Torpedoes are self-propelled guided projectiles that operate underwater and are designed to detonate on contact or in proximity to a target. For the U.S. Navy, the modern torpedo enables submarines to defeat surface and undersea threats and gives surface ships and aircraft the means to reach beneath the surface and attack submarines. Once the torpedo reaches the target location, the collision of the torpedo against the target results in an explosion. During World War II, submarine and aircraft-dropped torpedoes sank hundreds of merchant ships and warships. Unlike the numerous aerial bombs or cannon shells required to sink large warships, just one or two torpedo hits could and sometimes did suffice to sink huge aircraft carriers and battleships.
Torpedoes are the primary weapons of most submarines and anti-submarine warfare platforms. They are typically classified as being either lightweight (air dropped or rocket-delivered) or heavyweight (submarine or ship launched); with lightweight torpedoes carrying smaller warheads and having reduced range and endurance; the primary distinction being not a particular weight but the mode of delivery, warhead and endurance. Lightweight torpedoes are designed to be delivered by aircraft or rocket, and typically have a warhead in the 40kg range. A lightweight torpedo is designed to be dropped very close to the target, and therefore has no wire guidance and frequently begins a programmed search pattern on entering the water. Lightweight torpedoes are typically ASW-only weapons with no ability to engage surface targets. Torpedoes primarily vary in their guidance mode and performance characteristics such as maximum depth, maximum speed and sensor performance. Torpedoes can be countered with acoustic countermeasures, decoys and even anti-torpedo torpedoes.
A heavyweight torpedo is typically fired from a submarine or surface ship and may have wire guidance. Compared to a lightweight torpedo a heavyweight torpedo will usually be much larger and have a warhead in the 300kg range. A heavyweight torpedo is designed to be launched from its firing platform and then travel to the target at high speed before acquiring the target and homing in on it (with the exception of WW2 era ‘straight runners’). Heavyweight torpedoes tend to be dual purpose ASW and ASuW weapons, however some older types are clearly more suitable for one type of target than any others (e.g. the Mk 36 is able to outrun submarines of its era but is unlikely to catch up to a surface ship). Submarine launched heavyweight torpedoes may be guided with a wire that connects them to their firing platform. This allows the firing platform to retain control over the torpedo, setting waypoints, changing speed and depth and even selecting a different target after launch.
The torpedo propulsion system is used to drive the torpedo, whereas the guided system helps the torpedo reach the target location. Torpedoes can travel several miles on their way to the target, and therefore they need a propulsion system that can run for 10 to 20 minutes. Most missiles that fly through the air use either rocket engines or jet engines, but neither of these work very well underwater. Most engines that we are familiar with, like car engines and jet engines, draw their oxygen from the air around the engine and use it to burn a fuel. A torpedo cannot do that, so it uses a fuel that either does not need an oxidizer, or it carries the oxidizer inside the torpedo. OTTO fuel has its own oxidizer mixed with the fuel. Hydrogen Peroxide (as discussed on this page) does not need an oxidizer. Torpedoes use one of two techniques for propulsion: thermal and electric.
Vickers, Ltd. in England, revolutionized torpedo propulsion by devising an alcohol-burning “steam” torpedo powered by a small turbine. This innovation yielded higher speeds and much longer ranges. In 1907, the U.S. Navy built “Bliss-Leavitt” torpedo and by adding their own improvements, notably injecting water into the combustion chamber for more steam resulting in 21-inch diameter weapon that could travel at 36 knots to a range of 3,500 yards.
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.
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.
The UGST torpedo is powered by a liquid monopropellant axial piston engine. A water-jet propulsion device is connected to the engine directly, without a reduction gear. The torpedo has a unique hydrodynamic layout with biplane ruddersfolded out as soon as the torpedo leaves the launch tube.
US Navy’s MK48 Advanced Capability (ADCAP) Heavyweight Torpedo, along with the MK46 Mod 5 and the MK50 Lightweight Torpedoes, are currently the workhorses of the fleet. The heavyweight torpedo is the submarine’s key multi-mission underwater weapon, capable of performing both anti-submarine and anti-surface roles.
Mark 50 torpedo is a U.S. Navy advanced lightweight torpedo for use against fast, deep-diving submarines. The Mk 50 can be launched from all anti-submarine aircraft and from torpedo tubes aboard surface combatant ships. The torpedo’s stored chemical energy propulsion system uses a small tank of sulfur hexafluoride gas, which is sprayed over a block of solid lithium, which generates enormous quantities of heat, which generates steam. The steam propels the torpedo in a closed Rankine cycle, supplying power to a pump-jet. This propulsion system offers the very important deep-water performance advantage in that the combustion products—sulfur and lithium fluoride—occupy less volume than the reactants, so the torpedo does not have to force these out against increasing water pressure as it approaches a deep-diving submarine. The torpedo can reach depth of 1,900 ft (580 m) and has maximum speed > 40 kn (46 mph)
In 1969, the Applied Research Laboratory at Penn State began work, under U.S. Navy sponsorship, on a lithium-based thermal energy system for torpedo application. The system, known as the Stored Chemical Energy Propulsion System (SCEPS), was applicable to the high-power, short-duration mission of a torpedo. In a subsequent effort to further torpedo capabilities, DARPA subsequently selected the SCEPS heat source for use with an engine design that could be suitable for deployment in a long-endurance undersea vehicle.
