Sea Power 2010: Size matters | ADM Apr 2010

Yet, in a Defence White Paper which explicitly calls for a future frigate which will be larger than the Anzac class vessels, an offshore combatant vessel that will be larger than the current Armidale class patrol boats, a large strategic sealift ship and large UAVs, there is no explicit call for a larger submarine.

But, rather for one that has greater range, longer endurance on patrol, and expanded capabilities compared to the current Collins class submarine.

Perhaps there are well founded reasons for the omission of this word.

These reasons are now examined.

Rex Patrick | Woolooware

It's been a year since the UK submarine, Vanguard, collided with the French submarine, Le Triomphant.

Both were on routine national deterrent patrols somewhere in Bay of Biscay, both were travelling under four knots and neither submarine heard the other.

Whilst mathematicians might argue about the statistical likelihood of two submarines being in the same spot in the ocean at the same time, the fact is they were.

From here it is the physicists that can provide insight into what probably happened.

In 1987, Tom Stefanick wrote a book entitled Strategic Anti-Submarine Warfare and Naval Strategy which is probably still the most comprehensive diatribe on strategic anti-submarine warfare publicly available.

In it he wrote a comprehensive annex on submarine radiated noise, detailing the noise source levels for a range of both US and Soviet submarines.

In the 1980's an Ohio's broadband radiated noise levels were between 90 and 110 dB.

It is reasonable to assume both Vangaurd and Le Triomphant have similar, or quieter levels.

Let's use 110 dB.

Physics tells us that sound pressure reduces six dB every time one doubles the range from the source.

Using those numbers, after propagating 1000 metres a 110 dB British or French submarine's radiated noise source level will have reduced to 50 dB.

At the sorts of frequencies submarines generate noise, the ambient noise might typically be 70 dB, which places these submarines signature well below the background noise and suggests they are unlikely to ever be detected passively at ranges beyond 1,000 metres.

Modern Submarines
But hang on - what about all those submarine trailing war stories from the Cold War?

Stefanick also detailed the radiated noise levels of Soviet submarines.

He estimated an Alfa's broadband noise level to be between 150 and 170 dB and a Victor III's to be between 130 and 150 dB.

The numbers get worse for their older submarines.

Using the same ambient noise levels as in the example above, a 150 dB Victor III would show 20 dB of signal above background noise at 1,000 metres, 14 dB at 2,000 metres, eight db at 4,000 metres and two dB at 8000 metres.

Yes, in both cases I'm ignoring cylindrical spreading and directivity index - it is meant to be illustrative.

The bottom line is that the Soviets could be detected at much greater ranges back then and therefore could be (re) detected by submarines and trailed.

Stefanick suggested that each generation of submarine gets about seven dB quieter.

It is highly likely that most modern submarines would have noise levels below the Ohio numbers stated above, making it increasingly harder to detect submarines.

Sure, against some older boats, submarines can still claim a useful ASW role.

Until fairly recently there has been a somewhat limited uptake for submarines in our region.

This situation has changed which has meant a proliferation of essentially modern boats: the older boats that are currently operating in the region are disappearing and by 2020 there will almost be none.

ASW in the 2020s
What I have revealed should come as no surprise to those current in the art today.

The ASW community knows it.

There has been a noticeable shift away from passive sonar as a means for detecting submarines.

Most modern ASW forces are moving to low frequency active sonar as a means for searching and localising submarines.

Low frequencies are being employed on account of the low absorption co-efficient which assists with long range active detection.

Most new ship hull mounted sonars operate below five kHz and most ASW ships now deploy Variable Depth Sonars (VDS) operating at frequencies well below this.

Ultra's solution for Australia's Air Warfare Destroyer transmits at 3.2 and 1.8 kHz.

Thales' FLASH helicopter dipping sonar operates below five kHz and L3's HELRAS operates at 1.3 kHz.

Both Thales' RASSPUTIN and Ultra's ALFEA sonobuoys operate between 1-2 kHz and the new US SQQ-125 sonobuoy is spec'd for 950 Hz.

The other technique being employed is "multi-statics".

Multi-statics is a Network Centric approach to ASW which sees each ASW unit capitalising on geometrically separated transmitters (hull mounted sonars, VDS, dipping sonars and sonobuoys) and receivers (hull mounted sonars, towed arrays, dipping sonars and sonobuoys).

Even the submarine community is starting to accept that active may be the way of the future for submarine detection and tracking.

Modern submarine active sonars are also moving lower in frequency.

Australian submarines will, in 2025, be confronting ASW forces with modern equipment, operating in the lower portion of the acoustic frequency spectrum and employing multi static active techniques.

In the active world, "target strength" is a key number.

Target strength increases as a function of size.

A four metre diameter sphere has a target strength of 0 dB.

