Satellite achieving forced convection of a refrigerant fluid with evaporation-condensation between several structural panels and dissipative equipment
A refrigerant fluid circulation loop with forced convection and evaporator-condenser heat exchangers addresses inefficiencies in satellite thermal management, enhancing heat evacuation and flexibility in geostationary satellites.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- THALES SA
- Filing Date
- 2024-06-10
- Publication Date
- 2026-06-12
AI Technical Summary
Existing thermal management systems in geostationary satellites face inefficiencies due to complex heat pipe architectures, limited thermal coupling, and inflexibility in accommodating changes in payload layout, leading to increased costs and schedule impacts.
A satellite design incorporating a refrigerant fluid circulation loop with forced convection, featuring evaporator-condenser heat exchangers on radiator panels, allowing flexible arrangement and efficient heat dissipation.
Enhances heat evacuation efficiency, maximizes payload capacity, and provides flexible layout options for dissipative equipment, reducing the impact of layout changes on satellite performance and cost.
Smart Images
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Abstract
Description
Title of the invention: Satellite achieving forced convection of a refrigerant fluid with evaporation-condensation between several structural panels and dissipative equipment
[0001] DOMAIN
[0002] The present invention relates to a satellite, in particular a telecommunications satellite, for example geostationary, comprising a plurality of structural panels defining an internal volume of the satellite, and dissipative equipment producing heat.
[0003] The invention also relates to a method for thermal control of such a satellite. EARLIER ART
[0004] When dissipative equipment is placed on the inner face of a structural panel, it is known to use heat pipes to distribute the heat produced onto the structural panel, which also acts as a radiator panel. The heat is conducted transversely to an outer face of the structural panel and is dissipated by radiation.
[0005] In the case of a geostationary satellite, a so-called "north" face, in reference to the Earth's axis of rotation, is constantly illuminated by the sun in summer and never in winter. For the south face, the situation is reversed. In other words, the illumination of these faces is periodic, with a period equal to the Earth year. For the east and west faces, located respectively on the side of the rising and setting sun, the illumination reverses twice a day. The illumination is therefore periodic, with a period equal to one Earth day.
[0006] Consequently, it is also known to thermally couple two structural radiator panels using coupling heat pipes (also known as crossing heat pipes), for example, between the north and south panels of a geostationary satellite. However, the installation of coupling heat pipes is not always feasible, and the thermal coupling between radiator panels is not optimal. This limits the satellite's ability to dissipate the heat generated by the payload.
[0007] Heat pipes perform evaporation (heat sink) and condensation (heat source) within the same enclosure. Furthermore, the sizing and layout of heat pipe networks (primary network and coupling or crossing network) remains complex. Their architecture is not easily adaptable to changes in the payload (equipment layout, local heat dissipation), and coupling possibilities are limited due to the internal dimensions of the satellite.
[0008] In fact, the sizing of the heat pipes is closely linked to the layout of the equipment in the satellite, which itself takes into account thermal constraints, even if this degrades the performance of the radio frequency chain. Thus, all Changes in this arrangement or in the dissipation properties of the equipment during a satellite development program call into question the architecture of the heat pipes, which has a significant negative impact on the costs and / or schedule of the program.
[0009] In the end, the architecture of heat pipes with coupling between radiator panels is relatively inefficient, due to the difficulty of arranging coupling heat pipes linked to the internal bulk of the satellite and the need to have several heat pipes in series to compensate for the limited transport distance of a heat pipe, while the coupling path between the panels is on the order of several meters.
[0010] More recently, forced circulation loops of a refrigerant fluid have been implemented in high-power telecommunications satellites. These loops achieve evaporation of the fluid at one or more internal shelves of the satellite carrying dissipative equipment, followed by condensation of the fluid in dedicated radiators, for example, deployable ones, which do not support dissipative equipment. This has increased the satellite's capacity to dissipate heat from the dissipative equipment.
[0011] However, this solution is not satisfactory if the compactness of the satellite requires dissipative equipment to be fitted on the radiator panels themselves.
