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1.Cesic® interesting Key propertiesCeramic materials present a strong interest for space instruments due to theirs mechanical performances : and thermal and thermo elastical performances :
Among different ceramic materials, Cesic® from ECM (G) has been selected by AlcatelAleniaSpace as the most favourite candidate for the development of structures and mirrors of space instruments. Cesic® is a composite ceramic made of SiC, Si and C. This material is obtained from the transformation of Carbon in SiC, due to the reaction at high temperature between C/C greenbody and liquid silicon (Figure 1). Cesic® is now a mature technology offering high structural capacity thank to its high mechanical performances associated to very high manufacturing capabilities :
2.Cesic® High manufacturing capabilitiesThe use of C/C greenbody manufacturing is one of the key technologies of the Cesic® process : during this manufacturing step ECM has a lot of positive features to create complex stiff and lightweight structures. Through the use of a large CNC controlled milling machine, ECM is able after the joining of C/C blocks, to manufacture by milling large lighweighted panels of flight structure. The C/C being not fragile, the panel lightweighting on CNC machine is easy and fast with no risk of greenbody failure. Large ribs with less than 1,5 mm thickness are easily shaped, and the machining time for a full reinforced panel is less than 15 days for 1 m2 Due to the specific joining technology, it is possible to manufacture very easily and fast C/C parts with conventional tools and join them prior to the silicon infiltration process. Such process allows to obtain in one part a very stiff monolithic structure with great geometrical inertia like structure in “I” shape. Through the infiltration process this joined structure will become afterwards a monolithic piece without discontinuities and differences of thermal and mechanical properties. External extension like mirror support mounts could be machined separately from others bocks and then joined to the central structural part to obtain the final structure shape in one element allowing to have a full monolithic complex structure. After the assembly the greenbody part is machined to final structure shape especially for the I/F area. Greenbody part is then infiltrated at high temperature with liquid Silicon which penetrates inside the C/C and then react with the C to form the SiC. The Cesic® manufacturing process is a direct to shape process, greenbody structure will be manufactured close to the final CeSiC structure in shape and size. In fact, during the infiltration process, the material has a controlled low shrinkage of 0.2% and the transformation from C/C to SiC is a transformation solid to solid with all the time a high structural capabilities (no deformation due to gravity or others effect). The low shrinkage allows the manufacturing of high precise structures and mirrors. Especially for the manufacturing and preparation of high accurate interface areas for big and large structures, this small dimension change is an important advantage of Cesic® compared to other silicon carbide materials. The current existing ECM XL furnace allows the infiltration of part up to a diameter of 2,4 m (Figure 4). After infiltration the structures are cleaned by sandblasting to remove the remaining silicon from the structure’s surfaces. According to the properties of Cesic® it is possible to use different methods for final machining for the I/F preparation or other functional areas : The possibilities of EDM machining and wire erosion gives the opportunity to have a precise method with no risk to realize good surface qualities at such areas. Therefore the low dimensional change during infiltration process combined with the possibility of EDM machining is a big advantage of Cesic® compared to other ceramic materials. I/F surfaces are finalised by EDM with precise pad, giving the flatness and the location of the I/F (flatness of less than 10µm with a position of +-20µm are easily achievable on large size). Holes could be precisely located by EDM machining through cost effective wire erosion process and without any risk for the piece (typical accuracy are H7 on hole with a locating +- 10 µm). We can therefore summarize the manufacturing properties of Cesic® process as :
3.Cesic® Optical bench structure development for space instrumentDuring development phase for future cryo instrument projects, large development have been launched under ESA contract supported by Alcatel Space and ECM own funding. For this purpose an optical structure fully representative of an instrument optical bench except in size was designed, sized & successfully manufactured. The manufactured bench size was 1045 mm x 600mmx 100mm The bench was designed and sized vs optical and launch loads constraint but also taken into account all the manufacture capabilities offered by the Cesic® process (half closed back design maximising stiffness, thin ribs with lightweight holes, C/C blocks junction I/F and holes made by EDM …) (Figure 5). The full bench has been then tested under vibration to validate its behavior and strength. For this last aim the bench has been conducted successfully up to the maximum sinus level allowed by the shaker (more than 80g at shaker level), such level exceeding largely by a factor 2 the qualification loads see by the full size flight bench, qualifying with margins such bench for flight application (Figure 6). FEM prediction was fully in line with test results, confirming material properties, and the availability to predict stiffness, stress and strength behavior. Bench eigen frequency is above 500Hz and the amplification factor measured during OBB sine test is less than Q=30, limiting loads induced on critical equipment under vibrations and notching needs. The stability of the optical bench has been also demonstrated by a cryo test from 300 K down to 30 K for which the in plane contraction and the out of plane stability has been measured by an accurate speckle holographic interferometric methods (accuracy around about 1 µm on the whole cycle with a resolution of 0,1 µm), measures performed by HOLO3. The test was performed in CSL (Belgium) in focal V chamber. The bench was cooled through cryo helium panels by radiative and conductive link (Figure 7). During the test all success criteria have been met :
Cesic® structure are fully compatible for a cryo application with very high demanding stability. Figure 9 shows the quasi null out of plane deformation observed on the whole bench from 300 K to 30 K. This will allow to keep during the cold down from 300K to 40 K relative good optical alignment. 4.Cesic® Mirror development resultsCesic® features are also of great interest for mirror application :
