Technical Note. PLM Configuration Trades. Prepared by: Ulrich Johann, Walter Fichter Date: 06/30/2003

Transcription

1 Technical Note HYPER Title: PLM Configuration Trades Prepared by: Ulrich Johann, Walter Fichter Date: 06/30/2003 Project Management: Ulrich Johann Distribution: See Distribution List Copying of this document, and giving it to others and the use or communication of the contents thereof, are forbidden without express authority. Offenders are liable to the payment of damages. All rights are reserved in the event of the grant of a patent or the registration of a utility model or design. Doc. No: HYP-3-03 Page 1

2 Change Record Issue Date Sheet Description of Change Release all all all first issue construction techniques added minor modifications Doc. No: HYP-3-03 Page A-I

3 Table of Contents 1 INTRODUCTION OBJECTIVE OF DOCUMENT ABBREVIATIONS AND ACRONYMS 2 2 DOCUMENTS APPLICABLE DOCUMENTS REFERENCE DOCUMENTS 4 3 PAYLOAD MODULE PHYSICAL ENVELOPE 5 4 PAYLOAD MODULE CONFIGURATIONS CONFIGURATION OPTIONS ONE DRAG-FREE SENSOR TWO DRAG-FREE SENSORS THREE DRAG-FREE SENSORS FOUR DRAG-FREE SENSORS BASELINE SELECTION CONFIGURATION WITH INTEGRATED 2-DIMENSIONAL ATOMIC SAGNAC UNIT PRINCIPLE AND FEATURES ADVANTAGES AND DRAWBACKS 15 5 OPTICAL BENCH CONSTRUCTION TECHNIQUES VACUUM ENCLOSURE CONCEPTS ASU RAMAN LASERS PHASE FLUCTUATIONS CAUSED BY WINDOW TEMPERATURE FLUCTUATIONS THERMO-MECHANICAL INTERFACE BETWEEN PAYLOAD AND PLATFORM 19 Doc. No: HYP-3-03 Page B-I

4 1 INTRODUCTION 1.1 Objective of Document The HYPER spacecraft design is heavily driven by the configuration of the payload module. The configuration of the payload module on the other hand is strongly affected by the maturity of the atom interferometry technology (e.g. beam temperature). Several design solutions are possible, where all of them have pros and cons with respect to the overall spacecraft design and the feasibility on payload module level. In this technical note the major payload configurations are presented and traded against each other. The baseline approach for the course of the industrial feasibility study is selected and justified. For the details in justification, reference is made to other technical documents, in particular to the Secondary Design (HYP-2-01) and the Updated Spacecraft Design and Feasibility Assessment (HYP-7-01). 1.2 Abbreviations and Acronyms AA ASU BOL CoM DFACS DFP DFS DoF FEEP FI FOV HYPER IRS LT MLI OB PM Atomic Assembly Atomic Sagnac Unit Begin-Of-Life Centre of Mass Drag-Free and Attitude Control System Drag-Free Point Drag-Free Sensor Degree of Freedom Field Emission Electric Propulsion Fiber Injector Field of View Hyper-Precision Cold Atom Interferometry in Space Inertial Reference Sensor (same as DFS, Drag-Free Sensor) Lense Thirring Multi Layer Insulation Optical Bench Proof Mass Precision Star Tracker Doc. No: HYP-3-03 Page 2

5 RRM SA SC SD SSM Retro-Reflecting Mirror Solar Array Spacecraft Spectral Density Secondary Surface Mirror Doc. No: HYP-3-03 Page 3

6 2 DOCUMENTS 2.1 Applicable Documents None 2.2 Reference Documents RD Doc. No. Title 01 CDF-09 Hyper CDF Study Report (as ammended by Errata Corrigum ref HYP-CDF-E/C-1) 02 HYP-7-01 Updated Spacecraft Design and Feasibility Assessment, Version 2.5, HYP-2-01 Secondary Design, Version 4.1, HYP-2-02 Secondary Performance, Version 2.1, HYP-2-05 Performance Requirements Breakdown, Version 4.0, HYP-3-01 and Optical bench Design Report, Version 4.2 Doc. No: HYP-3-03 Page 4

