For the latest on Sweden vs Germany see June 11, 2014’s Australia's Future Submarine - Swedish vs German Claims http://gentleseas.blogspot.com.au/2014/06/australias-future-submarine-swedish-vs.html . It is unclear whether Germany or Sweden hold the strongest intellectual property rights to the Stirling AIP.
Australia's future submarine decision making process is at the very basic stage of what do we want, when and why? Concerning when? - the usual complex decision making cycle will mean the first of the new submarines will probably be launched on or after 2030 the last in about 2040. The six or more submarines will have the usual thirty year operating life meaning they will be with us until 2070.
It was assumed that such basic issues had been ironed out under the previous Labor government. However Australia new (since September 2013) Coalition Government claims nothing has been resolved other than the future submarine will be: conventional diesel-electric; probably larger than the Collins; not necessarily 12 submarines; and the submarines will probably be assembled in Adelaide, South Australia.
The major challenge for the future submarine will be improvements in anti-submarine sensor technology in the operating areas of Australian submarines. Australian submarines would be most vulnerable to detection if they were on the surface. Surface movement might have been possible in World War Two, but today surface movement would be too detectable even at night. This is due to the increasing presence of efficient sensors on satellites, ships, UAVs, fixed undersea arrays and manned aircraft. Diesel electric submarines must "snort" for short periods - that is suck in air while the submarine is close to the surface. The air is used principally to drive the diesel engines to recharge the batteries. The period of the snorting process (known as the "indiscretion rate") renders the submarine vulnerable to detection by many types of sensors because the snorkel must be run on the surface, the submarine is running shallow and the diesel engines are relatively noisy when running.
Longer periods and longer travel distances of quiet operation is thus desirable and likely to be present in Australia's future submarines. The Australian solution might be AIP achieved by Stirling, Fuel Cell or MESMA and/or increased battery capacity (possibly Lithium ion). All these solutions add to the weight and size to the submarine.
Australia also has to consider the 3,000 km each way transit distances to travel from the major bases at Fleet Base West, Rockingham, Western Australia and Sydney 6,000 km to northern Australian waters. For a conventional submarine to transit those distances quietly takes much longer than SSNs travelling at 30+ knots. Due to secure transit requirements 2,000 ton European theatre submarine designs are inadequate. Hence European MOTS won't meet Australian needs. Japanese MOTS in the shape of the 4,000 tonne Soryu might meet Australian needs. The HDW 216 if it is a sufficiently developed design (perhaps present in Singapore's HDW 218SG) might possibly meet Australia's range-discretion-endurance needs.
If Australia could select ANY submarine design it would probably be a current US, UK or French SSN MOTS. The most efficient energy source in terms of power to weight and discrete operation is nuclear. This would address Australia's transit speed, range, endurance, and other operational needs (not only in littoral waters) while more compatible with US alliance requirements. However, for Australian domestic political reasons, the nuclear option is banned.
In terms of comparative cost the higher capabilities of nuclear propulsion, along with shorter maintenance rotations, would mean that fewer SSNs would be required - perhaps down to 4 blue-gold crewed SSNs instead of 6-8 single crew SSKs. This may make an SSN force equivalent in cost to a more numerous SSK force. The rising nuclear propulsion capabilities of regional powers, China and India, must also be considered and, to an extent, countered. Japan could also develop nuclear propulsion rapidly.
It has long been envisaged that Australia's future submarines might field Tomahawk cruise missiles for land attack. It is conceivable that during these submarines' operating lives (through to 2070) that the warheads might not all be conventional. It would be risky to entrust Australia's possible future missile arsenal to the limitations of SSK operation.