sign in sign up
<<
Home , Arianespace, BepiColombo, Exploration of Mercury, Mariner 10, RUAG Space

49th International Conference on Environmental Systems ICES-2019-97 7-11 July 2019, Boston, Massachusetts

The Thermal Commissioning of BepiColombo MPO

Andrea Ferrero 1, Domenico Battaglia 2 and Tiziano Malosti 3 , I-10146 Torino,

Daniele Stramaccioni 4 (ESA-ESTEC), NL-2200AG Noordwijk, the

and

Jürgen Schilke 5 EADS , D-88039 Friedrichshafen,

BepiColombo was launched aboard an 5 rocket on 20 October 2018 at 01:45 UTC, with arrival on planned for December 2025, after a flyby of , two flybys of , and six flybys of Mercury. It is the first European mission to Mercury and will place in around the two separate : Mercury Magnetosphaeric Orbiter (MMO), provided by the Japanese Space Agency (JAXA), and Mercury Planetary Orbiter (MPO), provided by the European Space Agency (ESA). The cruise from Earth to Mercury is being ensured by a dedicated module, Mercury Transport Module (MTM). MPO, the subject of this paper, is the European scientific contribution to the BepiColombo mission. Upon cruise completion, when orbiting Mercury, it will be 3-axis stabilized, planet oriented, with a planned lifetime of 1 year, and a possible 1- year extension. The mission will perform a comprehensive study on Mercury, by means of several instruments, including a laser altimeter, different types of spectrometers, a and radio science experiments. This paper describes the activity performed on MPO during pre-launch, launch and commissioning, besides it evaluates the first in orbit thermal results and where possible compares them with the behaviour expected from the analyses and from the results of the environmental thermal test program, in order to provide a first assessment of the spacecraft thermal design and performance.

Nomenclature AOCS = Attitude and Orbital Control System RCS = Reaction Control System AU = Astronomical Unit (= 149.60×10 6 km) RSA = Ruag Space GmbH CPS = Chemical Propulsion System S/C = SpaceCraft ESA = European Space Agency STM = Structural Thermal Model GFRP = Glass Fiber Reinforced Polymer TAS-I = Thales Alenia Space Italia HGA = Hi gh Gain Antenna TB = Thermal Balance JAXA = Exploration Agency TCS = Thermal Control System LEOP = Launch and Early Orbit phase TLL = Temperature Lower Limit MCS = Mercury Composite Spacecraft (stack of TMM = Thermal Mathematical Model MOSIF, MMO, MPO and MTM) TUL = Temperature Upper Limit MGA = Medium Gain Antenna TV = Thermal Vacuum MMO = Mercury Orbiter UTC = Coordinated Universal Time MOSIF = Magnetospheric Orbiter Sunshade and Interface UV = MPO = Mercury Planetary Orbiter VDA = Vapor Deposited Aluminum

1 Thermal Verification Specialist, B.L. Space Infrastructures and Transportation, Strada Antica di Collegno 253 10146 Torino. 2 Passive Thermal Design Specialist, B.L. Optical Observation & Science, Strada Antica di Collegno 253 10146 Torino. 3 Senior Thermal Analyst, B.L. Space Infrastructures and Transportation, Strada Antica di Collegno 253 10146 Torino. 4 MPO Module Manager, BepiColombo Project, ESTEC, Keplerlaan 1, 2200AG Noordwijk, The Netherlands 5 Thermal Architect, AET42, D-88039 Friedrichshafen

Copyright © 2019 Thales Alenia Space MTM = Mercury Transfer Module VUV = Vacuum UltraViolet NECP = Near Earth Commissioning Phase αS = Solar Absorptance PFM = Proto-Flight Model εIR = Emissivity

