A new technological step for SiC mirrors preparing OTOS

Transcription

1 A new technological step for SiC mirrors preparing OTOS ICSO 2014 Session N 6B Mirrors and Materials 1 Wednesday 8 October 2014 Michel BOUGOIN and Jérôme LAVENAC BOOSTEC michel.bougoin@mersen.com jerome.lavenac@mersen.com Claude COATANTIEC claude.coatantiec@astrium.eads.net Vincent COSTES vincent.costes@cnes.fr AIRBUS DEFENCE & SPACE CNES ICSO th International Conference on Space Optics, Tenerife 1

2 SUMMARY 1. Boostec SiC material for mirrors 2. Boostec SiC manufacturing technology for mirrors 3. Experience in large Space SiC mirrors 4. OTOS program 5. SiC mirror design for OTOS 6. Developing and manufacturing a 0.52m mock-up 7. Conclusion ICSO th International Conference on Space Optics, Tenerife 2

3 1. Boostec SiC material for mirrors Boostec SiC is the trade name of our material It is a Sintered Silicon Carbide (SSiC) For space optics, the key properties of Boostec SiC material are High specific stiffness ( 420 Gpa / 3.15 g/cm 3 ) High thermal stability (180 W.m/K / /K) Single SiC phase, highly homogeneous Microstructure and then physical properties perfectly isotropic (in particular the CTE) No mechanical fatigue No outgassing No degradation by space radiations Perfectly stable in time No moisture absorption, nor release Perfectly reproducible inside a part and from batch to batch no CTE mismatch measurable with accuracy in the range of ppm/k Physical properties kept or even enhanced down to cryo temperature qualified for Nirspec instrument to be operated at 30 K ICSO th International Conference on Space Optics, Tenerife 3

4 2. Boostec SiC manufacturing technology for mirrors Manufacturing Monolithic Parts Near-net shaping large size and complex 3D shapes Cost-effective, reliable and quick process New size capability up to Φ 1.25m or 1.70mx1.20mx0.60m Isostatic pressing blanks Green machining Sintering Grinding / Lapping Prooftesting ICSO th International Conference on Space Optics, Tenerife 4

5 2. Boostec SiC manufacturing technology for mirrors Manufacturing 3m class parts with help of the Brazing Technique Starting from monolithic ground parts (previous slide) Giving highly stable assembly, cryo temperature Silicon + additives powder Molten silicon alloy SiC part N 1 SiC part N 2 Around 1500 C, the molten silicon alloy penetrates into the gap by capillary action. After cooling down, both parts are welded. SiC part N 1 SiC part N 2 ICSO th International Conference on Space Optics, Tenerife 5

6 2. Boostec SiC manufacturing technology for mirrors SiC CVD cladding is used for masking the fine remaining porosities of optical faces, which could be source of light scattering Available equipments in Mersen Gennevilliers, an affiliated company of Boostec Now qualified for space mirrors application, up to Ø 1.25 m Capacity of mounting saving tools, thus preventing from cladding areas where CVD SiC is undesirable Thanks to its high purity, the CTE of Boostec SiC fits very well with the one of the ultra-pure CVD SiC ICSO th International Conference on Space Optics, Tenerife 6

7 3. Experience in large space SiC mirrors 14 all Boostec SiC telescopes are (or have been) successfully operating in space From the OSIRIS NAC camera, embarked on Rosetta to the GAIA telescope which has been commissioned in July this year 6 others are being integrated or waiting for launch Boostec has a strong experience in manufacturing large space mirror blanks Monolithic with SiC CVD cladding (ex. GAIA M1 mirrors) Brazed assembly of segments ( ex. Herschel M1) but without any CVD cladding ICSO th International Conference on Space Optics, Tenerife 7

8 4. OTOS Program Folowing successful launch of 2 Pleiades satellites in 2011 and 2012 CNES is preparing the future generation of very high resolution optical Earth observation systems OTOS is a technological program of CNES Many new technological developments Leading to very compact and low cost satellites Then, very compact and lightweight telescopes The preliminary studies demonstrate The stringent optical sensitivity The stringent mechanical requirements The use of adaptive optics allows to Optimize the design of the primary mirror Reduce drastically its mass Technological demonstrators are developed by industrial partners, in the frame of OTOS The Ø 1.5m extremely lightweight SiC mirror of an adaptive optic is now developed by Boostec under Airbus D&S contract ICSO th International Conference on Space Optics, Tenerife 8

