4 Final Report - Emits - ESA

4 Final
Report
manoeuvres, the APE settling time is so long that the duty cycle becomes as high as 368%. This only
allows Geo-Oculus to perform 27.2% of the required manoeuvres over 24 hours, which is around the
same performance as a configuration using microHEMPT can achieve. The 2 mN option can perform
21.7% of the required manoeuvres and the 3 mN option can perform 26.5% of the required
manoeuvres, assuming that the low force manoeuvres are so slow that no settling time is needed after
the completion of the manoeuvre. In this context it can be seen that some fuel mass can be saved
using microHEMPT for manoeuvres.
Table 4.5-10: Total manoeuvre times for HEMPT 3050, microHEMPT and MBW based
configurations, duty cycle over 24 h
Manoeuvre [deg] HEMPT 3050 (30 mN) MicroHEMPT (2 mN)** MicroHEMPT (3 mN)** MBW (400 mN)
0.25 20 s 132.6 s 108.3 s 58 s
0.40 153 s 167.8 s 137.0 s 70 s *
2.00 300 s 375.2 s 306.3 s 70 s *
Duty cycle over 24 h 368% 463% 378% 89%*
*Assumption, performance currently not achieved in simulations
** Theoretical performance, no settling time included
Overall performance
A MBW configuration provides very good attitude control performance and is assumed to be able to
meet the manoeuvre requirements with some additional tuning of certain parameters. The EPS
attitude control performance is also very good, but the manoeuvre performance is far worse than what
is required. By using large EPS thrusters, the actual manoeuvre time is within the allocated time, but
the settling time after a manoeuvre is much to long for the 0.4° and 2° manoeuvres due to the low
available torque from the attitude control thrusters. This leads to the result that not all manoeuvres can
be performed which reduces the mission value. Also, using the HEMPT 3050 causes a high fuel and
power consumption. The microHEMPT option for manoeuvres has not been investigated in detail, but
it is clear that the fuel consumption will be lower, and that such a configuration will be able to perform
as many manoeuvres as a HEMPT 3050 system if the assumption that no settling time is needed after
the manoeuvre completion holds.
The MBW option is the clear favourite of the two, and is selected as a baseline nominal mode
actuator. The only drawback is that the MBW development is in an early phase, and might not be
available as expected in 2013. If indications of significant delays in the MBW development, or
shortcomings in the performance surfaces, the EPS option can be considered again. Figure 4.5-10
shows the baseline AOCS configuration and Table 4.5-11 summarizes in which operational modes the
various AOCS equipment is used.
The hybrid option discussed in [RD 7] has not been considered further, as it leads to unnecessary
high costs and system complexity. The CPS system can perform wheel offloading and East/West
station keeping every three weeks, with a total outage of less than 10 min each time. The North/South
station keeping is performed twice every year. If the total mass of the spacecraft should exceed the
capabilities of the desired launcher, the EPS system can again be considered as it has the potential to
lower overall system mass.
Doc. No: GOC-ASG-RP-002 Page 4-65
Issue: 2
Date: 13.05.2009 Astrium GmbH