Whilst forty years ago the torpedo was solely an anti surface ship weapon, over the years it has increasingly assumed a primarily anti-submarine role. The requirements and resulting technical specifications called for to engage the two targets are radically different and until recently achieving a weapon to perform both tasks was a major challenge. The air-dropped weapon, which was first developed as an anti-ship device, today poses fewer problems of compatibility. The heavy-weight torpedo can be either ship or submarinelaunched, but the ship-launched weapon has, in the main, only an anti-surface ship role to fulfil.
Because of these varied requirements, different nations have viewed the balance between these roles in a different light. For example, over the past thirty years whilst Germany and Sweden have developed 533mm torpedoes primarily to counter a surface vessel threat, both the U.S. and U.K. have regarded the submarine as the heavyweight torpedo’s main target. These differing operational requirements have had a significant effect on the propulsion systems chosen in different torpedo designs.
Propulsion Requirements
To achieve those roles, a modern torpedo design, be it lightweight or heavyweight, has to meet a number of requirements which are at times mutually exclusive. In particular, as far as the powerplant is concerned:
- The torpedo has to be fast enough to overtake and attack an evading target; o It must be quiet enough not to be detected itself, thereby allowing the target to launch effective countermeasures; o It requires sufficient range and endurance, to attack at the maximum practical range and to compensate for any inaccuracies in the target’s computed position;
- It should have sufficient endurance to re-attack if it misses first time — a capability unique to the “underwater missile;”
- Its combustion products should not produce a detectable wake;
- The power plant and propulsor contribution to self noise, which interferes with the torpedo’s homing capability, should be low;
- Engine start-up must be rapid, to ensure safe discharge from a torpedo tube or water entry after air drop.
All the above requirements must be achieved whilst leaving sufficient space to carry the necessary guidance system and a lethal warhead.
A torpedo propulsion system consists of three main separate elements; the energy source; the prime mover; and the thruster. Together, they form what is generally known as the torpedo after body.
Battery-powered Homing Torpedoes
The main thrust of U.S. developments during WW II and of the U.K immediately post war was to achieve effective homing weapons, for which a quiet, electrically-driven torpedo offered the most promising platform. In the U.K., the Mk20 passive homer became the Mk23 wire-guided version, to be replaced by the dual-mode active/passive-homing Mk24 (later TIGERFISH) in the late seventies. In the U.S., the wartime Mk18 anti-ship passive weapon went through successive changes via the Mk27 to be replaced eventually by the wireguided Mk37, a short Sm, 485mm-diameter weapon for antisubmarine use only (the contemporary anti-surface ship torpedo was the Mk45, capable of carrying a nuclear warhead.) France developed the F17, Italy the A184 and Germany the anti-surface ship SEAL, its shorter submarine-launched SEESCHLANGE companion (for ASW use only) and the dual-purpose swr export model. The later versions of these weapons are still in service, and will be for some time.
All these battery propulsion systems had common features, if many details are different. The lead/acid battery has been replaced by a silver/zinc battery (developing some 125kw in a full-size weapon) which was stored in sealed bags in each cell. The battery is primed immediately before firing and will develop full power within 20 seconds, the firing sequence thereby starting at launch minus 20 seconds. Typically the priming mechanism is a coiled rod which, on rotating, releases a plunger in each cell which ruptures the bag allowing the electrolyte to flow in under gravity. As silver/zinc batteries are temperature-dependent, it is necessary to warm tube-borne weapons to about 12-15°C.
Dual Speed, whereby the torpedo searches at low speed to enhance homing ~nd increases speed for the final run to the target, is a common feature usually achieved by the simple expedient of connecting the two battery stacks either in series or in parallel (a more sophisticated solution, adopted for instance for the Swedish TP43XO 400mm-dia. torpedo, is to have the battery supplying the main motor via a thyristor switch unit, giving access to three speeds selectable during the run). The power thus generated drives a series-wound DC motor with a contra-rotating field rotor and armature, driving the forward and after propellers, respectively, by direct shafts without the need of a gearbox. A performance in the order of 26/28 knots for 30,000m, or 36/38 knots for 15,000m, is achieved. Later versions may do slightly better.
