Why now?

Tony Blair's answer to the House of Commons was this:

The current Vanguard submarines have a service life of 25 years. The first boat should leave service in 2017. We can extend that for five years, so in 2022 that extension will be concluded, and in 2024 the second boat will also end its extended service life. By that time, we will have only two Vanguard submarines. That will be insufficient to guarantee continuous patrolling.

The best evidence we have is that it will take us 17 years to design, build and deploy a new submarine. Working back from 2024, therefore, that means that we have to take the decision in 2007.

So the Vanguard submarine has a maximum lifespan of 30 years.

Richard Garwin (whose title may be Mr, Dr or Professor depending on who you believe) gave evidence to the House of Commons Defence Select Committee yesterday. He said:

US Trident submarines operate two-thirds of the time at sea, the UK subs about one-quarter of the time. The lifetime of the US submarines has been extended to 45 years.

He said:

I would expect that the UK submarines, from the point of view of wear-out, would last 100 years.. I see no reason why they shouldn't last 45 years.

He said the steam generators would wear out but that they could be replaced easily and relatively cheaply.

He said:

If Britain wants to preserve a strategic nuclear choice, then taking a decision now to replace the Trident submarines is a highly premature and wasteful approach. Unless some grave error has been made in the design of the Vanguard, it should last 100 years.

So there's a problem and when it comes to matters of trust, Blair starts with an obvious disadvantage. Even if he didn't though, I'd still be inclined to believe that it's the nuclear physicist who's right.

Why now?

It might be possible to spot the answer in this article which manages to be on exactly the same subject and on an entirely different subject at exactly the same time.

(A note for clarity - the U.S. navy doesn't use the Vanguard. They have their own design, the Ohio class. Comparisons are similar for similar, not like for like.)