SUBMARINE PROPULSOR TECHNICAL DEVELOPMENTS, OPPORTUNITIES AND CHALLENGES
5. OPPORTUNITIES AND CHALLENGES 1 ‘MORE COMPOSITE’ PUMP JET DUCT
For reasons discussed earlier, the pump jet propulsor weight has a direct impact on cost. Propulsor weight reduction can therefore help to improve affordability, provided that the benefits are recognised at an early stage in the submarine design. A more stringent propulsor weight budget can only be fixed, however, if confidence in the technology needed to deliver the weight improvement is available. Rolls-Royce, sponsored by UK MoD, is therefore developing the manufacturing technology for production of Composite Stator Vanes (CSVs) for future submarine pump jet propulsors. Initial CSV development has been focussed on a typical ASTUTE stator vane geometry using a glass-fibre epoxy resin laminate system.
CSVs also offer affordability improvements since the parts can be produced to accurate tolerances by resin transfer moulding and, unlike their metallic counterparts, they require no further machining to attain the necessary tolerances and surface finish. There is also a broader, competitive UK supply chain for these parts than for the high integrity NAB castings that they would replace.
The structural design justification for introduction of CSVs into Astute class comprises a combination of elements, including stress analysis, validated by full-scale component testing. The component tests include:
• Start-of-life ultimate strength tests;
• Fatigue tests;
• Impact tests;
• Underwater shock tests;
• End-of-life residual strength tests (post fatigue, impact or shock).
A number of non-destructive examination (NDE) techniques, including laser shearography [Figure 5], are used before and after component testing to search for evidence of failure. The laser shearography technique can be applied to non-uniform surfaces, such as those of CSVs, to indicate changes in sub-surface condition of the material. Destructive microanalysis in Rolls-Royce laboratories of material taken from pre and post-tested CSVs also assists in the investigation of laminate failure mechanisms. Observations made at this stage of development are being used to help to improve the laminate design for the final application.
Figure 5: Laser Shearography Fringe Patterns for a CSV (Laser Optical Engineering Ltd, Loughborough)
The development project has culminated in successful underwater shock testing of full-size CSVs by Weidlinger Associates [Figure 6] and the results will be used for future validation of improved numerical shock models.
Figure 6: Underwater Shock Testing of Prototype CSV (Weidlinger Associates Ltd)
The development programme has also demonstrated the feasibility of embedding robust optical fibre strain sensors [Figure 7] within the CSV laminate, for potential use in condition monitoring, or for design validation.
Figure 7: CSV Flat Panel with Optical Fibre Strain Sensors (Insensys)
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© 2011: The Royal Institution of Naval Architects Basing the CSV design on the existing geometry of the
Astute pump jet propulsor NAB stator vane, however, introduces some challenges. For example, if a straightforward material substitution is implemented, where all NAB stator vanes are replaced by CSVs, the global stiffness of the duct/stator system is reduced significantly. This might impair the hydrodynamic efficiency when compared to that of the established metal design and may also introduce hydrodynamic noise sources. The reduction in modal frequencies of the duct/stator may also have an impact on the radiated noise signature if new resonances are introduced. Increased deflections during shock/whipping loading might also reduce margins against shock.
To illustrate these issues, simple design comparisons can be made between equivalent beams in different materials.
Consider the load/deflection equation for a tip loaded rectangular section cantilever beam:
Deflection: į = P / k Stiffness: k = (3.E.I) / l3 Second moment of area: I = (b. t3) / 12 From which: į = (4.P. l3) / (E. b. t3)
Two transversely loaded rectangular beams of identical length and breadth and with the same tip applied force will therefore have the same deflection if:
E1t13 = E2.t23
Assuming the NAB modulus is about six times that of a typical glass fibre vinyl-ester composite (approximately 120 GPa compared to 20 GPa respectively), then a composite beam would need to be about 1.8 times thicker than the equivalent NAB beam to give the same deflection under load.
