Post on 19-Feb-2023
VTC 1.5 THE NEW ESTEC HIGH FLUX SIMULATOR
Alexandre Popovitch, ESA-ESTEC
Mark Wagner, ESA-ESTEC, Mark.Wagner@esa.int
ABSTRACT
The Large Space Simulator LSS and the medium sized
PHENIX are the main thermal vacuum test facilities at
ESTEC used to simulate space environmental
conditions in the course of spacecraft test campaigns.
However the large dimensions of these chambers imply
high running costs and thus are oversized for equipment
and small payload specimen. In order to perform
thermal vacuum testing on equipment units and small
subassemblies under real and complex conditions it was
considered necessary to complement the ESTEC test
centre facilities with a small, modern and flexible
facilities capable to provide essential test data to be used
for the verification and validation of technology
developments and designs including the update and
correlation of thermal mathematical models. Future
spacecraft projects such as BepiColombo and Solar
Orbiter also were asking for the capability of long
duration high solar flux simulation testing. For this
reason it was decided to refurbish the vacuum vessel of
the 35 years old VTC 1.5 test facility and complement it
with a state of the art thermal control system and a high
flux solar simulator.
1. INTRODUCTION
ESA’s Space Test Centre is located on the premises of
the European Space Research and Technology Centre
(ESTEC) in Noordwijk, the Netherlands. It is one of the
main test centres for spacecraft level environmental
testing in Europe. As such it comprises indispensable
test facilities for qualification or acceptance testing of
spacecraft systems which are unique in terms of
dimensions and performances in Europe and, in parts,
for the rest of the World. To complement the services
offered by the test centre and also to meet testing needs
of future space missions it was necessary to develop a
small, modern and flexible thermal vacuum facility
which can be used for long duration tests and which
allows to simulate high solar fluxes or complex
temperature distributions on the specimen.
2. VTC 1.5 FACILITY OVERVIEW
The new VTC 1.5 test facility consists of:
horizontal vacuum vessel,
primary pumping system (oil free),
secondary pumping system,
thermal generator with one radiative channel and 3
liquid loops allowing 24/7 unattended operation,
new high flux simulator system.
Figure 1: VTC 1.5 facility front side
Figure 2: VTC 1.5 High flux Simulator
3. VESSEL
The vessel of the 35 years old VTC1.5 test facility
which was installed in the past in the test centre in
vertical configuration has been refurbished, repainted
and turned to horizontal orientation. Figure 3 depicts the
vessel main dimensions. The usable test volume is 3m
in length on a diameter of 1.7m and the horizontal axis
of the vessel is 1.5m above the floor which ensures a
comfortable working space for the operator. The vessel
is mounted on kinematic mounts allowing breathing of
the vessel and vertical adjustment. All internal surfaces
of the vessel are polished. The vessel and the related
flanges compatible with vacuum down to 5E-8mbar and
maximum leak rate including all doors and flanges is
lower than 2E-4mbar.l/s. The specimen is loaded via the
front door which can be slide away from the chamber
using an airglide system The interior features six
internal fixation points on its top side each capable of
supporting 150kg., two internal, parallel rails on bottom
side to support test setup of up to 200kg. The rear door
capable to accommodate a quartz window for sun
simulation. In case the sun simulation is not installed the
rear door will be equipped using a blind flange instead
of the window.
Figure 3: Vessel main dimensions
Figure 4: Photo of vessel at time of delivery
The VTC1.5 vessel has been refurbished and
complemented by AMOS (B).
4. PUMPING SYSTEM
Primary pumping system:
The VTC 1.5. facility setup allows two options:
1. Central Pumping Station (CPS) which is also used
for the primary evacuation of the LSS.
2. Local VTC1.5 pumping system
The CPS is based on 5 independent sets of large and
medium size vacuum pumps. Two sets are based on
1300m3/h IDX screw pumps associated to roots blowers
and three sets are based on 600m3/h claw pumps
(GV600) associated on EH4200 roots blowers. The CPS
is completely oil free. The local VTC1.5 primary
pumping system is based on a dual stage primary pump
Edwards EM80 associated to a roots blower.
High vacuum pumping system
The main pump is a DN630 cryopump from Leybold
running with two cold heads fed from a single helium
compressor. The cryopump can be isolated from the
vessel using a VAT gate valve. In addition a turbo
molecular pump ADIXEN ATH 2300 backed with a
two stage primary pump is used to improve the pumping
capability for light gases such as Helium and Hydrogen.
Also this pump is isolated from the vessel via a VAT
gate valve. The high vacuum pumping set is completed
by a scavenger panel cooled to liquid Nitrogen
temperature functioning as a cold trap for molecular
contamination.
Pumping performances
The empty chamber end vacuum pressure with the
shroud in liquid Nitrogen temperature is <5x10E-8
mbar.
