• The Bulletin of The Korean Astronomical Society
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• v.37 no.2
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• pp.162.2-162.2
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• 2012

한국형 발사체 KSLV-2의 발사능력을 고려하면 달 탐사선의 총 무게는 약 550kg이 된다. 따라서 달 탐사선에 탑재할 수 있는 탑재체 무게를 최대화하기 위해서는 지구에서 달로 가는 천이궤적을 가장 효율적으로 설계하여 연료 소모량을 최소화하여야 한다. 본 연구에서는 최근에 달 탐사 천이궤적으로 가장 많이 사용되고 있는 Weak Stability Boundary 천이궤적에 관해 연구를 수행하였다. Weak Stability Boundary 천이궤적은 지구로부터 출발한 후 원지점 약 1.4 km(지구-태양의 L1 점 근처)까지 비행한 후 태양풍을 통해 에너지를 얻어 근지점 거리를 지구-달 거리만큼 증가시켜 LOI(Lunar Orbit Injection)시 ${\Delta}V$를 최소화하여 달 궤도에 들어가는 방법이다. Weak Stability Boundary 천이방법의 TLI(Trans Lunar Injection) 값은 직접천이 방법의 TLI 값보다 더 크지만 달 궤도 진입에 필요한 LOI ${\Delta}V$값은 25% 정도 덜 든다는 장점이 있다. 이 방법은 일본의 Hiten 탐사선이 최초로 사용하였으며, 달에 도착하기까지 수개월이 걸리는 단점이 있다. Weak Stability Boundary 천이궤적 시뮬레이션을 통해 최대로 절약할 수 있는 연료 소모량을 확인할 수 있었으며, 다른 천이방법들과의 장단점 비교를 통해 한국형 달 탐사선의 지구-달 천이궤적 후보로 사용 될 수 있음을 확인하였다.

• Bulletin of the Korean Space Science Society
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• 2009.10a
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• pp.40.2-40.2
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• 2009

이 연구에서는 달천이(TLI: Trans Lunar Injection) 및 달포획(LOI: Lunar Orbit Injection) 기동 시 대전 지상국의 가시성을 고려한 최적의 임무를 설계하였다. TLI 기동은 탐사선이 지구 주차궤도에서 지구-달 천이궤적으로 진입하기 위하여 주어지는 기동이며, LOI 기동은 탐사선이 지구-달 천이궤적에서 달의 중력권으로 진입하기 위하여 주어지는 기동이다. TLI 및 LOI 기동 시 대전 지상국에서의 가시성의 확보는 실제적인 미래 한국의 달 탐사를 대비하였을 때 중요한 요소이다. 따라서 이 연구에서는 TLI 및 LOI 기동 시 대전 지상국에서의 가시성을 모두 고려하여, 최소연료로 지구 주차궤도에서 달 임무궤도 진입까지의 모든 단계에 대해 임무설계를 실시하였다. TLI 및 LOI 기동 시 추력은 순간 추력(Impulsive thrust)로 가정하였으며, KSLV-II 발사체의 성능을 적용하여 설계하였다. 임무 설계 시 태양, 지구, 달의 섭동력을 고려한 N체 운동 방정식을 탐사선에 적용하였으며, 지구의 비대칭 중력장, 태양 복사압, 달의 J2 섭동에 의한 영향도 고려하였다. JPL의 정밀 천체력인 DE405를 사용하였고, 상용 소프트웨어인 SNOPT(Spares Nonlinear OPTimizer)를 이용하여 비행 궤적의 최적해를 도출하였다. 임무 설계 결과를 통해, 대전 지상국의 가시성을 고려한 TLI 및 LOI 기동의 크기에 의한 임무설계의 분석을 수행하였다. 또한 최적화된 달 탐사 임무의 단계별 기동의 크기와 지구-달 천이 궤적의 형상 및 다양한 임무 요소들의 해석을 도출하였다.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.40 no.1
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• pp.69-77
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• 2012

After lunar explorations were restarted in 1990s, the world space advanced countries have been competing actively to preoccupy the Moon from the 2000s. Korea has been also conducting precedent study on lunar exploration to carry out that by ourselves in 2020. This study analyzed delta-V of various Earth-Moon transfer trajectories for minimization of fuel consumption. Through the simulation, the best Earth-Moon transfer trajectory for Korean lunar mission is suggested and it will be used as useful materials of Korean lunar mission.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.7
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• pp.684-692
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• 2010

Based on the planar restricted three body problem formulation, optimized trajectories for the Earth-Moon transfer are obtained. Mixed impulsive and continuous thrust are assumed to be used, respectively, during the Earth departure and Earth-Moon transfer/Moon capture phases. The continuous, dynamic trajectory optimization problem is reformulated in the form of discrete optimization problem by using the method of direct transcription and collocation, and then is solved using the nonlinear programming software. Representative results show that the shape of optimized trajectory near the Earth departure and the Moon capture phases is dependent upon the relative weight between the impulsive and the continuous thrust.

