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