A couple of months ago Andrew Davies offered some comments on the pump-jet propulsion system that’s proposed as a key feature of the Shortfin Barracuda. As he observed, there seems a clear case for propellers being more efficient at low speeds—which is very important for conventional submarines. Comments from Naval Group’s director of the future submarine program seemingly confirmed this.

Andrew raised two important questions that might help resolve the apparent contradiction between Defence’s confidence in the pump-jet solution and seemingly credible evidence that its low-speed inefficiency makes a pump jet an odd choice for a diesel-electric submarine.

The first was that the cross-over point—where a pump jet becomes more efficient than a propeller—was unknown. If the pump jet was actually more efficient at plausible higher transit speeds, on balance that may offer a benefit over the entire mission profile, even if the pump jet is marginally worse at very low patrol speeds. The second was the effect of the ‘hotel load’. Since the square law for drag means that the energy required for propulsion becomes extremely small at very low speeds, underwater endurance might be more affected by the power required to keep the lights, sonars and air conditioning running than by the power necessary to move forward.

Both of these arguments really require some better understanding of the likely efficiency curves in order to be answered with any confidence, as Andrew notes. But it’s quite possible that sufficient evidence does lie in the public domain to rule some possibilities in or out.

The first question requires the establishment of a plausible cross-over point. Here, all available evidence points firmly against the idea that the cross-over could lie anywhere near the plausible transit speeds of submarines (around 10 knots). For surface vessels, open propellers are adopted essentially universally by all ships that work at speeds up to around 30 knots, including relatively fast ferries. Only extremely fast vessels (35 knots plus) tend to have waterjets. Australian companies specialising in such very-fast vessels have spent considerable effort investigating the prospect of using jets in the 20–30 knot range and discovered that the efficiency penalty of jets in this range is substantial, up to a factor of two.

And at low speeds the penalty can be very high indeed. Other research shows that the propulsive performance of waterjets falls off towards zero as craft velocity decreases. Marine enthusiasts may observe that ‘waterjets’, which eject water on or above the surface, are not quite the same as the completely submerged pump jets often found on the aft end of submarines and torpedoes. But the more I dived into the literature on fully submerged pump jets, the less advocacy I found for efficiency in any speed range. In fact, it seemed that pump-jet efficiency had to be traded-off to eliminate cavitation at higher speeds.

The method by which this occurs is simple enough to warrant mentioning. The role of the duct in a pump jet is to slow the water down before it reaches the spinning rotor that adds energy to the flow. This slowing of the water stream also increases its pressure, and the elevated pressure helps reduce cavitation. Problems arise when you try to slow down water that’s not going very fast in the first place, which is what happens at low speeds. The result is predictable: some of the water stops, or actually starts moving in the other direction, spinning around in eddies and vortices in a phenomenon called ‘flow separation’. The slower you go, the more that occurs, and the worse the efficiency becomes. The more the duct acts to raise the pressure to eliminate cavitation, the more slowing occurs, and so the sooner flow separation begins, further worsening efficiency.

Consequently, it seems unlikely in the extreme that the acoustic advantage offered by pump jets at higher speed can be decoupled from an efficiency penalty at lower speeds. It also seems highly unlikely that pump jets are more efficient than propellers at submarine transit speeds of around 10 knots.

The second question requires some quantitative assumptions about the curve and hotel load, as well as a more generalised model to be built out of the algebra described in Andrew’s ‘geeky annex’.

A good reference for plausible curves is a 2008 BMT and Rolls-Royce study of a pump-jet concept for a very quiet anti-submarine warfare surface ship. While the precise jet design may not be identical, this jet was at least fully submerged, unlike most surface ship jets. The study replicates the same efficiency curve one would expect from theory, with efficiency rising to  its peak at around 30 knots, and declining towards zero at very low speeds.

With a hotel load of 100kW (consistent with Andrew’s assumption that it might equal propulsion power at patrol speeds), it appears that in general the impact of the low efficiency of a pump jet will still be very significant in most of the important speed ranges for a submarine, particularly around 3–7 knots. Altering a range of assumptions doesn’t generally change that conclusion, as can be seen in the full discussion here. Switching back to a propeller could realistically result in improvements in dived range and endurance of 60% or more at these lower speeds. This could amount to a couple of days dived endurance, or hundreds of miles additional dived range, and credibly five or ten times that difference if air-independent propulsion was included in the submarine.