U.S. Navy says that it is interested in giving its submarines the ability to launch small torpedoes. These weapons could offer added offensive firepower, as well as an all-new anti-torpedo defense interceptor capability. The mini-torpedos use a common body and future variants might also arm unmanned ships or submarines, as well as flying drones, act as naval mines, and more. The Navy has already developed multiple variants of the CVLWT, the best known of which is the Countermeasure Anti-Torpedo (CAT), also called the Anti-Torpedo Torpedo (ATT).
This is a defensive “hard-kill” interceptor that is supposed to destroy incoming torpedoes by either slamming into them or destroying them with its explosive warhead. The mini-torpedo has a Stored Chemical Energy Power Systems (SCEPS) power system for its propulsor in the rear. SCEPS works by bathing a solid block of lithium in sulfur hexafluoride gas, creating an extremely energetic chemical reaction that, in turn, produces steam to drive a turbine engine. In use in torpedoes for years already, this helps make the smaller CVLWT accelerate very fast, reaching fifty percent of its unspecified top speed in less than 12 seconds on average.
One of the engineering obstacles that the DARPA adaptation of the heat source overcame was the development of long-life injectors of SF6 (one of the SCEPS chemical ingredients) that could survive in the system’s molten lithium bath. The Navy SCEPS program, which had also been experiencing some difficulty with injectors, adapted the DARPA technology. SCEPS became the power plant for the MK 50 Torpedo, which the Navy first authorized for use in late 1992.
In Feb 2020, it was reported that Aerojet Rocketdyne will develop a propulsion technology for an anti-submarine warfare weapon under a $63.2M other transaction agreement with the U.S. Navy. The company said Monday it aims to build and integrate a prototype power plant, tail cone and afterbody of the Stored Chemical Energy Propulsion System into the service branch’s MK 54 Mod 2 Advanced Lightweight Torpedo. The service branch aims to increase the performance of its torpedo through the SCEPS program.
Batteries and an electric motor
This is the same technique that any non-nuclear submarine must use when running underwater. 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.
Hybrid propulsion systems offer increased efficiency over a broad range of power levels when compared to conventional systems, which tend to operate with reduced efficiency at off-design conditions. Hybrid propulsion systems offer better efficiency compared to conventional systems irrespective of the power band in use. A hybrid torpedo also works on the same concept of incorporating multiple powerplants or propulsion systems in which one of them is optimized for very low-speed operations. Increasing efficiency at low speeds helps to enhance the overall range with enough power remaining for a high-speed operation, such as an attack or rapid transit.
A hybrid torpedo also incorporates multiple powerplants and/or propulsors, with one of these systems optimized for very low speed operation. By improving efficiency at low speeds, it is possible to achieve tremendous increases in overall range while still retaining the ability for a high speed operation (i.e. for attack or high speed transit). These improvements in endurance and operating envelope should allow hybrid torpedoes to perform a wide range of current and future missions. This flexibility may offer increased weapon effectiveness (as measured by such metrics as probability of kill P), and will allow hybrid torpedoes to more effectively perform missions that take advantage of emerging technologies such as improved sensors, enhanced weaponplatform connectivity and advances in battle space awareness.
The tactical impact of both hybrid torpedoes a will probably be felt most strongly in missions that;
- Take advantage of the increased range of these systems to allow the host platform (SSN or SSGN) to operate at some distance from potential threats, particularly in environments that favor SSKs relative to SSNs
- Use the increased endurance of these systems to allow a single SSN or SSGN platform to position weapons over a broad area, effectively multiplying the effectiveness of the host platform.
Some examples of specific mission scenarios include;
- Long duration search at low speed followed by high-speed attack run against selected underwater or surface target.
- Fast transit from safe standoff range, followed by long duration low speed search (i.e. for bottomed SSK).
- Standoff launch and transit to “patrol” area followed by attack or second transit, possibly using third-party targeting and cueing information (i.e. from distributed sensor network or from aircraft monitoring sonobuoys).
- Launch of lightweight torpedo by aircraft standing off from target to minimize SUBSAM threat.
- Traditional ‘Blue Water’ type engagement with a relatively short duration acquisition period followed by a high-speed chase against a fast SSN opponent.
Because they operate efficiently over a wider range of power levels than conventional systems, hybrid undersea weapons (both torpedoes and weaponized UUVs) will be capable of more effectively performing this increasingly diverse mix of missions. This will translate into improved performance, as measured by the Pk and PCk. Since hybrids can be adapted to new tasks as the Navy’s mission evolves, they will offer life cycle cost savings relative to “single purpose” conventional weapons.
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