A one metre diameter mine has a target strength of -12 dB (into the -20's with clever shapes and anechoic tiles) and a small submarine has a target strength between 5-25 dB, depending on its aspect.

The larger the submarine, the larger the target strength.

Unfortunately, anechoic tiles don't work as well at the low frequencies now being introduced into the ASW world because the wavelengths at these frequencies are large (1.5 metre wavelength at one kHz) in comparison to the tile thicknesses (typically five cm), which are most probably optimised for torpedo frequencies.

In the 2025 multi-static low frequency ASW environment, large submarine hulls will be a liability.

Size implications
It is clear that a large submarine is at a disadvantage with respect to target strength when compared to a small submarine.

So what are the pros and cons of a large submarine hull?

Operational Stealth: With respect to the submarine characteristic of Operational Stealth (i.e. stealth in the context of the total operational area) hull size has
very little impact.

All submarines, through the act of submerging, have the advantage of covertness, initiative and operational survivability.

During the Falklands war, no-one in the Royal Navy cared that ARA San Luis was a 1,200 tonne submarine (as opposed to a 4,000 tonne submarine).

ARA San Luis caused significant consternation to Admiral Sandy Woodward's team.

They allocated one carrier, 11 destroyers, five submarines, one diesel submarine and over 25 helicopters to the ASW task, all but depleted available stores of sonobuoys and expended so many weapons, reportedly more than 100, that the US was called upon to provide replenishment for British inventory.

Operational Endurance: The link between a submarine's hull size and its endurance is very weak and indirect.

The factors that directly affect operational endurance are the fuel load, equipment efficiencies, the required payload for the total mission, the reliability and maintainability of the platform and mission system, the stores/spares carried and the stamina of the crew.

Whilst it is true that a larger-hulled submarine can have greater fuel storage capacity, it also requires more energy to propel the larger hull through the water.

This in turn creates a need for larger diesels, larger batteries, larger main motors, more auxiliary equipment, more personnel and greater stowage and spares capacity.

The reality is that the laws of diminishing returns kick in and empirical data supports this assertion.

There is a direct link between hull size, which generally determines the payload capacity, and what is referred to as mission endurance.

A submarine with more weapons should, given the same tactical encounters, be able to remain on task for longer before the weapon load is depleted.

Reliability and maintainability are not really affected by hull size.

A larger hull does not even guarantee greater redundancy - for example, a Collins has three diesels whilst a Type 209 has four.

Finally, an increase in hull size has a positive effect on endurance with respect to crew habitability.

A larger hull provides a better living environment for the crew (to be balanced against the fact that larger boats have larger crews).

In the 500 tonne Type 206 submarines operated by Germany, there is sufficient space for the entire crew only because one third of them are always on watch.

The same is not true for the Type 212s procured to eventually replace the Type 206s.

Additionally, the layout of the living space in larger submarines is normally better with the crews being accommodated in dedicated areas perhaps laid out across a couple of decks, most with dedicated cabins, as opposed to a smaller submarine where the crew areas are squeezed into spaces left over after operationally related space allocations have been made.

Any decrease in hull size for a defined capability represents an efficiency that will improve operational endurance.

Flowing from this, any technological advances that aid in the reduction of the size of submarines without reducing capability would, or should, be welcomed by operational authorities.

Operational Freedom Of Movement: All submarines, no matter what size, have Freedom of Movement within the operational area.

Large hulled nuclear powered submarines have an advantage in the speed at which they can move about the area, but this is attributable to the reactor, not the hull size.

Smaller hulled submarines have an advantage in their ability to operate in shallower water than large hull submarines.

This can be particularly important in the littorals.

Smaller submarines may experience difficulty operating close to the surface in heavy seas.

As a benchmark for "small", an 1,800 tonne German built Type 214 submarine is considered an ocean going submarine.

Submarines that are smaller than 1,800 tonnes can avoid heavy seas by remaining at or below safe depth, although this may not always be a possibility for a submarine in transit.

Operational Flexibility: Most submarines, whether they have a large or small hull, have the Operational Flexibility to perform (or threaten to perform) almost all of the roles and functions familiar to submariners (Interdiction and Strike, Land Strike, ISR, Mine Warfare, SOF operations and a range of peace time constabulary tasks).

The exception to this are the ballistic missile submarines which demand a large hull in order to embark, transport, conceal and launch large intercontinental ballistic missiles.

It would be true to suggest that different hull sizes will offer different pros and cons for each role and function, but this doesn't change the fact that basically all submarines offer the flexibility to perform each of them.

Operational Lethality: Hull size has little effect on the Operational Lethality of a submarine (i.e. a submarine's ability to effect unit kill, mission kill or limited damage to an adversary).

