour. Now there are other, easier, better ways of gaining intelligence. Send a drone. Use space assets. Do we really want to warp the structure of the forces just to retain a capacity that's "handy"? Every dollar can only be spent once.

The other - unspoken and unacknowledged - capacity submarines have is as the ultimate deterrent; to conduct a second strike. But we don't have the bomb, and nor (currently) do the submarines offer a delivery mechanism (missiles). Might we in future? As the cost of the subs spirals upward into the stratosphere, this is the only possible reason to continue pursuing this capability.

Even that's not good enough. There are other alternative paths to build strategic stability which are far more effective than going broke pouring money into submarines. It's worth bringing this debate into the open, because unless it's acknowledged it can't be dispatched for the furphy it is. The key essential is that technical developments mean conventional submarines can't achieve this task. They fail here for exactly the same reason as they can't offer conventional capacity. They are like knights in shining armour; beautiful, but militarily increasingly irrelevant.

Examine this project dispassionately for a moment. Technically, the battle between submarines and surface fleets has always sea-sawed, with the advantage swinging from side to side depending on scientific developments. Physically, however, trend lines now favour transparency over stealth, speed over lurking below the surface, and artificial intelligence over human command. Submarines always face one perennial problem: they have to carry people, underwater. Their hulls must combat incredible pressures the deeper they dive. Simple, cheap, and remotely controlled, artificially guided "torpedo carriers" will be deployed to destroy submarines. They can't be countered.

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None of this was considered, however, by the Australian National Audit Office as it released its detailed and damning report last month. This was confined to the conduct of the design program, but what it found is unlikely to inspire confidence: "Defence cannot demonstrate that its expenditure of $396 million on design of the Future Submarine has been fully effective in achieving the program's two major design milestones to date." Indeed.

The problem with slippage - inevitable in a project such as this, working at the very frontiers of science - is it derails the whole enterprise. Today's short delay becomes tomorrow's crisis; a minor gap in capability rips through the entire force. Already we need to engage in "life-of-type" extensions for the Collins class to keep them operational while we wait for the new subs. These have become spoilt children, demanding others work harder. This might be okay if we knew the wait would be worth it. It won't.

None of this should be read as criticism of the Naval Group design team or those working on the project. They're dedicated to doing the right thing, which is exactly why the blowouts occurred. Together they desire nothing more than achieving the best possible solution, but the task they've been given is impossible. One day we'll admit this. The sooner the better.

Let's call the submarine project what it is. An attempt to buy time for a government responsible for closing down the car industry while reassuring voters Australia is a big player on the international stage able to do the impossible: build a conventional submarine with nuclear capabilities. It was a big ask. It's great to have big ambitions, but if you want them fulfilled it makes sense to have realistic ones.