7 3 Payload Module Physical Envelope The payload module configuration has to accommodate the various sensors required to perform the measurements (see RD01): Two ASU assemblies (atom beam generation, drift tube, Raman laser interaction optics, detection) in perpendicular planes with intersection co-linear to the boresight One having a FOV towards the anti-sun side Various IS spanning a noise sensing envelope incorporating the ASU sensitive areas Thermo-stable optical bench structure Thermal insulation and electrical/fiber/mounting interfaces (optional launch locks ) The laser assembly and payload electronics units are outside the payload module configuration accommodated in the platform compartments. The atom interferometry measurement principle (2 Mach-Zehnder interferometers combined to a Sagnac interferometer for each measurement plane) sets principal limits on the minimum size of the assembly: The state of art performance in cooling down the atom clouds and controlling their drift velocity determines the atom beam cross sections and hence drift tube diameters and laser beam diameters. The ASU sensitivity is directly linked to the active area of the Sagnac interferometer spanned by the coherent beams. That is the lateral deflection and drift time as well as the distance between interaction zones of Raman laser and atoms. These parameters had been fixed for the purpose of this study (1µK temperature, 20 cm/s drift velocity, 60 cm longitudinal size of interferometer) however could be changed in the future when atom interferometry technology progresses (e.g Bose Einstein Condensates). Such a change would affect the PLM configuration and possibly would lead to different solutions. The dimensions are largely driven by the optical principle employed and by the required sensitivity (aperture); see RD06). Table 3-1 summarises the relevant physical parameters of the ASU assumed in this study. Most parameters affect the physical dimensions of the ASU in a direct or indirect way. Figure 3-1 illustrates the dimensions, envelopes and elements of the ASU assembly approximately to scale. Figure 3-2 illustrates a possible layout of the Raman laser launcher (serving as reference for the dimensions to be accommodated in the configuration). Doc. No: HYP-3-03 Page 5

8 Atomic cloud: Cs (132.9) 1 µk vtemp = 13.7 mm/s vdrift = 200 = 12 mm = 53 = 95 mm Raman laser: wavelength 852 nm pi pulse 20 µs? diam. (80%) 60 mm intensity? W/cm 2 freq. stability? Average power? W Detuning capability to keep ASU in control range TBD Fiber launcher: pol. maintaining fiber core 5 µm, NA =0.2 collimation optics diam 60 mm, feff = 130 mm QWP input mirror Table 3-1:Assumptions which have been made on ASU internal specifications in order to assess their impact on optical bench interfaces and accommodation. Some parameters are not specified yet. Cs/Rb oven, MOT, beam preparation, detection: 300 x 300 x 250 mm 3 MOT coupling optics included tube 200 mm(d) x 700 mm laser,window, mirror opt. diam. 60 mm vacuum housing µ-metal shield magnetic guide field solenoid RL1 RL2 RL3 100 mm 100 mm 300mm 300mm Astrium Figure 3-1: ASU: Beam generation/detection units, Raman laser and atomic beam dimensions and elements in the drift tube approximately to scale. The rapidly expanding atom clouds of both counterpropagating beams is determining laser beam and lateral size. Doc. No: HYP-3-03 Page 6

9 80% central intensity (truncated Gaussian beam) 60 mm diam. 200 mm TBC QWP Figure 3-2: Possible fiber launcher geometry to be accommodated on optical bench for each Raman laser beam. 300mm 300mm RL1 RL2 RL3 11 mm 100 mm 100 mm Astrium Figure 3-3: Geometry of the counterpropagating atomic beam Sagnac interferometer and the overlapping region approximately to scale. Doc. No: HYP-3-03 Page 7

10 4 Payload Module Configurations The options for the payload module configuration can be classified into two major groups: Configurations with an ASU based on RD01 This is the group of options that is mainly discussed below. The baseline option is selected out of this group. Configurations with advanced new ASU concepts still to be developed. A conceptual idea Integrated 2-Dimensional Atomic Sagnac Unit is briefly illustrated. This option offers several advantages from a configuration point of view however, it is based on a new design of the Atomic Sagnac Unit, whose feasibility and suitability still needs to be verified. It is not participating in the study concept trade-off. The criteria for the assessment of different configurations are: 1. Number and placement of the drag-free sensors The number and placement of drag-free sensors imposes constraints to the configuration design. Number and placement of DFSs depend on the need for an on-board gravity model, and the availability of measurement information (angular acceleration, gravity gradient). This is discussed in detail in RD03 and RD Mass of the optical bench It is a requirement to minimise mass, since the optical bench is one of the main contributors to the mass budget. 3. Thermo-elastic stability of the optical bench The level-0 requirement for -ASU has to be met. 4. Assembly and access to payload components The first point / constraint is given as an input to the configuration trades. A configuration with two drag-free sensors is a preferred solution however, although configurations with one, three, and four drag-free sensors are considered here for comparison and to illuminate their advantages/disadvantages. The configurations are traded off using criteria 2, 3, and Configuration Options In the following the design options are grouped according to the number of drag-free sensors One Drag-Free Sensor Doc. No: HYP-3-03 Page 8

