BAF Acceptance successful

Despite few remaining uncertainties, MSG-3 launch campaign carries on with enthusiasm. Today, another successful milestone was achieved: the BAF (Batiment d’Assemblage final/Final Assembly building) acceptance.

This blind tower is somehow mysterious as already explained in the last part of the message (from BIL to BAF). No pictures from inside can be published, and one will have unfortunately to read further this message for getting the details of what happened today. As a compensation, a cropped picture of the acceptance team was authorised to be released.

From left to right: Alina (Arianespace), Francois (ESA), Jean (Thales), Jean-Claude (Eumetsat) and Cyril (ESA)

It is always impressive to visit the BAF due to the close presence of the Ariane 5 rocket inside. Getting to the upper platforms with the lift gives an impressive view on the solid rocket boosters and the main tank. Seen so closely from the windows of the moving lift, the rocket seems to take off: fortunately this is just a visual illusion :-)

For memory, the Ariane 5 VA207 will be a double launch where Echostar-17 will get the top position and MSG-3 the lower one inside the fairing. Mating and encapsulation of these two spacecrafts will take place in this dedicated building. Most of the tasks are made by the Ariane space personnel. However, a limited number of MSG-3 team members will have to be present for specific operations.

The purpose of today acceptance was to check in advance that areas to be used by MSG-3 inside the BAF facility are ready and all specifications announced by Arianespace are properly implemented.

MSG-3 team inspected three specific areas: 

• The airlock on the groung floor, i.e. the area where MSG-3 will arrive at BAF

• The two platforms situated on the top of the rocket where MSG-3 will be mated on the launcher: i.e. PFEI and PFES located in the upper part of the BAF

• The COTE room at the bottom of the Launch table where the equipment will be connected on one side with the Spacecraft and on the other to the control room at EPCU S5C  (COTE stands for Check Out Test Equipment)

Just for curiosity, a bird cage was found in a corner close to one of the elevated platforms (at the upper of the tower): what was it used for?

The purpose is very simple: it is a trap. There is in fact always a risk to have birds entering the building between opening and closing of the external doors. This cage, full of food, will just permit to catch them before, trapped inside the fairing, they can get a free ticket to space and/or create some other incidents. For the same reasons, insects are not welcome in this area. Several fly catchers (with UV lamps) were placed around and proved to be rather efficient. For rodents, we did not find any visible counter measures. Do mice simply suffer from vertigo and therefore give up adventuring on the top of the rocket?

Whatever the answer is, the acceptance was concluded fully successful for all other technical aspects. This event was felt by the team as a positive achievement and a good sign for the steps to come.

MSG for dummies :-)

Note: This message is published as a living list of naive questions. It is of course not exhaustive of the MSG design.

A special thanks to Jean-Claude, Rolf and Francois for their support.

I - MSG PLATFORM DESIGN

What does "platform" means?

A spacecraft is normally made of two main elements; The platform which comprises all the support subsystems and the payload (SEVIRI and GERB for MSG) that will provide the mission data.
The MSG platform design comprises the following subsystems:

• Data Handling Subsystem (DHSS) + Software (SW)
• Electrical power Subsystem (EPS): 8 Solar Arrays, 2 Batteries, Power conditioning and distribution Units (PCU and PDU), and Pyrothechnic Release Unit (PRU)
• Attitude and Orbit Control Subsystem (AOCS). This comprises Control Electronic units (AOCE), Sun and Earth Sensor Units (SSU and ESU) and the passive nutation Dampers. The AOCS directly commands the UPS.
• Unified propulsion System (UPS). The UPS is a Bi-propellant system including the Liquid Apogee Motors (LAMs), the Reaction Control thrusters (RCTs), the pressurant and propellant tanks, all necessary valves, filters, pressure regulators and transducers and the Ultrasonic Gauge sensors (UGS)
• The Telemetry, Tracking and Command Subsystem (TT & C). This is supported by the Mission Communication Package (MCP) with all antennas and electronic units.
• The Thermal Control Subsystem (TCS), mainly insulation blanket (MLI) and optical sun reflectors (OSR). 
• The Structure Subsystem, mechanisms and Pyrotechnic devices

Where can I find a good description of the MSG-3 spacecraft platform functions?

About satellite architecture.

About Communications.

About propulsion: to be filled later

About  MSG facts and figures

Why MSG has a cylindrical shape?

Because the spacecraft is designed to rotate around its vertical axis during its mission at an angular speed of 100 rounds per minutes. The spinning increases the moment of inertia and therefore the gyroscopic stabilisation. It simplifies the the design of the attitude control sub-system compared to a 3 axis stabilised spacecrafts.

Why MSG is spinning and what are the general benefits of spinning satellites?

MSG rotation is 100 round per minute around its vertical inertial axis. It belongs then to the spin-stabilised satellite family. The main benefit is the attitude stabilisation by mean of the gyroscopic effect which makes the design of the attitude control much more simpler than a 3 axis stabilised spacecraft. The attitude stability is an essential parameter to get a sharp image by a “camera” at 36.000kms distance from Earth. At the time MSG programme was started in the seventies, the 3 axis technology was not able to meet the mission requirements. This new technology will appear on the third generation of Meteosat spacecraft (MTG) by end of the decade.

Why the SEVIRI solar array is slightly longer than the other ones?

 

There are eight solar panels mounted on MSG: one SEVIRI and seven "normal" panels. It is true that the SEVIRI Solar array (see above) is slightly bigger in size but the solar array surface covered by solar cells is the same for each panel. This oversize is made to compensate, with additional solar cells, the large SEVIRI aperture. As MSG is spinning the electrical current to be generated by the solar array in space should be the same for each solar array to prevent variations during the rotation.

What is the use of Earth and Sun Sensor Units  (ESU and SSU)  mounted on MSG?

 The Earth and Sun sensor units allow:

1. to determine the spacecraft attitude when  S/C, sun and earth  are not to close from  alignment (i.e. angle  between S/C earth direction and S/C sun direction  is above ca  15 deg and below ca  165 deg) 
2.   to measure the spacecraft spin rate (function done by the sun sensor unit only) 
3.   to elaborate a reference pulse used  a)  for synchronising the image taking by SEVIRI and GERB with the S/C rotation (ie to determine when the instrument is exactly facing the Earth), b) for synchronising the radial thrusters in order to deliver a radial force in a given direction (as the S/C is permanently spun) 
4. to synchronise the electronically despun antenna  (which counteract the S/C spin in order to ensure permanent pointing of this antenna towards the earth.

What is a despun antenna?

An Electronically Despun Antenna (EDA) is mounted on the top of the cylindrical MSG-3 spacecraft. The EDA purpose is to have the antenna continuously pointing at Earth whilst the spacecraft is rotating.

How MSG communicates with Earth?

This function is performed by the Mission Communication Package.
to be filled later

What are the propellants used on board MSG-3?

MSG propulsion is achieved by a classical bi-propellant system, i.e. the combustion of two components: a liquid fuel with a liquid oxidizer.

MSG system uses Monomethylhydrazine (MMH) as liquid fuel and Mixed Oxides of Nitrogen (MON) as liquid oxidizer.