9 6. SiC mirror design for OTOS Airbus D&S has designed a Ø 1.5m very lightweight SiC mirror to be manufactured as a demonstrator A Ø 0.52m mock-up has then been designed with same scale one cells and thickness in order to investigate and to validate the technological innovations, in a first step The Ø 1.5m demonstrator features: 6 brazed segments, a triangular network of very thin ribs and sub-ribs, 1.9mm thick optical face sheet, including 0.4mm thick CVD, spherical concave optical face, Rc = 3600mm The concept of 6 segments of 60 allows Boostec: To manufacture brazed SiC mirrors of Ø 1.3m up to Ø 2.5m To mitigate risks : before brazing, a possibly damaged segment can be easily replaced by a spare one ICSO th International Conference on Space Optics, Tenerife 9

10 6. SiC mirror design for OTOS The 0.52m mock-up features similarly 2 brazed segments; the 2 segments are perfectly the same, hexagonal outline, 520mm between opposite corners, a triangular network of very thin ribs and sub-ribs (in only 2 cells), 1.9mm thick optical face sheet, including 0.4mm thick CVD, spherical concave optical face, Rc = 4500mm 3 glued invar interfaces including helicoils, 3.4kg total weight including invar pads, equivalent to an aerial density of 16.5kg/m² on the only SiC part. ICSO th International Conference on Space Optics, Tenerife 10

11 7. Developing and Manufacturing a 0.52m mock-up ICSO th International Conference on Space Optics, Tenerife 11

12 7. Developing and Manufacturing a 0.52m mock-up Manufacturing sintered and ground segments At this stage the main challenges were green machining very thin and large ribs, green machining small radius at each pocket corner (R4 for 100mm deep pockets), grinding the optical face sheet with a thickness of only 1.5mm (taking into account the following addition of 0.4mm thick CVD cladding). ICSO th International Conference on Space Optics, Tenerife 12

13 7. Developing and Manufacturing a 0.52m mock-up CVD cladding and finishing the segments CVD cladding in a large furnace of Mersen Gennevilliers In 2 different runs, thus being representative of industrial condition for a large mirror Development and mounting specific tooling in order to prevent from SiC deposition on all back side areas After CVD, dismounting the savings and grinding some areas in order to be ready for brazing ICSO th International Conference on Space Optics, Tenerife 13

14 7. Developing and Manufacturing a 0.52m mock-up Brazing and finishing the assembly The main challenge of the brazing is the alignment of the optical faces; better alignment gives better SiC CVD layer thickness homogeneity on the final blank The brazed assembly is then ground in order to obtain: An optical face with required shape and CVD thickness Accurate lateral interfaces Brazed joint checked free of braze material voids (ultrasonic technique) SiC assembly checked crack free Gluing and proof-testing the invar pads on the SiC blank ICSO th International Conference on Space Optics, Tenerife 14

15 7. Developing and Manufacturing a 0.52m mock-up Main characteristics of the final mock-up, ready for polishing spherical optical face Rc # 4500 mm with shape defect of 0.044mm PTV (specified < 0.05mm), optical face thickness comprised between 1.83 and 2.04mm /- 0.25mm), SiC CVD thickness comprised between 0.35 and 0.41mm /- 0.10mm), weight # 3.46kg (expected 3.4kg) equivalent to 16.5 kg/m² on SiC part ICSO th International Conference on Space Optics, Tenerife 15

16 8. Conclusion In the frame of OTOS, a CNES technology program, BOOSTEC and AIRBUS Defence & Space are developing a new concept of extremely lightweight optical mirrors. This concept is breaking down the barrier of the manufacture of monolithic optical mirrors. It is capable of mirror diameter of 1.5m and far beyond. A 1.5m diameter mirror has been designed by AIRBUS Defence & Space. It is a 6 brazed segments mirror. A first mock-up has been successfully manufactured by Boostec, with an aerial density of only 16.5kg/m². It has validated the very new sequences of this brazed optical mirror concept. The manufacture of the 1.5m mirror is now on-going in Boostec. ICSO th International Conference on Space Optics, Tenerife 16

17 DGA, CNES and AIRBUS D&S teams have put their trust in BOOSTEC for carrying out this innovative OTOS technological project; we greatly appreciated. We also express our warmest thanks to Didier Castel (AIRBUS D&S) for his very constructive and fruitful collaboration on this project. Thank you for your attention ICSO th International Conference on Space Optics, Tenerife 17