EUSO – BALLOON
THERMAL ARCHITECTURE
EUSO-TA-INST-409-IRAP
EUSO-BALLOON THERMAL ARCHITECTURE
Name & Society
Prepared by
Date
Gustavo Medina Tanco (UNAM)
Frederic Trillaud (UNAM)
23/11/2012
Guillaume Prévot (APC)
27/11/2012
Peter von Ballmoos (IRAP)
27/11/2012
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INDEXATION NOTE
KEY WORDS :
TITLE :
AUTHORS :
SUMMARY
DOCUMENT STATUS:
Volume :
HOST SYSTEM :
Pages : 21
Luminaries pages:
Nb of annexes :
Language : EN
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MODIFICATION CHANGES
Ed.
Rev.
Date
1
0
23/01/2012
Name
Modified pages
Version 1.0
Version 2.0
TBC and TBD LIST
TBC/TBD
Paragraph
Brief description
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1 Scope of the document
The EUSO balloon instrument is described in details in EUSO-DF-INST-204-LAL and EUSO-TS-INST-206. It is an
imaging UV camera that, looking at Nadir, continuously monitors its field of view during the night. The
Instrument is a proof of concept for the JEM-EUSO mission: it shall be able to perform UV background
observations while possibly triggering and observing a few UV tracks induced by cosmic rays. The camera is
auto-triggered with capability to separate the searched signal from the background. As a whole, the EUSO
Balloon Instrument is expected to fulfill the mission requirements described in the EUSO-MS-INST-402-IRAP
V1.0.
This document describes the thermal behavior of the EUSO-Balloon instrument under a range of operational
conditions. The main objective is to show that, from an extreme cold case to an extreme hot case, a suitable
mechanical architecture, can comply with the thermal requirements of the instrument as defined in the
technical specification AD2 (-30°C / +50°C required for the electronic). .
The results of this analysis will also be an input for the thermo-mechanical analysis performed by IRAP, in
order to demonstrate that the safety requirement of “no falling parts” is fulfilled in any case for the
instrument.
2 Documentation
2.1 Aplicable documents
[AD1] EUSO Balloon instrument definition EUSO-DF-INST-204-LAL
[AD2] EUSO Balloon Technical Specification EUSO-TS-INST-206
[AD3] EUSO Balloon Mission Specification EUSO-MS-INST-402-IRAP V1.0
[AD4] Balloon environmental constraints BL-ST-0-3539-CN
[AD5] Specification d’environnement thermique externe projet pilot PILOT-SP-BORD-3035-CN_01
[AD6] EUSO Balloon Instrument Mechanical Architecture EUSO-MA-INST-406-IRAP_V1-2
2.2 Glossary
Main glossary defined in Figures 1, 2, 3 and 4.
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3 Mechanical structure of the instrument
The mechanical structure of the EUSO-Balloon instrument is described in the document [AD6]. Nevertheless,
the main elements relevant to the present analysis are shown in Figure 1.
Figure 1: Schematics of the mechanical structure of the EUSO-Balloon instrument. Relevant to the present
analysis but not shown here is a Styrofoam cover that envelops the 5 external faces of the Instrument Booth.
Relevant to the present analysis, but not shown in Figure 1 is a Styrofoam cover that envelops completely the
4 external lateral faces of the Instrument Booth and, either fully or possibly partially, the upper face
(RADIATOR plate) of the Instrument Booth. This cover is shown schematically in Figure 2.
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Neither the flotation devices nor the external IR camera are shown in Figure 1, but they are irrelevant to the
thermal analysis.
4 Thermal modelling
The document AD5 was used to define general thermal requirements for the EUSO Balloon Instrument.
The thermal environment of a stratospheric balloon is highly variable due to both seasonal and latitudinal
changes, as well as, keeping such variables fixed, due to the unforeseeable presence of clouds under the
instrument at any given time. Therefore, two limiting cases (HOT and COLD) were defined from the detailed
AD5 document to bracket the possible environmental range of conditions:
- COLD CASE: defined by the coldest conditions to be encountered at the Kiruna launching site, with a ground
temperature of -30˚C, a ceiling temperature of -100˚C and a minimum IR flux of 70 W/m2.
