45th International Conference on Environmental Systems ICES-2015-283 12-16 July 2015, Bellevue, Washington International Conference on Environmental Systems 1 Small-GEO Satellite: Electrical Propulsion Sub-System (EPPS) Thermal Design and Lessons Learnt Davide Rizzo 1 , Christian Vettore 2 , Alessandro Spalla 3 CGS Compagnia Generale per lo Spazio, Milano, 20151, Italy Marcus Gröller 4 , Dr. Frank Bodendieck 5 , Dr. Dieter Birreck 6 OHB System SE, Bremen, Germany Small-GEO is a general-purpose small geostationary satellite platform that is giving European industry the opportunity to play a significant role in the commercial telecom market. Small-GEO has been developed by an industrial team managed by OHB System SE. The Thermal Control System (TCS) Critical Design Review (CDR) close-out was successfully closed and the integration tasks have just been accomplished. The satellite is now on his way to Thermal Vacuum (TVAC) test facility. Small-GEO foresees three different types of propulsion systems, namely chemical, electrical and cold gas. This paper describes the Satellite Platform Electrical Propulsion Sub-System (EPPS) thermal control which uses mainly passive concepts complemented with heaters either thermostatically or software regulated. The sizing cases used to design the thermal control system will be presented as well as the final predictions both for transfer and geostationary phases. The EPPS design is mainly connected to the GEO phases. The EPPS provides, with electric propulsion thrusters, the impulse for orbit control, as well as for dissipation of excess angular momentum that is accumulated in the reaction wheels. An overview of the sizing firing scenarios will be presented; they involve not only the thrusters but the control units and the related piping too. Then the most relevant results will be shown and discussed. The last part of the paper deals with the problems encountered, the solutions adopted and the lessons learnt during the design and optimization steps. Nomenclature ADE = Actuator Drive Electronics ADPM = Antenna Deployment and Pointing Mechanism AVG = Average BOL = Beginning Of Life CDR = Critical Design Review CFRP = Carbon Fiber Reinforced Plastic CGTA = Cold Gas Thruster Assembly COP = Cold OPerating EOL = End Of Life EPPS = Electrical ProPulsion Sub-System EP = Electrical Propulsion EPTA = Electrical Propulsion Thruster Assembly EQ = Equinox 1 Senior Thermal Engineer, Thermal & Mechanical Dept, [email protected]. 2 Head of Thermal and Mechanical Department, Thermal & Mechanical Dept, [email protected]. 3 Thermal Engineer, Thermal & Mechanical Dept, [email protected]. 4 System Thermal Engineer, Thermal Design & Verification Dept, [email protected]. 5 Head of Thermal Analysis and Verification Department, Thermal Dept, [email protected]. 6 SGEO Project Manager, [email protected].
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45th International Conference on Environmental Systems ICES-2015-283 12-16 July 2015, Bellevue, Washington
- Xenon tanks are fully covered with MLI and equipped with heaters;
- Xenon piping is isolated from structure by low conductivity stand-offs;
- specific piping sections are heater controlled and covered by low-emissivity tape;
- HET assemblies are mounted on Al brackets, attached to a S/C radiator and to a CFRP panel;
- PPU is mounted on heat pipes, along with ADE and SCE, to reject its high dissipation;
- ETSU is heaters controlled and installed on an Al doubler;
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- PRP and CGCS are bolted on CFRP panel, isolated by Ti washers and heaters controlled.
Each of the above mentioned bullets are now fully explained. Here below some pictures of the integrated S/C are
presented.
A. XTA and piping
The two Xenon tanks are fully covered with MLI and equipped with heaters for maintaining the temperature
level during the cold phases. The parts of the Xenon piping that are thermally controlled are covered as a baseline
with low emissivity tape. No MLI is used. Low conductivity stand-offs keep the piping at few centimeters from the
walls.
Figure IV-1 Small SGEO Satellite EPPS components
Figure IV-2 Small SGEO Satellite EPPS piping
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B. Thrusters
The HET assemblies are mounted as shown in Figure II-4. The HET thruster is mounted on an aluminum bracket
(i.e. HET bracket) that also carries the XFC and HIB units, as shown in Figure II-3. All these components are
mounted without thermal filler. Heaters on the HET bracket maintain the thruster above the minimum non-operating
temperature in cold cases.
The HET assembly is mounted – again without thermal filler – on the HET Bracket at 6 fixation points.
