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ESA Document - Emits - ESA

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s<br />

HMM<br />

Assessment Study<br />

Report: CDF-20(A)<br />

February 2004<br />

page 375 of 422<br />

The attitude control actuators are a set of reaction wheels assembly (RWA), 4 clusters of 5<br />

hydrazine thrusters plus 4 clusters of 2 hydrazine thrusters of 10 N thrust each, and two<br />

ASTRIUM engines acting with 550 N force each.<br />

The RWA includes 4 Reaction Wheels (RWL) implemented on a skewed configuration. This<br />

configuration enables to perform most of the nominal operations of the mission with a 3 RWL<br />

configuration among 4. The Reaction wheels provide the GNC control torques during all the<br />

phases of the mission except the trajectory corrections, the attitude acquisition and back up<br />

modes. Each Reaction Wheel provides an angular momentum storage capacity of 12 Nms and a<br />

motor torque of ± 75 mNm in the speed range from + 6000 rpm to - 6000 rpm. Under nominal<br />

conditions, the RWA uses a 3-wheel configuration. The fourth wheel is used during critical<br />

mission phases to avoid any mission outage if one wheel fails and minimises the reconfiguration<br />

duration.<br />

During nominal modes hydrazine thrusters are used to perform wheel off- loading, while during<br />

safe mode they become the attitude control actuators.<br />

The 10 N thrusters for attitude control are all located in the second stage of the orbiter. A<br />

detailed analysis of the CoG and the controllability of the orbiter has not yet been done.<br />

However, the present configuration based on the ATV looks feasible in this first iteration.<br />

The LIDAR expected features are the following:<br />

• Validity range of the measurements shall be between 1 metre and 10000 metres<br />

• Field of view (FoV) shall be of no less than 10 degrees<br />

• The weight shall be of no more than 10 kg.<br />

• The average power consumption shall be of no more than 20 Watts.<br />

Even with the specifications above, the power consumption and weight values are slightly<br />

increased, according to the latest news about the LIDAR development process.<br />

The CAMERA features to be tested shall be as follows:<br />

• The validity range of the measurements shall be between 0 metres and 5000 metres to the<br />

target<br />

• The field of view (FoV) shall be of no less than 20 degrees<br />

• The weight shall be of no more than 2 kg.<br />

• The average power consumption shall be of no more than 3 Watts.<br />

In the same way as with the LIDAR, slightly more conservative values have been taken.<br />

Table 4-53 summarizes some of the characteristics of the GNC equipment.

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