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Baek S.-H.,KAIST | Kim W.-S.,KARI | Jang C.-J.,KAIST | Lee J.-J.,KAIST
Key Engineering Materials | Year: 2011

For most of structural failure in engineering structures, fracture often takes place due to a phenomenon called fatigue. Therefore, many studies about the effect of the various mode-mixities on fatigue characteristics have been performed. However, most of the former studies are about metal/metal interface or delamination of composite. In this study, the fatigue characteristics of a composite/metal interface are investigated. The fatigue tests were performed using single-leg bending (SLB) specimens bonded with composite and steel using co-cure bonding method. This paper focuses on the fatigue characteristics depending on different mode ratios (GΠ/GT). The overall results obtained in this study show that the crack propagation rate increases with the mode Π loading component. Source


Cho D.-H.,KARI | Bang H.,KAIST | Kim H.-D.,KARI
Proceedings of the International Astronautical Congress, IAC | Year: 2013

There are many kinds of strategies for the optimal lunar landing trajectory design. This lunar landing stage is usually divided into two parts, de-orbit burn phase and powered descent phase, and the optimal lunar landing problem is focused on the powered descent phase by using continuous thruster. These optimal lunar landing trajectories have a variety shape and the lunar lander frequently increased its altitude at the initial time to earn the enough time to reduce the horizontal velocity. Due to this increase, the lunar lander requires more fuel for lunar landing mission. This phenomenon is depending on its thrust-mass ratio. For the enough thrust, this increase in the altitude is reduced, but this huge thrust-mass ratio is sometimes not physically possible due to the cost, mass budget, thruster technology and so on. Therefore, we suggest the hybrid engine for the lunar landing mission and we also introduce the optimal lunar landing strategy for this engine. For this approach, we supposed that the lunar lander retro fired the impulsive thruster to rapidly reduce the horizontal velocity at the initiated time of the powered descent phase. Then the lander reduced the total velocity and altitude for the lunar landing by using continuous thruster. In contradistinction to other formal optimal lunar landing problem, the initial horizontal velocity and mass are not fixed at the start time due to the retrofiring maneuver in this problem. Thus, the initial free optimal control theory is applied in this paper and we can find out the optimal initial value and optimal lunar landing trajectory to minimize the fuel consumption. Source


Kim S.,Pohang University of Science and Technology | Choi C.,KARI | Kim J.,KARI | Park J.,KIMM | Baek J.,Pohang University of Science and Technology
Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy | Year: 2013

The objectives of the present study were to investigate the effects of tip clearance on the performance and flow characteristics of a turbopump inducer by using computational fluid dynamics. Three different tip-clearance sizes were analyzed for design and off-design conditions. The numerical results were validated by comparison with experimental results. Our detailed investigation of the simulated flow fields shows that backflow penetrates farther upstream for large-tip clearances, which means that performance declines rapidly. On the other hand, for small-tip clearances, the extent of backflow is reduced but the development of hub separation in the middle of the passage causes performance degradation. Our results confirm that a particular value of the tip-clearance limits the development of secondary flow in the passage. © IMechE 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav. Source


Cho D.-H.,KARI | Kim H.-D.,KARI
International Conference on Control, Automation and Systems | Year: 2014

To design the optimal lunar landing trajectory, the periapsis of de-orbit burn phase is usually used as a starting point of the powered descent phase. And the optimal problem is constructed based on the initial states at this point. For this optimal problem, some kinds of result trajectories have the increase in their own altitude at the early stage to earn the enough time to reduce their huge horizontal velocity for the low thrust, and this phenomenon increase the consumption of the propellant mass. For this reason, it is possible to exist another phase angle to reduce this phenomenon instead of zero for periapsis of de-orbit burn phase because the vertical velocity is not zero at this point. To find out this optimal phase angle, in this paper, the initial free state optimal problem method is applied without any additional assumption and change of hardware for the traditional optimal lunar landing problem. Using this approach, it shows that the optimal phase angle does not always equal to the periapsis for some cases, and it is possible to reduce the fuel consumption of the lunar lander. © 2014 Institute of Control, Robotics and Systems (ICROS). Source


Heo Y.,KAIST | Kim K.-J.,KAIST | Oh S.-H.,KARI | Kim D.-K.,KARI | And 2 more authors.
SAE Technical Papers | Year: 2011

Reaction wheels are used to control the attitude of a satellite in space in an almost static manner. Excitation forces at high frequencies as well, however, due to unbalance or bearing faults, can be transmitted to the satellite structure and work badly against missions of the satellite. Hence, counteractions such as vibration isolators are often employed in practice. In this paper, procedures are presented to design and test rubber vibration isolators based on characteristics of the transmission forces without isolators obtained from a previous study. First, a system consisting of reaction wheel, bearing, rigid cover and isolators was modeled with 11 degrees of freedom. Second, stiffness and damping of the isolators were designed such that the forces transmitted onto the satellite structure might satisfy given criteria. Finally, an actual isolation system fabricated using a rubber was tested to check the transmission forces. Copyright © 2011 SAE International. Source

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