Midget or mini-submarines, manned and unmanned, are making a comeback after years of poor performance relegated them to secondary status in many navies.

China, Japan, Russia and India are exploiting new hull, battery, sensor and communications technologies. Others – including the United States – also are talking about new mini-submarine projects.

Even the older generations, however, have been known to be effective. On March 26, 2010, a North Korean torpedo launched from a midget submarine exploded beneath the South Korean cruiser Cheonan, splitting the ship in half and killing 46 crew members. A total of 58 people were rescued.

One of the key features of small submarines, especially when operating on battery power, is that they are very quiet and hard to detect. Radars can sometimes pick them up when they are near the surface, and the North Korean sub was probably within 30 meters of the surface when it fired its torpedo. 

The Cheonan was carrying out an anti-submarine mission when it was struck, but its sonar system did not detect any threat. And this occurred despite the fact its hull-mounted sonar was a Signaal (Thales) PHS-32, a high-quality and capable active sonar.

Parts of the torpedo have been found and an investigation showed that the model was a North Korean-manufactured CHT-02D with a 250-kilogram explosive warhead and a proximity sensor. There have been some claims that this was a suicide mission, but no mini-submarine debris has been found.

Other combat uses of mini-submarines have been less satisfactory. On December 7, 1941, at approximately 3:30am, Japan sent five mini-submarines known as the Type A Ko-hyoteki (Ko-hyoteki ko-gata) toward Pearl Harbor, Hawaii. Launched from larger, mother submarines, each had a two-man crew. While it isn’t known for sure, the same large Japanese subs may also have launched seaplanes, which acted as spotters for the mini-subs. (Larger World War II Japanese submarines were capable of carrying and launching, as well as recovering a seaplane from an enclosure on its deck.)

Each of the Japanese mini-subs carried two reduced-size (450mm-diameter) torpedoes known as a Type 97. Of the five launched, two made it into the harbor. The periscope of a mini-sub known as I-20 followed the cargo ship Antares through anti-submarine nets protecting Pearl Harbor. It was spotted and at 6:37am, more than an hour and a quarter before the air attack on Pearl Harbor began, and DD-139 USS Ward, a US Wickes-class destroyer, attacked.

There had been doubts about the efficacy of the Ward’s attack, but in August 2002, the University of Hawaii located the wreck of the I-20. Scientists found that the sub had been hit by a shell that penetrated its conning tower, sinking it, putting an end to any remaining doubts.

What still is puzzling, however, is that although the Ward destroyed the I-20, and US warships chased some of the other Japanese mini-subs hours before Japan’s main air attack, Pearl Harbor’s naval commanders took no action – they didn’t move any of the docked ships out of harm’s way, or even warn the commanders. That allowed the “surprise” air attack to proceed as planned.

Other mini-submarine attacks during World War II took place as far afield as Australia, where a converted ferry was sunk in Sydney Harbor, and Madagascar in 1942, where a British Revenge-class battleship, HMS Ramillies, was sunk by a mini-sub torpedo. The Ramillies was later re-floated, towed to Durban and then to Plymouth, England, where it was repaired and put back in service.

What has changed

Today, Iran has been building knock-offs of North Korean designs (with perhaps some improvements). In turn, the North Korean mini-submarines are copies of a now-defunct Yugoslav mini-submarine called the P-4 or Yugo-class.

In addition, North Korea has Sang-O-class (built between 1991 and 1999), Yono-class and Sang-O II-class subs that are the first in the North Korean fleet that can reload torpedo tubes. The Yono-class SSM was the basis for Iran’s IS-120 Ghadir mini-submarine. Two Yugo-class mini-subs were also provided to Vietnam by North Korea. (SSM is the designation for a mini-submarine, where SS stands for submarine and M denotes a mini.)

But until recently, getting countries with advanced technology to invest in mini-submarines has been almost impossible, and in some cases is still so. Taiwan, for example, which desperately needs submarines, has refused to look at mini-subs as a solution. Taiwan’s navy may be ultra-conservative, though, as new technology and some highly significant changes are redefining the game and bringing mini-subs back into vogue.

The first change is in construction materials. Most mini-submarines are now made of specialty steel or a combination of steel and titanium. Some are double-hulled, like bigger conventional submarines, while others are primarily littoral in character with single-hull designs.  