Figure IV-3 HET bracket mounting
C. PPU/ETSU
The PPU, sitting on the South Platform (P/F) Radiator, that powers the HET thrusters has a dissipation of up to
136W during thruster operation. This very large dissipation is spread over the platform radiator by means of the heat
pipe network.
The ETSU, with its small dissipation, does not need heat pipes and is cooled by conduction to the radiator and
radiation to the environment.
The ETSUs are sitting on an aluminum doubler that serves mainly to spread the heat transferred by Reaction Wheels
Heat pipes.
D. SCE
This low dissipation electronic unit is functionally associated to the PSA-PRP. It is located on the same heat
pipes as the PPUs.
E. PSA PRP
The PSA PRP are mounted to internal CFRP panels with 19 bolts for PSA-PRP. Titanium washers are used at all
bolts and there is no contact between the two units and the supporting panels. The dissipation is evacuated mainly by
radiation to the environment. No special thermal control means are needed except cold case heaters.
V. Thermal sizing cases and EPPS operation scenarios
In order to derive the thermal sizing cases the complete mission profile has been analyzed and specific
dissipation cases have been selected depending on the scenario (e.g. GTO vs GEO).
Phase Description
Pre-launch Ground operation.
LEOP From liftoff (depending on mission, the S/C may be launched in an active/passive state) up to S/C separation. Depending on mission, the S/C may be launched in an active/passive state.
GTO This phase is entered when the spacecraft is transferred from the GTO to the GEO orbit.
Orbit Relocation This phase is activated when the satellite is to be manoeuvered in its orbital position for operational reasons. Payloads are deactivated in this phase.
On-Station The On-Station phase represents the nominal phase when the satellite has reached its geo-stationary position. Payloads can only be activated in this phase.
Graveyard This phase is entered when the satellite is retired.
Table V-1 Complete mission profile summary
The EPPS works mainly after the LEOP and GTO phases.
The driving factors and the criticalities for the EPPS firing sizing cases are:
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HET thruster firing duration;
orbital position and season;
reaction wheels power dissipation profiles;
PPU (South side, one is very close to the reaction wheels) temperature limits;
FU temperature limits;
The most critical cases for the SGEO EPPS are in hot environment, that is to say that the HET thrusters firing
occurs when the Sun is illuminating the North (Summer Solstice) or the South (Winter Solstice) side of the
satellite.
Figure V-1 On station Sun illumination
The other driving factors are the Payload (P/L) and P/F dissipation profiles. A quick synoptic summary of the EPPS
power dissipation is provided in section III.
A. EPPS EP configuration
Figure V-2 shows the Electrical Propulsion (EP) thrusters accommodation on the satellite. The EP system
consists of two branches, which are operated in cold redundancy in the baseline:
Branch A (EP1, EP4, EP5, EP8) made of HET/ SPT-100 operated at 75 mN
Branch B (EP2, EP3, EP6, EP7) made of HET/ SPT-100 operated at 75 mN
Only branch A is supposed to fire in the simulations.
Figure V-2 EP Thrusters accommodation on the Satellite
In the baseline operational concept, only one EP thruster is operated at a time at its nominal operating point
(nominal power, thrust and specific impulse). If a manoeuver requires the use of two thrusters, it is split into two
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sequential manoeuvers using only one thruster each at a time. To switch from one thruster to another, a minimum
gap of 60 s shall be assumed between two sequential manoeuvers.
B. Nominal operation – Station Keeping
During station-keeping, 4 HET thrusters are typically fired in total 5 or 6 times per day (i.e. each thruster is fired
once or twice). The specified firing scenarios have been somewhat modified in order to make the implementation in
the thermal model easier. The main differences are:
• no gap between thruster switching is considered, however the specified maximum burn time for each thruster is
considered as well as the preheating of the thrusters;
• the PPU as the driving factor is powered until the temperature limit is achieved, this is estimated to be 116
minutes of continuing firing;
• the FU and XFC as the driving factors are powered until the temperature limit is achieved, this is estimated to be
63 minutes of continues firing, the total amount of 116 minutes for two adjacent firings has been assumed;
• in order to catch the worst case for the thruster, the firing manoeuver starts at the hottest point in orbit, which
might deviate from the actual node for a given sequence. Here after are the firing sequences described applicable to
the different seasonal cases.
