Optimal Strategies for Precise Lunar Landing
- Paper number
IAC-08.A3.2.INT7
- Author
Mr. Jesus Gil-Fernandez, GMV S.A., Spain
- Coauthor
Mr. Pablo Colmenarejo, GMV S.A., Spain
- Coauthor
Mr. Raul Cadenas, GMV S.A., Spain
- Coauthor
Ms. Mariella Graziano, GMV S.A., Spain
- Year
2008
- Abstract
Future landers on the Moon will require precise landing capability in order to explore scientifically interesting regions. The descent and landing (DL) strategy must be designed to optimise the landed mass while permitting hazard avoidance manoeuvres and providing the required landing accuracy.
The DL starts with a de-orbit manoeuvre that places the pericenter at a given altitude. Then, the main-braking at maximum thrust cancels most of the relative velocity and finishes at the so-called high-gate. In the next sub-phase, the thrust is reduced (throttle-back) and there is visibility of the target. When the low-gate is reached, the terminal descent starts and there is no re-targeting capability, in addition there might be an additional throttle-back.
All the sub-phases are considered in the optimisation process. The optimisation algorithm is simple and robust enough to be carried out on-board, in order to be able to react to deviations from the nominal trajectory. The optimisation algorithm is based on a hybrid direct/indirect method that uses the optimal control theory to parameterise the thrust arcs. The non-linear programming (NLP) solver is a commercial package based on sequential-quadratic programming (SQP). To assure the convergence of the optimisation (would not be needed on-board) a continuation method is used providing the initial guess to the core optimiser by solving problems of increased complexity. The computation time is few seconds in the worst case, using a desktop and Matlab language.
Different DL strategies are analysed considering the constraints on the trajectory (e.g. visibility of the landing site, throttling capability, hazard avoidance) in order to trade them and find the best one for a specific mission. The DL strategies for precise lunar landing are classified in the following groups:
- Full-thrust minimum propellant mass: gives the maximum landed mass for comparison purposes
- Minimum propellant with throttle-back: gives the maximum propellant with one throttle-back and serves for sensitivity analysis of the throttle-back
- Quasi-vertical descent: maintaining a constant high elevation wrt the landing site, after the main-braking, to facilitate hazard mapping. Different flight-path angles and several throttle-backs are considered in order to analyse the impact on the landed mass and on the hazard avoidance.
- Shallow descent: minimum elevation wrt landing site permitting its continuous visualization. Several throttle-back are analysed.
Extensive optimisation campaign is performed to analyse the impact of the relevant design parameters in the different strategies. The results for a typical vehicle will be presented to justify design criteria based on the system architecture. The considered DL design parameters are:
- Periselenium altitude after de-orbit
- Location within the elliptic orbit at the start of the main-braking manoeuvre
- Full thrust
- Altitude at the end of the main-braking (high-gate altitude)
- Elevation angles for the visual approach and terminal descent
- Thrust in the different throttle-backs
- Abstract document
- Manuscript document
IAC-08.A3.2.INT7.pdf (🔒 authorized access only).
To get the manuscript, please contact IAF Secretariat.