11 In RD03 and RD04 it is shown that a drag-free control system design based on only one DFS is feasible although there is a need for an on-board gravity gradient model and there is no possibility to obtain any angular acceleration information, see RD03. Moreover, a one DFS solution imposes the least constraints with respect to configuration design and mass and is therefore an interesting option. However, configurations with one drag-free sensor only are not investigated in order to be conservative from a system design point of view. For drag-free control investigations the one DFS option is obtained by using one DFS of a two DFS baseline option. Note, that this approach also leads to redundancy for the DFS Two Drag-Free Sensors With 2 drag-free sensors, the connecting line between the DFSs (DFS axis) shall be coincident with the intersection line of the two ASU planes (ASU axis), see RD03 and RD05 for details. 2 DFS is the preferred and feasible solution from a 2 nd point of view. Option 1 This configuration option is shown in Figure 4-1. Pros: All components, i.e. DFS, ASU, and are accommodated on one line. mounted on optical bench. Cons: Large structure, relatively high mass. IRS1 IRS2 y z x Optical Bench Structure Figure 4-1: Configuration option 1. Option 2 This configuration option is shown in Figure 4-2. plate-mounted in front. Pros: Coincident ASU and DFS axes. More compact design compared to option 1. Cons: plate-mounted, i.e. is not directly mounted on optical bench structure. Doc. No: HYP-3-03 Page 9

12 IRS1 IRS2 y z x Optical Bench Structure Figure 4-2: Configuration option 2. Option 3 This configuration option is shown in Figure 4-3. Plate-mounted as option 2, but centred. Pros: DFS and ASU axes coincident. More compact than option 2. Cons: plate-mounted, i.e. is not directly attached to the optical bench structure. PS T IRS1 IRS2 IRS y z x Optical Bench Structure Figure 4-3: Configuration option 3. Option 4 This configuration option is shown in Figure 4-4. The DFS s are not on the intersection line of the ASU planes, drag-free point not even ASU axis. mounted in front. Doc. No: HYP-3-03 Page 10

13 Pros: DFSs not plate-mounted, but directly on optical bench Cons: DFS placement. y z x IRS1 Optical Bench Structure IRS2 IRS Figure 4-4: Configuration option 4. Option 5 This configuration option is shown in Figure 4-5. It represents the baseline of the ESA CDF report. The DFSs are not on the intersection line of the ASU planes, drag-free point on ASU axis. mounted in front. Pros: DFSs not plate-mounted, but directly on optical bench Cons: DFS and ASU axes not coincident. IRS1 PS T y z x Optical Bench Structure IRS2 IRS1 Figure 4-5: Configuration option 5. Doc. No: HYP-3-03 Page 11

14 Option 6 This configuration option is shown in Figure 4-6. Pros: Very compact design. mounted directly on optical bench. Cons: MOT access might be difficult. DFS axis not coincident with ASU axis. IRS1 IRS2 y z x Optical Bench Structure Figure 4-6: Configuration option Three Drag-Free Sensors With 3 DFSs a complete 3-axis angular acceleration information can be obtained. It is preferred that the ASU axis lies within the plane spanned by the DFSs, see RD03 for details. Since angular acceleration information around three axes is not required to design a feasible 2 nd, and gravity gradients can not be measured with 3 DFSs, this case is not treated here explicitly. Moreover, from a configuration point of view the 3 DFS case is similar to the 4 DFS case Four Drag-Free Sensors With 4 DFS, a 3-D space can be spanned basically offering a sensor volume embedding completely the ASU assemblies and measuring all relevant envelope data. Option 7 This configuration option is based on shown in Figure 4-7. Doc. No: HYP-3-03 Page 12