I. Introduction EPICOLOMBO is the ESA’s cornerstone Z Bmission to Mercury in collaboration with JAXA MMO and will enable a better understanding of the planet history, geology, composition, and Y X MOSIF magnetosphere. The mission is named after Italian mathematician and engineer Giuseppe “Bepi” Colombo (1920–1984), who suggested that the MPO Mariner-10 probe could perform three fly-bys of planet Mercury, thus extending significantly the coverage of the planet. It is the first European MTM mission in orbit around Mercury, the second overall after NASA’s MESSENGER ( actually orbited the ), and the first to send two spacecraft to make complementary measurements of the planet and its dynamic environment at the same time. With reference to Figure 1, Bepi Colombo Figure 1. Mercury Composite Spacecraft composed of consists of a spinning satellite, Mercury MTM, MPO and MMO inside its sunshade MOSIF . Magnetospheric Orbiter (MMO), provided by JAXA, a shield designed to protect the MMO by the intense Sun radiation during the cruise to Mercury 6 (MOSIF) and a 3-axis stabilized orbiter (MPO), both provided by ESA. Finally, the propulsion module (MTM), also provided by ESA, completes the Mercury Composite Spacecraft (MCS) that is currently cruising from Earth to Mercury. The MTM houses the electric propulsion system that is needed to propel the MCS during its cruise. During the 7-year cruise, the MCS stack is 3-axis stabilized and Sun-oriented to ensure sufficient electric power from the MTM solar arrays for the electric propulsion and protection from the Sun. Once BepiColombo reaches its destination, the mission will last for one year, with the possibility of extending its measurements for another year. The Mission is led by Deutschland Friedrichshafen (ASD), with Thales Alenia Space Italia Turin (TAS-I) responsible for the MPO and MOSIF thermal design. Ruag Space Austria GmbH (RSA) developed and provided the MLI. The MPO carries eleven payloads (P/L) for Mercury Figure 2. BepiColombo liftoff. Credits: ESA/ planet observation and remote sensing. It will provide a CNES//Optique vidéo du CSG – JM Guillon complete mapping of the entire planet surface with unprecedented resolution and will perform also other experiments of radioscience and fundamental physics 7. The specific challenges of the thermal design of this mission and, in particular, of the Mercury orbiter (MPO) stem clearly from the demanding thermal environment in combination with the chosen orbit and with the nadir pointing attitude of the spacecraft will be explained in paragraph II.

6 The MMO cannot withstand the Sun irradiation when it is not spinning. Hence, the MOSIF is designed to keep it in shadow during the cruise to Mercury. 7 For a description of the scientific mission, see ref. 7. 2 International Conference on Environmental Systems

II. Mission Overview During its journey to Mercury, BepiColombo will need to make several braking manoeuvres to adjust its orbit and counteract the gravitational pull of the Sun in order to slow down enough to be captured by the of Mercury. The cruise will total around 8.5 billion kilometres – the equivalent of travelling from the Earth to and back again, or 38 times the maximum distance between the Earth and Mercury. The BepiColombo mission will go through the following phases, with considerable changes of composite spacecraft configuration and thermal environment: • Launch and Early Orbit Phase (MCS); • Near-Earth Commissioning Phase, lasting 3-4 months and including commissioning (MCS). • Interplanetary Cruise Phase (MCS), with min/max distances from Sun 0.298/1.16 AU, and including Earth, Venus, Mercury fly-by’s. The will correspondently range from 1,020 to 15,400 W/m 2. • Mercury approach Phase: o Mercury capture (jettisoning of MTM); o Mercury Orbit Insertion; o MMO Orbit Acquisition, potentially including a Mercury Aphelion passage in nominal MMO orbit (jettisoning of MMO in its final orbit followed by jettisoning of MOSIF); o MPO Orbit Acquisition (MPO alone). • Mercury Orbit Phase (MPO).