The time to reach the 1x10E-5
mbar with empty chamber
is shorter than 1.5 hours. The subsequent figure shows a
typical pumping curve of VTC1.5.
Figure 5: Empty chamber pumping curve
Vacuum gauges:
The vacuum level is measured using Membranovac
pressure gauges for pressures above 10mbar, and two
Leybold ITR90 Pirani and Bayard-Alpert combined
gauges for pressures below 20mbar.
Re-pressurisation system:
The chamber re-pressurisation system allows to stop
and resume the chamber re-pressurisation and is able to
inject either nitrogen gas of filtered air following a
programmed venting rate.
5. THERMAL GENERATOR AND CONTROL
SYSTEM
The VTC 1.5 thermal generator and control system,
consists of a set of multi-segmented internal shrouds
and the thermal control unit (TCU) located in the
ground floor below the main chamber. The thermal
system provides 4 independent thermal channels:
One irradiative channel: One cylindrical and two
circular(doors) vessel shrouds - GN2 & LN2 mode
of operation.
Three independent liquid loop channels to be used
for specific MGSE setups or conductive plates.
1,E-06
1,E-05
1,E-04
1,E-03
1,E-02
1,E-01
1,E+00
1,E+01
1,E+02
1,E+03
1,E+04
15:43:1216:12:0016:40:4817:09:3617:38:24
Cryopump on
Turbomolecular on
Primary pumping on
Decontamination plate on
The radiative shrouds are painted with black paint
MAP-PU1 with following typical optical properties:
Solar absorptance: α2Πs= 0.96 ± 0.02
IR emissivity: εN,IR= 0.88 ± 0.04
The VTC 1.5 thermal subsystem has the verified
performances detailed in table below:
Figure 6: View inside the chamber
The subsequent figures show the shroud temperature
distribution in LN2 mode and GN2 mode.
Figure 7: LN2 cool-down
Figure 8: Typical GN2 mode temperature profile
Unattended operation:
One major requirement was to be able to operate the
facility unattended i.e. without any operator present
during the test duration. To achieve this a thorough
Failure Mode Effect and Criticality Analysis (FMECA)
has been performed. The FMECA covered each
potential hardware failure with the specific attention on
detection possibilities. Following this automatic
responses of the VTC1.5 system were systematically
designed. In case a failure is detected, alarm messages
are sent by email and SMS to the operators on-call and
the system is automatically brought into a safe mode. A
specific part of the validation test programme was
dedicated to check all FMECA items by simulating each
failure case. Based on the outcome the hardware and
software design was adapted to safeguard the facility
and the specimen. In addition the control and
supervision of the thermal control subsystem and of the
secondary pumping system is based on a hot redundant
PLC.
In the unattended mode of operation the thermal
subsystem follows pre-defined temperature profiles
which have to be imported prior to the test. The
unattended mode can be interrupted and modified
during the course of the test.
The validation and acceptance campaign impressively
demonstrated the functionality of the unattended mode.
Thanks to this thorough assessment and the related
design effort, VTC 1.5 facilitates long duration testing
and reduces operational costs. To the knowledge of the
authors this feature together with the given complexity
of the overall system is unique in the World of
environmental test facilities.
The thermal generator and control system has been
designed and implemented by ANGELANTONI (I).
GN2
mode
LN2
mode
3x
Liquid
loops
Max temperature +120 oC -190
oC +120
oC
Min temperature -150 oC -60
oC
Max gradient in
steady state 10
oC 5
oC 2
oC
Max change rate 2 oC/min
Max internal
thermal load 1kW 10kW 500 W
6. HIGH FLUX SUN SIMULATOR (HFS)
The VTC 1.5 test facility is equipped with a high flux
sun simulator to test samples at a high level of
irradiance (up to 20 Solar Constants).
The solar simulator flux is generated using one Xenon
short arc discharge lamp that has a light spectrum
similar to the sun spectrum outside of the atmosphere. A
lamp module can accommodate lamps of rated power up
to 30kW (700A-46V DC) and is able to provide the flux
previously mentioned over a circle of 0.5m-diameter.
The complete sun simulator consists of:
One 25kW Xenon short arc discharge lamp, (same
as used in LSS Sun Simulator),
a 1m-diameter optical quartz window currently
available as LSS spare window,
a water cooled tube connecting the window to the
lamp module trolley,
a lamp module including the primary reflector and
electrical gear to strike the lamp,
a lamp module trolley,
an electrical rectifier to feed power to the Xenon
lamp and control the lamp module,
a high pressure (20bar) water cooling system
required by the Xenon lamp,
a Nitrogen gas flushing device to make an oxygen-
free light path to avoid the ozone formation,
a water cooled shutter to manually close the optical
path.