- HOT CASE: defined by the hottest conditions encountered at Alice Springs, with ground and ceiling
temperatures of 10˚C and 0˚C respectively and a maximum IR flux of 340 W/m2.
The objective of the thermal architecture is to guarantee that the mechanical architecture is able to keep the
instrument at a safe operational temperature for the electronics, while keeping the mechanical integrity of
the lenses and/or to detect possible critical points and suggest the pertinent modifications.
Two analysis have been used: (a) a simplified analytical model and, (b) a detail numerical model. The first,
shown here in two independent approaches that cross validate each other, has been used to check the
coherence of the numerical model and to give confidence on the approach adopted for the mechanical
architecture. The second one is a more complicated and delicate process, which produces more precise and
detailed results with spatial resolution, as required for example for the thermo-mechanical analysis. The first
results of the numerical analysis have also been produced.
4.1 Analytical modeling
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Figure 2 shows simplified schematics of the EUSO Balloon instrument, which contains its most relevant
features from the thermal point of view, as well as its environment.
Figure 2: schematics of the EUSO-Balloon Instrument showing the most relevant mechanical parts and
environmental parameters for the thermal definition of an analytical model. A note on notation: I [=] W/m2,
while J [=] I x A [=] W.
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TU¥ and TL¥ are some significantly averaged temperatures for the characterization of the radiation received
by the instrument at the upper radiator plate and at the lateral walls ( IV = IVS + IVR and I L respectively).
The incoming radiation from the ground is called I G and, for convenience of treatment it will be divided into
two components: I GD , which impinges normal to the outer lens of the optics and I GL , which impinges the
lateral side of the booth at a properly averaged angle q eff , with respect to the normal of the lateral wall.
The Styrofoam blanket can be seen, fully covering the lateral sides of the Instrument Booth (of total area
ASL ) and partially covering the back plane, leaving exposed an effective radiating area AR = lR2 , at
temperature TR . The total upper area AU is larger than the area of the lenses, AD (cross section of the
Booth), since it includes the thickness dS of the Styrofoam) cover.
The external surface of the Styrofoam, of area AS (and where: AS = ASL + ASU = ASL + ( AU - AR ) ) is
characterized by a temperature TS .
The optical system (PMMA lenses, L1) faces the ground with an area AD and temperature TD . The heat
fluxes radiating from the instrument are FU = FR + FSV , FSL and FD respectively.
Finally, W is the power dissipated by the electronics inside the Instrument Booth, which is at an average
equilibrium temperature TI , the variable to be estimated.
Given this simplified model, two alternative treatments are presented, one that is purely radiative and
another that only takes into account conduction. Both are representative of the steady state of the system
and should be comparable if a consistent representation has been attained.
4.1.1
4.1.1 Pure radiation approach
In this approach, we reduce the model of Figure 2 to the one depicted in Figure 3.
From energy conservation:
W + JL + JGL + JG + JVS + JVR = FSL + FSV + FR + FD
This equation can be simplified significantly under several assumptions.
(1)
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We consider that the only relevant external heat flux is that coming from the ground, IG = IGD + IGL , and
neglect I L and IV respectively ( TL¥ º TU¥ º 0 ). Furthermore, we also assume that the Styrofoam has a certain
reflectivity  . Therefore, IGL ' º IGL ´ (1- r ) .
Figure 3: pure radiation analytical model.
We assume that the external temperature of the radiator (the exposed portion of the back plane) is the same
as the internal temperature of the Instrument Booth, i.e., TR º TI , and that it radiates like a black body at this
temperature: FR = s TI4 ´ AR .
The Styrofoam blanket is assumed to be radiating also as a blackbody at temperature TS , i.e,
FS = s TS4 ´ AS = s TS4 ´ ( ASL + ASV ) .