III. Launch and early LEOP Figure 3. BepiColombo mission profile. BepiColombo was launched aboard from the Credits: Loren Roberts for The Planetary Society , European in at 01:45:28 UTC on 20 October CC BY-SA 3.0 2018. Booster separation came just two minutes later, as the two solid rocket boosters of Ariane 5 detached themselves from the core stage; later, BepiColombo began its solo flight upon separation from the Ariane upper stage at around 2:12 UTC. At 2:20 UTC, the ESA deep space tracking station at New Norcia, Western Australia, picked up the first signals from the spacecraft, confirming that the launch was successful. Then BepiColombo started communicating bundles of engineering telemetry, as well as spectacular images from three low-resolution monitoring cameras located on the Mercury Transfer Module, that allowed flight controllers to visually verify the correct deployment of solar arrays and high and medium gain antennas. MPO is also equipped with a high- resolution scientific camera, which however is located on the side of the spacecraft fixed to the MTM during cruise, and therefore able to take pictures only after separation upon arrival at Mercury. The launch and early orbit phase was completed on October 22 with the deployment of solar arrays, confirming that the spacecraft and all systems were healthy and functioning. After separation and initialization of sensors, the sun was on the -X/-Y side and illuminated the radiator. As the sun was slightly above the XY plane, it was trapped between the fins. Figure 4. First image of BepiColombo from At 02:26 the attitude was recovered, with on on +Y (anti- space. Credits: ESA / BepiColombo / MTM, CC radiator side). As usual, the attitude control was switched off BY -SA 3.0 IGO 3 International Conference on Environmental Systems

during the deployment of the solar arrays. The S/C movements during the deployments can be clearly seen. The reaction wheels were switched on at 07:00, first warming up the shaft bearings. was started only once the shaft was sufficiently warm and the lubrication was good enough to reduce friction. At 13:30 the heater thresholds were increased.

Figure 5. S/C Attitude.

Figure 6. Reaction Wheels temperatures. Figure 7. Major Units temperatures.

Figure 8. Predicted heater power profile. Figure 9. Measured heater power profile.

4 International Conference on Environmental Systems

Figure 10. First image of the deployed MGA (top Figure 11. Image of the deployed HGA (top center) left) onboard MPO on 21 October 2018. Credits: onboard MPO on 21 October 2018. Credits: ESA / ESA / BepiColombo / MTM, CC BY -SA 3.0 IGO BepiColombo / MTM, CC BY -SA 3.0 IGO

IV. Near Earth Commissioning Phase The Near Earth Commissioning Phase (NECP) started right after the early LEOP Phase and lasted about three months, during which all initial commissioning activities were completed. It included: • deployment of the 2.5 m long magnetometer boom (successfully executed on October 25), that allowed to start measuring the on the way to Mercury; • commissioning of all the spacecraft systems, including deployment of the medium and high gain antennas; • payload activation and functional checkout, as far as possible in the MCS configuration; Final commissioning of the MPO payloads and the MMO will take place in Mercury orbit. Commissioning of the spacecraft systems was generally nominal, no major problems were highlighted. Commissioning of the Thermal Control System (TCS) is treated specifically in paragraph V.

V. TCS Commissioning The TCS commissioning, performed during the NECP phase, allowed to verify the main features of the thermal control system, particularly concerning the heater control software, as well as to perform some other activities in order to support the operations and commissioning of other systems. The main unexpected behaviour of the TCS subsystem was the reconfiguration of several heater loops from main to redundant side, which was linked to unadequate values of the software heating filters. This issue is treated in detail in paragraph VI. Concerning the sizing of the thermal design, in general, the temperature of MPO was slightly higher than predicted by the system thermal mathematical model (TMM). This discrepancy has been analyzed by analysis of the telemetry data and simulations performed with the TMM. Theoretically, several different physical parameters could cause the discrepancy, however it is essential to determine whether the root cause is a higher than expected heat flux coming from the environment or from the other components of the MCS stack, or a lower than expected efficiency of the heat transport and rejection path, because this will affect the thermal behavior once MPO will be in orbit around Mercury, when payloads will start to operate and temperature margins versus the maximum design temperature will be thin due to the extremely hot environment. The following parameters have been identified as possible candidates: A. degradation of MLI solar absorptance from the Begin of Life value B. degradation of radiator fins specularity and infrared emissivity C. higher than expected heat leak from MTM to MPO through the inter-module hardware