[0012] One object of the invention is to remedy all or part of the above disadvantages, by providing a satellite that is both efficient from the point of view of the evacuation of heat generated by dissipative equipment, so as to be able to maximize the payload, while being flexible from the point of view of the layout (arrangement) and / or nature of the dissipative equipment. Summary of the invention
[0013] The invention relates to a satellite, in particular a telecommunications satellite, comprising:
[0014] - a plurality of structural panels defining an internal volume of the satellite, said plurality comprising at least two radiator panels having an external face adapted to radiate heat into space, and an internal face, the two radiator panels being adapted to conduct heat from the internal face to the external face and having distinct orientations from each other, the orientations being defined respectively by the external face,
[0015] - dissipative equipment intended to produce heat, and
[0016] - a refrigerant fluid circulation loop,
[0017] the loop comprising:
[0018] - at least one pumping system to achieve forced convection of the fluid in the loop,
[0019] - at least one condenser adapted to achieve at least partial condensation, preferably total, of the fluid, the fluid transferring heat to the condenser, the condenser being adapted to dissipate said heat into space, and
[0020] - at least two evaporation-condensation heat exchangers for the fluid, each of the two heat exchangers extending respectively against the inner face (64) of one of the two radiator panels,
[0021] each of the two heat exchangers comprising:
[0022] - a first part in thermal contact respectively with at least one of the dissipative equipment, the first part being suitable and intended to receive heat from said at least one of the dissipative equipment by conduction and to transfer heat to the fluid by conduction-convection to vaporize a fraction of the fluid, and
[0023] - a second part in thermal contact with said inner face, the second part being suitable and intended to receive heat from the fluid by conduction-convection to condense a fraction of the fluid, the second part being suitable and intended to transmit heat to the inner face by conduction.
[0024] According to other advantageous aspects of the invention, the satellite comprises one or more of the following features, taken individually or in all technically possible combinations:
[0025] - the satellite is intended to be placed in a geostationary orbit, the satellite having a north face and a south face with respect to the Earth's axis of rotation, the north face and the south face being defined respectively by the two radiator panels;
[0026] - the loop is configured to achieve parallel fluid circulation in the two heat exchangers;
[0027] - each of the two heat exchangers comprises a plurality of pipes mounted in series and / or in parallel with each other and adapted to be traversed by the fluid, the first part and the second part of each of two heat exchangers being formed by walls of the pipes;
[0028] - several of the dissipative equipment are in thermal contact with several of the piping for each of the two heat exchangers;
[0029] - the pipes are distributed on the inner face of each of the two panels radiators;
[0030] - the plurality of pipes comprises several branches mounted in parallel from each other, each of the branches defining a winding path on the inner face of each of the two radiator panels;
[0031] - the loop includes at least one evaporator adapted to transfer heat to fluid and to vaporize a fraction of the fluid, the heat coming from at least one of the dissipative devices, said at least one of the dissipative devices being fixed to an internal shelf of the satellite or included in an external antenna of the satellite; and
[0032] - the fluid includes ammonia.
[0033] The invention also relates to a method for thermal control of a satellite as described above, comprising:
[0034] - heat production by dissipative equipment,
[0035] - forced convection of the fluid in the loop by the pumping system,
[0036] - heat conduction from at least one of the dissipative equipment to the first part of each of the two heat exchangers,
[0037] - a transfer of heat to the fluid by conduction-convection through the first part of each of the two heat exchangers with vaporization of a fraction of the fluid, and heat reception from the fluid by conduction-convection by the second part of each of the two heat exchangers with condensation of a fraction of the fluid,
[0038] - heat conduction from the second part of each of the two panels radiators on the inner face, and from the inner face to the outer face, and
[0039] - heat radiation from the outer face of each of the two panels radiators towards space. Brief description of the drawings
[0040] The invention will become clearer upon reading the following description, given solely by way of non-limiting example, and made with reference to the accompanying drawings, in which:
[0041] [Fig-1] [Fig.1] is a schematic view of a satellite according to the invention, in orbit around the earth,
[0042] [Fig.2] [Fig.2] is a diagram showing a fluid circulation loop refrigeration unit of the satellite shown in [Fig. 1], as well as structural panels, including two radiator panels, and dissipative equipment of the satellite,
[0043] [Fig. 3] [Fig. 3] is a schematic, perspective view showing side-by-side the two radiator panels shown in Figures 1 and 2, as well as two heat exchangers of the loop shown in [Fig. 2], and dissipative equipment in thermal contact with the two heat exchangers, and
[0044] [Fig.4] [Fig.4] is a schematic, sectional view showing one of the two radiator panels shown in figures 1 to 3, as well as a portion of one of the two heat exchangers and dissipative equipment shown in [Fig.3]. DETAILED DESCRIPTION Satellite
[0045] With reference to [Fig.1], an artificial satellite 10 according to the invention is described.
