• Bulletin of the Korean Space Science Society
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• 2009.10a
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• pp.40.1-40.1
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• 2009

미래 한국의 달궤도선 임무에 대비하여 달 근접 궤도 전파기인(orbit propagator) YSPLOP ver. 1(Yonsei Lunar Precise Orbit Propagator version 1)을 개발 하였다. 개발된 궤도 전파기의 성능은 상용 소프트웨어인 STK Astrogator를 이용하여 검증되었다. 개발된 궤도 전파기를 이용, 달 궤도선의 운용에 있어서 다양한 섭동력들이 궤도선의 수명(orbital decay)에 미치는 영향을 분석하였다. YSPLOP ver. 1은 정밀한 달 중심 탐사선의 위치산출을 위하여 M-EME2000 (Moon-Centered, Earth Mean Equator and Equinox of J2000) 좌표계, M-MME2000 (Moon-Centered, Moon Mean Equator and IAU vector of epoch J2000) 좌표계 그리고 M-MEPMD (Moon-Centered, Moon Mean Equator and Prime Meridian) 좌표계를 이용하여 탐사선의 상태(state) 정보를 산출한다. 또한 태양, 지구, 달, 화성, 목성의 중력에 의한 섭동력 및 태양풍에 의한 영향을 포함할 수 있도록 설계되었으며, 달 근접 궤도선의 궤도 운동에 가장 큰 영향을 미칠 수 있는 섭동력인 달의 비대칭 중력장에 의한 영향 또한 고려하도록 하였다. 달의 비대칭 중력장 모델 (Lunipotential model)은 LP165p 모델이 사용되었으며 행성의 정밀한 위치 산출을 위하여 JPL의 DE405 천체력이 사용되었다. 개발된 궤도 전파기를 이용, 달고도 100 km, 궤도 경사각 $90^{\circ}$인 달 중심의 극궤도를 약 30일 동안 전파한 결과, YSPLOP ver. 1의 성능은 STK Astrogator와 비교하여 보았을 때 약 수 m의 오차를 보이는 것으로 확인되었다. 달의 극궤도 탐사선의 궤도 수명을 분석한 결과, 최소한 달의 비대칭 중력장이 70 by 70 이상으로 고려되어야 함을 확인하였으며 이때 달 궤도선의 수명은 약 160일으로 나타났다. 아울러 달 근접 환경에서의 지구 중력에 의한 섭동력은 달 궤도선의 운동에 있어서 무시 할 수 없는 정도의 많은 영향을 끼치고 있음을 확인하였다. 이 연구를 통하여 개발된 궤도 전파기는 미래 한국의 달 궤도선 및 착륙선의 임무 설계시 사용 될 수 있다. 또한 이 연구에서 제시된 달 근접 환경에서의 다양한 섭동력들이 달 궤도선의 운동에 미치는 영향에 대한 해석 결과는 추후 달 근접 임무 설계시 고려되어야 하는 섭동력들의 기본 사양을 제공할 것이다.

• Journal of the Korean Society for Railway
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• v.18 no.3
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• pp.241-251
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• 2015

Compared with ballasted track (BT), ballasted asphalt track (BAT) has been increasingly adopted in many countries due to its more greatly reduced reinforced roadbed thickness and smaller cumulative plastic deformation, and its advantages in terms of maintenance. In this respect, the authors' previous research includes analysis of BAT sections that show performance similar to that of BT sections of the present specifications; reliability verification of the analysis results through real-sized static and dynamic train-load tests were performed. Based on previous research, this paper estimates the track lifetime using the strain of the lower roadbed according to reinforced roadbed thickness; using probabilistic LCC analysis, this paper presents a BAT section that satisfies the design lifetime and that has performance similar to or higher than that of BT.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.32 no.6D
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• pp.663-671
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• 2012

The demand of CWR is rapidly increasing because of the adaptation of concrete track, the need for rapid and comfortable ride, and the reduction of maintenance cost. Because of short applying period of the concrete track, there is not a case of CWR fracture in Korea caused by repeated load of the train, which makes it difficult to calculate replacement period of rail based on rail fatigue life using an actual field data. This study thus inspected the bending stress at rail bottom through analyzing the vehicle/track interaction, performed multiple regression analysis on the data, deducted the bending stress prediction equations by the speed and the surface irregularity. Finally, the fatigue life of CWR on the concrete track was predicted based on the prediction equations for bending stress at rail bottom.

• Aerospace Engineering and Technology
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• v.13 no.1
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• pp.184-192
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• 2014

In this paper, the lifetime of the artificial space objects in the LEO is analysed by using TLE data, which is provided by JSpOC. We observed the change of the number of space objects from 1957 and determined the reason of space debris generation. And then, we performed the analysis about present condition of space debris environment. The lifetime analysis includes a total of 11,792 artificial space objects and performed until the year 2050 by orbit propagation. We analyze the annual reentry frequency for the high RCS objects such as nonoperational satellites and rocket bodies, which have the possibility of earth ground impact through STK/Lifetime Tool for accurate and effective calculation. The results show that 9 payloads or rocket bodies will be decayed annually and 2 or 3 objects of total value have the possibility of ground impact. In addition, it is also shown that the 40% of a total analysed objects have the lifetime over 200 years.