Both large and small submarines carry a wide range of effectors including torpedoes, mines, land attack missiles, anti-ship missiles and Special Forces.

One stand out exception is the large hulled ballistic missile submarines which carry a load of the most lethal weapons on the planet.

Noting the Defence White Paper's "strategic strike" requirement for our future submarine, the impact of such a requirement on hull size warrants further examination.

Smaller hulled submarines can have a land strike capability.

Germany's Type 212 Batch II submarines will have a short range IDAS missile land strike capability and the Spanish S-80 will have a long Tomahawk capability.

In both of these cases, the missiles are launched through the torpedo tubes.

One disadvantage with the tube launched approach is the limit to the number of missiles that can be launched at one time (six in the case of the S-80) which means that two or more smoke datums are provided in the launch of a large salvo of weapons.

US submarines, for example, have 12 vertical launch tubes to overcome this limitation.

Vertical launch tubes require a minimum hull diameter that requires either a larger submarine or a "mound" somewhere on the hull of a smaller submarine - although there was a suggestion by HDW at Pacific 2010 that a vertical launch system could be inserted into one of their existing MOTs designs.

Tactical Manoeuvrability: Smaller submarines are more manoeuvrable than larger submarines.

They have a smaller draft, and respond to helm movements more quickly, particularly when coupled with cross configuration control surfaces.

This can be important in shallow water and certain
tactical scenarios.

During the Falkland's War, HMS Onyx, an Oberon class conventional submarine, was deployed to the Islands because she was more suitable than the larger nuclear powered submarines, already in the area, for certain littoral operations such as shallow water reconnaissance and Special Forces insertion and extraction.

Tactical Stealth: Large submarines require more power and therefore larger equipment which in turn will almost certainly produce more noise than a smaller submarine of similar generation.

Large submarines are also likely to have stronger magnetic signatures and have a greater chance of visual counter detection, particularly in shallow water.

A smaller diesel electric or AIP equipped submarine provides very small acoustic, thermal, magnetic, electrical and visual signatures.

When operating on electrical propulsion its radiated noise is virtually negligible making it difficult to locate acoustically.

The issue of active sonar target strength has already been discussed.

The US submarine force has, in a variety of forums, admitted that they perform poorly against smaller, stealthier submarines.

It is an issue they are trying to address through joint exercises with operators of small capable submarines, e.g. through the leasing of a Swedish Gotland class submarine and requests to get Italian 212 submarines (claimed by the German Navy to be the quietest submarine in the world) operating in US waters on a regular basis.

Tactical Survivability: Survivability is relatively independent of hull size.

Technically a larger submarine can carry a larger number of acoustic and torpedo countermeasures, though in practice this is not necessarily true, but any improvement in this regards is almost certainly offset by the downsides a larger hull has with respect to manoeuvrability and tactical stealth.

In Essence
Summarising the effect that a large hull has on various operational and tactical characteristics;
• Operational Stealth - Neutral
• Operational Endurance - Overall Slightly Positive (but not $36 billion worth)
• Operational Freedom of Movement - Negative (limitations in the littoral)
• Operational Flexibility - Neutral
• Operational Lethality - Neutral
• Tactical Manoeuvrability - Negative
• Tactical Stealth - Negative
• Tactical Survivability - Neutral

Even in the context of Australia's "unique" requirement to operate in the far field, a large submarine offers our submariners very few advantages.

On the Positives
Payload: A larger submarine does offer the Navy the ability to carry more weapons into the far field.

However, this advantage is eroded when translated from the theoretical to operational domain.

At the height of WWII submarine operations, German submarines were sinking on average 120 ships per month.

Our future submarine, with between 20 and 30 weapons embarked, would be depleted of all weapons within a week (particularly if a land strike or mine lay were conducted).

It would make very little operational sense to have the boats return to Australia and then transit back to the operational area - we will have to rely on support bases, tenders or other approaches to ensure that the maximum value of the asset can be exploited.

Note that the US Navy still operates two tenders - USS Frank Cable operates out of Guam and USS Emory S. Land will soon be operating out of Diego Garcia - which are maintenance hubs in what the US Navy refers to as an expeditionary maintenance model.

Those that still think the payload advantages of a large hull should dominate our thinking need to understand that the payload benefit gained is offset by the fact that larger submarines require more materiel and equipment and therefore have a greater cost to build than a smaller submarine and they also require more fuel and maintenance which results in higher through life cost.

For the cost of procuring and fielding one large conventional or AIP submarine, it is possible to procure and field two smaller submarines.

These two smaller submarines, between them, provide an operational commander with a greater total payload in the field, plus the flexibility of having the submarines, uncompromised by the downsides of a larger hull, in two different locations within an operational area.

It is also worth noting that, from an adversaries perspective, two submarines greatly complicate operational planning and split ASW forces.