[0046] The satellite 10 is in orbit around the Earth 12, whose proper axis of rotation 14 has been shown, the Earth moving around the Sun 16 in the plane of the ecliptic 18.
[0047] The satellite 10 is advantageously geostationary, that is to say, substantially fixed in a frame of reference linked to the Earth. The satellite 10 moves relative to the Sun in an equatorial plane 20 of the Earth, flies over the same point on the Earth's equator, and always presents the same face 22 towards the Earth.
[0048] Satellite 10 is for example a telecommunications satellite, comprising equipment 24 specific to this function, known in themselves and which will not be detailed.
[0049] According to variants not shown, the satellite 10 is not geostationary and / or is not a telecommunications satellite, but fulfills one or more classic missions for an artificial satellite in Earth orbit, such as an observation mission.
[0050] Depending on the time of year, the Sun 16 defines a variable angle a with the equatorial plane 20, which can range from +23°27' to -23° 27' (angle of the ecliptic).
[0051] The satellite 10 comprises a plurality of structural panels 26 defining an internal volume 28, dissipative equipment 30 (in particular Figures 2 and 3) producing heat when in operation, and a loop 32 ([Fig.2]) for circulating a refrigerant fluid 34 ([Fig.4]).
[0052] Being geostationary in the example, the satellite 10 notably has a face north 36 (lit by the Sun 16 only in summer) and a south face 38 (lit only in winter) with respect to the Earth's axis of rotation, as well as an east face 40 (on the side of the rising Sun 16) and a west face 42 (on the side of the setting Sun).
[0053] In the example, the satellite 10 comprises a substantially parallelepiped-shaped body 44 defining the aforementioned faces 22, 36, 38, 40, 42 in a strict manner.
[0054] If the body 44 is not parallelepiped, the aforementioned faces, in particular north and south, can be defined by analogy, for example as deviating by less than 20° from strictly north and south faces.
[0055] In the example, the satellite 10 also includes two shelves 46, 48 ([Fig.2]), an external antenna 50, and two external radiators 52, 54 advantageously deployable relative to the body 44.
[0056] According to variants (not shown), the satellite 10 includes only one external radiator, and / or one or more than two shelves. Satellite components
[0057] The structural panels 26 comprise two radiator panels 56, 58, defining for example the north face 36 and the south face 38 respectively.
[0058] According to variants not shown, the radiator panels 56, 58 are more than two, and / or define faces other than the north and south faces.
[0059] As can be seen in [Fig.4], each of the two radiator panels 56, 58 has an external face 60 adapted to radiate heat (arrows Fl) towards the space 62, an internal face 64, and for example a honeycomb structure 66 extending between the external face 60 and the internal face 64.
[0060] The two radiator panels 56, 58 are adapted to conduct heat from the inner face 64 to the outer face 60.
[0061] The two radiator panels 56, 58 have distinct orientations from each other, defined respectively by the external faces 60. In the example, the two radiator panels 56, 58 are oriented north and south respectively.
[0062] The external antenna 50 constitutes one of the dissipative equipment 30.
[0063] Shelves 46, 48 extend into the interior volume 28 and carry some of the dissipative equipment 30.
[0064] The two external radiators 52, 54 are adapted to dissipate heat into space 62. Loop
[0065] The loop 32 includes a pumping system 66, two condensers 68, 70 for example formed by the two external radiators 52, 54, and two heat exchangers 72, 74 for evaporation-condensation of the fluid 34, each of the two heat exchangers extending respectively against the inner face 64 of one of the two radiator panels 56, 58.
[0066] Advantageously, the loop 32 includes three evaporators 76, 78, 80 included for example in the two shelves 46, 48 and in the external antenna 50.
[0067] According to variants not shown, the loop 32 comprises a single condenser or more than two condensers.
[0068] According to variants not shown, the loop 32 comprises one, or two, or more than three evaporators.
[0069] According to other variants, the loop 32 includes more than two evaporation-condensation heat exchangers, for example located on the inner face of other radiator panels.