• Journal of Astronomy and Space Sciences
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• v.26 no.2
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• pp.171-186
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• 2009

Various Earth-Moon transfer trajectories are designed and analyzed to prepare the future Korea's Lunar missions. Minimum fuel trajectory solutions are obtained for the departure year of 2017, 2020, 2022, and every required mission phases are analyzed from Earth departure to the final lunar mission orbit. N-body equations of motion are formulated which include the gravitational effect of the Sun, Earth and Moon. In addition, accelerations due to geopotential harmonics, Lunar J2 and solar radiation pressures are considered. Impulsive high thrust is assumed as the main thrusting method of spacecraft with launcher capability of KSLV-2 which is planned to be developed. For the method of injecting a spacecraft into a trans Lunar trajectory, both direct shooting from circular parking orbit and shooting from the multiple elliptical intermediate orbits are adapted, and their design results are compared and analyzed. In addition, spacecraft's visibility from Deajeon ground station are constrained to see how they affect the magnitude of TLI(Trans Lunar Injection) maneuver. The results presented in this paper includes launch opportunities, required optimal maneuver characteristics for each mission phase as well as the trajectory characteristics and numerous related parameters. It is confirmed that the final mass of Korean lunar explorer strongly depends onto the initial parking orbit's altitude and launcher's capability, rather than mission start time.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.38 no.10
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• pp.998-1011
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• 2010

For preparing Korean lunar missions, an Earth-Moon transfer trajectory is designed and analyzed using finite thrust. To be a more realistic scenario, kick motor's performance which is used for TLI (Trans Lunar Injection) maneuver is assumed to have a certain maximum capability. Under this assumption, optimal Earth-Moon transfer trajectory analysis is made from the beginning of Earth departure to the final lunar closest approach. As a results, optimal Earth-Moon transfer trajectory solutions with finite thrust are compared to those of designed with impulsive thrust in previous study. It is confirmed that if the trajectory solutions derived with impulsive burn is directly applied to estimate the finite burn trajectory solutions, careful consideration for finite burn losses must be paid as for TLI maneuver. Presented algorithm and various results will give numerous insights into the future Korea's Lunar missions using finite thrust engines.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.39 no.11
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• pp.1021-1032
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• 2011

This paper presents the results of a parametric study for the design of optimal Earth-Moon transfer trajectory using mixed impulsive and continuous thrust. Various types of the optimal Earth-Moon transfer trajectories were designed by adjusting the relative weight between the impulsive and the continuous thrust, and flight time. Two very different transfer trajectories can be obtained by different combination of design parameters. Furthermore, it was found that all thus designed trajectories permit the ballistic capture by the Moon gravity. Finally, the required thrust profiles are presented and analyzed in detail.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.37 no.9
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• pp.843-854
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• 2009

For preparing Korean lunar missions, optimal Earth-Moon transfer trajectory is designed using continuous low thrust. Using both constant and variable low thrusting method, "End-to-End" mission analysis is made from beginning of the Earth departure to the final lunar arrival. Spacecraft's equations of motion is expressed using N-body dynamics including the gravitational effects due to the Earth, Moon, Sun and also with Earth's $J_2$ effects. Planets' exact locations are computed accurately with JPL's DE405 ephemeris. As a results, optimal thrust steering angle's characteristics are discovered which showed almost tangential direction burns at the near of central planets. Also, it is confirmed that variable low thrusting method is more efficient than constant thrusting method, and can save about 5% of fuel consumption. Presented algorithm and various results will give numerous insights into the future Korea's Lunar missions using low thrust engines. Also, it is expected to be used as a basis of more detailed mission analyzing tool.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.36 no.4
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• pp.357-367
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• 2008

Preparing for future Korean Lunar missions, preliminary Lunar mission design software is developed using a impulsive thrusting method. Developed software is capable of design and analysis every required mission phases to design Lunar mission, including the Earth departure, Lunar transfer, Lunar arrival and mission operation phases. Also, assuming that KSLV-II is selected as a launch vehicle, future Korean Lunar explorer's mass budget is estimated based on driven optimal trajectory characteristics. Tracking analysis is also performed using Deep Space Network including angle geometry analysis between Earth - Moon - Lunar explorer - Sun which are very important for communication, solar panel pointing strategy and eclipse analysis when Lunar missions are under designing phase.

• Journal of the Korean Society for Aeronautical & Space Sciences
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• v.35 no.7
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• pp.647-654
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• 2007

A Discrete continuation Method/homotopy approaches are studied for energy/fuel optimal low thrust Earth escape trajectory by solving a two point boundary value problem(TPBVP). Recently, maneuvers using low thrust propulsion system have been identified as emerging technologies. The low thruster is considered as the main actuator for orbit maneuvers. The cost function consists of a energy/fuel consumption function, and constraints are position and velocity vectors at the terminal escape point. Solving the minimum energy/fuel problem directly is not an easy task, so we adopt the homotopy analysis. Using a solution of the minimum energy, which is solved by discrete continuation method, we obtain the solution of the minimum fuel problem.