But steel may be a material of the past. Littoral and expositional mini-sub hulls now can be made of composites, which have a number of advantages over steel. For example, the hull does not have to be demagnetized, since it isn’t made from a magnetic material, which means that magnetic mines are not a threat to a composite mini-sub.  Composites naturally dampen sound, so special sound-absorbing tiles are not needed to fool sonars. Because it is a composite structure, welding sections together is minimized and there is consequently less rust and corrosion and less risk of buckling under pressure.

A second major change is in power plants. Mini-submarines, like their bigger brothers (other than nuclear-powered submarines), are diesel-electric-powered, which means they combine both a diesel engine and a battery system for propulsion. The diesel engine is used to charge the batteries and the batteries drive an electric motor for propulsion. In combat operations under silence protocols, the diesel engine is turned off and the sub moves on battery power.  

A number of attempts were made to extend the “silent” range of diesel-electric submarines. One of the most noteworthy attempts was in Italy by a company called Maritalia headed by engineer Giunio Santi. He designed a mini-submarine called the 3GST, which included a toroidal pressure hull that allowed a diesel engine to operate underwater, significantly extending the underwater combat range because the batteries could be charged while in combat mode.

The design was sold to Italy’s largest shipbuilder, Fincantieri, but a market for a toroidal-hulled mini-sub was never found. The 3GST ended up sitting on a display platform in front of the Fincantieri submarine sheds at Muggiano shipyard in La Spezia, Italy.  

The United States much more recently designed, built and deployed (for a short while) an Advanced Seal Delivery Vehicle (ASDV), which was a mini-submarine transported over a long range by a mother nuclear submarine. 

The ASDV was an all-electric sub designed to have an underwater battery range of about 200 kilometers (about half the range of a Tesla automobile). Its original silver-zinc batteries proved problematic, however. The US Navy offered a contract to replace the batteries with lithium-ion cells, similar to those now being used in electric cars. But the move never happened because of soaring costs and performance issues, and because of a fire caused by the subs’ batteries. 

Overall, the ASDV was not popular with the nuclear-oriented US Navy and, in the view of many submariners, the ASDV – being in essence a tactical SEAL-insertion warfare platform – interfered with more important strategic missions such as tracking Russian and Chinese nuclear submarines.

Extended range

But the future for all electric mini-submarines is far brighter thanks to technology developed for electric cars, especially the leader Tesla.  Using quick-charging stations underwater (like the air-to-air refueling of fighter planes and bombers), mini-subs can have significantly extended range, especially in the role of port, harbor and coastline defense. With much-improved electric motors and controllers, a new generation of mini-subs will be far more efficient.

In addition, advanced electronics and GPS (Global Positioning System) coupled with acoustic and signature-based obstacle-avoidance systems (a version of what is used in robot automobiles) can make mini-submarines capable underwater defenders able to differentiate actual threats from the normal sea-traffic background of commercial and friendly vessels.

Just as automobiles and trucks are transitioning from human drivers to robot-operated autonomous vehicles, unmanned mini-submarines or UUVs (unmanned underwater vehicles) will soon prevail, just as has happened with UAVs (unmanned aerial vehicles).  

UUVs should soon be able to communicate in multiple ways: from their charging stations, where they can be fed high-resolution imagery data leading them to targets; connections to satellites for location updates; and near the coast access to mobile-phone towers. Furthermore, UUVs can carry sonars and other sensors and can be armed with torpedoes and anti-ship missiles, including cruise missiles.

Advanced versions can even have manipulator arms to remove enemy sensors from local waters. The Russians have already shown such a capability in using manipulator-arm submersibles to find and possibly disable telecommunications cables.

UUVs can be multi-role, with capabilities for mine hunting and destruction, for surveillance and for counter-surface ship and counter-submarine duties.

Because unmanned underwater vehicles have no crew on board, there is no risk to seamen and women. An unmanned vehicle is also far cheaper to build and, with artificial intelligence and data links, just as effective as a manned boat.

Because of tensions in the Pacific and China’s pressure on its neighbors, including military threats aimed at Taiwan, the mini-sub’s modern reincarnation (whether manned or UUV) is exceedingly timely.