[0070] The loop 32 is advantageously configured to achieve a circulation of the fluid 34 in parallel in the two heat exchangers 72, 74 ([Fig.3]).
[0071] According to an unrepresented variant, the circulation takes place in series in the two heat exchangers 72, 74.
[0072] Advantageously, the condenser(s) 68, 70 are located downstream (from the point of view of the circulation of the fluid 34) of the two heat exchangers 72, 74, themselves located downstream of the evaporator(s) 76, 78, 80.
[0073] The pumping system 66 is adapted to achieve forced convection of the fluid 34 in the loop 32, in particular in the heat exchangers 72, 74.
[0074] The evaporators 76, 78, 80 are adapted to transfer heat to the fluid 34 and to vaporize a fraction of the fluid 34, the heat coming from the dissipative equipment 30 concerned, in the example the external antenna 50 and those located on the shelves 46, 48.
[0075] The fluid 34 is adapted to be two-phase in a range of pressures and a range of temperatures corresponding to normal operation of the satellite 10.
[0076] Advantageously, the fluid 34 is two-phase from the moment it enters the heat exchangers 72, 74.
[0077] The fluid 34 comprises, for example, ammonia, advantageously at least 95% by mass.
[0078] The condensers 68, 70 are adapted to achieve at least partial, preferably total, condensation of the fluid 34, the fluid 34 transferring heat to the condensers, the condensers being adapted to evacuate said heat to space 62. Heat exchangers
[0079] The two heat exchangers 72, 74 are advantageously analogous to each other.
[0080] Each of the two heat exchangers 72, 74 comprises a first part 82 ([Fig.4]) in thermal contact respectively with at least one of the dissipative equipment 30, and a second part 84 in thermal contact with the inner face 64 of one of the two radiator panels 56, 58.
[0081] Each of the two heat exchangers 72, 74 comprises, for example, a plurality of pipes 86 mounted in series and / or in parallel with each other and adapted to be traversed by the fluid 34, the first part 82 and the second part 84 of each of the two heat exchangers 72, 74 being formed by walls 88 of the pipes 86.
[0082] The first part 82 is suitable and intended to receive heat from said at least one of the dissipative equipment 30 by conduction, and to transfer heat to the fluid 34 by conducto-convection so as to vaporize a fraction of the fluid 34.
[0083] The second part 84 is suitable and intended to receive heat from the fluid 34 by conduction-convection so as to condense a fraction of the fluid 34, and to transmit heat to the inner face 64 by conduction.
[0084] The pipes 86 are advantageously distributed on the inner face 64 of each of the two radiator panels 56, 58.
[0085] The plurality of conduits 86 comprises, for example, several branches 90 ([Fig.3]) mounted in parallel with each other, each branch defining, for example, a sinuous path on the inner face 64 of each of the two panels radiators 56, 58. The pipes 86 wind advantageously on the inner face 64, in the example forming right angles.
[0086] Advantageously, several of the dissipative equipment 30 are in thermal contact with several of the pipes 86 of the two heat exchangers 72, 74, for example by straddling several branches 90. Pumping system
[0087] The pumping system 66 includes, for example, electronically controlled pumps 92, 94, a particle filter 96, and a booster tank 98.
[0088] The pumping system 66 includes a plurality of valves, for example manual valves 100 and a throttling valve 102.
[0089] Advantageously, the pumping system 66 includes a temperature sensor 104, and two pressure sensors 106, 108 located upstream and downstream of the pumps 92, 94.
[0090] In an alternative (not shown), the pumping system 66 comprises only one pump. Functioning
[0091] The operation of the satellite 10 derives from its structure and will now be briefly described to illustrate a method according to the invention.
[0092] If it is summer, the north face 36 of the satellite 10 is illuminated by the Sun 16, while the south face 38 is in shadow.
[0093] The dissipative equipment 30 is in operation and produces heat.
[0094] The pumping system 66 causes a forced convention of the fluid 34 in the loop 32.
[0095] The fluid 34 is partially vaporized in the evaporators 76, 78, 80 arranged on the shelves 46, 48 and in the external antenna 50, which absorbs heat generated by some of the dissipative equipment 30.
[0096] The fluid 34 enters each of the two heat exchangers 72, 74, while it is advantageously already two-phase.