15 Pros: Best possible angular acceleration and gravity gradient measurement. Very compact design Cons: 4 DFS, large mass, accommodation interfaces, volume, costs. IRS3 IRS1 y IRS4 z x Optical Bench Structure IRS2 IRS1 IRS4 Figure 4-7: Configuration option Baseline Selection Since a 2 DFS solution is feasible from a 2 nd point of view, the 3 and 4 DFS (option 7) cases are discarded. Options 4, 5, and 6 do not fulfil the requirement of coincident ASU and DFS axes. From the remaining options 1, 2, and 3, the option 3 is the most preferable solution from a configuration point of view, since it is most compact (mass) and it turns out that the thermal alignment stability requirement can be fulfilled. Therefore, option 3 is selected as baseline. 4.2 Configuration with Integrated 2-Dimensional Atomic Sagnac Unit Principle and Features The actual size envelope of the active area of the Mach-Zehnder interferometer is about 600mm x 11mm. The two perpendicular Sagnac interferometers could be significantly compactified by integrating them (four Mach-Zehnder IF) into one assembly and one drift tube. This could be done by deflecting the atom clouds subsequently released by the MOT in one atomic beam by a set of perpendicular Raman lasers subsequently fired (see Figures). The magnetic guide field in the drift tube would have to be adiabatically switched for the interactions. Doc. No: HYP-3-03 Page 13

16 ASU Y IF Y 1 counterpropagat ing atomic beam not shown ASU z IF z 3 y x z Mirror frames Figure 4-8: Principle of integrated 2-D Atomic Sagnac Unit. Only one of the counterpropagating interferometers is shown. Atom clouds produced by the MOT at double rate are deflected sequentially at each interaction zone, spanning the vertical and the horizontal Mach-Zehnder interferometer, respectively (ASU IFy and ASU IFz) by activating the appropriate Raman laser group. 0s 1s 2s 3s 4s MOT + detection ASU IFx ASU IFy pi/2 RL1 pi RL2 pi/2 RL3 MOT + detection Possible B field orientation collinear to active laser system Figure 4-9: Timing diagram of two sequential counterpropagating beam cloud pairs deflected into the two perpendicular ASU planes. The magnetic field orientation at each interaction could be switched adiabatically and appropriate to each deflection direction. Drift time is to the right and drift positions are along the vertical axis. Doc. No: HYP-3-03 Page 14

17 IRS1 IRS2 ASUxy Figure 4-10: Configuration of integrated 2-dimensional ASU concept. A linear arrangement of all elements on a common sensible axis is possible. The sensibility to differential distortions on ASU interferometers is minimized by the proximity of the active areas. The structural members are ULE/Zerodur plates bonded together Advantages and Drawbacks Pro s: Integrated (one) ASU assembly for both measurement planes (4 IF) maximally symmetric configuration all sensitive axes are collinear (DFS,, ASU axis) DFC area as compact as possible (gg-effects, etc.), ASU planes as close together as possible lowest ASU planes rot displacement arround intersection laser beams (both systems) can be routed to for on-board internal alignment monitoring (e.g. 48 min period) and/or for on-ground calibration or testing vacuum chamber for ASU drift tube and IRS could be omitted: no windows, permanent vent, good vacuum, however AIVT problem, launch contamination easy accessability during testing Con s: Bad aspect ratio 1:4 to 1:5 (launch loads) Doc. No: HYP-3-03 Page 15

18 Integrated ASU concept requires ASU technology development and verification Assembly, integration, verification may be more complicated Doc. No: HYP-3-03 Page 16

