World naval development October 2018
By Norman Friedman*
In October the French held the biannual Euronaval show, displaying a wide variety of current European and other (including American) naval technology. As at previous shows, the French Naval Group (formerly DCNS) displayed a model of a concept submarine (SMX 31), broadly analogous to the concept cars at car shows: an attempt to show what future technology might offer, but not a firm proposition for an operational submarine. This one dramatized the potential of lithium-ion batteries and artificial intelligence. It is interesting as an indication of what may be possible in the near future.
In its publicity, Naval Group emphasized the use of artificial intelligence to reduce crew size and to limit the crew to vital functions. That means integrating combat system and platform management functions. Such integration may be inevitable, because future submarines may have to maneuver nearly automatically both to clarify the situation around them (by what amounts to multiple target motion analysis) and to evade enemy weapons. Integration may also make it easier for a submarine to exploit local oceanographic conditions. In the case of SMX 31, the designers may be hinting at such exploitation when they emphasize the use of offboard sensors. For example, a submarine on one side of a current (which blocks sonar) might run a sensor through to the other side to detect targets there.
The Naval Group claims that automation reduces the required crew to fifteen. This is not a new idea; in the 1970s the Russians drastically reduced the crew of the Alfa-class (Project 705) nuclear submarine (to 32) by accepting a high level of automation, despite the rather poor state of their computer industry. Crew reduction was motivated in that case by a desire to shrink the size of the submarine, so that it could achieve high speed using a high power density reactor and a lightweight (titanium) pressure hull. Alfa combined too many innovations in a single hull, but accounts of the design generally do not single out automation as a major flaw.
SMX 31 is an all-electric submarine, exploiting the increased energy density of lithium-ion batteries. They already dominate the commercial electronics market and are widely used elsewhere. Their energy density is far greater than that of conventional lead-acid batteries. The Japanese are installing them on board a new submarine (Oryu) and abandoning their current air-independent propulsion by Stirling engine. Reportedly these batteries will give the submarine eight day endurance at low speed (they may triple endurance at high speed). Their great advantage over Stirling and other air-independent powerplants is that they can be recharged by the submarine’s diesels. All other AIP powerplants require special oxidants, the capacity of which limits total AIP endurance. Too, eliminating a separate AIP plant would considerably simplify submarine design and operation, and it might also reduce vulnerability to shock. At least in theory it should considerably improve reliability. Reportedly other submarine manufacturers are considering taking the same step away from separate AIP powerplants.
The French concept submarine takes this a step further. Its lithium batteries would presumably be charged before it left port; Naval Group claims that SMX 31 would have a 30 to 45 day endurance, presumably at low speed (maximum speed would be about 20 kts). In a way this is a throwback to the early days of French submarines, when the French built small short-range all-electric craft. They were distinguished from submersibles, which had separate surface powerplants and could recharge their batteries while underway. These craft ran mainly on the surface but could dive as required. Since it took a surface powerplant to gain much endurance, the all-electric submarines were abandoned. Submersibles have been the rule ever since, except for nuclear submarines, which do not need any contact with the atmosphere (modern submersibles are true submarines thanks to their ability to snorkel).
The great question about big lithium batteries is safety. They can burn and explode. There is a reason that airline passengers are cautioned against placing them in packed luggage. In a few cases electric cars (which use lithium batteries) have burned fiercely after crashing. To make matters worse, lithium reacts violently with water, so lithium fires are very difficult to extinguish. It is only fair to point out that conventional lead-acid batteries also present hazards; many submarines have been lost due to explosions caused by hydrogen gas emitted by their batteries (it appears that such an explosion sank USS Scorpion).
SMX 31 is to displace about 3400 metric tons submerged; it is about 70 meters (230 ft) long with a beam of 13.8 meters and a hull depth of 9.7 meters. Instead of a conventional propeller or pump-jet, it has a pair of side nacelles carrying rim-driven propulsors. Rim drive, which would be based on fixed electro-magnets around the edge of the propulsor, is often suggested as the natural step beyond current shafts. It eliminates hull penetrations for shafts, but on the other hand it saddles a submarine with a much larger magnetic signature. In the past few years the Chinese have publicized the supposed development of a shaftless propulsor, but there is no evidence that they have adopted it. Publicity for SMX 31 emphasizes that eliminating the usual propeller shaft leaves the after end of the submarine free for torpedo tubes and UUV ejectors. Apparently there are two large doors fore and aft, each covering a vertical pair of torpedo tubes. There are also six vertical launch tubes forward. Claimed weapon capacity is 16 to 46 full-size torpedoes. A crew module is immediately abaft the forward tubes, so apparently the main weapon stowage space is aft. In addition to weapons, the submarine supports divers, with a special diver trunk.