5 International Conference on Environmental Systems

D. degradation of MLI thermal insulation performance E. sunlight trapping at the interfaces between MPO and the other modules (+Z and – Z panels), where the gap closure compresses the MPO MLI and could create local cavities. Parameter A. has been deemed as very likely to play a role, as from available flight data from other missions a large portion of the solar degradation occurs during the first months of flight. Parameter B. may have contributed to some extent, but it is considered unlikely to be a major factor. Parameter C. has been ruled out, as Table 1 . Comparison between flight telemetries and predicted the MTM and MPO interface temperature. Case 1 is set with TMM, adjusted to actual flight conditions, α temperatures have been found to be Case 2 with partial S degradation. very close and no major interface flux can therefore occur. Parameter D. is also considered unlikely, as the MLI performance was extensively tested and correlated on ground. Parameter E. is considered a possibility, especially at the interface with MTM, since the two modules have not been thermally tested together, such interface was not present in the thermal balance test of MPO and the heat into MPO simulator was not of interest in the thermal balance test of MTM. At this interface the MLI could form a V- groove, leading to sunlight trapping. Additionally just at this interface the MPO MLI is compressed and locally degrades insulation performance. The measured flight temperatures of the payloads located at the MPO -Z side support this theory, but cannot confirm it. To verify the impact of these parameters, several analysis runs have been carried out using the MPO system TMM, updated to reflect the actual flight conditions of the NECP Phase in terms of distance from the Sun, S/C configuration, equipment thermal dissipation and heaters settings. Degradation of MLI is certain to have played a role, with an actual value between 0.32 and 0.40 (probably closer to the latter), but is not sufficient to explain the discrepancy. After the thermal balance test of MOSIF, the MLI materials of the gap closure were changed to all metal. Therefore there is no risk of material damage even at 0.3 AU. On the basis of these results, the two more likely causes for the slight discrepancy between flight data and model results are: • additional heat flux coming from the MLI because of degraded solar absorptivity (or local insulation deficiencies), surely present but not sufficient to explain the phenomenon • additional heat flux coming from the MPO/MTM gap because of sun trapping and MLI compression (to be confirmed) In order to conclusively discern how much these two parameters contribute to the temperature discrepancies, a second flight-model comparison is planned for the Venus fly-by stage, when the warmer environment (equivalent to 2 solar constants on Earth) will allow to extrapolate the results to hot Mercury orbit conditions with greater confidence.

6 International Conference on Environmental Systems

VI. TCS Design and Heater Control Software Overview

A. TCS Design The thermal design of MPO is mainly driven by the Mercury approach and orbit phase, when extremely high solar and planetary fluxes will occur. Being the orbit of Mercury quite elliptic, the solar irradiance is a function of the Mercury true anomaly varying from 6,290 W/ m2 at Mercury aphelion to 14,500 W/ m2 at Mercury perihelion. Because of the slow rotation of Mercury around its axis and of the properties of its surface, the surface of the sunlit face is essentially in equilibrium with the solar flux, and radiates as IR all the absorbed solar flux. To reduce the maximum intensity of orbital fluxes, the MPO orbit apocentre is positioned nominally above the sub-solar point at Mercury Perihelion, in order to fly at the maximum altitude from the planet surface and therefore to minimize the planetary fluxes when the solar irradiance is at its maximum. The opposite happens at aphelion and, consequently, the planetary fluxes are Figure 12. MPO inside the LSS chamber. relatively constant between Aphelion and Perihelion, but naturally very different between day side and night side of the planet. In such an environments, the high emissivity of a standard radiator would cause an unbearable absorption of infrared energy from the planet; even the low solar absorptance of a standard radiator would still be too high with the Mercury solar irradiance and albedo. Consequently, the choice of a surface where to locate the S/C radiators was not obvious and the S/C is configured to accommodate the radiator surface according to the following criteria: • The radiators are positioned only on one side of the spacecraft (-Y), which is anti-Sun oriented during cruise and parallel to the orbit plane during Mercury orbit. • During half a Mercury year, the β- Figure 13. Heat Pipes Network. angle ranges from 0° (at perihelion) to 180° (at aphelion) and the Sun is always in the half-space opposite to the radiator. A flip-over maneuver executed twice a year keeps the radiator always in shadow of the S/C. • All the surfaces of the S/C except the radiator are insulated from the extreme external environment using an extremely efficient MLI stack, composed of three different blankets designed to different temperature levels (high, intermediate and standard). • The radiator is protected from direct view to the planet (and thus possible exposure to albedo and IR) by means of a system of high reflective louvre blades.

