# Design and Analysis of the High-Speed Underwater Glider with a Bladder-Type Buoyancy Engine
## 1 Introduction  
In recent years, discovering marine resources has become a significant issue, as the world has utilized limited resources despite energy shortages [1]. The ocean contains vast resources, including minerals and energy, a substantial part of which has remained underutilized. As the sea has many life-threatening elements that are dangerous for humans, unmanned ocean exploration robots are essential for direct ocean exploration.  
Autonomous underwater vehicles (AUVs) are well-known as representative unmanned marine exploration robots. AUVs are underwater robots used for military and academic research because they can autonomously function with minimal operator intervention [2]. AUVs collect information about the marine environment and perform essential missions such as surveillance and reconnaissance activities. This information can be used for exploration in the deep sea, which is difficult for humans to access. However, as the propeller generates propulsion, the hull would be heavy, and the volume would be significant. In addition, as the amount of energy stored for developing propulsion is limited by the capacity of the battery, it is disadvantageous for long-term exploration.  
In contrast, underwater gliders (UGs) are unmanned marine exploration robots designed to travel between the ocean's depths and the ocean's surface, and their roles are not remarkably different from those of AUVs. Nevertheless, they are more efficient in energy consumption because they do not use separate propellants [3]. The propulsive forcein UGs is generated by buoyancy control by fluid inflow and outflow and by moment change using internal mass movement and wing movement. Therefore, they are suitable for long-time and long-distance ocean exploration because the power requirement is relatively small [3; 4].  
Currently, UGs are widely used in oceanographic research [5; 6; 7]. UGs can navigate by changing the ballast water amount in the buoyancy engine periodically to have good merit of using minimal energy [8; 9]. Several researchers have set up the dynamic models of UGs, including the mass shifter motion [10; 11; 12]. Motion stability and accuracy analysis were studied to study the performance of UGs [13; 14]. Many optimization designs for UGs have been proposed in various fields, such as hydrodynamic shape [15], pressure hull [16], and control parameters [17]. A study also uses back-stepping technology and direct adaptive control for UG pitch control [18]. Multidisciplinary design optimization (MDO) was applied to realize multidisciplinary co-optimization in UG systems, a preliminary applied study of model-based systems engineering (MBSE) [19; 20]. Recently, a new type of UG has been proposed that is less affected by ocean currents and increases cruising speed. [21].  
Slocum, a representative torpedo-type UG, can cruise up to 1 knot. In this study, we designed a UG that can move at a maximum speed of 2 knots, faster than Slocum, in line with the characteristics of the Korean Peninsula. Through this, the limitations in driving the UG were offset, making it possible to operate the UG even in areas with fast ocean currents. In addition, the payload in the middle of the hull is designed by increasing weight and buoyancy compared to the existing underwater glider so that various marine observation sensors can be mounted. The buoyancy engine of the UG was designed based on the purpose of operation, and the relationship between the control value of the ballast emitted from the buoyancy engine and the speed of the UG was analyzed. In addition, the reliability of the design was verified by performing dynamic modeling. After designing and manufacturing the UG system, we confirmed the maximum UG speed via sea trials.  
In the following section, we present the design and analysis of the hull of the UG. Section 3 introduces the mathematical model of the UG. Section 4 introduces the sea trials, and Section 5 presents a summary and conclusions.