The ARIEL Thermal Control Hardware (TCHW) includes all passive or active components that are used to reach and maintain the operating temperatures of the PLM units within their required ranges and stability.
The general list of the PLM TCHW items is composed by:
PLM passive thermal control system
The V-Grooves (VG) system design is a key issue of the ARIEL thermal performance as they represent the first cooling stage of the PLM. VGs are high efficiency, passive radiant coolers, whose performances in a cold radiative environment such as L2 has been definitely demonstrated by the Planck mission . The ARIEL V-Grooves system consists in a set of three specular shields working in sequence, composed by six semi-circular panels arranged in a “V-shaped” configuration, each one with a specific inclination with respect to the S/C X-Y plane (Fig. 5). Their objective is to intercept radiative and conductive heat leaks from warmer sections of the S/C and reject it to deep space after multiple reflections between each VG pair.
The VGs are light-weight sandwich panels with Aluminium 5056 alloy hexagonal honeycomb core (3/16–5056-0.0007) and Al 1085 0.25 mm thick external skins. Each VG shield is tilted by a certain angle with respect to the adjacent one, creating a divergent radiative path for the reflected thermal rays. Figure 5 shows a very simplified schematic of this concept. In the present baseline of the ARIEL PLM thermal configuration, the three VG angles are 7°-14°-21°. The three radiators are mechanically supported on the SVM interface by means of a total of 8 low conductivity struts. These struts are hollow tubes made of GFRP with Ti alloy (Ti6Al4V) end fittings.
The thermo-optical properties of the VGs are of essential importance since they are the key parameters for thermal isolation and heat rejection to space. To achieve the required performances, VGs surfaces must have a very low emittance coating, a high reflection/mirroring material needed to reflect thermal radiation. A Vapour Deposited Aluminium (VDA) layer can reach an emissivity in the IR band of less than 0.05, as measured on the Planck PLM , with a specular reflectivity as high as 95%. The upper surface of VG3, exposed to the cold sky, is coated with high emissivity black paint (e.g. MAP PUK, or Aeroglaze Z306). It has been demonstrated experimentally that for this type of coatings the IR emissivity at cryogenic temperature can decrease by a factor of 2. For this reason, in order to maintain the required emissivity at low temperatures, the properties and performances of several possible solutions to increase the emissivity of coatings at low temperatures are under investigation. As the VG3 heat rejection capability is key to the whole PLM thermal performances, in order to maximize the radiative coupling to deep space at low temperatures, a black painted open aluminium honeycomb configuration is baselined. The Planck mission has demonstrated that this solution is able to maintain the surfaces IR emissivity well above the required value, ε > 0.8.
The harness from the warm electronics in the SVM to the cold units should be thermally coupled to each VG in order to reject its conductive load and minimize the leak to the coldest stages. In the present baseline configuration, as already reported, this is true only for the thermal and M2M control harness as the detectors control cables exchange heat only with the first V-Groove and not with the other two. This solution is required to maintain the channels SIDECAR above their safe operating temperature.
The JT cooler piping dissipates the gas pre-cooling load on the V-Grooves by means of a heat exchanger located on each stage. The assumed loads on the three V-Grooves for thermal analysis purposes are, at this stage, 65–180, 20–120 and 40–140 mW (Cold – Hot Case), respectively for VG1, VG2 and VG3. In these conditions, the present VG thermal design is sufficient to limit the temperature gradient over the shields to less than 3 K between the hot spots (cooler & harness heat exchangers) and the rest of the panel. The actual precooling loads depend on the required mass flow and on the VGs temperature. For this reason, a more realistic assumption, based on analytical fit functions, will be integrated in the payload Thermal Mathematical Model (TMM) in the next phase.
The bipods (Fig. 6) are hollow tubes made of low conductivity material. At this stage, CFRP is baselined as the tubes material with a diameter of 48 mm and 2 mm wall thickness. Ti alloy (Ti6Al4V) end fittings connect them to the Aluminium alloy feet and heads. The rear and front bipods leg length is respectively ~625 mm and ~ 400 mm.
The main task of the bipods design is to maximise the mechanical support and performances of the whole Telescope Assembly while limiting the thermal conductance across the stages. Each bipod leg tube is filled with rigid foam (or IR filters at various temperature stages) to avoid internal reflections, minimizing any possible radiative coupling between the warm and cold ends. During flight operations they will always face the cold sky so their external surface shall be black painted to maximize heat rejection to space. This configuration ensures a very limited heat leak to the TOB across the full length of the bipods (on the order of few mW).
The bipods, as well as the VG struts, must be thermally coupled to the V-Grooves, to minimize heat leaks to the PLM colder stages. Each bipod leg is connected to each VG by means of thermal straps, as shown in Fig. 6.
