Note that a Scorpene (2,000 tonne "small" conventional submarine (SSK)) is depicted with a pump jet. Was the Scorpene pump jet only an idea in 2005 that was phased out/cancelled? Or is pump jet a possible future inclusion for Scorpene? Pump Jets have been on French submarines since the first Triomphant class SSBN was launced in 1994. (Artwork courtesy DCNS Australia)
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Here the Scorpene has no Pump Jet, on the Shipbucket graphics website (circa 2010).
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Shortfin concept displaying its proposed pump jet. Also note its X-plane rudders. (Artwork courtesy Navy Recognition)
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DCNS' 2016 "pitch" for the Shortfin stated: "Pump jet propulsion means the Shortfin Barracuda can move more quietly than submarines with obsolete propeller technology. In a confrontation between two otherwise identical submarines, the one with pump jet propulsion always has the tactical advantage."
Will a pump jet (which appeared then disappeared from DCNS' Scorpene SSK) disappear from the Shortfin SSK? Will the Shortfin then have the bare propeller which practically all SSKs have?
The submarine speed threshold (14 knots? 20 knots? somewhere in between?) of when a pump jet becomes tacticly advantageous depends on the situation and needs to be weighed against the downsides of pump jets.
High pump jet weight compared to a bare propeller is a common downside. But wouldn't pump jet weight be scalable? That is would a pump jet for an average 1,800 tonne SSK be proportionatly smaller and lighter than a pump jet for a 5,000 tonne Shortfin?
If the scalability argument is valid - what is the main thing distinguishing SSKs from SSNs? Engine power and resulatant speed.
An SSN can operate at 20+ knots for weeks-months while a SSK fully submerged on battery (and even AIP) can only operate at 20+ knots for (probably) 2 days or less.
(POSSIBLE) ADVANTAGES AND DISADVANTAGES OF PUMP JETS OVER BARE PROPELLERS
Advantages:
- In the rare but crucial tactical situation where high speed is required to fight another submarine or flee from a surface ASW threat, a pump jet can allow a higher speed before the onset of cavitation. This means lower acoustic signatures.
- the shroud of a pumpjet can protect the rotating element (the impeller) from striking hard objects (like rocks or the seafloor). This can assist littoral, shallow water operation.
- If the pump jet is steerable it may make the submarine more maneuverable at slow speeds.
Disadvantages compared to Bare Propeller
- Can be less efficient than a propeller at low speed, leading to higher consumption of limited fuel (not a concern for unlimited nuclear reactor). This may well include an SSK's typical efficient submerged speed (5 knots?) on battery or AIP.
- Inability to efficiently reverse or reverse at all to slow down or reverse the submarine? Therefore the sub needs a bow thruster? Or would there be a bow thruster anyway for a Shortfin?
- heavier, more expensive, complex?
- Punp jet intake grill can become clogged with debris; e.g., seaweed. (Can be mitigated by being able to reverse?)
The Kilo submarine B-871 ("Alrosa") (launched in 1989 (with pump jet? or retrofitted?) has a large pump jet with 7 stators and 11 propulsors. It spends more time in dry dock for repairs and upgrades to its pump jet than at sea. The pump jet appears to be of excessive size - perhaps implying Alrosa is a test vehicle for a pump jet intended for much larger SSN's or SSBNs.
COMMENT
It is unclear why pump jets have not been used for SSKs - leading to more questions than answers, at this stage:
- are the usual 2,000 tonnes or less size SSKs too small?
- do the relative lower power of SSK diesels limit their ability to reach pump jet effective speeds?
- only used once(?) for a larger 2,350 tons (surfaced) SSK (that being Kilo B-871 Alrosa)
- artwork of a pump jet included on a DCNS Scorpene (2,000 tonne SSK) but no evidence (?) it has been adopted for Scorpene.
- are pump jets are a recent, expensive, high end, technology only used in already expensive nuclear submarines?
- pump jets have not been retrofitted on DCNS' small (2,400 tonne surfaced) Amethyste-Rubis class SSNs, so does that prove small size eliminates pump jets or cost of retrofitting on Amethyste-Rubis would be too high?
It is very difficult to nail down whether the pump jet proposal for the Shortfin is of sales value but a technology that may be of marginal practical value.
There is the argument that the Barracuda SSN can serve as a prototype to iron out any technical problems before the Shortfins are built? But then, will the speed envelope of the Shortfin be similar enough to the Barracuda for Barracuda to be a pump jet prototype for Shortfin? Ultimately much will rely on how many knots Shortfin can reliably move - something that may only be apparent in about 2031.
Artwork courtesy DCNS.
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- https://en.wikipedia.org/wiki/Pump-jetand https://en.wikipedia.org/wiki/Propulsor
and KQN's comments at https://www.blogger.com/comment.g?blogID=19245896&postID=3070342239950420359
Pete