[0097] In each of the two heat exchangers 72, 74, the first part 82 receives, by conduction, heat from the dissipative equipment 34 with which it is in thermal contact.
[0098] The first part 82 gives up heat to the fluid 34 by conduction-convection and this vaporizes a fraction of the fluid 34. The second part 84 receives heat from the fluid 34 by conduction-convection and this condenses a fraction of the fluid 34.
[0099] It is noted that the fluid 34 is, concomitantly, partially vaporized by the first part 82 and partially condensed by the second part 84, which creates a heat transfer from the first part 82 to the second part 84 via the fluid 34.
[0100] The fluid 34 carries heat away from the two heat exchangers if its vapor quality increases during its passage through the heat exchangers 72, 74. Conversely, the fluid 34 brings heat to the heat exchangers 72, 74 if its vapor quality decreases during its passage.
[0101] The inner face 64 of each of the two radiator panels 56, 58 receives heat from the second part 84 by conduction, and heat is conducted from the inner face 64 to the outer face 60.
[0102] Heat is radiated (arrows Fl) towards space 62 by the outer face 60 of each of the two radiator panels 56, 58.
[0103] The fluid 34 is at least partially, preferably totally, condensed in the two condensers 68, 70 before returning to the pumping system 66. Advantages
[0104] Thanks to the characteristics described above, the satellite 10 is both efficient with regard to the evacuation of heat generated by the dissipative equipment 30, so as to be able to maximize the payload, while being flexible with regard to the arrangement or nature of this dissipative equipment.
[0105] Indeed, the two structural panels 26 north and south in the example become radiator panels 56, 58 capable of supporting some of the dissipative equipment 30. The loop 32 advantageously establishes a thermal coupling between the two radiator panels 56, 58, which makes it possible to benefit from the advantageous thermal conditions (low temperatures) which prevail in at least one of them (north face in winter for example).
[0106] Each of the heat exchangers 72, 74 carries out evaporation and condensation at the level of the same radiator panel 56, 58, like a primary network of heat pipes, but with a fluid 34 circulating in forced convection mode and not by capillarity, which increases the efficiency of heat exchanges and flexibility.
[0107] The heat exchangers 72, 74 are connected to each other according to a series or parallel strategy consistent with hydraulic constraints (limiting pressure losses). The mass flow rates per unit area, which can be chosen to be much higher than in a heat pipe, guarantee good heat exchange by evaporation and condensation.
[0108] Each of the heat exchangers 72, 74 provides local power extraction from the dissipative equipment 30 through evaporation and ensures a regular release of these various heat sources through condensation along the path of the fluid 34 in contact with each of the radiator panels 56, 58. The dissipative equipment 30 can be freely arranged in contact with each of the heat exchangers 72, 74, even concentrating them in one area for specific needs in radio frequency, and moved over the course of the satellite development program 10. The flexibility thus obtained with regard to the arrangement of dissipative equipment 30 constitutes a fundamental advantage.
[0109] The invention makes it possible to equip two structural panels and radiators 56, 58 facing different environments, advantageously the north and south panels of a telecommunications satellite, and to hydraulically couple these networks, advantageously in parallel, in order to maximize the heat rejection capacity of the assembly. Each radiator panel 56, 58 is no longer sized independently of the others by considering its worst-case external environment (for example, facing the Sun, or constantly illuminated), but by taking a more favorable average case (for example, one of the panels is in the shade while the other faces the Sun).
[0110] Advantageously, the fluid 34 can remain in fairly similar states between the inlet and outlet of the heat exchangers 72, 74, especially if they are mounted in parallel with each other, the flow of fluid 34 then being a vector for the exchanges by evaporation and condensation which more or less compensate each other.
[0111] Furthermore, it is entirely possible to have an asymmetrical distribution of the dissipative equipment 30 within one of the radiator panels 56, 58 and between them, which is not the case with heat pipes. Indeed, circulation in the heat exchangers 72, 74 allows for better heat distribution within a radiator panel, and a greater quantity of steam can be produced in one panel compared to the other (or the others).
[0112] Furthermore, the addition or removal of one of the dissipative equipment 30 or the increase in its level of dissipation has little effect on the overall thermal operation and the general sizing of the satellite 10, which is not the case with a network of heat pipes.
