AFAIK an UUV is basically a 53cm torpedo, launched by a submarine. They have generally a run-time of 30-45 minutes and a top speed of some 5-6 knots. They are wire-guided. They have both passive and active sonar modes.
The 32.4cm (12”) torpedoes based designs are also common. The choice probably depends mostly on the endurance/range requirements. Almost all designs use electric battery propulsion, for obvious reasons.
UUVs are used in three main situations:
- Navigation
- Detection
- Defense
In Navigation mode, a UUV is highly effective in mine-fields. I have never heard of UUVs actually disarming the mines yet (how would they do that?). But what they do is detecting and giving an exact position of the mines so that the parent sub can safely move through the mine-field. In other words, offering a “clear path” for the boat to follow.
Sea mines are almost never disarmed, unless the deminers desperately need a sample for study. The usual procedure for sweeping (clearing) mines are:
- For moored contact mines you cut the cables so they come to the surface and can be detonated with gun fire
- Bottom influence mines are typically detonated in place by simulating the target characteristics they look for (magnetic field, sound, pressure, etc.)
- Recent designs of bottom mines use combinations of sensors and software to make them harder to sweep. For these you need to place an explosive charge on them, or in close proximity, to detonate the mine (preferred) or disable the operating mechanism.
- Torpedo deployed mines have been around for a couple decades.
UUV perform option 1 using hydraulically driven bolt cutters and option 3 with a small detachable charge placed on the mine. Option 2 is usually carried out using helicopters. But the survey work is probably more important, first to find and confirm the existence of the mines, and then to confirm their destruction.
In Detection mode, a UUV can be launched and deployed on a specific course and depth, while the parent sub takes a totally different position at a different depth... In other words, it’s used for enlarging the coverage envelope. In passive mode, the UUV might detect an enemy sub for instance. Turned active, it can offer a clear picture for a firing solution while the parent sub would still be undetected and at a relatively safe range.
Communications is the restriction on this activity. A fiber optic line is currently the best option, but the length of the line and drag of the water restricts the total movement of the sub and drone.
Sensitivity of passive sensors is proportional to the array size, so UUVs will probably be of limited use for extending the range of a submarine in this regard, they could however be used to generate a cross bearing for determining range, allowing a shot to be setup using entirely passive means. There is also the possibility of using ‘pelagic travelers’ to set up a passive tracking network to monitor submarine movement. These are extremely low power UUV that use wings and slight differences in buoyancy for movement, only 1 or 2 knots, and surface every few hours to download any data to satellites. No good for setting up a shot, but they can tell you where a sub has been.
The real value of drone will be any use requiring low power active sensors, that may escape enemy detection but have ranges measured in 10s of meters, for anti-mine warfare and detailed seafloor mapping in enemy controlled areas.
Finally, in the Defensive mode, a UUV can be used as a decoy by an attacked submarine. The UUV’s sound signature will be the same as the parent submarine at that specific speed of 5-6 knots. The sonar operators in the attacking sub will suddenly have two identical contacts splitting from the same spot! If the UUV is launched after the enemy torpedoes were fired, there would be quite a challenge for the weapons officer to guide his torpedoes towards the real target. Confusion being the key element here.
These are the big pluses of UUVs. The disadvantages are their slow speed, limited endurance and being detectible.
As far as USVs are concerned, I see them as effective (or less) than an actual ASW ship. If we look at the US Fleet Class USVs, other than a considerable smaller scale, they can be as easy detected (reaching speeds of 35 knots) by a sub as a full-scale destroyer or frigate. The only advantages being that they can create confusion (like the UUVs), are a lot cheaper and would not cost human lives if destroyed.
Less effect, sort of ... Small size is a key element for drones that have to be loaded by a mothership. ASW USVs will probably be about as effective as antisubmarine helicopters, and use much of the same equipment and tactics. Sprint ahead, stop and drop the sonar array, listen a while, and then retract the array and sprint to the next spot. They would be slower than helicopters by have the advantage of greater endurance, and would probably work together synergisticly. They would have the ability to carry 1 or 2 ASW torpedoes, and probably a RWS for a machine gun to carry out patrol duties while the ship is anchored.