Telescope baffle and instrument radiator
The V-Groove-based design provides a cold and stable environment for the telescope, instruments and cryocooler cold end. In this cold volume all main surfaces exposed to space work as radiating units to increase performances and margins of the PLM passive design. The two main surfaces operating as passive stage reference for the instrument and the telescope are the Instrument Radiator and the top half of the Telescope Baffle. The exposed areas of the Optical Bench can also help in this direction, providing an extra heat rejection surface.
The baseline thermo-optical design of the external surfaces is based on high IR emissivity coatings (black paint, such as MAP PUK or Aeroglaze Z306, with εIR ≥ 0.9 at room temperature) to maximize radiative coupling to cold space. As already mentioned, further studies are in progress to investigate the expected behaviour of the IR coatings at cryogenic temperatures and to evaluate possible solutions for an improved emissivity. For this reason, at this stage, it is assumed that the thermo-optical design of the Instrument Radiator external surface is based on a black painted open honeycomb structure. The internal surface of the Baffle, the TOB and the Instrument Radiator, facing the telescope and the instrument cavity, is black painted also for stray-light control purposes.
At present, the Instrument Radiator is mounted directly on the TOB and operates as a lid for the Instrument Cavity, bolted to the bench. Its function is to operate as an efficient radiating surface to help maintaining the temperature of the TOB and the units on it in their operating range.
In the present thermal configuration, the radiator is able to operate at temperatures in the 50 – 60 K range while rejecting up to 200 mW to deep space. At present, the Instrument Radiator approximated top face area is around 0.4 m2. In case a larger margin is required, a wider surface (in the 0.4–0.5 m2 range) could be easily fit on top of the allocated volume on the optical bench, if the relative mass increase can be allowed in the budget. The radiator orientation is parallel to the TOB with an angle around 7° with respect to the vertical direction.
If needed, in the next design phase of the project, the Radiator can be used either to provide a stable reference for the FGS detectors (now directly mounted on the module box), to reject the cryo-harness heat leak (now dissipated on the TOB) or to dissipate the heat load due to the discrete CFEE of the AIRS module. In all cases, the Radiator shall be thermally decoupled from the TOB to minimize the impact on the TOB temperature and stability.
At present the Baffle is designed as a simple black painted shroud made of a 2 mm Al6061 alloy layer. This thermo-mechanical configuration reduces mass while ensuring at the same time a good thermal conductance and mechanical stiffness. The baffle is mechanically supported on the optical bench and on the two stiffening arms of the telescope structure by brackets that minimize any possible stress to the telescope structure due to thermo-elastic effects. The thermal analysis results show that using the top half of the Telescope Baffle as an extra PLM radiator, given its large surface, offers a great chance of improving the passive thermal performances of the mission. For this reason, the baffle is also thermally connected with high conductance straps to the Optical Bench to operate as a single large passive stage (TIF4) for all instrument units.
The main conductive links of the ARIEL PLM units are based on high purity 5 N Al braids (wires or foils). Because of the high thermal conductivity of pure Al in the 40–60 K range (around 1000 W/m-K) and its low density, it is possible to maintain dimensions, and mass, of the braids within allocations. The straps are used to thermally connect:
the bipods to the VGs for conductive parasitic leaks interception: at least one straps per bipod leg per VG;
the Telescope Baffle to the Optical Bench (four straps);
the instruments’ FPAs to their temperature reference (radiator or cooler cold end): one per detector assembly.
At this stage, in the thermal analysis, the straps are simulated by dedicated conductors that simulate the total conductance across each strap. This conductance evaluation includes: the combination in series of the conductance through the flanges and braids and the contact conductance at the interfaces, plus some efficiency factors that take into account realistic inefficiencies in the density of wires (or foils) per unit area, in the effective length of the link (due to turn and bends) and in the welds of the braids to the end flanges.
PLM active thermal control system
Neon JT active cooling system
The Active Cooling System (ACS) is being used to cool both channels of the Ariel Infra-Red Spectrometer (AIRS) detector focal planes to a temperature of 42 K or below. The ACS is a closed cycle Joule-Thomson (JT) mechanical cryocooler using Neon as the working fluid. A schematic of the ACS is shown in Fig. 7.
The ACS provides active cooling at the cold tip thermal interface by performing a Joule-Thomson expansion of the working fluid across a restriction, in this case an orifice. The cooler is operated sub-critical, collecting the liquid produced after expansion in a reservoir such that the heat exchanger return-line pressure above this reservoir determines the temperature. The cooling power achieved depends upon the fluid mass-flow through the orifice as well as the initial and final states of the fluid before and after the expansion process. In general, greatest cooling occurs when the fluid is initially pre-cooled well below its inversion temperature, with its initial pressure close to the inversion curve boundary and its final pressure much lower.