Claims
1. Demands Satellite (10), particularly for telecommunications, comprising: - a plurality of structural panels (26) defining an internal volume (28) of the satellite (10), said plurality comprising at least two radiator panels (56, 58) having an external face (60) adapted to radiate heat into space (62), and an internal face (64), the two radiator panels (56, 58) being adapted to conduct heat from the internal face (64) to the external face (60) and having distinct orientations from each other, the orientations being defined respectively by the external face (60), - dissipative equipment (30) intended to produce heat, and - a loop (32) for circulating a refrigerant fluid (34), the loop (32) comprising: - at least one pumping system (66) to achieve forced convection of the fluid (34) in the loop (32), - at least one condenser (68) adapted to achieve at least partial, preferably total, condensation of the fluid (34), the fluid (34) transferring heat to the condenser (68), the condenser (68) being adapted to dissipate said heat into space (62), and - at least two evaporation-condensation heat exchangers (72, 74) for the fluid (34), each of the two heat exchangers (72, 74) extending respectively against the inner face (64) of one of the two radiator panels (56, 58), each of the two heat exchangers (72, 74) comprising: - a first part (82) in thermal contact respectively with at least one of the dissipative equipment (30), the first part (82) being suitable and intended to receive heat from said at least one of the dissipative equipment (30) by conduction and to transfer heat to the fluid (34) by conduction-convection to vaporize a fraction of the fluid (34), and - a second part (84) in thermal contact with said inner face (64), the second part (84) being suitable and intended to receive heat from the fluid (34) by conduction-convection to condense a fraction of the fluid (34), the second part (84) being suitable and intended to transmit heat to the inner face (64) by conduction.
2. Satellite (10) according to claim 1, intended to be placed on a geostationary orbit, the satellite (10) having a north face (36) and a south face (38) with respect to the Earth's axis of rotation (14), the north face (36) and the south face (38) being defined respectively by the two radiator panels (56, 58).
3. Satellite (10) according to claim 1 or 2, wherein the loop (32) is configured to achieve a parallel circulation of the fluid (34) in the two heat exchangers (72, 74).
4. Satellite (10) according to any one of claims 1 to 3, wherein each of the two heat exchangers (72, 74) comprises a plurality of pipes (86) mounted in series and / or in parallel with each other and adapted to be traversed by the fluid (34), the first part (82) and the second part (84) of each of the two heat exchangers (72, 74) being formed by walls (88) of the pipes (86).
5. Satellite (10) according to claim 4, wherein several of the dissipative equipment (30) are in thermal contact with several of the pipes (86) of each of the two heat exchangers (72, 74).
6. Satellite (10) according to claim 4 or 5, in which the pipes (86) are distributed on the inner face (64) of each of the two radiator panels (56, 58).
7. Satellite (10) according to any one of claims 4 to 6, wherein the plurality of pipes (86) comprises several branches (90) mounted in parallel with each other, each of the branches (90) defining a sinuous path on the inner face (64) of each of the two radiator panels (56, 58).
8. Satellite (10) according to any one of claims 1 to 7, wherein the loop (32) comprises at least one evaporator (76) adapted to transfer heat to the fluid (34) and to vaporize a fraction of the fluid (34), the heat coming from at least one of the dissipative devices (30), said at least one of the dissipative devices (30) being fixed on an internal shelf (46, 48) of the satellite (10) or included in an external antenna (50) of the satellite.
9. Satellite (10) according to any one of claims 1 to 8, wherein the fluid (34) comprises ammonia.
10. A method for thermally controlling a satellite (10) according to any one of claims 1 to 9, comprising: - heat production by dissipative equipment (30), - forced convection of the fluid (34) in the loop (32) by the pumping system (66), - heat conduction from at least one of the dissipative equipment (30) to the first part of each of the two heat exchangers (72, 74), - a transfer of heat to the fluid (34) by conduction-convection through the first part (82) of each of the two heat exchangers (72, 74) with vaporization of a fraction of the fluid (34), and a reception of heat from the fluid (34) by conduction-convection through the second part (84) of each of the two heat exchangers (72, 74) with condensation of a fraction of the fluid (34), - heat conduction from the second part (84) of each of the two radiator panels (56, 58) to the inner face (64), and from the inner face (64) to the outer face (60), and - heat radiation from the external face (60) of each of the two radiator panels (56, 58) into space.