Future submarine torpedo options.

In ASPI’s well-respected 2011-2012 Defence Budget Brief a suggestion was made that Defence would mandate equipping Australia’s future submarines with US AN/BYG-1 combat systems and US Mk 48 Mod 7 CBASS torpedoes. In fact, the use of the Mk 48 was “a given”. The source of this claim was not provided. Neither was an explanation as to “why”.

31st Aug 2011


 SEA 1000


 Future submarine torpedo options.

Introduction

In ASPI’s well-respected 2011-2012 Defence Budget Brief a suggestion was made that Defence would mandate equipping Australia’s future submarines with US AN/BYG-1 combat systems and US Mk 48 Mod 7 CBASS torpedoes. In fact, the use of the Mk 48 was “a given”. The source of this claim was not provided. Neither was an explanation as to “why”.


Torpedo Role and Requirements

Torpedoes are the traditional tool used by submariners to engage both surface targets and submarines. They are the most potent ship killers; missiles may cripple, but torpedoes sink ships. Additionally, torpedoes are the only choice for encounters with other submarines.
But how are they employed and what are the key requirements for submarine-launched torpedoes?


The torpedo’s employment story starts well before it is launched.  Using the combat system the submarine’s crew first compiles the tactical picture and decides which of the contacts, if any, need to be targeted. Once raised to target status, the submarine’s command then determines a strategy for engagement, moves the submarine to the best possible firing position and obtains a relatively accurate solution to the question of the target’s range, course and speed. In some circumstances, this pre-launch process will necessarily happen in a very short period and so the torpedo must either be continuously available or have a very short warm-up period.
After the torpedo is warmed and has been loaded with targeting data, it is launched. The launch should ideally be silent, although getting a torpedo weighing one and a half tonnes from stationary to more than 25 knots in a very short time frame inevitably generates noise. Whether the torpedo is swim-out or starts after it has been discharged positively can impact on launch noise levels.


After launch the torpedo then needs to run-out to the target, possibly via waypoints. The run-out can vary in range from very short to over-the-horizon, although it must be understood that most nation’s rules of engagement would require visual identification of a surface target before engagement. Run-out should be done at the lowest speed tactically possible, which will vary depending on the launch geometry, in order to minimize the amount of noise that the weapon generates. In the case where a high speed target is opening, the maximum speed capabilities of the torpedo must be sufficient to catch up to it; a nominal speed ratio of 1.5:1 is desirable. The run-out would normally be as deep as possible to minimize cavitation but with due consideration as to the best overall counter detection depth.
The run-out to the target is carried out in a relatively “dumb” mode. Once close to the target the torpedo will come to a best search depth and speed and then switch on its sensor circuits. A search for the target will commence; passively against a noisy surface contact or actively against a quiet surface contact or a submarine. Target acquisition should occur almost immediately after the search has commenced if the search point has been determined correctly. The torpedo must then be able to track multiple targets, including other own submarine torpedoes operating in the vicinity for situations where a salvo has been used. It must be able to do all of this in deep or littoral water environments.


Assuming the torpedo acquires and holds the target, it will increase speed and commence homing. It must resist any countermeasures employed by the enemy; but it should be understood that the best anti-countermeasure technique is to avoid counter detection in the first place. Once the torpedo arrives at the target, the warhead, triggered by influence sensors or contact, will explode and sink the vessel. If the torpedo misses the target or has been fooled by countermeasures, it should attempt a re-attack on the target. Sufficient reserves of energy for conducting re-attacks is a consideration in the design of the torpedo.
A wake-homing search is also possible against a surface ship; wake-homing allows for firing at targets without the need for an accurate solution or in datum fleeing situations. These torpedoes are aimed astern of the target with instructions to turn either left or right after wake is encountered such that it can be followed to its source. Wake homing torpedoes are relatively immune to most soft kill countermeasures.
It should be noted that most modern torpedoes are wire guided. This allows the torpedo to receive command guidance and pass information back to the submarine throughout its run-out, search, homing and re-attack phases.


