BMT SSGT (Ship Submersible Gas Turbine)


Donald McKelvy
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14 August 2009
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From February 2004, BMT Defence Systems unveiled the concept of the SSGT (Ship Submersible Gas Turbine):

BMT Defence Services Ltd unveiled its concept for a high mobility submarine at the Pacific 2004 maritime exhibition in Sydney, Australia, at the start of February.

A model of the SSGT (Ship Submersible Gas Turbine) was displayed on the BMT Defence Services stand at the show, attracting interest from naval delegations and defence sector professionals from many Pacific Rim regions and countries including Australia and New Zealand; south-east Asia, Canada and the USA during the four day event that began on 3rd February 2004.

Marketing Director for BMT Defence Services, John Davis, explained the key benefits of the design to those viewing the disruptively-patterned submarine model: “SSGT is designed to provide effective mobility approaching that of an SSN without the financial and political costs of ownership associated with having a nuclear reactor onboard”.

Conceived by design engineers at BMT Defence Services in Bath, UK, the SSGT sees the first serious proposal for using gas turbines in a conventional submarine. Though gas turbines can be very compact they are voracious consumers of air and submarine designers have to date been unable to arrange sufficient volumes of air to feed gas turbines buried inside the hull of a submarine.

The innovation proposed by BMT Defence Services, in consultation with gas turbine specialists Rolls Royce, is to locate two independent gas turbine-electric alternator sets in individual containments located in a bulb at the top of the submarine fin where they can draw sufficient air through an 8m tall induction mast. When operating on gas turbines, the submarine runs semi-submerged, with just the gas turbine bulb above the sea surface. In this mode, SSGT is able to travel up to 6,000 nautical miles at 20 knots, far in excess of the capability of a conventional diesel-electric submarine.

The SSGT design trades off tactical covertness of the submarine against strategic mobility on the assumption that the submarine will meet few, if any, threats during transit. Once in-theatre, SSGT shuts down its gas turbines, dives and can operate fully covertly for up to 25 days in an Air Independent Propulsion mode (AIP). A mixture of fuel cells and advanced ZEBRA batteries provide power for systems and permit submerged operations up to 10 knots and short tactical sprints at 30 knots respectively. Kerosene is used to fuel the gas turbines and (via reformers) the fuel cells thus giving deep flexibility between transit and in theatre operations. Liquid oxygen is stored to enable the fuel cells to operate when the boat is submerged. SSGT may also run its fuel cells at the surface taking air using a conventional snort mast. In this way the boat may be more covert whilst in transit and preserve the stored liquid oxygen to maximise discretion in theatre.

Well resourced in onboard power, SSGT supports a highly capable and comprehensive combat suite including chin, fin and flank sonar arrays, six heavyweight torpedo tubes, eight vertical launch missile tubes and stowage for four large unmanned underwater vehicles (UUV) or swimmer delivery vehicles (SDV). Accommodation for up to 12 special forces personnel and a six-man lock-in lock-out chamber complete a flexible design capable of undertaking a range of high and low level missions.

BMT white paper on the SSGT:


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Saw this some years back and nothing heard since. Appears doomed to be a paper project, the prospect of having 3rd world navies running a gas turbine drive versus a diesel one is rather unattractive, me thinks.
The SSGT is not conceived as a replacement for conventional diesel-electric SSKs operated by nations with short deployment ranges. But rather an alternative to long range fleet submarines; diesel-electrics like the Collins class SSG and nuclear powered SSNs. The idea is the semi-submersible SSGT would use the higher power of a gas turbine to sustain a high transit speed to quickly get to the patrol area and then revert to more conventional battery-snorting patrolling.
Thanks for the feedback Abraham. Do you believe that the BMT SSGT would be attractive to the Royal Navy, Royal Australian Navy, and Armed Forces Canada?
Triton said:
Thanks for the feedback Abraham. Do you believe that the BMT SSGT would be attractive to the Royal Navy, Royal Australian Navy, and Armed Forces Canada?

The SSGT was more of an exploration of concepts - how to get SSN type transit speed without bouncing neutrons around the ship. Because of the very high air flow requirements of gas turbines (compared to diesels) it needs to keep the top of the fin above water. I doubt (well I know) this isn't very attractive to submariners.

BMT has shelved the SSGT and focused on the technology that is really exciting to future conventional submarines. That is high power density batteries (Li-Ion, Molten Salt, etc) and high temperature super conductors (HTS). With this kind of technology the next generation of diesel-electrics will be much faster and be able to stay under water much longer. On paper not in the league of an SSN but tactically very, very close (not including the CSG ASW mission) and without the high noise levels of running a reactor.