19 5 Optical Bench Construction Techniques 5.1 Vacuum Enclosure Concepts A vacuum enclosure is required for ASU ground testing, cleanliness and during the general assembly and integration process as well as launch. The vacuum enclosure concepts are: Enclosing the whole payload module keeping the aperture open. Enclosing the individual ASU assemblies by dedicated vacuum vessels. Integrating the ASU elements into the optical bench so that the drift tube area and MOT s can be kept in vacuum at ambient pressure. Obviously the first option leads to large and heavy vessels given the large size of the payload module. This is not acceptable. The second option is more favourable, but will require optical windows between the atom beam and the Raman mirror group (see Figures). Analysis shows, that differential temperature fluctuations in the window material between interactions will change the optical path and hence the laser phase. This effect mimicries an acceleration or rotation rate if not sufficiently supressed (10-4 K fluctuations). See analysis below (in paragraph 5.2). The third option which has been chosen as baseline for this study has the advantage, that the critical windows can be avoided all together and that no drift tube dedicated vacuum enclosure is required. Instead, the optical bench material itself, required for thermal stability anyway is also serving as vacuum enclosure. The price to pay is a more complex integration process of the ASU assemblies. 5.2 ASU Raman Lasers Phase Fluctuations Caused by Window Temperature Fluctuations. A differential laser phase fluctuation between Raman interaction areas or at drift times of 1.5 s mimicries a rotation rate or acceleration rate signal. This phase fluctuation can be caused by laser frequency noise, by differential acceleration of Raman mirrors, by magnetic field noise etc. A further effect, to our knowledge not investigated previously, is laser optical path length change between atom beam interaction and mirror group caused by thermal fluctuations in a vacuum window in between at the critical time scale. It would have the same effect as mirror acceleration. A first analysis for BK7 or quarz as window material indicates significant path changes for differential temperature steps as can be expected at the level of the payload module stabilisation (10-3 K), see Table 5-1. The analysis shows, that both, geometrical length change and index of refraction change with temperature, respectively, contribute in a strongly material dependent fashion. To investigate, whether the dynamic effect is relevant, a step response of the window material to ambient temperature change has been modelled for the case of an isolated and copper or beryllium mounted window. The mount serves as a thermal capacitor to damp out fast temperature changes from outside. Doc. No: HYP-3-03 Page 17

20 Figure 5-1 shows the result and a marginal behaviour, which justifies further investigations. Possible remedies are dedicated temperature sensing at 10-4 K accuracy level and post correction of signals, avoidance of windows between interaction zone and mirror altogether, self compensated window material. The latter proposal relies on material or material combinations selected so that the index of refraction and geometrical expansion effects compensate at the laser wavelength. The sign of index of refraction changes with temperature may be changeable in vicinity of absorption lines of doping material. Whether such a material can be found needs further investigation. The dynamic effects induced by laser absorption has not been analysed so far. The configuration baseline selected for the present study avoids the windows altogether and hence does not require investigations in the direction outlined. Assuming a temperature instability of DT~10-3K at the IRS window the optical pathlength deviation (Dl=Dlex+Dln) for single transmission using fused silica windows: Dlex = n ex thickn. DT = /K 5mm 10-3K = 4.38 pm Dln = dn/dt thickn. DT = /K 5mm 10-3K = 55 pm Total (2 passes) = 119 pm using BK7 windows: Dlex = n ex thickn. DT = /K 5mm 10-3K = pm Dln = dn/dt thickn. DT = /K 5mm 10-3K = 14 pm Total (2 passes) = 149 pm For the whole measurement: 2passes More than 100pm error for fused silica and BK7 if window temperature changes 10-3K (in 3 s) More severe than mirror acceleration d 2 x/dt 2 = m x Lm x d 2 DT/dt 2 Best option: no windows -> vacuum concept and AIVT isuue affected Possible way out with windows: 1.Thermal capacity at window big enough to limit thermal fluctuations in critical time scale (3s), e.g. by copper shield 2. Thermal stability requirement enforced to 10-4 K 3. Thermal fine sensors and post signal correction Table 5-1: Effects on the optical path lengths and hence laser phase due to a temperature step of 10-3 K of the window material. A further possible way out is a thermally compensated window material composite. Doc. No: HYP-3-03 Page 18

21 tem p erature [ C] Temperature vs. Time, GL(2,3) = 0 270, , , , , , ,00070 T_Glas 270,00065 T_Be 270,00060 T_Env 270, , , , , , , , , , , , time [s] Figure 5-1: Dynamic response of a window (BK7) to a temperature step of 0.01 K (red) for isolated glass (blue), and window with Beryllium heat sink enclosure. The temperature gradient is marginal for the time 1.5 s between interactions. 5.3 Thermo-Mechanical Interface between Payload and Platform see RD02 Doc. No: HYP-3-03 Page 19

22 Distribution Sheet Name Giorgio Bagnasco Phil Airey Ruedeger Reinhard Ernst Maria Rasel Philippe Bouyer Arnaud Landragin Ulrich Johann Walter Fichter Giovanni Cherubini Dep./Comp. ESA/ESTEC ESA/ESTEC ESA/ESTEC IQO IOTA SYRTE Astrium Astrium Galileo Avionica Doc. No: HYP-3-03 Page 20