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• Although there will be no shaſt thrust block, there must still be a mechanism of
However, it is unknown whether this will have a negative, positive or neutral effect.
• EM signatures: it is unknown at this stage what level of EM signature might
transferring thrust to the hull. Tis will likely be built into the forward bearings of the rotor, which will then be transferred to the hull via the propulsor supporting structure. However, this may impact the design and maintenance of the rim drive so must be borne in mind.
be produced by a rim driven device and whether these can be mitigated.
In order to carry out a first stage feasibility
investigation, a rim drive was sized for an appropriate submarine. The submarine design chosen was a 5,000tonne SSN used on UCL’s Submarine Design Exercise, which has a length overall of 85m, pressure hull length of 45m, pressure hull diameter of 10m, shaſt horsepower (SHP) 15,000, maximum speed of 30knots and a conventional propeller diameter of 5.8m. It was determined that for the required
SHP of 15000 (equivalent to 11 megawatts, MW), a rim driven propeller diameter of 4.5m would be appropriate (this is the inner diameter of the thruster and does not include the duct thickness). An rpm of 91 was selected for the thruster and, in this relatively simplistic way, it is demonstrated
that a rim driven thruster of feasible size can be produced for the concept submarine. Another option for sizing the rim drive
is to use a contra-rotating arrangement and use two thrusters of half power. Using the same sizing procedure, this would give two 5.5MW thrusters of 4.2m and an rpm of 106. Tis is thought to be a particularly promising possibility and at this early stage is considered worthy of further investigation.
Challenges and risks As shown above, although rim drives show potential benefits for use on SSNs and at this stage appear feasible, more work needs to be done to establish the technical performance, integration and cost aspects. In terms of performance there are two
primary areas of risk: the first is the issue of whether a rim drive of adequate power can be manufactured and whether it will have comparable efficiency along with other operational characteristics. The second area is the level of acoustic and electromagnetic signatures that may be achieved by the device. Tere is a high degree of uncertainty regarding this aspect, although some technical issues are known, such as the challenges in the design of large diameter bearings Te structural integration of the rim drive
will require some assessment. In addition to the traditional ‘add-on’ arrangement, the lack of a shaſt leads to the potential for blended body or wrap around configurations. Te advantages and disadvantages of which should be
In a rim driven propulsor the outer duct and blades form part of an electric motor and the blades are rotated from their outer edge as opposed to from a central hub
investigated. Other aspects to consider include: the transfer of thrust, hub or hub-less blades; electrical supply routes; and the effect of compression on the gap between rotor and bearing. Although it has been highlighted that the overall submarine architecture will be disrupted, it is not known whether this will have a positive or negative effect. Rim driven thrusters are complex to
design and build, primarily due to their aspect ratio and increased air gap size. Tis results in high labour and manufacturing costs. Current propulsive devices are also expensive but it is unknown whether the costs are comparable at a component level. Research is being done into alternative topologies for reduced manufacturing difficulty. A particular risk for PM machines is the availability and cost of PM materials, there is a potential for these to increase to a point where they become prohibitive.
Conclusions Te increasing pressure of affordability and the changing nature of future conflict are forcing a change in the way the underwater capability is fulfilled. However, the current trends in the UK submarine enterprise constrain the path of future SSN development. Future boats must be smaller and cheaper to achieve adequate hull numbers and they must be more flexible and adaptable to meet emerging threats and exploit new technologies. Tere is an opportunity for a more revolutionary approach to design to be taken as the current evolutionary approach has become unsustainable. Rim drive technology shows potential
for use on submarines as an enabler for smaller, more flexible SSNs, applicable both to current propulsion power levels and also reduced levels that may be necessary to sustain the submarine flotilla. Current trends in commercially available rim driven thrusters indicate the potential feasibility of this concept. However, none have currently reached the required size and power for SSN propulsion. If rim drive technology is a serious contender for future propulsion, there are technical issues both of detailed design and integration with the whole boat to be addressed. Research is needed to support the development decisions that will need to be taken now to ensure a suitable technological road map is created to reverse undesirable trends in submarine cost and numbers. WT
Warship Technology October 2014 35
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