You speak with some certainty on this project and the company; do you happen to have anymore information on the project? I enjoy modeling submarines from scratch (currently working on a 1:144th LA Class) for radio control. I have searched for information on this boat, and have only found a few papers on it, with only small not very detailed drawings. All efforts to contact the people that are listed in the papers have not responded back. (Yeah, I know sounds like the typical spy fishing story, though that I am not, just a modeler that is anal about getting things to look right on the outside).

All I know about the BMT SSGT is in the context of their presentations to the Pacific 2005 (SSGT) and Pacific 2007 (Vidar) naval technology conferences and further discussions with their staff in relation to the Vidar design concept. You're not going to get much more out of BMT in relation to the SSGT other than what's in the data sheet linked above. It was expressed to me that SSGT was very much their radical outside the box exercise to get their design team thinking laterally and seeing what emerges from such an exercise for more conventional submarine technology.

However just like diesel electrics have spent the past 100 years coasting of all the investment in lead acid batteries thanks to the car/truck industry and alternator/generator technology thanks to the power industry they can now look forward to the next 100 years coasting of the technology investment in high density batteries thanks to the personal electronics industry and compact alternator/generators thanks to the hybrid car industry.
Do I understand correctly that the maximum patrol endurance of the Upholder/Chicoutimi-class SSK is 56 days with a maximum continuous-submerged endurance without snorkeling of ten days? A maximum range of 8,000nm at 8 knots?

While the SSGT has a patrol endurance of 60 days and a continuous-submerged endurance, via Air Independent Propulsion, of twenty-five days? A maximum range of 6,000nm at 20 knots and 13,000 nm at 10 knots?

The SSGT seems superior to SSK. Also sounds like it offers some of the capabilities of an SSN at a lower unit and operational cost. It would probably be attractive to navies running diesel boats and a lower-cost supplement to a navy with an SSN force.
Triton said:
Do I understand correctly

You can certainly quote some open source figures but the way you string them together indicates you don't understand how a submarine operates, what those figures mean and what is important and what not. I'm not about to spend the next five hours of my life explaining those things to you so perhaps you should start reading about submarine operations, watch some TV documentaries or even better find a sympathetic submariner with some time to spare and get a good understanding of the dynamics before quoting figures that are not related as if they are.
There's an excellent model of the SSGT in BMT's offices in Bath. From speaking to some of the engineers there, the current focus is more on cheap modular designs that are able to actually be sold to countries like South Korea. The SSGT was a nice exercise but is pretty much just BMT advertising what they are able to do.
It is unfortunate that there is not more information available on this concept. I guess that if I ever do it, I will just have to extrapolate the data from what I have. Thanks for the interesting info Abraham.

On SSGT, I have a very interesting PP presentation in PDF format I found in the web three or four years ago. It has some detailed data of the concept and several good pictures. If you are interested give me a mail address and a couple of days...

I'm a part time surface puke and I don't have much (read any) training or knowledge in sub ops but a few points about the SSGT strike me as problematic.

Gas Turbines require a LOT of air. The designers SEEM to have dealt with this via the perfectly straightforward technique of making the intake really really big. This however means that instead of a snort/snorkel protruding, a good bit of the sail is above the waterline....which would seem to kill the stealth features that is a submarines reason for being. The high O2 consumption would seem to have another downside. A diesel boat that is snorkeling can deal with the air intake being occasionally covered up by a wave in a high sea state, closing up the boat for a moment with a gas turbine running could lead a dangerous loss of pressure in the boat very quickly. There are probably work arounds for this but it seems like a non trivial issue to me.

Gas Turbines are HOT. This means exhaust shouting "Hi IR sensors! Kill me please!" Again, there are ways to mitigate this but as I understand it from open source references they generally require space and running water...which is noisy...which is another blow against stealth.

The blurb says that the turbines are for transit but one of the strengths of submarines is that no one knows where the hell they are. The heat plumes from gas turbines associated with wakes of surfaced sails at 20 kts would likely be very observable from the air or orbit. Satellites are unlikely to be rare birds in the future. Aircraft in theater or IR sensors on a mountain would extend the area that is "in theater" considerably. Even if the sub went quiet a hundred miles out it might have announced its entry into play....reducing its advantage

The big sail looks like it might add to generated noise.

The blurb mentions the higher power capacity of the boat, but if that is due to the very high generation capacity of the turbine powered dynamoes then it is illusory as the turbine is not something you'd want to light off in a war zone unless it was to flee...on the surface...which would indicate that the sub had failed in its mission (be sneaky) and would likely do exactly no good except give a good IR image to facilitate the killing blow.

On the other hand the kerosene based fuel cell, if it works, would seem to be a real boon to submersibles. Even without the O2 tanks the fuel cell would seem to afford a huge reduction in noise while snorting over a diesel. The addition of O2 tanks would seem to give some flexibility. I'd say forget the turbine and put the fuel cell in an off the shelf SSK.

Just my two cents.
Brickmuppet said:
Just my two cents.