7 International Conference on Environmental Systems

Most of the equipment are mounted on the internal +X and –X panels and then connected to the radiator with a network of heat pipes: embedded heat pipes on +/–X panels to distribute the heat; high capacity heat pipes that run on top of the +/–X panels and that, after a 90° turn, are clamped on the heat pipes of the radiator; heat pipes embedded in the radiator panel and on its surface for final heat spreading. Payloads with strict pointing stability requirements are mounted on an optical bench and are cooled by heat pipes linking the units either to the radiator or to MPO main structure. MPO battery and instrument cold detectors are linked to dedicated local radiators, mounted within cut-outs of the main radiator and thermally decoupled from it and from the S/C main structure, to be able to stay at lower temperatures. Specific coatings had to be developed to limit and to withstand the high temperatures of external items. Also the high temperature High Gain Antenna has a dedicated thermal control, able to handle over 550 °C whilst providing the demanding performance required by the Radio Science Experiment. The high temperature Solar Array is able to survive up to 220 °C operational limit. The passive design is completed by a computer controlled heater system that keeps the temperatures above the minimum requirements during the cold cases of the cruise and the eclipses. The overall mass of the MPO thermal control is around 130 kg, most of which is the MLI mass.

B. Thermal Control Software Figure shows an example of thermal control involving a heater control when the initial unit or area temperature (e.g., launch pad temperature) is above the operational temperature range (Temperature Upper Limit TUL , Temperature Lower Limit TLL ) and includes also the situation when the measured temperature Tmeas < T LL .

Figure 14. Heater control with initial temp above operative range; heating filter exceeded.

The heater processing is as follows:

• The cooling filter – counting the number of times the temperature is found above T UL – is automatically reset upon initialisation of the heater line control. The corresponding heater switch state is commanded to "OFF" if T meas > T UL . The cooling filter will be automatically enabled. • This cooling filter and the heating filter – once enabled when the operational temperature range is reached – allow detecting the failure of an element of the actuation chain • If the heating filter is exceeded a switch over to the redundant heater branch will be performed • If the cooling filter is exceeded and the heater switch state is "ON" while already commanded to "OFF" this indicates the heater switch has failed in the closed position. Therefore the current LCL has to be open 8 International Conference on Environmental Systems

to stop power getting to the current heater. That also means that all other heater related to this LCL have to switched to the redundant LCL. Otherwise the filtering will be reset always until the temperature monitoring limit is exceeded

In case the initial unit or area temperature (e.g. launch pad temperature) is lower than the operational temperature range (T UL , T LL ) the control behaviour is equivalent: • The heating filter – counting the number of times the temperature is found below T LL – will be automatically enabled after reaching T LL . The counter is reset at initialisation. • If the initial temperature T meas < T LL the heater shall be set to [SHON main] immediately (i.e. before enabling of the heater filter)

• If the cooling filter is exceeded (i.e. T meas > T UL ) and the HS status is "OFF" only the cooling filter will be reset, to avoid switching to redundant heater branch in case the temperature remains high due to the hot environment • If the cooling filter is exceeded and the heater switch state is "ON" while already commanded to "OFF" this indicates the heater switch has failed in the closed position. Therefore the current LCL has to be open to stop power getting to the current heater. That also means that all other heater lines related to this LCL have to be switched to the redundant LCL.