Peacetime missions in the far field do not require large payloads.

Australia has never conducted peacetime operations in the far field without visiting a foreign port at least once during a deployment.

Vertical Launch: The ability to have a vertical launch land strike configuration, which has some advantages over tube launched land strike configuration, needs to be considered in the context of its likely usage by our submarines and the downside effects that hull size have on a submarine's tactical capability.

Even though chartered with a new strategic strike role, our submarines will very rarely conduct such a role.

Other missions will have a greater propensity of occurrence throughout the future submarine's life.

According to Woolner and Yule, in simple terms, intelligence gathering was the most important reason [for Australia to have submarines].

Crew Stamina: A larger submarine does provide for better crew habitability.

Chung, a former submariner and the founder of the Korean Institute for Maritime Strategy, commented on this "If possible we should secure a better living environment in the submarine, but we should be very careful so that the survivability of our submarines is not impaired for that reason ... the choice of viability in battle or habitability is optional, but the former should be given with higher priority over the latter".

On the Negatives
There are a number of negatives associated with a larger hull - particularly in the area of freedom of movement, manoeuvrability and tactical stealth.

These become particularly relevant in any littoral, ISR or submarine vs submarine operation and in a 2025 ASW active sonar environment - these traits will dominate outcomes.

Smaller submarines provide their crews with significant advantages, particularly at the tactical level.

Large nuclear powered submarines provide their crews with a number of advantages that tend to trade off some of the large hull tactical disadvantages.

A large-hulled conventionally powered submarine seems to sit in operational and tactical no man's land.

Rex Patrick, a former submariner with sea experience on Oberon, Collins, Los Angeles, Ula and Type 214 class submarines, is the director of Acoustic Force, a provider of generic undersea warfare training to Australian and international military personnel.

The views expressed here are his own.

Updated submarine comms?

Julian Kerr | Sydney

Senior Royal Australian Navy (RAN) officers have been briefed on Raytheon's Deep Siren system, a long range acoustic tactical paging capability that addresses a critical shortfall in submarine communications.

Deep Siren allows an operational commander to reliably send messages to a submerged submarine in near real-time, regardless of the submarine's tactical operating environment, and over ranges of more than 100 nautical miles.

Rick Smith, Director of Maritime Communications Solutions at Raytheon, told ADM the company would be continuing discussions with the RAN under the terms of a Technical Data Licence that allows it to discuss the system's attributes with the UK, Germany, Norway, Sweden, Australia and Canada.

Deep Siren consists of an expendable buoy with a built-in Iridium antenna for data uplink to satellite, connected via a cable to a low-frequency acoustic transmitter that can be lowered hundred of feet below the surface.

A standard laptop receive station is carried in the submarine and integrated with its sonar arrays, and a command station is installed in a surface ship or shore station.

The buoy is placed inside an underwater launch vehicle and ejected through the submarine's waste disposal unit, rises to the surface, and activates when the submarine is at a safe distance.

An acoustic projector descends to a depth determined by water temperature and salinity and, the buoy, using its Iridium uplink, connects to the command station and alerts an operator to receive a pre-loaded message.

The command station, for its part, can send hundreds of messages to the submarine during the buoy's 72-hour mission life, which includes 180 minutes of acoustic transmit time.

Single word messages have the highest probability of reception.

These are constructed from simple drop-down lists arranged by category.

According to Raytheon, current shortfalls include the need for the submarine to be at or near periscope depth, and for a faster broadcast cycle for integrated operations.

The data rate is currently circa two bits per second.

Although the system is still under test by the US and Canadian navies, Smith said he was hopeful of a sale this year to the UK following sea trials with the Royal Navy in the Mediterranean in March 2009.

During the trials, the Trafalgar-class nuclear attack submarine HMS Talent was positioned at increasing ranges from a Deep Siren buoy and the acoustic signals were received through omni-directional and directional sonar arrays at distances of up to 150 nautical miles.

The tests also saw the amphibious assault ship HMS Bulwark transmit accurate targeting coordinates to the submarine, allowing it to identify which hostile surface ships to attack.

Raytheon is now developing an air-launch capability which it hopes to demonstrate by mid-2010, Smith said.

The system has been developed by Raytheon with UK-based RRK Technologies and Canada's Ultra Electronics Maritime Systems.

Raytheon's Submarine High Data Rate (SubHDR) multi-band satellite communication (SATCOM) system is already deployed on US, UK and Canadian submarines and Australia's Collins-class fleet.

SubHDR allows submarines to transmit and receive mission-critical information by raising a mast just above the ocean's surface while the boat is submerged at periscope depth.

The system transmits secure wideband multi-media, secure and non-secure Internet access, voice and data traffic, imagery and video teleconferencing.