The cooler is required to provide 88 mW (including margins) of cooling at a temperature of ≤35.0 K in order for the AIRS detectors to operate at <42 K. Neon is selected as the working fluid because its boiling point (27.05 K at 1 atm) is well matched to the temperature requirement. In nominal operation the return line pressure is designed to work at ~3.5 bar which gives a cold tip temperature of ~32 K, and the inlet line pressure is designed to be around 20 bar, which is very much lower than the inversion curve boundary (~300 bar at 100 K), but is accessible for reciprocating compressors.
The compressors (CPA) circulate the Ne gas around the system by a set of reciprocating linear motor compression stages, with an arrangement of reed valves and buffer volumes, to produce a DC flow through the Joule-Thomson orifice whilst maintaining the pressure ratio across it. Two compression stages in series (CPA-S1 and CPA-S2) are needed in order to produce the required high and low pressures.
The gas must be pre-cooled prior to the Joule-Thomson expansion taking place, there are three pre-cooling stages available from the spacecraft V-Groove radiators and, to reduce the heat rejected at these pre-cooling interfaces (CHX-IFn), counter-flow heat exchangers (CHX-n) are used between them.
The ancillary panel (CAP) carries gas handling and measuring equipment, as well as particulate filters and a reactive getter to ensure gas cleanliness, which is critical to the long term operation of the cooler. The disconnection plates and connecting pipework (CLA) allow the system to be separated into several pieces to aid integration. This allows the heat exchanger assembly to be delivered and integrated with an instrument independently from the CPA and CAP, with a final purge and gas fill procedure being carried out after installation of the CLA to re-connect the CPA/CAP to the CHX.
The cooler is controlled by a set of drive electronics, housed in the Cryocooler Control Electronics (CCE) unit, which perform all commanding and controlling functions as well as providing the electrical input power for the compressors and returning the cooler housekeeping data.
The cooling power as a function of high pressure and mass-flow is shown in Fig. 8.
Several operating points satisfying the cooling power requirement (88.0 mW plus 20% margin) have been identified for the ACS. A range of combinations of these operating parameters is given in the table below: the nominal operating configuration (highlighted) is selected on the basis of a trade-off between cooler parameters on one side and precooling stages dissipation on the other, as the ACS must be compliant with maximum heat rejection allocations for the full range of the V-Grooves temperatures (Table 2).
Thermal monitoring: Thermistors
The knowledge of the payload units’ thermal conditions during flight operations is a key issue for the evaluation of the mission technical and scientific performances. A detailed temperature monitoring is achieved by the combination of direct measurements with thermal analysis results, correlated with all ground test data at sub-system and system level. For the instruments units and the cooler cold end monitoring, Cernox thermistors are the baseline. For the telescope and other PLM passive units (such as the V-Grooves) diodes sensors can be used, using Cernox only for critical interfaces or thermal control purposes.
The total number of sensors needed to monitor the PLM is still under definition. The present estimation is around 40 fully redundant thermistors. This number must be kept as low as possible in order to minimize the number of wires and the read-out electronics complexity. At the same time the thermistors shall be enough to ensure, in combination to the thermal maps resulting from the correlated TMM, a complete monitoring of the PLM during flight operations. A thermistor is installed in correspondence of each unit’s TRP, main thermal interface or critical item. At this stage, all thermistors are assumed to be fully redundant. The thermometers inside each module box, for detector, CFEE or optical units’ thermal control are monitored by the associated DCU. The JT cooler sensors are read by the cooler electronics. All other PLM thermistors are read and acquired by the TCU.
The reading/acquisition rate of the temperature of critical items (such as the instrument radiator or the optical bench) shall be 1 Hz. The other passive units (such as the V-Grooves or the Baffle) can be monitored at a relaxed rate (with periods of tens of seconds), especially if they are dominated by low frequency variations. All thermistors read-out is based on 4-wires measurement with connections to the readout electronics arranged in shielded twisted pairs to minimize EMI from external sources. A resolution of at least 25 mK and an accuracy of 50 mK are required for the units on the TOB, for the Instrument Radiator and for the Primary Mirror when they are in their operational range. The thermistors used for thermal control should have a resolution of at least 10 mK for the detectors and 25 mK for the M1 in the operating temperature range.
