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Deployable sunshield

Deployable sunshield

The Gaia spacecraft will carry a large deployable sunshield attached to the base of the service module. This disk-like structure, with a diameter of 10.2 metres, shields the spacecraft and payload from direct illumination by the Sun and helps to provide a stable and low temperature environment. Preventing thermal instabilities is crucial for the mission's scientific objectives as they would affect the final accuracy of the microarcsecond astrometry measurements.

The sunshield's large diameter is driven by the fact that the Gaia spacecraft maintains an angle of 45° between its central axis and the anti-Sun direction. To fit inside the launcher fairing the sunshield will be folded against the sides of the spacecraft during launch, forming a cylinder around the spacecraft. It will deploy after the spacecraft has separated from the launcher, opening 90 degrees to form a disk.

 
Quick facts of sunshield
Diameter 10.2 metres when fully deployed
Framework 12 rectangular rigid frames (0.8m × 3.2m).
Made of CFRP tubes joined by metallic fittings.
Frames are hinged at the base, for deployment.
Thermal shields 2 parallel MLI blankets, each comprising:
   -   12 fixed rectangular sections, and
   -   12 foldable triangular sections
Solar flux is reduced by a factor of 280
Planarity Sun-side blanket kept flat within 10mm throughout mission
Solar panels 8 solar array panels installed on 8 of the 12 frames


Structure and composition

The structural framework for the deployable sunshield is formed by 12 H-shaped rigid frames, made of carbon-fibre reinforced plastic (CFRP) tubes that are joined by metallic fittings. This framework supports two parallel blankets of multi-layer insulation (MLI) that act as the thermal shields. The blanket on the Sun-side of the framework is made of 2-layer MLI and the one on the shadow-side is a 5-layer MLI.

The two parallel MLI blankets both consist of fixed rectangular sections that cover the 12 frames, plus foldable triangular sections that cover the space between the frames. The foldable sections are rolled up when the sunshield is stowed for launch, and unroll during deployment. Together the fixed and foldable sections form the complete sunshield.

 

Two views of the sunshield, semi-deployed, during testing of a qualification model in July 2009. Left: the Sun-side MLI is not yet fully installed so the H-shaped CFRP frames are visible (only three frames make up this qualification model). The outer two frames are equipped with the base plate for a solar panel. Right: all MLI is in place, including the foldable triangular sections between the frames. Credit: ESA.

For the shadow-side blanket, the part covering each frame is made up of a single section of MLI. For the Sun-side blanket the MLI over each frame is made up of a lower part and an upper part. Eight of the 12 frames of the deployable sunshield will be equipped with a solar panel. These frames will only have the upper part of the Sun-side MLI, with the lower part of the frame covered by a different type of MLI for insulation underneath the solar panel.

All the rectangular sections and the triangular sections of the sunshield MLI are attached to the frames through tensioning devices, which hold the sections at their corners and keep the two parallel blankets of the sunshield in place.

Deployment of the sunshield

 

The qualification model of the sunshield, made of three frames and with the two parallel MLI blankets in place, seen in fully deployed configuration. 
Credit: ESA

When in the stowed configuration, the 12 frames of the sunshield are held in place along the spacecraft's sides. For this, each of the 12 frames has a fitting in the centre of the intermediate horizontal tube of its H-shaped frame. This fitting is part of the hold-down/release mechanism that fixes the frame to the thermal tent - the protective structure which surrounds the Gaia telescopes. The fitting contains a release nut, which holds a separation bolt that extends from the corresponding fitting on the thermal tent.

The controlled deployment of the sunshield is initiated by the firing of small pyrotechnics in the release nut fitting on each frame, resulting in the release of the separation bolt (which is then retracted and retained into the fitting on the thermal tent) and thereby the release of the frames.

All frames have two hinges at their base, with one at each corner of the frame. Each hinge has two loaded springs that provide a force in the deployment direction. It is these springs that drive the sunshield's deployment.

The hinges of adjacent frames are connected by a flexible metal coupling that spans the 30° angle between two frames. Together the 12 flexible couplings ensure a synchronous deployment of the sunshield frames. 

After release of the 12 frames from the spacecraft sides the deployment is controlled by a regulator installed on what is called the master frame. The regulator is connected to one of the master frame's hinges. It is a cylinder filled with an alloy that has a low melting point. Initially the alloy is solid and prevents the part inside the cylinder that is attached to the hinge from moving, thereby keeping the sunshield from deploying. By heating, the solid alloy melts and the resistance against deployment is reduced. This allows for a controlled and smooth deployment.
 
When the sunshield is fully deployed, the regulator is switched off so that the alloy is left to cool again and solidify. Together with the hinge springs this will keep the sunshield steadily in its deployed configuration throughout the mission.

Last Update: 1 September 2019
6-Dec-2021 12:08 UT

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