Submarine Matters provides an expanding database on submarines worldwide. Australia should contract in 2016 to only buy a batch of 6 Shortfins - then, in the 2030s, decide whether to buy: 6 more Shortfins or 6 Barracuda SSNs or 4 Virginia SSNs. With increasing numbers of Chinese, Russian and Indian SSNs in Australia's region Australia's Shortfins cannot attain any 2016 Defence White Paper goal of being "regionally superior". Australia would need to buy SSNs to be "superior".
April 29, 2016
Shortfin's Pumpjet Propulsor - A Sales Feature?
Note that a Scorpene (2,000 tonne "small" conventional submarine (SSK)) is depicted with a pumpjet. Was the Scorpene pumpjet only an idea in 2005 that was phased out/cancelled? Or is pumpjet a possible future inclusion for Scorpene? Pumpjets have been on French submarines since the first Triomphant class SSBN was launced in 1994. (Artwork courtesy DCNS Australia)
Shortfin concept displaying its proposed pumpjet. Also note its X-plane rudders. (Artwork courtesy Navy Recognition)
DCNS' 2016 "pitch" for the Shortfin stated: "Pumpjet 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 pumpjet
propulsion always has the tactical advantage."
Will a pumpjet (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 pumpjet becomes tacticly advantageous depends on the situation and needs to be weighed against the downsides of pumpjets.
High pumpjet weight compared to a bare propeller is a common downside. But wouldn't pumpjet weight be scalable? That is would a pumpjet for an average 1,800 tonne SSK be proportionatly smaller and lighter than a pumpjet 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 PUMPJETS OVER BARE PROPELLERS
- In the rare but crucial tactical situation where high speed is required to fight another submarine or flee from a surface ASW threat, a pumpjet 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 pumpjet 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 pumpjet? or retrofitted?) has a large pumpjet with 7 stators and 11 propulsors. It spends more time in dry dock for repairs and upgrades to its pumpjet than at sea. The pumpjet appears to be of excessive size - perhaps implying Alrosa is a test vehicle for a pumpjet intended for much larger SSN's or SSBNs.
It is unclear why pumpjets 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 pumpjet effective speeds?
- only used once(?) for a larger 2,350 tons (surfaced) SSK (that being Kilo B-871 Alrosa)
- artwork of a pumpjet included on a DCNS Scorpene (2,000 tonne SSK) but no evidence (?) it has been adopted for Scorpene.
- are pumpjets are a recent, expensive, high end, technology only used in already expensive nuclear submarines?
- pumpjets 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 pumpjet 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 pumpjet prototype for Shortfin? Ultimately much will rely on how many knots Shortfin can reliably move - something that may only be apparent in about 2031.