Thank to assembly at greenbody and to EDM capacity Cesic® offers the possibility to have integrated filtering mirror fixation device (MFD) : SiC CVD Polishable cladding on Cesic® has been developed and qualified through :
Mirrors of 300 mm have been also successfully coated and polished (Figure 13). All these tests have showed the good behaviour of the coating and WFE of less than 20 nm with a roughness lower than 0,2 nm has been reached. During the Sic CVD application at Schunk the shape of the mirror has been carefully measured and no noticeable deformation has been observed. In order to demonstrate the ligthweigthing capabilities of large mirror, the study & manufacturing of a large Cesic® mirror demonstrator has been performed under Alcatel Space and ECM self funding. The mirror has been designed in order to guarantee a WFE lower than 20 nm vs polishing quilting effect, large I/F deformation, to be able to sustain up to 20 g and to offer a high stiffness with an eigen frequency above 280 Hz including MFD softness. The mirror design has the following features :
The full greenbody machining & infiltration has been mastered and successfully performed in less than 2 months (Figure 15). Geometrical measurement performed after infiltration have showed no deviation vs the shape design (radius of curvature, tolerance vs best shape) The mirror is now ready for future operations : SiC CVD cladding deposit and polishing operation. 5.Cesic® focal plane development resultsCesic® is also an interesting material for the realisation of large focal plane holding high number of dissipative detectors. Indeed Cesic® assure a high stiff and stable focal plane interface to detectors and thank to its high thermal conductivity ensure through thermal conduction the heat evacuation issued from the detectors. Thank to EDM manufacturing capabilities, flat & complex detectors I/F could be machined, including calibrated shims. Detectors could be directly in plan located thank to small precise locating holes performed by EDM after infiltration. A full size demonstrator holding 16 CCD has been successfully manufactured, then detectors integrated and tested. The focal plane is 650 mm length by 350 mm width with integrated stiffeners having numerous I/F, the plate having more than 100 I/F holes. The Cesic plate has only a mass of 4,1Kg for an equipped mass of 17 Kg with eigen frequencies of 220 Hz and 317Hz for the first 2 modes. The equipped focal plane has been successfully tested under qualification vibration level : ie 28 G sinus and 18 G Rms under Random on the shaker. Thermal test has been also performed confirming the good thermal transfer inside the focal plane. Very Large focal plane could be done also in Cesic® holding more than 180 matrix CCD (Figure 16). To demonstrate its ability, a full scale very large focal plane demonstrator for future missions has been designed and successfully manufactured (Figure 17). Its huge shape stability during manufacturing process allows to obtain as fired a quasi plane surface allowing a very limited final machining of the detectors surface. 6.Cesic® High Ultra stable structureCesic® has been selected by AlcatelAleniaSpace for the most demanding structural elements in terms of stability. Among them a large monolithic structure holding two large entrance mirrors with a stability of few picoradians has been designed using Cesic® features. To validate this stability, under an ESA contract, a Cesic® flight representative 3D monolithic structure has been already successfully manufactured and then tested to demonstrate its stability (Figure 18). The tests have been done under ultra stable vacuum environment and the structure has been instrumented with very precise optical device (Michelson interferometer with phase shift determination). Results have demonstrated the sufficient homogeneity of Cesic® to ensure a very stable line of sight of the instrument, measured deformation being well below the requirements. 7.Cesic® todays & future applicationsAll the development activities have now concluded to the high maturity status of Cesic® technology for the application of space instruments for structure parts as for mirrors or focal planes. Therefore Cesic® technology has been selected for the realisation of a small full integrated space flight instrument, structure and mirrors being made in this material. Thank to the high integration capabilities of Cesic, the structure is made in one single piece and M1 & M2 also, therefore a full telescope has been manufactured and will be assembled with only 3 main parts (Figure 19). The monolithic structure hold all the elements (mirrors, detection units and dioptres) and have therefore numerous I/F which will be fine machined thanks to EDM (Figure 20). In order to be stiff enough and notably in torsion, the central part of the structure has a closed box design. This has been made possible only thank to the geenbody assembly capabilities before infiltration Flight Mirrors have been also manufactured. As they are off axis mirror, the front shape has been carefully and very precisely machined by EDM in close relation vs the I/F. Then the mirrors have been coated by Sic CVD before to be polished (Figure 21). Thanks to all its high positive features Cesic® will be also one of the most interesting technology to save the high technology breakthrough requested by the next large future space missions like BEPI COLOMBO and Darwin projects. For Bepi, thanks to its high stiffness, low mean CTE and high manufacturing capabilities, we are in position to propose a very lightweight accurate and very stable High Gain Antenna made in Cesie® (see Figure 22). The main part is a Cesic® monolithic main reflector 1 m diameter with a skin and ribs of less than 2 mm in thickness. Such antenna is able to sustain temperature change from –180°C up to 350°C with thermal gradient up to 250°C on the surface, while maintaining a very accurate shape (less than 100 µm Rms). Such 1m main reflector will weight less than 8 kg and will be nevertheless be able to sustain vibro-acoustic and 25G quasi-static loads due to the launch condition. In order to demonstrate these performances, AAS/ECM, under ESA/KDA TDA contract has started the design and the manufacturing of a scale 1 fully flight representative of the 1 m main reflector. In order to validate all the manufacturing process a 60° segment has been already successfully manufactured, the mass being only 2,4 kg (see). The C/C greenbody of the reflector is under final lightweighting stage (ribs being 1,5 mm and skin 2,2 mm before final front face machining) and will be soon infiltrated (see Figure 24). After manufacturing the antenna main reflector will be submitted to vibration and thermoelastical test sequences. ConclusionThank to large development effort performed conjointly by AlcatelAleniaSpace and ECM and supported by ESA, Cesic® is now a mature technology offering:
Cesic® is available for all future space applications or others. |
