Page 22 - Australian Defence Magazine August 2018

UNDERWATER TECHNOLOGY
PROPULSION
“No evidence had been found of a pumpjet of the type used on submarines ever being recommended as superior for overall efficiency.”
The final configuration of the Future Submarine won’t be available for years to come.
speeds while snorkelling at periscope depth. “When you’re on station and you’re there for six weeks, if you’re ever doing more than four knots there’s something wrong. If you can be doing two or three knots that’s much better. It’s only when you get above double digits or the high teens that the pumpjet starts to deliver
greater efficiencies.”
RADM Sammut’s intervention also con-
tradicted several conclusions reached in a report entitled “Australia’s Future Subma- rine: Getting this Key Capability Right” produced by the independent consultancy Insight Economics.
The report, headed by Michael Keat- ing, a former head of the Australian Pub- lic Service and one-time Secretary of the Department of Prime Minister and Cab-
inet, suggested the choice of pumpjet propulsion cre- ated a technical risk for the future submarine.
A pumpjet was much heavier than a propeller system and, unless Naval Group had made a break- through, used more energy at the speeds at which diesel- electric submarines gener-
ally travelled, the report said.
Technical justification for these concerns
was provided in a separate paper written by Aidan Morrison, managing director, with background in physics and engineering, of a defence consultancy.
While acknowledging he had no access to classified material, Morrison noted that it was possible to confidently know some of the fundamental bounds and constraints within which hydrodynamics and tur- bomachinery operate.
Although it was broadly acknowledged that propellers had superior efficiency at lower speeds but pumpjets might be more ef- ficient at higher speeds, the precise point of cross-over seemed unclear in public debate, Morrison commented.
“One informed commentator suggested
that the crossover point was likely to be below or near the transit speed of the sub- marine, and that the pumpjet might offer enhanced range and endurance over the whole mission profile as a consequence,” his report said.
However, no evidence had been found of a pumpjet of the type used on submarines ever being recommended as superior for overall efficiency. A lower acoustic signature was likely at higher speeds but improving a pumpjet’s propulsive efficiency at low speeds would necessarily involve trade-offs in terms of acoustic advantage.
“In a comparison between two otherwise identical submarines, the one with the pump- jet will always have a lower dived endurance, a lower dived range, a worse indiscretion ratio, a lower overall endurance, and a lower overall range, than the one with a propeller,” the re- port postulated. “This will confer a substan- tial tactical and strategic advantage on the conventionally-propelled submarine in a very broad range of operational scenarios....
“The scale of the probable impact on range and endurance (of a pumpjet) is quite probably so substantial that it is difficult to see how such a performance penalty is con- sistent with the government’s stated aim of acquiring a regionally superior submarine”.
22 | August 2018 | www.australiandefence.com.au
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