This propulsion system is thereby relatively simple, inexpensive and reliable. The battery-driven torpedo offers another advantage: it makes “swim-out” tubes possible.
Thermal Powered Torpedoes
The advent of the SSN capable of speeds of up to 30 knots and hitherto unanticipated diving depths threatened to outpace the weapons. It was necessary, therefore, to recreate the classic torpedo speed advantage of 1.5:1, and at the time there was no battery available which could generate the necessary power (75kw) within the space constraints of the light-weight torpedo dimensions. Secondly, U.S. homing technology had advanced to the state which made active homing at 45 knots a practical proposition.
The result was the Mk46 torpedo. In its initial Mod 0 trial version the engine was driven by hot gases generated by the burning of a solid, cordite-type charge. This system was, however, too noisy to optimize homing and was soon replaced by the Mod 1 variant, which entered service in 1965. A mono fuel known as “Otto fuel” powers a five-cylinder reciprocating engine, which drives two contra-rotating propellers via a gear box.
Otto fuel, a propriety compound, contains its own oxygen. It is relatively energy-efficient, safe and easily handled. Once ignited by a pyrotechnic charge, combustion is self-sustaining, the high resulting temperatures being reduced by sea water injected into the combustion chamber; the resulting gas and super-heated steam drive the engine. The fuel storage and handling system therefore, is simple and does not need complex pumps or pressure vessels.
When the U.S. replaced the Mk37 with a dual-role torpedo, they decided, at the start, on the thermal propulsion system to achieve the required specifications of 900m depth, a variable speed (55 knots top) and a range approaching 40km.
Range is linearly related to fuel capacity for a given speed, but increased speed demands an inexorable rise in power following a cube law. An increase from 45 to 55 knots, therefore, calls for the doubling of the power transmitted to the propulsor: better propeller design goes some way to achieving these power levels, but the key is an efficient fuel engine combination.
A Gould weapon powered by Gould’s own swashplate engine, competed against a Westinghouse turbine driven torpedo, which was much quieter. But the Gould engine was more efficient, particularly at maximum operating depths where the combustion products are ejected against very high back pressures. As torpedo noise was not, at the time, considered to be a problem when U.S. submarines had both the sonar and weapon advantage, the Gould design, the Mk 48, was selected.
New Systems
The British STINGRAY is not strictly new in that it has been in service now for some four years, but it most certainly has an effective, improved sea water battery.
STINGRAY’s sea water battery consists of stacks of magnesium alloy/silver chloride cells using sea water as the electrolyte, the water being circulated by pump. The voltage is controlled during battery discharge by regulating the sea water intake, making perfonnance sensibly dependent of sea water temperature and/or salinity. The battery-powered motor provides auxiliary power, both hydraulic to power actuators and AC for the nose sonar. The DC motor is contra-rotating with the field coupled to the forward propulsor via a hollow shaft, and the annature coupled to the after propulsor via the central shaft. The contra-rotating propellers are ducted, which both reduces noise and enables the weapon to run closer to the surface at full power without cavitating. Only one speed is used — full power — matching the thermal engine’s speed.
Perhaps the most critical feature of the sea water battery is to achieve rapid fill on water entry – otherwise, battery fires can result. Though not strictly part of the propulsion system design, careful parachute design is an essential element to ensure controlled water entry and pull out.
Battery development has continued and both France, with MURENE, and Italy with the A290 lightweight torpedoes due in service in the early nineties are using an aluminium/silver oxide battery with potassium hydroxide dissolved in sea water as the electrolyte. In comparison to Mg-AgCl, AJ-AgO provides a somewhat better energy density, is unaffected by salinity, and is less critical in its start-up or fill requirements. A separate lithium battery powers the electrolyte pump, delivering the electrolyte at constant rate via a closed-loop circuit which includes a heat exchanger and a gas separator to inject the generated hydrogen. Battery temperature is thereby adjusted to provide constant voltage but the electrolyte management system at present is battery.