SMX 31 is sail-less, with a hull form based on the shape of a whale, fat at the bow and slim aft. It has an X-stern and a pair of retractable bow planes. In the past, both the United States and the Soviet Union have considered sail-less submarines, which offer some advantages in reducing drag and which should also be more or less immune to snap-roll. On the other hand, near-surface sea-keeping is far more difficult. In the U.S. case, the idea was abandoned because near-surface operation was so important for reconnaissance. The sail was also wanted because the submarine needs a bridge for navigation near her base. In the French case, the solution to reconnaissance is for the submarine to launch buoys containing the relevant sensors, which can operate from depths as great as 100 meters (328 ft). Such buoys could, in theory, substitute for periscopes, since they can carry the same electro-optic and ESM sensors that modern periscopes do. No solution to the inshore navigation problem was presented, and the submarine seems too full of equipment to provide for a retractable bridge. A cross section showed a concentrated crew module forward and an escape module abaft and above it. It was not, incidentally, clear where the large battery was.
The buoys are not likely to include any sort of snorkel, and it is difficult to imagine how an electric submarine with limited power can maintain a liveable atmosphere for the claimed operating time. It may be that the dramatic reduction in crew size (to fifteen) makes it possible for the submarine to carry sufficient oxygen in tanks, but that seems unlikely. An earlier concept submarine (SMX 26 of 2012) had a flexible hose to the surface, which it would use to obtain air when bottomed in fairly shallow water. Such a hose would probably be unusable from a moving submarine. It is possible that SMX 31 would have a rigid snorkel mast, but in that case poor near-surface seakeeping would make its operation problematic. It is possible that the designers expect automation to solve the near-surface problem.
SMX 31 is to rely heavily on remote sensors it can deploy: both tethered remotely-operated vehicles (ROVs) and free-running UUVs. In addition, it will be able to launch and control UAVs from buoys it launches. It is not clear to what extent the submarine is to have organic sensors; all Naval Group says about its skin is that it is a special anti-sonar formulation (the model showed the skin in hexagonal elements).
Test depth is given as more than 250 meters (820 ft).
SMX 31 is a considerable departure from the concept submarine shown in 2016, SMX 3.0 (or 30). It was a more or less conventional 3000 ton submarine whose main unusual external feature was vertical missile launchers. Its other unusual feature was a fuel cell for air-independent propulsion. It had a sail (with planes), an X-stern, and a conventional single propeller. SMX 30 was apparently a reduced version of the previous show’s SMX Ocean, a 5000 tonner with a fuel cell which was credited with the ability to allow it to patrol for a month without snorkeling.
The last really exotic concept submarine shown by the French submarine designers was SMX 26 of 2012. Unlike SMX 31, it emphasized littoral operations, which at the time seemed to be the future of submarine warfare. The submarine was in the form of two parallel bodies, merged into each other, with a very shallow ridge above in place of a conventional sail. Each body had its own pumpjet propulsor, and the submarine had four thrusters in addition to its conventional bow planes and X-stern. It showed a flank array. This was a much smaller submarine (about 1000 tons, about 39.5 m [130 ft] long) intended specifically to support a 6-member special forces team with a RHIB for fast deployment. SMX 26 was intended specifically for very shallow water (depths as little as 15 m [49 ft]); it would spend a lengthy period bottomed. For that purpose it was provided with retractable wheels, a feature not seen in full-size submarines since Lake’s boats of the early twentieth century. DCNS said a the time that it could sit on the bottom for 30 days, using a flexible pipe to draw in air. Armament would be two full-size and eight smaller-calibre torpedo tubes. The difference between SMX 26 and SMX 31 is a measure of how the naval world has moved since 2012. SMX 31 is advertised as a submarine for operation in the face of intense ASW activity, particularly air ASW, in deep water. Both submarines can support special forces, but that was the main focus of SMX 26, and now it is fairly secondary.
*Norman Friedman is author of The Naval Institute Guide to World Naval Weapon Systems. His column is published with kind permission of the US Naval Institute.
