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Abstract. The US Navy's 2004 Unmanned Underwater Vehicle (UUV) Master Plan outlines the Navy's aim to expand the role of UUVs, and one of the key areas of interest is the increase in UUV range and endurance. A class of UUVs known as underwater gliders achieves this objective by cyclically modifying its buoyancy and covering horizontal distance with a climb/dive pattern. The present study proposes the use of shape memory alloys (SMAs) in a buoyancy heat engine (BHE) where the oceanic thermocline would be exploited to produce martensite-austenite phase transformations that in turn change the buoyancy of a piston-cylinder prototype. The working principle of the device involves transitioning between the following two states. At low temperature (at depth) the SMA wires are tensioned into a detwinned martensitic state by a parallel compressed spring. This moves the piston within the cylinder to increase the chamber dry volume and device buoyancy. At higher temperatures (near the surface) the SMA wires undergo a martensite-toaustenite phase transformation, recover part of the applied strain, and reduce the volume and buoyancy of the piston-cylinder. This paper presents the analysis, design, fabrication, and testing of a prototype device. The prototype was immersed in a water bath, and it was demonstrated that its volume would change, as expected, with change in temperature of the water bath. Simulation results showed good correlation with test data. I. IntroductionIn 2004, the United State Navy released its UUV Master Plan for Unmanned Underwater Vehicle (UUV) integration into naval missions through 2050. The plan outlined classes of UUVs ranging from 3"-9" in diameter to over 3' in diameter. The UUV Master Plan also aims to expand the role of UUVs in the US Navy and recommends increasing UUV experimentation across the board [1]. The 2004 UUV Master Plan outlined several UUV classes. Engineering UUVs to populate the classes specified by the master plan and complete the myriad of missions tasked to them will take the integration of new technologies. Previous studies have considered the implementation of adaptive structures into UUV design. Rufino et al. demonstrated that a morphing hull design could improve UUV performance in specific missions outlined in the UUV Master Plan by reducing drag as fuel is burned [2] [3].Among the missions described in the UUV Master Plan were surveillance, communication/navigation aid, and oceanography. Technologies specifically developed to increase UUV endurance and range would bridge an important gap to allow UUVs to complete these tasks. Figure 1 shows the interior hull of a contemporary UUV glider. This glider uses a pump to move low density oil from the internal reservoir to the external bladder. Another type of UUV termed a "thermal glider" uses the surface to depth

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The US Navy's 2004 Unmanned Underwater Vehicle (UUV) Master Plan outlines its aimsto expand the role of UUVs in Navy missions, and one of the key areas of interest is the increase in UUV range and endurance. A class of UUVs known as underwater gliders achieves this objective by cyclically modifying its buoyancy, and covering horizontal distance with its climb/dive patterns. The present study proposes the use of shape memory alloys (SMAs) in a buoyancy heat engine (BHE) where the oceanic thermocline would be exploited to produce martensite-austenite phase transformations that in turn change the buoyancy of a piston cylinder device. The working principle of the device involves transitioning between the following two states. At low temperature (depth) the SMA wires are tensioned into a detwinned martensitic state by a compressed spring in parallel. This moves the piston in a cylinder to increase the chamber volume and device buoyancy. At higher temperature (surface) the SMA wires undergo a martensite-to-austenite phase transformation, recover part of the strain, and reduce the volume and buoyancy of the piston cylinder. This paper presents the analysis, design, fabrication, and testing of a prototype device. The device was immersed in a water bath and it was demonstrated that its volume would change, as expected, with change in temperature of the water bath. Simulation results showed good correlation with test data. I. IntroductionIn 2004, the United State Navy released its UUV Master Plan for Unmanned Underwater Vehicle (UUV) integration into naval missions through 2050. The plan outlined classes of UUVs ranging from 3"-9" in diameter to over 3 ft in diameter. The UUV Master Plan also aims to expand the role of UUVs in the US Navy and recommends increasing UUV experimentation across the board [1] . The 2004 UUV Master Plan outlined several UUV classes. Engineering UUVs to populate the classes specified by the master plan and complete the myriad of missions tasked to them will take the integration of new technologies. Previous studies have considered the implementation of adaptive structures into UUV design. Rufino et al. demonstrated that a morphing hull design could improve UUV performance in specific missions outlined in the UUV Master Plan by reducing drag as fuel is burned [2] [3] .Among the missions described in the UUV Master Plan were surveillance, communication/navigation aid, and oceanography. Technologies specifically developed to increase UUV endurance and range would bridge an important gap to allow UUVs to complete these tasks. Figure 1 shows the interior hull of a contemporary UUV glider. This glider uses a pump to move low density oil from the internal reservoir to the external bladder. Another type of UUV termed a "thermal glider" uses the surface to depth temperature difference in a large body of water to modify its buoyancy in a cyclic pattern [4] . Figure 2 shows the climb/dive profile that results from this buoyancy cycle. The oceanic thermocline has a temperature difference near 20°C from the s...

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