Unless the French have achieved a quantum leap in pump-jet technologies in the past few years and none of the previous physics or experimental results apply, it seems that the choice of a pump jet is curious indeed. Exactly what kind of advantage would justify accepting such a penalty in terms of dived range, dived endurance, indiscretion ratio and overall range is quite hard to imagine when building a ‘regionally superior’ submarine. Defence has made crucial errors of judgement with grave long-term consequences in acquisition projects before. We would do well to make sure that the same doesn’t happen with the future submarine.

After reading Peter Briggs’ excellent posts on why Australia should build its own submarines (here and here), it occurred to me that we may be missing a more important element of the discussion. Recently, I was involved in considering the nature and implications of disruptive or game-changing technologies in warfare. A common theme was the unpredictable interplay of various and sometimes unrelated technologies in creating a capability leap. Often, the technologies driving a leap had existed or been in development for a long time prior to their convergence.

The discourse here in Washington is that we’re on the verge of a tectonic shift in the way military operations are conducted, and that there are important lessons from history that we should heed. Failure to do so risks our platforms becoming prematurely redundant or having to make costly design changes throughout the process.

Major military platforms are getting more expensive and taking longer to develop than ever before. In his article, Peter assumes initial sea trials in 2025 and full replacement of Collins by 2028. To give some perspective on how the world might have changed by then, consider that the kids graduating from university at that point will likely have been born after the release of the first iPhone. Assuming a traditional life cycle of major platforms, the subs will then need to meet Australia’s operational needs until sometime around 2050. In other words, over 100 years after the end of World War II. Read more

ASPI’s ‘Submarine Choice’ conference has highlighted much more than the central dilemma confronting Australia: what sort of submarines do we need and how should we acquire them? Various speakers have spoken of the broader consequences of submarine proliferation in the Indo-Pacific. Greater numbers of submarines are simply a fact of life for the region’s future. But in acquiring submarines for the first time or building up their submarine fleets, regional countries may be underestimating the risks of submarine operations. More submarines in the region pose challenges for maritime confidence building and ensuring submarine safety.

Submarines are inherently dangerous systems. Even a relatively minor accident onboard can have catastrophic consequences. Then there are the navigational risks associated with having more submarines operating in relatively confined waters with a high level of fishing activity and dense shipping traffic. Most seriously, more submarines in the region are potentially destabilising, particularly as they may be employed on covert surveillance and intelligence gathering missions in disputed waters.

Those problems can be accentuated because submarines suffer from severe command and control limitations. A submarine may be out of radio contact for extended periods of time. Radio waves don’t penetrate sea water to any extent, and a submarine has to put itself, or an antenna, close to the surface to make radio contact. In many operational circumstances, that may not be possible. Read more

We saw yesterday how geography helps determine the number of submarines required for a credible deterrent capability. But that’s not the end of the story. The characteristics of the submarines themselves are also important. For example, the speed of advance is the critical factor in determining how long it will take a submarine to complete the transit to and from a patrol area. This speed is determined not only by the submarine’s own design, but also by external factors such as weather, ocean currents, the need to remain covert to achieve the mission and level of ASW surveillance/threat. Design features of the submarine, such as hull shape and the rate at which it can recharge its batteries (and their capacity) will determine how it performs in those environments. Not all designs are equal; these features are all critical attributes that need to be balanced and optimised in the design of the future submarine. My brief summary understates the challenge and complexities involved in achieving this.

The external factors will vary during the course of a transit and the mission profile will be adapted ‘on the fly’ to accommodate these variations. Typically the submarine will ‘snort’ (run its diesels to recharge the batteries) at a slow speed and for a limited time, exploiting local acoustic and environmental conditions where possible to reduce counter detection risks, before going deep to run at higher speed using power from the battery to cover the ground. To avoid snorting in a high threat/surveillance situation in the choke points enroute to the patrol area, it’s possible that an air independent propulsion system may have to be used—though that’s generally a limited resource. Read more

It’s another public holiday here in sunny Canberra, but we thought these posts from our archives would be topical to revisit.

First up, submarines: last week saw Cameron Stewart publish articles on the prospects for life extension program for the Collins class submarines. Below is what we had to say about the public statements on the program last year.

Second is a look at Tony Abbott’s view of Australia’s foreign policy in the middle of last year, from a speech he gave to to the Heritage Foundation. Read more