• Journal of the Korea institute for structural maintenance and inspection
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• v.20 no.2
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• pp.110-121
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• 2016

In the decision-making, such as selection of structure, construction method, or time and scheme of maintenance, the evaluation of life-cycle cost(LCC) is of great importance. The maintenance cost occupy a large portion of the LCC of the railway track as well as the initial construction cost. Futhermore, the proportion of the maintenance cost is much higher in the ballasted track. Thus, the importance of the LCC evaluation is higher than in any other engineering structures. In this study, a LCC model that can consider various design parameters such as the type of track structure, annual traffic volume, axle load, train speed, and proportion of curve sections and engineering structures has been developed. Fundamental data for calculating costs also have been presented. Based on the model and data proposed, the trends in the variation of LCC according to the design parameters were examined and the most important design parameters in the LCC analysis of railway track were investigated. The results show that the proportion of renewal and operational costs is much higher in the ballasted track than in the concrete track, and the annual traffic volume and ballast taming period are most significant factors on the LCC of the ballasted track. On the contrary, it is revealed that the proportion of the initial construction costs in the concrete track is much higher, and the LCC of the concrete track is less sensitive to the traffic volume, train speed, and axle load.

• Aerospace Engineering and Technology
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• v.5 no.2
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• pp.205-212
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• 2006

Atmospheric drag is the most significant factor effecting the low Earth satellites under the altitude of 800 km Although the atmospheric density of the low Earth orbit is very low compared to that of the sea level, the accumulated effect of the atmospheric drag slowly lowers the satellite velocity at the perigee. Decrease in velocity at perigee directly causes decrease in altitude at apogee which changes the eccentricity of the orbit. The orbit finally reaches a circular orbit before reentering the Earth. This paper states the methods of calculating the atmospheric drag and the lifetime of the satellite. The lifetime of the kick motor and the satellites which will be used on KSLV-L are calculated by Satellite Tool Kit.

• Journal of the Korea institute for structural maintenance and inspection
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• v.14 no.4
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• pp.86-93
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• 2010

The wear of railway track affects loss of rough ride, noise or vibration of train and traveling safety. Moreover as the track is worn away, this promotes destruction of structural mechanism of rail track which can bring about increasing of rail track maintenance cost drastically. For this reason, it is very important and interested research subject to design railway track structure and to analyse train movement mechanism based on systematic analysis of the reasons causing rail wear possible in real field. In this research, for the efficient maintenance, Life Cycle Performance of rail track and maintenance characteristics are computed considering some track components such as track type, contracting type, sleeper type and roadbed type. Time - Wear probabilistic distribution relationship as well as multiple regression analysis based on time, curvature and wear data are computed to predict the service life remainder of railway track and to be adapted to safety assessment.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.35 no.5
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• pp.1191-1198
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• 2015

In this study, vehicle/track interaction analysis by the Saemaul powered vehicle was carried out. The prediction equation for the bending stress of rail was estimated using the rail bending stress by the rail surface irregularities at welds. Also, the fatigue analysis using a S-N curve of welds in the conventional railway was carried out. We estimated the fatigue life of CWR by the fracture probability. By the rail grinding, the fatigue life of CWR was evaluated in consideration to reduce the rail bending stress through removing the rail surface irregularities. Therefore, it presented the fatigue life of CWR according to the rail grinding execution plan in the conventional railway.

• Journal of the Korean Society for Railway
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• v.17 no.3
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• pp.193-200
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• 2014

In this study, fatigue tests on existing continuous welded rail (CWR) on a concrete track were carried out. Based on the test results, a S-N curve expressing the remaining life of the CWR at a fracture probability of 50% was obtained using weighted probit analysis suitable for small-sample fatigue data sets. As rails had different histories in terms of accumulated passing tonnage, the test data were corrected to average out the accumulated passing tonnage. The remaining service life for the CWR on the concrete track in an urban railway was estimated using the prediction equation for the bending stress of rail developed in the past to estimate rail base bending stress and taking the surface irregularities into consideration. Estimating the remaining service life of the CWR in an urban railway showed that the rail replacement period could be extended over 200MGT. In addition, comparing the concrete track to the ballast track, the fatigue life of rail was analyzed as approximately 300MGT higher than. Therefore, the rail replacement criteria needs to distinguish between the ballast track and the concrete track, and not the criteria needs to be changed as a target for the maintenance, although it is necessary to remove longitudinal rail surface irregularities at welds by grinding.

• Journal of the Korean Society for Railway
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• v.20 no.5
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• pp.649-657
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• 2017

In this study, fatigue testing was carried out for long-term use of rail according to track type. From S-N curves for 50%~0.01% failure probability, the fatigue life of the long-term use rail for each track type was derived using the weight probability analysis technique on the experimental data. Because the rails used in the fatigue test have different cumulative tonnages, the number of repetitions was modified by averaging the cumulative tonnage. In addition, the bending stresses of rail bottoms, considering rail surface irregularities, track support stiffnesses and train speeds, were evaluated using the predicted rail bending stresses derived from existing studies. As a result, for rail fatigue life evaluation, the fatigue life of rail on the ballast track was found to be more than 200 million tons higher than the standard value for rail replacement. Also, the fatigue life of rail on concrete track is more than 300 million tons higher than that on ballast track. The Haibach rule is adaptable for the fatigue life evaluation of rail for stress range under fatigue limit.