Using a similar approach, we can estimate the necessary beam thickness in each material needed to maintain the same natural frequency in air.
Considering the expression for the fundamental natural frequency of a simple mass-spring system:
Natural frequency: f = (1/2.ʌ).(k/M)½ Mass of beam: M = b.t.l.U
From which: f = (1/4. ʌ).((E/U).(t/L)2)½ For the same natural frequency:
t1 x (E1/ȡ1) ½ = t2 x (E2/ȡ2) ½
Assuming the same values for Young's modulus as used in the previous calculation, and typical density values of
7,600 kg/m3 and 2,200 kg/m3 for NAB and composite, the composite beam would need to be about 1.3 times thicker than the NAB beam to give the same natural frequency. Similar calculations can be made to estimate the relative frequency shifts of the different materials when submerged in water (to allow for the entrained mass effect).
These types of calculation can provide a useful insight at the earliest stages of design to assess the characteristics of components made from different materials and to assist the material selection decision. They also illustrate that it is potentially misleading just to focus on the weight saving benefits of composites without considering other performance impacts. Problems can be avoided if the use of composites is recognised from the onset of the hydrodynamic design, since it is then that geometric and structural changes can be most easily accommodated and trade-offs made. Again, the full involvement of the manufacturer in the design process is important to ensure cost effective manufacture and to avoid exotherm problems that can affect thicker section composite parts during thermosetting of the resin.
The stark contrasts highlighted by these calculations suggest that there may be some benefit from attained by alternating the metallic NAB and composite vanes in a pump jet propulsor duct/stator. Work completed by Rolls-Royce as part of the CSV development indicates that this may provide a lower risk route to technology insertion on current platforms, where hydrodynamic shape is fixed. It also allows greater scope for optimisation of hydrodynamic, signature and weight characteristics in future applications. The projected future weight saving benefits from CSV introduction for a future attack submarine are shown in Figure 5. The projections assume that just over 50% of the NAB stator vanes can be substituted by CSVs.
Figure 8: Masses of TRAFALGAR, ASTUTE and projected future attack submarine pump jet propulsor ducts
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© 2011: The Royal Institution of Naval Architects 5.2 ALTERNATIVE ROTOR MATERIALS The benefits of using alternative rotor materials to reduce weight include diminished loading of the submarine tail shaft, bearings and support structure, which may provide spin-off benefits in terms of size and complexity of interfacing systems/structures.
A recent study by Rolls-Royce shows that a lightweight rotor system is unlikely to influence the sizing of the tail shaft, but reducing the overhanging mass reduces tail shaft deflection, benefiting the tail shaft bearings and the sealing of those bearings. The reduced deflection would allow the shaft to run more concentrically within the bearing with a more uniform bearing pressure.
Furthermore, a more lightweight rotor system reduces the bearing loads, such that it may be feasible to consider shorter, more compact bearings, or to maintain hydrodynamic lubrication at reduced shaft speeds, which may help achieve lower noise signatures.
Reductions in rotor weight are principally realised by using materials with higher specific strengths (strength/density ratio). This and other parameters used to compare the relative performance of candidate rotor materials are summarised in Table 1.
Material Vy/ MPa
E/
GPa U/
Kg m-3
Specific Strength
Vy/U
Specific Modulus
E/U
NAB 180 120 7,600 1 1 Composite1 200 20 2,200 3.8 0.57 Ti-6Al-4V 834 113 4,430 7.9 1.65 Notes:
1. Typical equivalent allowable stress for glass fibre vinyl ester composite
2. Specific strength and specific modulus are normalised using NAB as the reference.
5.2(a) Composites
Composites are candidate materials for future pump jet propulsor rotors. Composite propellers are available from a number of commercial suppliers for a range of sizes up several metres in diameter. QinetiQ have also developed technology for a carbon/glass fibre composite propeller, resulting in trials on the R V Triton research vessel [5].