Spot on. The 24 hours a day in transit sail above water would completely change the radar detection dynamic of a conventional submarine. Not to mention the air flow and IR signature issues. A typical diesel electric submarine needs to keep its snorkel up for around 8 hours a day in transit (night time) which is an extremely difficult radar target. To detect a submarine mast you need a very high scan rate radar at a very acute angle to the surface (ie low altitude) and even then detection ranges are 5-15 NM. The speed boat sized gas turbine sail structure would be detectable by a high flying aircraft with conventional radar. Making detection and tracking of the transiting submarine as easy as detecting and tracking a corvette.

Knowing when and where the submarine has deployed to the operational theatre takes away its strategic and operational surprise. The enemy can deploy their ASW assets and take other countermeasures. The Falklands War showed how important strategic and operational stealth for submarines is with the Royal Navy's submarines destroying the Argentinian surface fleet while thanks to an intelligence leak the patrol box of the Argentinian submarine was known to the British completely defeating its effort. Its a testament to the silence of a diesel electric submarine in littoral waters that the Argentinian Type 209 boat was able to survive the war considering how much the British threw at it knowing the area it was in.
From what I recall of speaking to some of those responsible for the SSGT a year to eighteen months ago, the gas turbines themselves are in the 'bulb' at the top of the fin. Which solves the problem with fitting the uptakes and downtakes in the fin, at any rate. Probably doesn't do many favours with the centre of gravity though, and I suspect that it could use all the help it could get there.

It was, as Red Admiral says, basically an exercise in pushing the limits of the state of the art, as well as demonstrating the design team's capabilities. As I recall, the main hull was conceived to run at speed just below the surface of the water, which is a regime that nothing much else (if anything at all) is designed for. Which meant of course that the whole thing had to be designed from scratch, making it a great way of demonstrating what you can do.
If gas turbines typically have higher power density, couldn't you run in a profile similar to a conventional diesel-electric boat with a short surface runtime at night? If the power increase is significant, couldn't you also shorten the exposure time itself? Though it's hard to get away from the IR signature problem, short of dousing the exhaust with pumped seawater. Well, unless you are crazy enough to try to do a heat recovery via a steam generator in the turbine exhaust like a land based coal power plant.

I wonder if there is any merit to using a small snorkel mast and precharging the necessary air into storage, similar to AIP? There's a related bit of technology in development now for a gas turbine that replaces the combustor with a fuel cell, using the compressor to get air up to the pressures needed for the fuel cell, then running the exhaust through a conventional turbine section. Instead of using a conventional matched gas turbine compressor, you use something that better matches a small snorkel's mass flow requirements and precharge your air storage. If the turbine runtime for equivalent energy output is substantially below a diesel, the idea may have legs.

A setup like that does create some problems though. You would have two masts unless you are willing to take the operational flexibility hit of exclusively requiring precharged air to run the turbine. There is the related problem of the mass flow of the exhaust. The provided images show the turbine housing as a bulb at or above sea level. I assume there are some very difficult engineering problems with trying to keep the the turbine running below sea level. You would either use a thick snorkel and pipe the exhaust way up (which puts you back into the thick snorkel/surfaced turbine bulb problem), or have some sort of annular blocking valve and keep the exhaust below sea level. If the exhaust can be kept below sea level, then the thin snorkel precharge could have merit as it's possible to have a conventional sub's profile (well, short of the enormously loud gas turbine exhaust dumping directly into the ocean), but it wouldn't necessarily solve the turbine bulb exposure transit problem.

I wounder why the turbines aren't mounted vertically, either inside the fore or aft part of the sail, rather than a top mounted bulb?

Someone pointed out to me an interesting gas turbine technology being developed by a company called RamGen. It's basically a ramjet mounted on the edge of a disk, slightly angled so it intakes air from the top of the disk and exhausts towards the bottom of the disk. The idea being you spin up the disk so the intake is supersonic, so the ramjet inlet can compress intake air, shove the air through a ramjet combustor, then exhaust through a ramjet exhaust to increase the speed of the exhaust to supersonic speeds. Since it's a rotating frame of reference, the exhaust usually ends up being subsonic. Efficiency and compactness is very interesting, in the case where it's a full ramjet on the edge of the disk, including combustor (the company has been recently focusing work solely on the ram compressor stage for use in gas compression projects since that seems to be a faster path to money). There was some side talk that tweaking the exhaust nozzle could drop the exhaust temperature significantly, due to simple gas dynamics.
Artist's impression of BMT SSGT.



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Problem is, you can't drive very fast at periscope depth without showing a lot of wake, and you will run into control issues keeping depth.

You're NOT going to do 20 knots broached like that. You'll be lucky to do 14, and even going that fast will beat the crap out of the screw from cavitation and let everyone in the world know where you are via radar and wake.

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