The Moon is recognized as an important destination for space science and exploration. To find a satisfactory answer for mystery of the universe and to make use of the lunar resources for welfare of human beings, several space agencies are planning manned and unmanned missions on the Moon. As a result, the concept of lunar vehicles has begun with advanced descent scheme to perform the function of safe and precise landing on the Moon surface. The task of lunar descent scheme is to take a horizontally oriented spacecraft from orbital speeds at a point of hundreds of kilometers from the desired landing point to the landing point at an almost vertical orientation and very low speed. Existing schemes for lunar descent and landing are date back to the Apollo epoch and cumbersome to achieve a precise landing. Precise landing technology is one of the most important technologies for future lunar exploration missions. It will help to investigate the specific and scientifically interesting places such as central peaks of a big crater. To achieve a precise landing, an improved descent solution and real-time applicable trajectory generation scheme is necessary that should fulfill the goal of these future missions being highly robust, cost effective and safely landing as possible. Reducing the complexity for developing the descent trajectories will certainly reduce pre-flight analysis cost and will increase the robustness for a precise lunar landing mission. Any scheme developed to replace the conventional schemes can have as its core the same element structure: a descent solution element and a real-time applicable reference trajectory generating element. Solution methods of lunar descent available in the previous researches are complex, iterative, numerical manner. Reference trajectory generation schemes followed the same style too. Moreover, some researches implements pre-flight reference trajectory generation techniques as well, which made the schemes having very poor robustness. Although a few literatures investigated real-time reference trajectory generation scheme but that is only for the portion of terminal descent phase. Again, a common limitation is observed for all the previous researches is that, after earth-moon transfer, several complicated steps are included to reach till the terminal descent initiation point, these are: Hohmann transfer, de-orbit maneuver etc. None of them proposed a straightforward descent directly from parking orbit conditions in an advanced manner.

This thesis proposes a scheme of a qualitative descent solution to the equations for spacecraft speed, horizontal span, vertical range and cross range as a function of velocity vector pitch angle and develops an analytical algorithm for dual step reference trajectory generation. The new proposed scheme satisfies the vertical terminal landing condition to confirm a safe lunar landing mission. Entire descent solutions and comparisons are represented here for both two dimensional and three dimensional illuminations. Mathematical derivations of new descent solution scheme are verified in terms of conventional scheme and comparative simulation results for a fully integrated solution, conventional schemes and a proposed advanced scheme are demonstrated to test the performance. Some suitable assumptions are made during this advanced lunar descent solution to avoid complexity. As in the conventional solution methods, there exist some poor assumptions such as during descent, constant vertical gravitational acceleration is the only other force acting on the descent vehicle. This inadequate postulation limits the validity of the system solutions within a very low altitude terminal descent area; that is, close to the lunar surface. In this thesis, an advanced descent solution is proposed where the centrifugal acceleration term is retained along with the gravitational acceleration term. It allows a complete representation of the descent module motion from orbital speed conditions down to the final landing state.

A trajectory optimization study is also performed for a lunar soft landing on Moon. Legendre Pseudospectral method is used in this investigation because it is easy to use and is capable of solving a wide variety of problems. Dual step reference trajectory generation scheme is developed from equations obtained through that advanced lunar descent solution scheme to provide an accurate descent path starting from a circular or elliptical parking orbit conditions. Robustness of the proposed reference trajectory generating algorithm is studied as well. Then, the doctor thesis proposes an advanced descent scheme and trajectory generation scheme for pin-point lunar landing. Analytical studies and simulations showed the effectiveness and usefulness of the proposed schemes for future lunar mission.

本論文は「Study on Advanced Descent and Trajectory Generation Scheme for Precise Lunar Landing Mission(高精度月面着陸ミッションのための先端的降下及び軌道生成手法に関する研究)」と題し、将来の月着陸探査ミッションにおいて、探査機の高精度な着陸を実現するために、動力降下フェーズに焦点を当て、降下および軌道生成手法について研究したものである。特に、実利用を念頭に、実時間処理およびロバスト性を重視し、動力降下中に軌道修正可能な手法について、理論および数値実験の両面から研究したもので、7章からなる。

第1章は序論として、月着陸探査の科学的意義、高精度着陸の必要性、先行研究およびその問題点を説明し、本研究の目的と基本的考え方をまとめている。

第2章では、高精度着陸に必要な技術課題を明確にし、その中で動力降下中に注目した背景および意義を述べている。

第3章では、月着陸探査機のモデルを定義し、そのダイナミクスを導出し、理論的考察のための定式化を行っている。

第4章では、従来の着陸降下手法を紹介し、今まで提案された手法の前提条件や数値解析手法の問題点を指摘し、実用化という観点から周回軌道からの着陸降下軌道を求める新しい手法を提案している。本手法は実時間処理可能でかつ精度よく近似解を得る手法となっている。2次元および3次元の数値シミュレーション検討を行い、その有効性を示している。また処理時間を評価し、宇宙機搭載の演算処理装置でリアルタイム処理が可能であることも示している。

第5章では、燃料消費を最少にする最適な軌道を生成する手法について、従来の手法を活用し、効率良い演算手法を組み合わせ、その性能を評価している。繰り返し演算を伴う最適化問題では、現在の演算装置では実時間処理が難しいという知見が得ている。

第6章では、実時間処理可能でセンサノイズ等にもロバストな軌道生成手法を提案している。これは軌道を数ステップに分割し生成する手法であり、その有効性を数値シミュレーションで示している。

そして、第7章では結論としての総括と今後の課題を具体的に記述している。

以上要するに、本論文は、月面高精度着陸の実現をめざして、動力降下フェーズに着目し、周回降下軌道の近似解を実時間で求める手法、および降下中に修正軌道をリアルタイムに自動生成する手法を新規に提案し、数値シミュレーションによりその有効性を示し、ピンポイント着陸の実用化への道筋を示したもので、電気工学、制御工学、宇宙工学への貢献が少なくない。

よって本論文は博士(工学)の学位請求論文として合格と認められる。