For commercial applications, the benefits of composite propellers include:
• Weight reduction;
• Reduced vibration; smoother, quieter running;
• Reduced cost of ownership compared to monobloc propellers (particularly in designs which facilitate replacement of individual blades);
• Increased efficiency over a wider speed range (where hydro-elastic deflections can be designed into the product).
Lower electromagnetic signatures may also present a benefit for future naval submarine applications, since the composite material can be effectively isolated from the seawater using integral non-conductive glass fibre layers, with less reliance on external coatings.
There are however, significant challenges to be overcome before composites can be introduced for submarine pump jet rotors. The duty is significantly more demanding in terms of the hydrodynamic forces involved and the kinetic energy that the blades would need to absorb if impacted by a foreign body is an order of magnitude higher than for static parts of the duct/stator.
Composites are also less resistant to the effects of cavitation damage, which could present through-life cost and availability issues if blades need to be replaced periodically.
Despite these difficulties, developments in aerospace and marine market sectors are likely to bring rapid advances in technology readiness for a composite submarine rotor application. The race to develop reliable and efficient marine current turbines, for example, provides a route to the design and justification of large composite marine blade structures. Similarly, in aerospace, composite fan blade development is being driven by the need to reduce the weight of gas turbine components for increased fuel economy. Here, the feasibility of composites is realised by a simultaneous reduction in fan speed to meet noise emission requirements, which reduces relative kinetic energy under bird strike conditions. Despite this reduction, the energies that such composite structures must be capable of withstanding dwarf those met in a typical submarine pump jet rotor. Rolls-Royce is involved in composite product development in both these market sectors and continues to support university research at various centres throughout the UK. It is envisaged that these developments will provide the manufacturing technology and design assessment methodologies needed to produce a robust composite rotor in the near future.
5.2(b) Titanium Alloys
Propulsor pump jet rotor blades provide an interesting potential application for titanium alloys. The use of titanium brings the advantages of improved corrosion resistance and high specific strength compared to NAB, the traditional material of construction, allowing blade designs to be optimised to minimise weight. The intrinsic non-conductive and non-magnetic properties of titanium would also help to reduce electromagnetic signature.
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© 2011: The Royal Institution of Naval Architects The major barrier to use of titanium is the high cost of
material and its processing. Low cost methods for manufacturing titanium parts using powder metallurgy offer some promise and are currently under development in Rolls-Royce. Initial work is based on recovery of Ti-6Al-4V titanium off-cuts from aerospace part manufacture.
Ti-6Al-4V exhibits a much higher specific strength (nearly eight times higher than NAB), indicating a significant potential weight saving if blades are designed to take full benefit of the enhanced strength. The modulus of titanium is also comparable to that of NAB, so increased deflection under hydrodynamic load should not be of significant concern.
Studies by Rolls-Royce into the potential weight benefits of using titanium alloy for pump jet rotor blade parts suggest that about 75% of the potential weight reduction would be attributable directly to the lower density of the material compared to that of NAB, with the remaining 25% attainable from blade geometry optimisation to take account of the material strength increase.
A strength based design approach may, however, be under-conservative since the material is significantly less ductile than NAB and other failure modes, including fatigue and fracture, may present more onerous limits.
When other failure modes are taken into account, it is possible that the weight of the rotor blades could be reduced by up to 50%.
These predictions assume a solid rotor blade construction. It is possible, however, to produce hollow (free-flood) titanium blade structures by the Superplastic Forming and Diffusion Bonding (SPF/DB) manufacturing technique used to produce aerospace fan blades. This may give additional scope for trade-off between hydrodynamic shape and weight than a solid blade design.
Other barriers to introduction of titanium include isolation of the material from other less noble materials in the shaft line to prevent galvanic corrosion of the latter in seawater. An integrated approach to ensure compatibility of all materials in the system would be necessary to avoid introducing significant corrosion problems.