BEOSAT: A Student Initiative
- Paper number
IAC-05-E2.3.09
- Author
Mr. Jörn Pfingstgräff, Experimental Raumfahrt-Interessen Gemeinschaft e.V., Germany
- Year
2005
- 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
BEOSAT is a micro-satellite for Earth-Observation and Space Debris research. The satellite is fully managed and designed by students of the ERIG at the Technical University of Brunswick, Germany. The ERIG is building sounding rockets, innovative hybrid rocket motors and BEOSAT which stands for Brunswick’s EarthObservation Satellite. Its two experiments micro-SCIA and AIDA have extensive impacts on the satellite bus. micro-SCIA is a micro-spectrometer in order to measure nitrogen dioxide of the troposphere. The satellite has to be nadir-oriented during measuring campaign. It has to be ensured to measure at last 10 minutes an orbit. AIDA which stands for Advanced Impact Detector Assembly is an impact detector for space debris and micro-meteoroids. The sensor is working all over orbit. The impact detector is mounted in flight direction and is almost as big as the whole front panel. BEOSAT is cubic shaped with an edge-to-edge-length of 40 cm and has a weight of 45 kg. Its dedicated orbit is sun-synchronous with an altitude of about 650 km. A design lifetime of 2 years is required. The bandwidth of required Power comprises 27 – 42 W regarding which operational mode is used. It was my work to figure out concepts for the solar generator, the battery and the power control and distribution unit (PCDU). The battery consists of four LVP9 Li-Ion-batteries. This results a total capacity of 9 Ah at a battery voltage of 14,4 V. Including cell balancing the battery unit will weight less than 2 kg. To reach a lifetime of 11000 cycles a maximum depth-of-discharge of 20 The solar generator will be build up by one panel which is body-mounted fixed on the zenith site of the satellite. The panel’s area is about 0,32sqm and comprises 96 triple-junction solar cells with an average efficiency of 26,6 The PCDU consists of two parts in one single box. The Power Control Unit PCU is build up with a micro-controller and a Maximum Power Point Tracker. A shunt will dissipate exceeding power. Yet it is not decided whether the PCU also has to control heaters actively or not. Beside this a battery charge and discharge unit (BCDU) is needed. The PCU communicates with the on-board-computer and the cell-balancing. The Power Distribution Unit (PDU) is build up centralised. Eight voltages has to be distributed. For each voltage an own DC/DC converter is required. In case of active heater control 50 power output connectors has to be placed on the PDU. The whole PCDU consists of eight modules on simple plug-in cards in order to give maximum flexibility with minimum weight. The total weight of the PCDU may not exceed 1 kg. The last part of the study was to create the procedures within the micro-controller. Procedures for all yet known input and output data were created like battery’s state-of-charge or solar cell temperatures.
- Abstract document
- Manuscript document
IAC-05-E2.3.09.pdf (🔒 authorized access only).
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