4 Final Report - Emits - ESA

4 Final
Report
Table 4.5-4: Comparison of HEMPT 3050 and microHEMPT
HEMPT 3050 microHEMPT
Force 30-50 mN 100-500 µN
Mass flow 1.2 mg/s 12 µg/s
For the final suggestion of preferred actuators for the Geo-Oculus mission, the properties of above
options in terms of pointing performance, agility and fuel consumption is checked for attitude control
tasks and manoeuvres in the following.
4.5.6.4 Attitude control performance
The attitude control performance of Geo-Oculus has been derived from simulations using a high
performance attitude control simulator as already applied in a similar project. The baseline sensor
suite has been used for simulations for both the MBW and EPS option. Below follows a summary of
the attitude performance figures for steady state attitude control and manoeuvres.
MBW system and performance
The configuration of a MBW based actuator system is equal to that of a ball bearing reaction wheel
system. Four or five MBW can be selected, depending on the impact zero-crossings has on the
pointing accuracy. Five wheels are needed if zero-crossings are judged to be of importance, as a five
wheel system will not experience zero-crossings even if one of the wheels should fail. A four wheel
configuration is the minimum for redundancy, but will have wheel zero-crossings if one wheel should
fail.
The worst values of the analysed performances are
• APE 14.1 μrad, AME 11.3 μrad, PDE 0.32 μrad/100ms (all values for 100% probability).
All performance values are better than the requirements of Table 4.5-3.
EPS system and performance:
The EPS analysis is based on the recently finished HOPAS-3 study, investigating the use of EPS as
the sole actuator on spacecraft in GEO. The study has been done for DLR by EADS Astrium GmbH.
The baseline EPS configuration is derived from this study.
The EPS based attitude control system will have a 12 thruster configuration based on the
microHEMPT. A HEMPT 3050 based system has been analysed and rejected based on its high fuel
consumption (~100 kg over 10 years) and problems with meeting the given minimum lever arm and
torque level requirements. Also, the high power consumption of the HEMPT 3050 (1.2 kW per active
thruster) does not allow more than 4 thrusters for attitude control to be active at the same time. This
impacts the attitude control performance, especially after the completion of manoeuvres, when only
attitude control thrusters are used for settling. In comparison, the microHEMPT can operate six
thrusters at the same time, with a total power consumption of 0.2 kW.
The attitude control thrusters are configured with four thrusters around each axis, as can be seen in
Figure 4.5-8. The special configuration has been developed especially to meet the lever arm and
thruster plume direction requirements, and at the same time provide decent torque levels and fuel
consumption.
Page 4-60 Doc. No: GOC-ASG-RP-002
Issue: 2
Astrium GmbH Date: 13.05.2009