Juho Lee

A rocket sled is a ground-based test facility used to evaluate impact, aerodynamic characteristics, and dynamic behavior in high-speed environments. During operation, significant dynamic loads can reduce sled stability, leading to lower test reliability. To address this, it is essential to design structures that minimize dynamic loads and reduce weight, thereby improving the sled’s ability to achieve the required velocity. In this study, dynamic loads on a supersonic rocket sled were predicted using the multibody dynamics software RecurDyn. A parametric study was conducted to investigate design parameters that reduce vertical and lateral dynamic loads, and the results were compared with predictions from the conventional Lambda method. Aerodynamic loads, including drag and lift, were obtained through computational fluid dynamics. The results confirmed that the proposed parameters significantly influence dynamic loads and that the RecurDyn-based approach is more suitable and efficient for achieving lightweight designs compared to the Lambda Method. These findings are expected to be incorporated into structural analysis, contributing to the optimal design of rocket sleds.

This review presents an in-depth examination of the advancements, current state, and future trends of separation and release devices in aeronautical and astronautical systems, focusing on their classification into one-point and linear devices as well as the move from pyrotechnic to nonexplosive methods. For one-point separation devices, this review discusses the evolution from traditional explosive bolts, which use high explosives to operate, to nonexplosive actuators such as shape memory alloy devices that offer resettable, low-shock options. It also highlights devices that rely on pyrotechnic initiators without using high explosives as key developments for reducing shock and increasing safety. In the section on linear separation devices, the review examines flexible linear shaped charges and mild-detonating fuses, which are crucial for precise separations such as stage and fairing detachments. Introducing expanding tubes as a new method to avoid the usual debris and damage from pyrotechnics is a big improvement. In addition, the use of clamp-band joint systems for satellite separation, secured by either pyrotechnic bolts or nonexplosive dual-initiator devices, is a common approach. The recent focus on developing pneumatic linear separation devices marks a shift toward systems that can be reused and reduce shock without explosives. Moreover, the emergence of nonexplosive satellite separation systems represents a shift toward safer and more adaptable deployment technologies. These fully mechanical actuation methods facilitate shock-free separations, which are crucial for the integrity of sensitive payloads. This review suggests that continuous innovation in nonexplosive separation technology will significantly change mission planning and system design in the aeronautics and astronautics fields.

Pyroshock is a large mechanical response with a wide frequency range that occurs instantaneously in a structure owing to the detonation of pyrotechnics or explosives. This response can cause critical damage to the surrounding electronics of aerospace systems. Therefore, designing a structure to prevent damage to electronic equipment from pyroshock is necessary. However, the characteristics of pyroshock change depending on the shape of the structure, boundary conditions, and materials; hence, it must be quantitatively analyzed. In this work, a pyroshock simulator was used for the characteristic study of shock propagation in various structures. A typical aerospace structure is a connected structure that uses rivets or bolts to connect two separate structures. Therefore, a study was conducted using numerical analysis and a shock simulator to characterize the pyroshock propagation in these joint structures. First, numerical analysis and experiments were performed by varying the bolt torque to observe the shock propagation characteristics according to the bolt load. Second, to observe the shock propagation characteristics according to the angle of the joint structure, the responses of 90°, 120°, and 180° joint structures were compared. Third, the characteristics of shock propagation through SUS304 and AL 6061-T6 were compared to observe the change in propagation characteristics due to material change. The results of investigation of the shock propagation characteristics in various structures in this study could be used to mitigate electronic equipment malfunctions due to pyroshock and in designing effective shock-attenuating structures.

This study intend to verify the role tasks of leaders considering the influence of leadership types and understanding motivation at the level of individual needs to prevent the departure of existing members in the volunteer organization. For purpose, this study analysis on Liongs Clubs with historical traditions and social contribution achievements. As a result of this study, the desire for existence, relationship, and growth of individual members were all identified as factors that increased individual organizational participation. At the same time, leadership was also identified as a factor that induces the participation of members of the volunteer organization. In particular, in the case of volunteer organizations, it is meaningful to confirm that the continuous participation and activities of members are important, which affects the intention to remain, unlike individual satisfaction and immersion. However, unlike the hypothesis of this study, the fact that leadership has not been confirmed to moderate between individual participation motivation and organizational participation is a task to be solved through follow-up studies in the future.

고속화된 기계 및 다양한 국방 시스템의 설계를 위해 충돌 및 폭발과 같이 극한 환경에 대한 구조물의 응답에 대한 이해가 필요한 경우가 많다. 이를 정확하게 예측하고 해석하기 위해서는 높은 변형률 속도에서의 재료의 거동을 잘 모델링할 수 있는 모델과 물성값이 필수적이다. 물성값을 얻기 위한 실험들은 많은 시간과 비용이 소모되기에 상대적으로 간단하고 높은 변형률 속도까지 실험이 가능한 Taylor impact test를 활용하고자 한다. 최적화 기법을 활용하여 Taylor impact test 실험 결과를 가장 정확히 예측하는 Johnson-Cook model 물성값을 얻는 방법론을 제시하였다. 상용 hydrocodes인 AUTODYN을 활용하여 Taylor impact test 수치해석을 수행하였으며, 유전 알고리즘 기반 최적화 기법을 적용하여 최소한의 수치해석으로 Taylor impact test를 더 정확히 해석하는 물성값을 획득하였다. 본 기법을 이용하면 적절한 고속 물성이 없는 경우보다 정확한 물성값을 유추할 수 있으며, 이를 관련 해석에 활용할 수 있을 것으로 기대된다.

Pyrotechnic-separation devices are widely used in the separation mission of satellites and projectiles. The pyroshock generated by the pyrotechnic-separation device can cause serious damage to surrounding electronic equipment owing to its high-frequency characteristics, which leads to mission failure. Therefore, solving the pyroshock problem is necessary. Typically, attenuation of the pyroshock propagation based on the understanding of the shock-propagation characteristics of a structure is possible. However, as pyrotechnics (or explosives) cannot be used for every pyroshock-propagation experiment due to the high cost and risk, a device for simulating a pyroshock environment that does not use pyrotechnics is required. In this study, a pyroshock simulator was developed, which could generate the desired shock environment by controlling shock environment-generation variables and be tested for any test structure. For this purpose, a resonator attached to the test structure and a pneumatic launch device was designed and fabricated. A resonator that generates a desired shock environment was designed by predicting the shock generation through impact analysis. A pyroshock simulator that generates a shock like an actual pyroshock was developed through comparison with the shock-response spectrum of a pyrotechnic initiator. The repeatability was verified, and the frequency and magnitude of the shock generated by the pyroshock simulator could be controlled by adjusting the collision velocity of the steel ball and the thickness of the resonator disk.