# Analysis of the Propulsion Performance and Internal Flow Field of an Underwater Launcher
## 1 Introduction  
Tremendous resistance makes it difficult for underwater vehicles to navigate at high speeds. To address this vexing issue, supercavitation technology was developed, which can greatly lower the navigation resistance of underwater vehicles [1, 2, 3, 4, 5]. However, supercavitation technology is only applicable to vehicles that have been fully submerged in water, and it is incapable of reducing resistance during the vehicles' underwater launching processes. As a result, finding a way to reduce resistance in the launching process is critical. The most ideal method for underwater launchers is to keep the barrel free of water to ensure that the projectile always moves in a low-resistance gas environment, achieving propulsion performance similar to that of an air launcher. To achieve this goal, previous researchers preferred to add additional devices to keep water out of the barrel. [PERSON] [6] proposed sealing the muzzle using a baffle and a member and then rotating the baffle to realize continuous sealing during the firing interval. Another two sealing methods were put forward by [PERSON] and his team. One makes sure the pressure of gas inside the barrel is higher than the pressure of external water by an external gas source with a pressure regulator, thus preventing water from entering the barrel [7]. The other seals the barrel with a coaxial inner barrel and an outer barrel [8]. In addition to the above sealing devices, [PERSON] [9] designed a plurality of movable baffles which involves magnetic and waterproof properties to realize sealing the muzzle. By pushing the baffleto transition between the sealing and firing positions, a continuous waterproof effect is achieved. Although the above sealing devices help the purpose of muzzle sealing, they also make the underwater launcher more complex. Obviously, the complex structure will reduce the reliability of the underwater launcher and make it unable to meet the complex underwater continuous shooting environment. Therefore, researchers are looking for more suitable solutions. [PERSON] [10] proposed firing a blank ammunition firstly to generate gas and steam to replace water in the barrel, resulting in a low-resistance launching environment for succeeding projectiles. However, serious accidents may happen if water gets into the barrel before live ammunitions are shot. To solve the shortcomings of existing underwater launching methods, such as poor reliability or low muzzle velocity [11], the gas-curtain launch for underwater launchers is proposed. By introducing a portion of the chamber gas to the projectile front, water within the barrel will be forced out. Then, in the barrel, a low-resistance gas route is created for the projectile, resulting in higher propulsion performance than that of usual fully submerged launchers without increasing structural complexity [12; 13; 14]. For the gas-curtain launch, predecessors conducted some mechanistic research, focusing mainly on the gas-water interaction mechanism in a confined circular tube. [PERSON] [15] conducted experiments to investigate the impact of nozzle size and number on the expansion properties of a high-temperature and high-pressure gas jet in water after developing three different rectangular sidewall grooves. The numerical model and simulation analyses were conducted on this foundation. [PERSON] [16] investigated the effect of nozzle size and injection direction on the expansion characteristics of a gas jet in water and examined the distribution law of characteristic parameters using conical spray nozzles. By opening 4-8 strands of grooves in the inner wall of a circular tube, [PERSON] studied the interaction features of numerous gas jets with water [17]. [PERSON] [18] conducted gas-curtain simulated launching experiments to further clarify the mechanism of the gas-water interaction within the barrel, as other research had not included projectile motion.