Most modern submarine-launched torpedoes are dual-purpose, meaning they are able to sink a ship or submarine, but they have different characteristics and methods for achieving those goals. Single-purpose torpedoes have a very specific method of attack and can be difficult to evade. In this article, we will cover the capabilities of both kinds of submarine-launched torpedoes and how they actually work, which is very different than what you have probably seen in the movies. Modern submarine torpedoes come in two variants: thermal and electric. Thermal torpedoes use a fuel, such as OTTO Fuel II, which can be burned without an external oxygen source. A gas turbine or axial piston engine converts this fuel into torque that spins counter-rotating propellers, propelling the torpedo up to speeds in excess of 60 knots. Higher speeds can be achieved if Hydroxylammonium Perchlorate (HAP) is injected during fuel combustion. A HAP boost gives thermal torpedoes a speed advantage over electric torpedoes.

Thermal torpedoes can have a much longer-range at higher-speeds than their electric counterparts. Liquid fuel stores more energy and can be burned more efficiently in modern gas turbines engines, giving these lethal weapons the engagement range and speed required to hit any target from outside detection range. Gas turbine engines have replaced older external combustion, axial piston-driven engines in some modern torpedoes. The higher RPM of a gas turbine engine coupled with sound silencing modifications to the torpedo chassis and exhaust have made thermal torpedoes as quiet as the submarines that launch them. It is likely that if a modern torpedo uses passive sonar for homing, a target will never know it’s being attacked until just before it explodes.

Public Domain The propulsion section of a Chinese Yu-6 Torpedo. Note the counter-rotating propellers and the wire hanging out of its central hub.

Electric torpedoes are more common because they are easier to make, maintain, and are less risky to handle. They also have some capabilities thermal torpedoes do not. These high-torque, permanent magnet electric motor torpedoes ramp up to speed in under a second. They go from sitting in a torpedo tube to 50 knots in a near-instant because they don’t have the mechanical lag and inertia thermal torpedoes must overcome during startup. Another big advantage of electric torpedoes is that they can be modular in design, such as Germany’s DM2A4 Sea Hake Mod 4 torpedo. The batteries are connected in series allowing each weapon to have 2, 3, or 4 batteries. More batteries give the weapon more range. Fewer batteries make the weapon much lighter and more agile, but at the cost of range. Both can maintain 50 knots and, like modern thermal torpedoes, are very quiet.

Atlas Electronik

High energy zinc-oxygen Batteries and some types of energy cells are also used in submarine torpedoes today. They provide much more sustained power than standard electric batteries. Specific capabilities of high energy batteries are closely guarded secrets, but Israeli contractor Electric Fuel Limited has been working with Germany to develop heavyweight torpedo batteries since 1995. Submarine movies such as Crimson Tide and Hunter Killer use torpedo chase scenes for dramatic effect. The reality is that a torpedo maneuvering and hunting submarines that are frantically trying to evade is the least likely scenario in a modern submarine attack. As already noted, in a 21st Century torpedo attack, the target will likely never know it’s about to be destroyed. Modern submarine torpedoes have sound silencing built into their design and, unless they use their active sonar modes, they may not be detected until the moment before detonation. A common event observed in naval exercises is two submarines passing within a few hundred meters of each other, detecting each other at the same time, and racing to get a shot off before the other. The other type of engagement is when one sub detects the other sooner, and often at range, resulting in a first shot, first kill. So, the underwater prolonged dogfights that are such beloved set pieces of modern submarine thrillers are just not the reality. Actual underwater combat occurs silently with very little reaction time to fend off an impending attack. In addition, many modern torpedoes have a command wire or fiber optic cable that reels out from behind the torpedo and establishes a data link with the submarine’s fire control system. Before a torpedo is launched, it must know three things: What are the torpedo’s course and depth after launch? At what range will it enable a search for a target? What are the kill box boundaries? With command wire capabilities, the weapon can change its attack geometry or even shut down if directed by the fire control operator. Detected targets can be changed, depth and range limitations can be set, and countermeasures, such as decoys and jammers, can be ignored using the submarine’s sonar data instead of the torpedo's lower-fidelity onboard sonar data. If the data link is lost, the weapon will follow its last given command and execute pre-programmed countermeasure defeating profiles, if necessary.

Raytheon Virginia class SSN firing a Mk48 Mod6 torpedo.

After launch, the weapon will do a short dive below the submarine, so the submarine doesn’t run into the command wire, potentially tangling it around the submarine’s sail and propeller. The wire or fiber optic cable is fed from a dispenser that is either mounted in the torpedo tube or from the torpedo feeding out as it moves through the water. In some cases, there is wire fed from a dispenser and the torpedo simultaneously. This decreases the chance of wire stretch or a break. The submarine’s fire control system has given the weapon digital boundaries, or a "kill box." These boundaries are designed to prevent the weapon from attacking the firing platform or any other target outside the designated area. These boundaries shape a three-dimensional cube of water space and can be very large or small as determined by the Weapons Officer before firing. The torpedo will run out on a predetermined course and depth to the kill box. During this transit, the weapon is measuring ambient background noise and getting to its search depth, unless otherwise directed. It can calculate how strong it will transmit its high-frequency active sonar without reverb, distortion, or saturating detections with background echos. During the search, the torpedo will lower its speed and its sonar transmit power level to maximize detection capabilities. This is especially critical in complex, shallow water, noisy, and icy environments.

BAE Systems BAE Systems' Spearfish heavyweight torpedo in profile and in action during a SINKEX drill.

When the weapon reaches this kill box, it activates its own sensors and begins hunting for a target. If the data link between the submarine and the torpedo is maintained, the fire control operator can change the size and dimensions of the kill box at any time. They can also manually steer or shut down the weapon on command. If the weapon ever leaves the kill box, it will inert its warhead, shut down its engine, and sink to the bottom of the ocean. A torpedo cannot be ‘command-detonated’ as seen in the movie Hunt for Red October. It is possible for the firing platform—the submarine that fired the torpedo—to enter the kill box during a torpedo attack. If not shut down, the torpedo would consider it a valid target. Situational awareness is key during a torpedo attack to prevent this. During wargame exercises, submarines have crossed into their own kill boxes while evading counter-fire torpedoes.

National Archives MM2 (SS) Joe Hackett inspects the MK 48 ADCAP torpedo for proper alignment as it enters the torpedo tube during exercise RIMPAC '98.