Within the chamber an 80cm-diameter opening through
the thermal shrouds will allow the light source to reach
the specimen under test.
Figure 9: Xenon lamp and reflextor inside lamp module
New Lamp power supply unit:
A new rectifier power supply has been designed and
manufactured by CONVERGY (F) with the following
characteristics:
Power : 70 kVA
Current : <100A
Lamp voltage : 20V to 75V at 700A
Start-up ignition current : 400A to 500A internally
adjustable
Accuracy : < 1% Imax
Resolution: 1A
Load regulation < 1% for load voltage variation
from 20V to 75V
Ripple: < 1% Imax (RMS value)
Long term stability : ±0,5% Imax
The power supply unit allows to control the light flux
output either in open loop i.e. via the lamp current or in
closed loop using a solar flux sensor inside the VTC1.5
chamber.
New flux control sensors:
To control the flux during the test, two sensors have
been developed by the ESA testing division based on
CAPTEC flux meters. The sensors are temperature
controlled and operate in a closed loop mode. The
sensors enable the operator to control the sun in flux
mode such that the fluctuation of the lamp output,
e.g. due to ageing, is compensated.
HFS main features:
The high solar flux simulation system provides the
following main features:
Light intensity from 5SC to 20SC over diameters
<500mm to >700mm. With a uniformity of ±20%.
Intensity distribution needs to be tuned case by case
according to customer requirements.
Flux uniformity: bell shaped intensity .
Spectral distribution of Non-filtered Xenon lamp.
Maximum de-collimation angle <5 degrees.
7. SUN CHARACTERISATION CAMPAIGN
The solar flux intensity distribution needs to be tuned
case by case according to customer requirements. In
addition the lamps have a limited lifetime and therefore
need to be replaced on regular basis. To ensure traceable
and reliable solar simulation results the following
activities are performed after each lamp replacement
Alignment of the lamp bulb in its parabolic
reflector,
Mapping of the solar flux distribution,
Calibration of the flux control sensors using a
KENDALL MK IV radiometer,
Measurement of the spectral content.
Lamp alignment
The lamp bulb alignment in its parabolic reflector is
performed against the measurement of a dedicated
alignment cross. The cross consists of two 1m-long
perpendicular beams which are cooled by a water pipe.
Each beam is fitted with 50 20mmx20mm flux sensors.
The output of the solar cells is recorded and set in
correlation with its location on the cross. A typical
result plot is shown in figure 12. The alignment
objective is on one hand to obtain the most symmetrical
light beam and on the other hand to modify the
divergence of the beam by moving the lamp arc along
the beam axis.
Figure 10: Alignment cross
The lamp module provides fine mechanical adjustment
possibility of the bulb in three orthogonal directions
enabling locating the lamp arc at the focal point of the
parabolic reflector .
Figure 11: Lamp alignment device
Figure 12: Typical achievable solar flux distribution
Spectral irradiance
The measured spectral irradiance is shown in below
together with sun AM0 spectrum (small picture blue
curve). As it can be seen the UV part of the sun
spectrum is very well simulated whereas there is a peak
on the IR wavelength which is characteristic for lamps.
The use of a filter was not considered since this would
reduce the overall solar flux output and since the project
are more interested to simulate the UV part of the solar
irradiance.
Figure 13: Spectral irradiance of 25kW Xe lamp
Absolute total irradiance:
The absolute total irradiance is measured against the
ESTEC radiometer standard Kendal MK Iva. The
obtained values are used to calibrate the flux control
sensors installed in VTC1.5.
Figure 14: Flux control sensors in front of shutter
Flux uniformity mapping:
The flux uniformity mapping is conducted using a high
speed video camera recording the reflections from a
water cooled white painted, reflective surface which is
installed inside the chamber. A special software package
allows to correct for the camera view angle. An average
picture is obtained by averaging 600 pictures over 10
minutes to get rid of the arc flickering instability. Figure
16 shows the flux distribution in the 700-mm diameter
circle centred on the chamber axis. The relative
standard deviation over this circle is equal to 21%.
Flux sensors
Window Water cooled shutter
Figure 15: Picture taken with video camera
Figure 16: Flux mapping in the chamber middle plane
8. CONCLUSION
The VTC 1.5 thermal vacuum test facility provides:
A small TV test facility.
A high level of safety for specimen and facility to
allow unattended 24/7 operations in LN2 and GN2.
High flux solar simulation up to 20SC.
State of the art thermal control system with 4
independent thermal channels.
A design which is compliant to up to date standards
and directives and regulations e.g. CE, PED etc..
Demonstrated its outstanding performances in a
thorough acceptance and characterisation
campaign.
Was already in use for a first BepiColombo
preparation test by Astrium.
For futher technical information please do not hesitate
to contact Mark Wagner, ESA TEC-MXP:
Mark.Wagner@esa.int