Thermal control: Heating lines
Stable conditions during observations are a key requirement for the scientific objectives of ARIEL. For this reason, thermal stability is one of the key drivers of the mission design. There are several possible thermal noise sources in the PLM: electrical dissipations instabilities, due to changes in the operating processes or modes, radiator temperature oscillations and cooler cold end fluctuations. The main cause of thermal fluctuations on radiators is due to attitude changes associated to the mission observation strategy, when repointing between two targets observations (on timescales of 10 h or so), or to seasonal variations (typically on longer periods like weeks, months, years). Experience on previous missions , testing and simulations show that low frequency oscillations due to active loads variations or to cryocooler instabilities, over a timescale of 10 h, can be controlled down to a level such that the thermal background stability does not represent a major contributor to the instrument noise budget. The most significant temperature variation will happen when the Sun aspect angle changes while repointing the S/C to observe a new science target. The SAA changes may cause variations of the temperature of the SVM radiators or of the PIP interface. The warm units’ radiator changes could introduce thermal instabilities transmitted throughout the read-out chain either by changes in the parasitic fluxes or due to the electronics thermal susceptibility properties. Constraints on the maximum slew angle between successive targets are set to limit the SAA changes and the induced temperature variation to less than 10 K in the SVM top floor. This variation is further damped by more than 2 orders of magnitude at the PLM level, well below the temperature stability requirement as demonstrated by analysis and simulations. As a result, it is not anticipated that significant temperature regulation is needed for the units inside the SVM beyond the nominal regulation to keep the units in their allowed operating range.
In a JT cooler, instabilities at the reference heat exchanger temperature are due to compressor modulation, with its typical high frequency spectrum (30-40 Hz range), to cold-end internal mass flow 1- or 2-phase dynamics (on the order of tens of seconds) and to precooling stage variations (low frequency).
Thermal stability of the optical modules directly connected to the optical bench is not expected to be a major concern given the typical instabilities of passive radiators in L2 either on the timescales of the ARIEL detectors average exposure or of the seasonal variations. The high thermal inertia of the instrument bench and modules can damp the typical fluctuations of both timescales well below the requirement.
On the basis of the present knowledge of the possible thermal fluctuation sources in the ARIEL spacecraft, the telescope passive control design is more than enough to keep the M1 well below the required stability. A 10 K over 10 h’ linear variation at SVM level induces a change of less than a mK on the M1 against a 2 K peak-to-peak requirement (see Chapter on the model results). At present, the thermal analysis indicates that there is no need of an active temperature control system for the primary mirror.
For all these reasons, the present baseline for PLM active thermal control is limited to survival and decontamination heaters. The design and implementation of the Decontamination and Survival Lines is under Consortium responsibility but they are operated by the S/C, as their activation must be triggered by operational phases, modes and conditions in which the PLM electronics is not operational.
The Decontamination Lines are activated in the cool down phase during transfer to L2 (first few weeks of flight). For decontamination purposes the temperature of the telescope mirrors, the TOB and the Instrument Radiator should be kept around or above 170 K (TBC), for approximately the first two weeks of flight operations. With the present thermo-mechanical design, the TMM prediction for the telescope mirrors and optical bench max decontamination power is around 170 W (with a 50% margin). Approximately 50 W are needed to take M1 temperature to 170 K once the mirror is already operating at 50 K, in case a decontamination run is needed during cold operations. The control logic for decontamination is based on a simple proportional loop.
The present baseline solution is to integrate the decontamination heaters directly on the back surface of the Al 6061 mirror as the possibility of integration on a radiative SLI panel behind the mirror to avoid direct contact does not seem a viable option due to mass (and power) allocation. In any case, the best solution will be analysed and evaluated in Phase B2. For this reason, the current baseline for the M1 decontamination heaters is to install them on the three whiffletree mountings to minimize stresses on the mirror during the cooldown phase. The use of Kapton film heaters glued and/or taped is preferred due to the better heat distribution and lower mass. Film heaters are widely used for cryogenic applications and a search for space qualified solutions related to their use on Al alloys will be carried out in the next phase of the project. In case no technical solution will be identified, a qualification campaign may be started within the TA development or the use of cartridge/Al cased heaters, bolted to the mirror mountings, can be evaluated as a possible solution. Another possibility is to use the TOB as a heating stage for the M1. The feasibility of this solution in terms of power allocation, efficiency and mirror response time will also be investigated in the next phase.
The Survival Lines are required to ensure that the Instrument CFEEs temperature does not fall, in any Operational Mode, below their safe temperature limit, 130 K. The SIDECARs temperature during Routine Operations is ensured by their active load dissipated on the dedicated radiators and by their thermal configuration. Every time the SIDECARs load is deactivated, the survival heating lines must take over and supply the required power to keep the units safe. Two Nominal Survival lines (plus two Redundant) are required, each one capable of providing power in the 0–1 W range.