SPEARFISH: Back to Thermal Propulsion
In deciding to develop a new torpedo rather than buying the Mk48 in its updated Mk5 ADCAP version, the U.K. were concerned that the reciprocating engine could never be made sufficiently quiet to achieve “stealth” at slow speed. To sustain a 1.5:1 speed advantage over the ALPHA. a top speed of over 60 knots would be needed, demanding a power output in the region of lOOOhp. Also, with the reported diving depth of the ALPHA exceeding l,OOOm, sustained high speed at this depth would be needed. Batteries were out of the question; a turbine was essential if “stealth” was to be achieved, and a fuel with greater energy density than Otto fuel was required.
The solution was to adapt the Sundstrand engine, originally developed for the Mark 48, double its power output, and enhance the thermal efficiency of Otto fuel by 40% by mixing it with an oxidizing agent, hydroxylamine per·chlorate. Great care has been taken to ensure that on no occasion does this agent come into contact with Otto fuel until intended.
Seawater is added at the combustion chamber and the resulting hot gas and superheated steam mixture drives a single rotor via the turbine and gearbox, operating in a duct with a rear mounted stator. The auxiliary power alternator is driven via the gearbox. Quiet operation has been achieved by careful duct and propulsor design, effective suppression mounts, exhaust·silencing and hull baffling. The combustion products are nearly all soluble thereby giving a wakeless track.
Closed Cycle Thermal Engines
The main disadvantage of thermal engines is that the exhaust gases have to be ejected outside the torpedo.
The U.S. Mk50 Advanced LightWeight Torpedo is nearing the end of its development. The required speed of 55 knots and long endurance call for some 150kw of power, which could not be met within lightweight dimensions from any conventional source, thermal or battery. The Garrett closed cycle engine was eventually selected as the technical risks of the advanced battery development were assessed as being too high.
The principle of this engine is simple, but the technical complexities of achieving a reliable torpedo engine are not. Metallic lithium is melted by pyrotechnics in a boiler, whose internal and external boundary is a coiled stainless steel tube through which water/steam is passed. Gaseous sulphur hexafluoride is injected into the lithium and the resulting violent but controlled reaction generates steam at very high temperature to drive a high-speed turbine. The steam is then passed through a hull section condenser, and recirculated through the boiler. The combustion products take up less space than their original constituents, and therefore there is no exhaust — both fuel and steam are sealed systems. Performance is, thus, independent of depth.
The final weapon has emerged some lOOkg heavier than the Mk46 it is to replace. The same diameter has been retained, but the weapon is slightly longer. A number of major problems had to be resolved. Variable speed is essential, but the residual boiler heat makes quick acceleration or deceleration difficult. Quick start-up on water entry causes similar problems.
Both France and the U.K. are carrying out feasibility studies and demonstrators of closed cycle engines as potential power plants for their own next generation weapons. Closed cycle technology promises increased power, quieter propulsion, full performance at depth, no exhaust (silent and wakeless running) — all features that would dramatically improve the Mk48’s performance.
Whilst there is no actual torpedo which calls for an advanced lithium battery, research continues and the lithium/thionyl-chloride battery is still the most likely contender from a number of lithium-based options. Overall efficiencies similar to that of closed cycle engines are likely. A lithium anode, coated with lithium chloride by the resulting reaction, acts with liquid thionyl-chloride serving as both cathode and electrolyte. Thionyl-chloride is corrosive and lithium potentially dangerous, and units are, therefore, hermetically sealed, but it is a flexible battery and varying the ratio of anode to carbon current collector surface area makes cells with very variable discharge rates. The battery has a specific energy density some seven times that of Al-AgO. It is perhaps not surprising that high rate batteries still present considerable safety problems.
The future path of torpedo propulsion is by no means decided. There will be very few opportunities for major developments other than the planned U.S. Mk48 update. In the mid-1990s Germany plans to install a new power plant in her silver/zinc battery-powered DM2A3 torpedo. Beyond that, STINGRAY, MURENE, A290, Mk50, SPEARFISH and other modem weapons will assuredly be updated, but no new torpedoes are apparently planned before 2015 — except, perhaps, in the Soviet Union. Torpedo propulsion is perhaps reaching a plateau of high capability, beyond which it will not significantly advance.
[This article is condensed from an article by Brian R. Longworth in Military Technology 9/BB~ and is published with the permission of that publication.]