Best Weapon


In relation to the Mk 48 being considered “a given”, in reality it would be misleading to argue that it is the best or even the correct torpedo choice for all situations. (KYM TO TASHA, BPS) To illustrate this it is worthwhile looking at the continuously evolving Mk 48 against a couple of new entrant torpedoes; Atlas Elektronik’s DM2A4 and the WASS Black Shark.
The Mk 48 has some real strengths in the ultimate torpedo quest. Some of these strengths come from its cold war heritage, where it served as the principal weapon for use against high speed Soviet nuclear powered submarines. A fundamental decision was made in the early stages of the Mk 48 program to use a thermal engine for propulsion as opposed to an electric motor. This decision was made because the thermal engine had much greater power density than a battery/electric motor solution; a thermal engine was critical in meeting the speed and endurance requirements of the USN. This decision was further amplified when, as a result of a competition held between a turbine-based propulsion plant and a mono propellant Otto II fuel engine, the Otto II engine was selected on account of its efficiency; the trade-off being the noise made by its swash plate reciprocating design and associated exhaust. Noise was less of a consideration for the USN on account of the acoustics superiority it enjoyed at the time but as this superiority eroded, it became the Achilles heel of the weapon.

In 1988 the USN began developing a new closed cycle Mk 48 propulsion system but the program was terminated in 1991. A fallback engine noise quieting program, which proposed dampening of the engine and fitting an exhaust muffler, was then initiated. Although this program proceeded, a 1995 US Accounting Office report opposed it on the grounds that it was unlikely the noise could be reduced sufficiently. Noise presents two problems to the torpedo user. It can have significant effect on the counter detection ranges of the torpedo and can affect the performance of the torpedo’s own sonar, particularly in shallow water when own noise reflects back off the surface and sea bed.
Very few recently developed torpedoes use thermal engines; the Russian UGST being the exception. Both the DM2A4 and the Black Shark employ electric motors fed by batteries. The DM2A4 uses a modular and operator configurable set of silver zinc oxide (Zn-AgO) batteries and a 90% efficient permanent magnet motor that develops 300 kW. This motor can vary its rotation speed continuously and without any steps over the entire range from slow to extremely fast drive, and the variation is virtually noiseless. This feature is useful throughout the entire torpedo run including during the launch phase, where cavitation can be avoided by carefully controlling acceleration, a particularly important capability in shallow water. A similar configuration is found on the Black Shark although it uses a slightly different battery technology; aluminium silver oxide (Al-AgO). Whilst radiated noise details are classified, it is reasonable to assume the electric propulsion of these two European designed weapons is quieter than that of the Mk 48. Additionally, the power to weight advantage of the thermal weapon has all but disappeared. Electric torpedoes are now achieving speeds only a few knots slower than the Mk 48, but with improved noise characteristics, greater speed control and no exhaust gases to contribute to what should otherwise be a minimally visible wake trail.


Returning to Mk 48 positives, Cold War reasons also have blessed it with a competition-winning 800 metres maximum operating depth. The Mk 48 was modified in the early 80s to enable it to attack deep diving Alpha class submarines, with the engineering trade off being greater weight. The current Mk 48 weights around 1700 kg. By comparison, the Black Shark weighs around 1200 kg and the DM2A4 sits somewhere between the two. Underway, torpedoes must use speed to generate enough body lift to overcome their negative buoyancy. This has a couple of significant implications. Firstly, higher speeds come at the expense of range. Range is linearly related to fuel capacity, but increased speed demands a rise in power following a cube law. For example, an increase in speed from 45 to 55 knots calls for a doubling of power delivered to the torpedoes’ propellers or propulsors. A relatively light Black Shark is capable of speeds below 20 knots with associated gains in range. The second weight implication relates to noise. A relatively heavy Mk 48 must have a faster minimum speed, which makes an already noisier propulsion system noisier.
The next big differentiator between torpedoes is the onboard processing. The reality is that all torpedo designers are similarly constrained by the size of the weapon with respect to sonar performance and processing capabilities. The size of the sonar array on a torpedo is limited by the 533 mm diameter of the weapon and further by hydrodynamic constraints at the front of the torpedo. Given the need to have sonar beamwidths smaller than 10 degrees to achieve reasonable targeting accuracy and resolution along with reasonable detection ranges, the frequency range of heavyweight torpedo sonar is generally constrained to between 20 kHz and 30 kHz. Signal processing can make some differences. All modern torpedoes have steered beamformers, multi-frequency capabilities, various filtering techniques, returning pulse analysis and other signal processing techniques; both in the passive and active domains which they can all easily switch between. The Mk 48 designers have sought to go broadband with CBASS processing in order to improve signal-to-noise in a reverberation limited environment; something necessary to improve performance against modern submarines operating in the littorals.