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An additional complication is the radiation out of the Instrument Booth through the optical system. We can
assume that this flux FD has two components: the proper emission of the lens L1 at temperature TD and the
radiation generated inside the Instrument Booth at temperature TI and transmitted outward through the
optical system. The latter is difficult to characterize, but can be written as a fraction
(
maximal case). In that case we can write, FD = s TD + bT
4
4
I
)A
b of a blackbody (the
D.
With these assumptions, eq. 1 can be written in term of TI as:
ìïì 1
üï
ü
1
TI = íí {W + IG éë AD + ASL (1- r ) cosq eff ùû} - TS4 ( ASL + e AD ) - TD4 AD ý
ý
þ (1- e + b ) AD ïþ
ïîîs
1/4
where
(2)
r is the reflectance of the Styrofoam.
Equation 2 can be further simplified if we neglect the radiation emitted by the Styrofoam ( TS » 0 ) and by the
outer lens ( TD » 0 ) when compared to that emitted by the radiator and the one emerging from the
Instrument Booth through the Optical Bench:
ïì W + I G éë AD + ASL (1- r ) cosq eff ùûïü
TI = í
ý
s (1- e + b ) AD
ïî
ïþ
1/4
(3)
Figure 4 shows the expected temperature inside the Instrument Booth as a function of the Styrofoam
covering factor , a design parameter for the mechanics, for a perfectly reflecting Styrofoam (white painted,
r » 1), for two extreme cases of the parameter . Both cases, COLD and HOT are shown. The green band in
the figure marks the interval -30 £ TI £ +50o C , which is the thermal operational range driven by the
electronics [AD2]. Figure 5 is the same as Figure 4, but for clarity, only the case =50% is shown, which is
likely to be a conservative value for .
It can be seen that, under very general conditions, both COLD and HOT cases can be accommodated to satisfy
the requirements just by varying the Styrofoam covering factor .
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Figure 4: Internal temperature inside the Instrument Booth for =1 (white painted Styrofoam), and two
extreme  cases (1% and 90%) as a function of the Styrofoam covering factor, e £1
. The green band
shows an acceptable operation range.
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Figure 5: same as Fig. 4, but for = 50%, which is probably a good guess about the true value. It can be seen
that the operation range requirement (green band), can be globally met by the strategy of changing the
Styrofoam covering factor.
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4.1.2
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Conduction approach
An independent validation of the temperature ranges obtained in the previous section, comes from the fact
that CNES balloon division has some measurements of the external skin of other instruments under cold
conditions, and they are of the order of -70oC.
Figure 5: Simplified model using pure conduction combined with available equivalent skin temperatures for
other stratospheric experiments.
The conduction flux across a wall with a temperature difference, DT , can be expressed as:
F=
lA
d
(TI - TS )
where  is the thermal conductivity and R =
d
is the so called resistivity. Values of interest of  are:
lA
lStyro = 0.03
lPMMA = 0.21
lair (low pressure) = 0.025
Taking dS = 200 mm , d1 = 270 mm , dL = 8 mm y W =120 W , an internal temperature TI » 32 oC is
obtained, in rough agreement with the values presented in the previous section.
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4.2 Numerical modeling
A detailed thermal numerical model, based on a somehow simplified model of the mechanical architecture,
has been achieved in a close collaboration between UNAM-México and CNES.
4.2.1
Model
The model has been developed in FEMAP 10.1.1, and includes: (i) internal and external convection, (ii)
internal and external radiation, (iii) heat dissipation of the equipment and (iv) conduction and thermal
contacts between parts. Parameters used in its definition are summarized in the Tables 1 to 4 below.
Two stationary cases are considerd so far (see, Table 4): (i) COLD environment flight at 3 hPa and (ii) HOT
environment flight 3hPa. These cases bracket all the range of possible environmental conditions to be
encountered by the instrument.
Table 1: Thermal properties of the materials.
Material
Aluminum
Fibrelam
Styrofoam
PMMA
Fibreglass
Conductivity [W/m2 C]
210
0.062
0.028
0.21
0.04
Table 2: Thicknesses of the materials used.
Material
Aluminum
Fibrelam
Styrofoam
Thickness [mm]
3
10
250
Emissivity
0.15
0.89
0.75
0.85
0.90
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PMMA
8
Table 3: Power dissipation.
