Quiet Steering Control: All-Electric Turning Control System for Submarines, UUV's and Surface Combatants

    Current systems for turning and maneuvering control for naval vessels have a range of limitations. For submarines operating in a littoral environment, enhanced turning capability at low forward speed is highly desirable, and improved maneuvering and performance would also benefit the coming generation of Unmanned Underwater Vehicles (UUVs). For surface combatants, the ability to maneuver while minimizing or eliminating cavitation-induced noise is a high priority. In all these applications, development of technologies that minimize the use of high-maintenance actuation (e.g., hydraulics) in favor of electrical systems are an important long-term priority for cost savings and for enabling new designs that save space and weight and minimize hull penetrations.

    Ducted propulsors have been assuming a growing role in naval combatants, and the concept of a steerable ducted propulsor - one whose outflow can be redirected to enhance low speed maneuvering and augment the performance of (or replace) conventional rudders or sternplanes - is an attractive prospect. However, mechanical implementation of such a concept through hydraulic or electro-mechanical actuation would be prohibitively difficult. The extension of rapidly maturing smart materials technology, however, offers a realistic path to achieving this goal. In particular, the development of practical Shape Memory Alloy (SMA) actuation devices for underwater use has opened the way for a deformable 'Smart Duct'.

    The Smart Duct illustration shown in the figure above depicts the overall concept of this system: a ducted propeller with a deformable shroud that redirects the propeller wash to provide direct steering force. The deformation is provided by an electrically-actuated structure - embedded in flexible, hydrodynamically smooth sheathing - whose prime movers are a set of high strength nickel-titanium SMA actuator cables. Key advantages of this technology for Navy systems are:
  • Enhanced submarine low speed maneuvering for key combat and surveillance roles;
  • Minimization of noise due to rudder cavitation or unsteady loading;
  • Reduction in size and - potentially - elimination of conventional steering surfaces;
  • Elimination of heavy, high-maintenance hydraulic actuation hardware; and, providing support for the development of a low-maintenance More Electric Ship.

    The Smart Duct system is an outgrowth of a series of successful demonstrations of Shape Memory Alloy (SMA) technology in recent years. SMAs offer a very high strain capability (up to 5-8%), which leads to the potential for large amplitude, high-force actuation with greatly reduced volume, weight and moving parts relative to competing concepts. SMAs exist in one of two stable crystalline phases: a high temperature 'austenite' and a low temperature 'martensite'. Since the most common SMA, NiTi, is a good conductor, transition between the austenite and martensite phases is typically accomplished through direct electrical heating. Phase transformation temperatures are typically in the range 70-90 deg. C, and in practice, 3%-4% strain levels are sustainable for long-term cyclic operation.

    Following design and construction of a Smart Duct demonstrator (described in detail in Quackenbush et al. 2005, AIAA Paper 2005-1077), principal testing of the demonstrator took place at the 36" water tunnel at Naval Surface Warfare Center/Carderock Division during January of 2004. The Smart Duct demonstrator was mounted in the water tunnel on an aluminum strut with a trailing edge fairing to minimize drag and protect wiring supplying power to the SMA actuators (shown in the following figures). The strut attached the model to a four-component balance permitting measurements of forces and moments. In addition to these integrated forces and moments, a laser-based measurement system was used to sense changes in the position of the deformable portion of the model.

Rear oblique view of the demonstrator, showing the observation window used for the laser position measurement system.

Oblique view the deflected Smart Duct model in empty duct mode.

    The first test runs consisted of "empty duct" cases at a flow speed of 12.5 fps (dynamic pressure of 150 psf). Starting from a neutral, undeflected position, a t.e. deflection of 0.73 inches was achieved (average duct angle of 7.5 deg.); a peak side force of 47.4 lbs was measured for this case. A follow-up test at a flow speed of 14.7 fps (dynamic pressure 210 psf) produced net trailing edge deflection of 0.70 in. and a peak side force of 69 lbs. The deflection achieved closely matched that observed in separate testing prior to this entry at CDI (figure on right). A number of runs were executed with a combined duct+propeller system, both with the tunnel at near-static conditions and with a representative mix of values of advance ratio J and propeller thrust coefficient KT. The propeller used in these tests is shown in the figure below. It is a 3-bladed design with a diameter of 12" and a design J of approximately 1.0. The first step in active duct+propeller testing was to explore the effect of duct deflection at near-static conditions with the propeller operating; follow-up tests at flow speeds of up to 14 fps and RPM levels of 1195 were then conducted. Peak trailing edge deflections very close to the empty duct test results were achieved in this case, and effective flow turning angles of up to 15 degrees were measured at thrust levels representative of operational submarines.

Left - Sketch of 12" diameter NSWCCD test propeller (face view). Right - side view of installed propeller + duct combination.

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