The DM2A4 camp claims advanced processing centred on the torpedo’s conformal array as does the Black Shark camp with its Advanced Sonar Transmitting and Receiving Architecture (ASTRA) technology, although there is little information about either of these in the public domain. The truth is that it is hard to assess what the differences are in processing performance across these torpedos – although noting that signal-to-noise ratio is the main parameter affecting the performance in all applications of target detection and parameter estimation, one could easily surmise that the noisier Mk 48 is starting with a handicap.
It is known that both European weapons have wake homing capabilities which can be combined with acoustical homing and communication wire related command guidance to assist in better tactical decision making. (KYM TO TASHA, BPS) The Mk 48 does not have this capability at present.
One clear area of difference between the US and the European torpedoes is the communication link between the submarine and the weapon. The MK 48 has a copper wire guidance system while the DM2A4 and Black Shark have fibre optic systems. In addition to the increase in cable length possible given its smaller storage volume and lack of signal attention, the higher data rate of the fibre optic link allows full bandwidth sonar data to be transmitted back to the combat system in the submarine.
This brings a number of benefits. It provides the opportunity for more advanced sonar processing to take place on-board the submarine. The highly variable speed control of permanent magnet motors allows torpedo speed to be set in response to the sonar search and homing performance being observed or listened to by the submarine operator. Information from the torpedo, which may have a different aspect to the target, can be fused with data from the combat system. Shaft and blade rates and other identification data can be compared with the data from the submarine’s on-board sensors to ensure the torpedo’s acquired contact is in fact the intended target. Finally, when countermeasures are used, it allows for the submarine operator to assist in deciding what is the countermeasure and what is the target using additional combat system information. A recent Navy News article revealed that the RAN has been assisting the US in Mk 48 fibre trials so there is respite here for the Mk 48 advocates, but it is clear that at this stage other torpedo manufacturers are well ahead in this regard.


Best Choice


The performance of any torpedo will be a compromise between all possible design parameters like size and shape limitations, required thrust, efficiency, range of operations speeds, cavitation conditions and noise reduction.
But in the end, it’s not always just about the performance of the torpedo. Other factors will weigh into the selection. Australia is part of a torpedo armaments cooperation program with the US and that no doubt brings benefits, not the least being access to an evolving design (albeit at a price). Then there is the fact that the Mk 48 is the incumbent weapon. Investment has been made in relation to infrastructure such as the Torpedo System Centre at DSTO in Edinburgh and the Torpedo Maintenance Facility at Garden Island in Western Australia. RAN personnel are very familiar with the weapon and that counts for something.
And the reality is that submarine warfare is asymmetric. Only in very few circumstances will the full capabilities of a modern torpedo be put to the test. Perhaps reliability of the torpedoes is far more important, as has been discovered in wartime by both the Germans and the US. Perhaps the 8,600 Mk 48 in-water exercise firings really count for something.


Options and Competition


A quiet weapon for a quiet platform; is it now time to make the switch to an electric torpedo?
Perhaps Raytheon could tender the Mk 48 Mod 8 for Australia coupled with a program for an indigenously designed and developed Mk 48 “Lite”.
The Italian Type 212 submarines have a Combat System capable of firing the DM2A4, the WASS A184 Mod 3 and the Black Shark. Maybe a mix of weapon firing capabilities for the SEA 1000 design is something to be considered?
Perhaps an option put on the table by WASS to build and support both Australian and regional Black Shark would be attractive to Defence, Industry and politicians alike?
Maybe we should all wait until there is greater clarity in the torpedo capabilities being designed into the F21 by DCNS, possibly in co-operation with Atlas Elektronik, for the Barracuda program and already sold to Brazil?


Summary

Noting the torpedo capability issues and other logistic aspects presented in this article, it would be prudent for Australia to consider all options for Australia’s future submarines. We want the best for our submariners. Advocates for any torpedo should not fear a competition, Mk 48 included. A competitive analysis should be conducted.
Our submarines will be used for a number of different roles. Even if the decision is made that Australia’s future weapon will be the Mk 48, that decision need not, nor should it, dictate the designer or design of boat that we select. That would be like having the tail wag the dog.

 

 

 

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