Summer solstice
1st thruster Start time 2nd thruster
First sequence EP4 17430s EP5
Second sequence EP1 70930s EP8
Winter solstice
1st thruster Start time 2nd thruster
First sequence EP5 17830s EP4
Second sequence EP8 72130s EP1
Winter solstice: PPU combined with the hottest RW
1st thruster Start time 2nd thruster
First sequence EP5 50380s EP4
Second sequence EP8 137140s EP1
Equinox
1st thruster Start time 2nd thruster
First sequence EP8 31740s EP1
Second sequence EP5 62910s EP4
Table V-2 HET firing sequences
The firing scenarios and associated dissipation are hereafter presented for equinox (winter and solstice profiles are
identical with different firing starting points):
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0 6 12 18 24 30 36 42 480
50
100
150
200
250
300
Time [hrs]
Po
we
r [W
]
EP1
EP2
EP3
EP4
EP5
EP6
EP7
EP8
0 6 12 18 24 30 36 42 480
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time [hrs]
Po
we
r [W
]
FU1
FU2
FU3
FU4
FU5
FU6
FU7
FU8
0 6 12 18 24 30 36 42 480
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time [hrs]
Po
we
r [W
]
XFC1
XFC2
XFC3
XFC4
XFC5
XFC6
XFC7
XFC8
Figure V-3 Equinox – HET, FU and XFC dissipations during firing
0 6 12 18 24 30 36 42 480
20
40
60
80
100
120
140
Time [hrs]
Po
we
r [W
]
PPU 1
PPU 2
0 6 12 18 24 30 36 42 480
1
2
3
4
5
6
7
8
Time [hrs]
Po
we
r [W
]
TSU 1
TSU 2
ADE
SCE
Figure V-4 Equinox PPU, TSU, ADE and SCE dissipations during firing
C. GEO Station acquisition and repositioning
During this case, the S/C rotates slowly around its Z axis (4 rotations per day), which remains pointed to the
Earth. The HETs are firing 16 times per day, in a sequence such that the firing direction is always in the same
quadrant of the S/C, Figure V-6.
Figure V-5: Attitude during Repositioning Thruster Firing Scenario
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Orbit path
Flight direction
72 min firing
9 min gap
9 min gap
Spin rate around Z 1°/min
Figure V-6: Thruster Firing Scenario
Also the cases with 60 minutes firing instead of 72 has been considered. The gaps are 15 minutes each.
The firing scenarios and associated dissipation for the specific on station acquisition/repositioning case (i.e. the HET
are firing 16 times per day) is hereafter presented.
0 6 12 18 24 30 36 42 480
50
100
150
200
250
300
Time [hrs]
Po
we
r [W
]
EP1
EP2
EP3
EP4
EP5
EP6
EP7
EP8
0 6 12 18 24 30 36 42 480
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time [hrs]
Po
we
r [W
]
FU1
FU2
FU3
FU4
FU5
FU6
FU7
FU8
0 6 12 18 24 30 36 42 480
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Time [hrs]
Po
we
r [W
]
XFC1
XFC2
XFC3
XFC4
XFC5
XFC6
XFC7
XFC8
Figure V-7 Winter Solstice Repositioning – HET, FU and XFC dissipations during firing
0 6 12 18 24 30 36 42 480
20
40
60
80
100
120
140
Time [hrs]
Po
we
r [W
]
PPU 1
PPU 2
0 6 12 18 24 30 36 42 480
1
2
3
4
5
6
7
8
Time [hrs]
Po
we
r [W
]
TSU 1
TSU 2
ADE
SCE
Figure V-8 Winter Solstice Repositioning – PPU, TSU, ADE and SCE dissipations during firing
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VI. Thermal analysis results
The main objective of the thermal analyses presented here is to demonstrate that the SGEO EPPS TCS complies
with the thermal performance requirements. These can be summarized as follows:
the TCS must maintain at all times all the parts and equipment within the applicable temperature limits, with
the applicable margins;
this must be achieved within the available resources (mass and power budget, number of heater lines, …).
The presented temperatures figures are predicted ones, that means calculated values plus (for the maximum
temperatures) and minus (for the minimum temperatures) the estimated uncertainties.
The Cold Operating (COP) and the Safe Mode (SM) are not sizing cases, that is why firing scenarios have not been
taken into account for these ones in the simulations. All the EPPS units remain within their limits in cold cases
thanks to the heating power coming from EPPS heater lines. Table VI-1 summarizes the duty cycles all over the