Media playback is unsupported on your device Media caption Unite's John Fussey described the dismantling as 'barbaric vandalism'

The Ministry of Defence (MoD) is taking £4bn worth of new spy planes and smashing them to pieces. Why?

One of the very first cuts Prime Minister David Cameron announced in October's Strategic Defence Review was to terminate the construction of nine Nimrod MRA4 reconnaissance aircraft.

Three of the planes are 90% finished. One is complete, but is still grounded due to safety issues.

But these Nimrods will never patrol for submarines. Instead, they will be stripped of their components, sliced up in an industrial shredder, and their aluminium melted.

The MoD has begun sawing the aircraft into pieces. It says it will save £2bn of operation costs by axing the Nimrods and using existing aircraft to perform their duties.

But why could these new planes not be sold? Or at the very least, mothballed? Is the UK, to quote one defence analyst, "flushing £4bn down the toilet"?

The answer is that the most expensive aircraft ever made for the RAF has almost no commercial value.

"These planes were designed 10 years ago. They are no longer state of the art. Cheaper alternatives are emerging," says Keith Hayward, head of research at the Royal Aeronautical Society.

THE ANSWER It will save £2bn on running costs

Selling the planes would be difficult, and mothballing too expensive Scrapping Nimrod fleet 'perverse'

"Who would want to buy them? The only countries who might be interested, we would never sell them to."

Iran, for instance, would not be the ideal customer. And Nato countries are unlikely to need a plane designed to patrol long distances over the Atlantic.

India might be an option. But even so, they would be buying an aircraft "effectively without a warranty", says Tim Ripley, a defence analyst at Jane's.

"If something goes wrong, you need someone back at the factory who can fix it for you. So unless you buy all those engineers back at BAE Systems and the RAF test centre, you are buying a plane without a guarantee."

Mothball costs

Another option - popular with campaigners who are trying to save the Nimrod - is to mothball the planes, keeping them in storage until such a time as the MoD can afford to fly them.

But while the ministry is preparing to do this with an aircraft carrier, it will not keep the Nimrods on ice.

"Storage still incurs a lot of the costs associated with the capability," said an MoD spokesman.

"It was therefore not a cost effective option."

Image caption The Nimrod was to patrol for submarines in the Atlantic

The trouble with storage, is that you can't just put the planes back in the hangar and fill them with foam. You also have to pay to keep their crews trained up, says Peter Felstead, of Jane's Defence Weekly.

"Not only the dozen aircrew for each of the nine planes, but also the supporting engineers, plus full technical support and spare parts for all the systems," he explains.

"That cost might be plausible if the Nimrods were part of a larger fleet, some of which were still flying, but they are not."

More importantly, by the time the MoD could afford to bring the planes back out of the hangar, they would be virtually obsolete, says Keith Hayward.

"The electronics on these Nimrods are already out of date. You wouldn't put them in last year's Playstation," he says.

"And of course, the targets you wish to spy on may well have changed. They could have a greater degree of stealth."

For the money it would cost to restore the Nimrods, the government might be better off buying a new generation of spy planes, says Elizabeth Quintana, of the Royal United Services Institute.

"In the not too distant future, you could do the same job as a Nimrod with a UAV (unmanned aerial vehicle)," she says. "There may be cheaper alternatives."

Scrap metal

So what is to become of the Nimrods - the last British combat aircraft ever to be built?

Are there any nuts and bolts from 100,000lb (45,000kg) of hardware that can be salvaged, re-used, or sold on?

"Not a great deal, I'm afraid," says Hayward. "The Nimrod's electronic systems are not very advanced compared to what is available today.

"That is the paradox of military development - you will always be outpaced by civil technology."

You might be able to "cannibalise" the engines, he says. "But basically there's nothing there of value bar scrap aluminium."

WHO, WHAT, WHY? A part of BBC News Magazine, Who, What, Why? aims to answer questions behind the headlines

Once the insides are gutted, the remains of the aircraft will be taken to an aeronautical "abattoir", says Tim Ripley.

"First, you take a giant industrial cutter, like a pile-driver. Then you hoist it up on a gantry, and smash it down onto the airframe," he explains.

"You gather up the pieces, load them onto lorries and drive them off to the aluminium smelter.

"A few weeks later, they are baked bean tins."

The images of £4bn of British engineering being mashed into a pulp are likely to cause consternation for some.

"The government... don't want this to become their iconic image - the sight of this plane being chopped up - like the BSE cull," says Ripley.