The initial settings (counts) for the heating and cooling filters have been individually derived for each heater line during the MPO and MTM TB/TV tests The current in-flight experience has shown that multiple heater line reconfigurations (nominal ‰ redundant side) have occurred, i.e. the (heating) filter counts had to be adjusted many times. Reason for that is • Several heater lines are running at 100% duty-cycle in cold thermal environment (no margin in heater power during S/C transients) • The thermal environment being different than tested (limited simulation capability in LSS for external loads, heat pipe functionality, missing solar array) • Physical effects of “wet” CPS in flight compared to test (gas “bubbles” migrating through pipework caused by pressure gradient, sloshing effects of tank propellant at start of maneuvers), disturbing the temperature control • Flight configuration having different thermal capacities compared to test configuration ‰ tanks! Especially the latter 2 points have strongly affected the heater control of the tanks causing many filter count updates: The (heating) filter counts by default are set to 5 before testing. They have been increased to 180 for (empty) tanks during TB/TV tests, i.e. about 90min delay before reaching the switch-on temperature again. In flight with full tanks the (heating) filter count had been increased up to 18,500 corresponding to about 6.4 days. Today it has to be stated that the thermal control S/W concept used on BepiColombo (being successfully used on or ) is not optimum. It is much more suited for a stable thermal environment.

VII. Conclusion During commissioning the TCS functionality has been successfully verified, by demonstrating that all heater lines could be operated on the nominal side and that they activated and deactivated correctly in correspondence of the on/off thresholds. No risk of hardware damage has been highlighted, based on the analysis of telemetry data. The performance of the Thermal control subsystemhas generally been as expected, with the exceptions of: 1. several reconfigurations from main to redundant heater line, which have been dealt with by progressively tuning the heating filters of the TCS software to appropriate values; 2. MPO temperatures generally higher than expected. Concerning point 2., a preliminary investigation has been carried out and its results have been presented in this paper. No impact is expected on cruise operations, since MPO has sufficient temperature margins. For the on orbit conditions, the operation of the most marginal payloads could be affected in the worst hot conditions. In order to better understand the impact on payload operations, a second flight-model comparison is planned for the Venus fly- by stage with a warmer environment (equivalent to 2 solar constants on Earth). If the hypothesis of a local effect at the MPO/MTM interface is confirmed by such analysis, the on orbit impact will be small.

9 International Conference on Environmental Systems

References

Proceedings 1Schilke, J., Natusch, A. and Ritter, H., “BepiColombo Radiator Breadboard Performance Test”, Proceedings of the 38 th International Conference on Environmental Systems , San Francisco, USA, 2008. 2Moser, M. et al., “High Temperature Multilayer Insulation of the BepiColombo Spacecraft – a design at the edge of material capability”, Proceedings of the 40 th International Conference on Environmental Systems , Barcelona, , 2010. 3F. Tessarin, D. Battaglia, T. Malosti, J. Schilke, D. Stramaccioni, “Thermal Control Design of Mercury Planetary Orbiter”, Proceedings of the 40 th International Conference on Environmental Systems, Barcelona, Spain, 2010. 4J. Schilke, F. Tessarin, D. Battaglia, D. Stramaccioni, “BepiColombo MOSIF 10 SC Solar Simulation Test”, Proceedings of the 41 st International Conference on Environmental Systems , Portland, USA, 2011. 5J. Schilke, F. Tessarin, A. Ferrero, D. Battaglia, T. Malosti, C. Ranzenberger, D. Stramaccioni, “Development and Qualification of Bepi Colombo MLI”, Proceedings of the 43 st International Conference on Environmental Systems , Vail, USA, 2013. 6A. Ferrero, D. Battaglia, T. Malosti, J. Schilke, D. Stramaccioni, “The Challenges of The Thermal Design of BepiColombo Mercury Planetary Orbiter”, Proceedings of the 46 st International Conference on Environmental Systems , , Austria, 2016. 7A. Ferrero, D. Battaglia, T. Malosti, J. Schilke, D. Stramaccioni, “The Thermal Balance/Thermal Vacuum Test of BepiColombo Mercury Planetary Orbiter”, Proceedings of the 46 st International Conference on Environmental Systems , Vienna, Austria, 2016. 8Benkhoff, J., et al., “BepiColombo-Comprehensive : Mission Overview And Science Goals ”, in Planetary and Space Science, Vol. 58, Is. 1-2, January 2010.

10 International Conference on Environmental Systems