F EW independence debates generate such heated discussion as that around defence.

Recent developments have given more of an insight into what the future shape of a Scottish Defence Force might look like. Recruits have been promised a job for life, troop numbers set out and billions of pounds worth of public spending has been pledged,

However, many of the key issues remain hotly contested between the two sides. Chief among these is, of course, the future of the nuclear deterrent on the Clyde.

In its independence White Paper last month the SNP included a commitment to try to remove Trident from Scotland by 2020.

But how and exactly when any move might actually take place remains unclear, not least because the ageing nuclear weapons system is expected to be a key element of any independence negotiations between the Scottish and UK Governments.

Estimates on how long it might take to move the nuclear deterrent out of the Clyde vary widely.

The Scottish Campaign for Nuclear Disarmament (CND) has calculated it could be done in as little as two years. But a number of defence experts have suggested a much longer timeframe - potentially even decades.

One central problem is there is no immediate place for Trident to go. Creating a home for it in Wales or England risks running into local opposition and potentially costing billions of pounds.

The Ministry of Defence (MoD) has already warned the price of any such move would be "enormous", while Lord West, the former First Sea Lord, has even suggested it could trigger unilateral disarmament for the rest of the UK.

One potential solution is to keep Trident on the Clyde even after independence, a "treaty port" solution that would keep "a little piece of Scotland forever in the UK".

This has specifically been ruled out by the SNP but could be theoretically an option, at least for a short period, if another party were to win elections in an independent Scotland.

Professor Malcolm Chalmers, from the highly respected defence think tank the Royal United Services Institute (RUSI), suggests that what to do with Trident is one of the great unknowns of the entire process.

"The UK would want to study how long it would take and how much it would cost to move Trident," he said. "Even studying that would itself take a significant amount of time.

"It might prove impossible to move, it might not. We don't know because that study has never been done".

The White Paper also outlines plans to "progressively build to a total of 15,000 regular and 5000 reserve personnel".

Other proposals include to spend £2.5 billion a year on defence, to retain Faslane as a naval base and joint headquarters of a Scottish Defence Force, to operate 12 Typhoon fast jets from Lossiemouth and to take two frigates, four mine counter measure vessels and four to six patrol boats "from the Royal Navy's current fleet".

The gradual approach is part of the SNP's strategy to ensure a managed transition to a new Scottish Defence Force. But even the equipment it has identified as its "inheritance" has divided experts.

Some academics, such as Dr Colin Fleming, Research Fellow and Project Leader on Defence and Security at the Scottish Centre on Constitutional Change, believe much of what the Scottish Government has asked for is reasonable and could be in the UK's best interests to provide, not least to ensure its near neighbour is protected.

Others warn, however, it would be costly to operate equipment such as fast jets and an independent Scotland could end up forced to rely on UK expertise for things such as maintenance. Another train of thought is there is an argument to be made for Scotland taking the cash equivalent instead in many cases.

This could allow it to better tailor defence equipment to its own needs, rather than accept hand me downs from the UK Armed Forces, although this potentially also raises questions about timescale.

Think tanks such as The Scotland Institute have also questioned an independent Scotland's ability to recruit to the new Scottish Defence Force. The argument goes that soldiers will be attracted to the wider opportunities offered by the UK Armed Forces.

Dr Fleming believes these fears are overblown. He believes a Scottish Defence Force, which would be unlikely to find itself in protracted operations such as Afghanistan and Iraq, would find it easier to retain staff than the UK Armed Forces, adding: "They are also finding it extraordinarily difficult to recruit reserves (at the moment) because those who might join up know that they will find themselves in a battle ground very very quickly.

"In a Scottish context I don't think that would be such a problem."

Supporting any new defence force - and being supported by it - is the Scottish defence industry. But here again there is intense disagreement over what impact independence would have on the future of the sector. Just last month hundreds of shipbuilders on the Clyde found out that they would lose their jobs amid a large-scale restructuring.

The UK Government has warned the situation would only worsen under independence. Ministers have suggested orders for warships would likely be pulled from Scottish yards, pointing to the fact the UK Government has never before built such complex and important vessels outside of its own waters.

The Coalition Government's defence analysis paper also suggested that up to 12,000 defence industry jobs could be lost if Scotland were to leave the UK.

Defence analyst Francis Tusa has predicted that the knock-on effect of the SNP's plans would be to "support a far smaller work force", while yards build smaller less complex ships. He has also questioned whether defence facilities currently based in Scotland will gradually migrate south of the border after independence.

For its part the Scottish Government argues independence will allow it to spend more money on defence in Scotland than the UK currently does, getting rid of what it describes as a multi-billion pound "shortfall".

The SNP also insist that Scottish skills will allow the industry to win international contracts.

There is much Scottish ministers also say they are prepared to share, including defence bases, which they argue would be again be in the best interests of the rest of the UK.

But the MoD has already rejected one shared vision contained in the White Paper, that of ships being built jointly with an independent Scotland's navy.

Prof Chalmers believes Scotland's defence industry is divided into essentially two parts, "shipbuilding and everything else". He describes much of the rest of the defence industry as "niche capabilities often within broader multi-national companies" and unlikely to move if Scotland becomes independent.

However, on shipbuilding he asks: "Would it really be credible that a UK Government would want to place orders in Scotland? Much will depend on the defence relationship between the two countries".

One of these issues, he suggests, would be how many defence contracts the Scottish Defence Force was placing with defence companies in the rest of the UK.

Another would be Trident.

"This is one of the reasons why the nuclear option is so important," he said. "If there is a rancour, well then the UK may not play ball".

Of course, all plans for a Scottish Defence Force are dependent on negotiations with the UK Government. And there is an understanding that much may be subject to change during talks.

Such a crucial subject as defence is, after all, unlikely to be separated from other areas of potentially fraught discussion - such as Scotland's share of North Sea oil or its share of UK national debt when it comes to the crunch.