Subsystem
DP
HUB
ICDV
PDM
SIREN
Power [W]
50
3
10
60
13
Table 4: Extreme ambient cases, COLD and HOT defined from the PILOTE mission used as case studies for
EUSO balloon.
Cases
Ground
temperature
[˚C]
Ceiling
Downward
temperature [˚C] Infrared
flux [W/m²]
Upward
Infrared flux
[W/m²]
COLD
-30
-100
0
70
HOT
10
0
0
340
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Modelizations have been performed without and with a radiation window on top of the Instrument Booth
(see, Figure 6 left and right respectively).
Figure 6: Global view of the model (grid) of two versions of the mechanical structure: (i) Left: upper plane
completely covered by Styrofoam, =1, and (ii) Right, Styrofoam with a window for increased radiation, <1.
The three lenses and part of the electronic shelf inside the Instrument Booth can also be seen in the left
model.
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Figure 7: Detail of the parts as they are included in the thermal numerical model inside the Instrument Boot,
including the Styrofoam outer layer and lenses L2 and L3.
EUSO-BALLOON Thermal analysis
50
P12_LENSES
P11_LENS-FRAMES
P10_BACKLATCH
P9_BATTERY
P8_PDM
500
P7_ICDV
500
P6_HUB
P3_SIREN
P5_DP
P2_FIBRELAM
P4_PDMBOARD
P1_STYROFOAM
P2_FIBRELAM
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P1_STYROFOAM
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50
500
500
P3_SIREN
P4_PDMBOARD
500
500
500
500
500
500
P5_DP
P6_HUB
P7_ICDV
P8_PDM
P9_BATTERY
P10_BACKLATCH
P11_LENS-FRAMES
500
P12_LENSES
Figure 8: Matrix of thermal contact (W/m2 C) for the thermal model. For nomenclature see Fig. 7.
4.2.2
Results
Figures 9 and 10 show the results for the COLD and HOT cases respectively, in one particular realization of the
numerical model in which the upper part of the gondola has a window leaving an enhanced radition surface.
The Styrofoam covering fraction usewas ALU=0.9 of the whole Styrofoam upper surface (i.e, =0.75 in the
analytical model).
It can be seen from Figure 9 that for the COLD case, the thermal requierement can be met even without
problems.
The HOT case, on the other hand, requires more fine-tunning. The two important parameters are Styrofoam
thicknes and covering factor . The Styrofoam thicknes have been fixed here to 250 mm in order to improve
boyance of the gondola. With the Styrofoam in its present configuration the PDM is too hot inside.
Nevertheless, a suitable range of temperatures can be found for both PDM and DP by playing with the
thickness of the Styrofoam and the covering factor .
Furthermore, it must be noted also that the thick layer of Styrofoam is used for boyance. However, the hot
case where the thermally insulating effect of the Styrofoam is too large, corresponds to Alice Spring,
Australia, where buyance is actually not necessary.
For the first flight in Timmings, conditions will be intermediate and therefore easy to accommodate
thermally.
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Figure 9: COLD case for a Styrofoam covering fraction ALU=0.9 of the whole Styrofoam upper surface (i.e,
=0.75 in the analytical model).
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Figure 10: HOT case for a Styrofoam covering fraction ALU=0.9 of the whole Styrofoam upper surface (i.e,
=0.75 in the analytical model).
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The numerical model still needs some improvements at present, as well as further tuning of design
parameters, such as Styrofoam thickness and upper window size for the hot case in particular. These
enhancements will be implemented later, in order to find the best design (windows’s size) for the launch site
thermal environment when this will be defined.
5 Conclusions
The thermal studies presented here, demonstrate clearly that the mechanical design is able to accommodate
the worst cold and hot extreme environmental conditions. That is, the design is adequate whatever the
launch conditions.
The tuning of design parameters, such as Styrofoam thickness and upper window size for the hot case in
particular, will be done when the environmental conditions are be clearly known (first semester 2013), and
after that for each particular flight.
Furthermore, this model can also be used later to find an optimized configuration in case of long duration
flight