ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Figure 15. Structural model of the Sphere  The following calculations were applied:  0  0  0  0 Modal analysis to check the stiffness of the structure  Quasi-static load at 2009 to simulate the shock (pessimistic case)  Loading under pressure at 1 bar on the covers to simulate the vacuum set up  Thermo-elastic to simulate the storage (AT of -1 40°C) and oven drying (AT of +1 OOOC)  Several iterations were carried out to obtain the best compromise between mass, rigidity and thermal  resistance. For example, much work was done on the insulating block. To withstand the shock in the  transversal direction a bigger section was necessary but this decreased dramatically the thermal  resistance. A compromise was found by widening the block in the upper part and especially by  optimizing the blend radius (Figure 16). m I  Initial design Final design  Figure 16. Insulating block optimisation  Specific tests were made to validate the use of the composite material (TorlonTM) for the insulating  block as a structural part. Static tests on the whole insulating block assembly showed good results  even after thermal cycles at +120"C and -1 20°C.  Finally, the results of the mechanical modelling gave positive margins for all structural parts.  Thermal desicln  A modelling of the thermal design of the sphere was developed on I-DEAS TMG (Figure 17).  180
Figure 17. Thermal model of the Sphere  The analysis gave the thermal behavior inside the sphere for hot and cold dynamic cases. Figure 18  shows the efficiency of the insulation: in the cold case with only 70 mW inside the sphere  (electronics), the minimum temperature is -93°C for an ambient temperature at -122°C and the  thermal amplitude during a Martian day is reduced from 1 10°C to 45°C.  However, Figure 19 shows higher thermal amplitude inside the sphere with 58°C. This is a potential  problem because of the thermal sensitivity of the VBB sensor. With such thermal amplitude, the  mobile part could come on the final stop and not allow measurement for several days.  Dynamic cold case  +Ambient I  Figure 18. Dynamic cold case  Dynamic hot case  -,m  Time (s)  Figure 19. Dynamic hot case  181
Testing  Seismoloaical performance  The first Breadboard model of the VBB sensor (Figure 20) was tested in a seismic cellar at St Maur  near Paris by the IPGP team.  Figure 20. Breadboard model of the VBB sensor  With several earthquake detections, for example the Sandwich Islands one on September 6, 2004  (Figure 21), the IPGP characterized the VBB sensor performance.  Figure 21. Sandwich Islands earthquake detection  The measured performance is presented in Table 2. They show results relatively close to the  modeling. Furthermore, the comparison measurements between the VBB sensor and the STS2  seismometer, the current reference on earth, confirmed the high performance of this Martian  seismometer.  Thermal & Mechanical aualification  Shock test on a pivot mock-up  A preliminary mechanical qualification test with random vibrations and shocks was performed on a  pivot mock-up (Figure 22). The aim of this test was to validate as soon as possible the pivot design  and the use of the final stop on the mobile part.  182
Table 2. Testing results of the VBB sensor  Leaf spring TModelino I Testing  -[0.50-  I Testing - -.  iensitivitv  4.1 pmPK 4.8 pmPK  p = +I.IO-~ K-' p = +i.ia6 K' Pendulum 0.35 Hz 3 I 1.39 10"s2 I 1.73 10-2s2 I I sensitivity  1 m  Figure 22. "Pivot" mock-up during shock testing  The shock test performed at CENCESTA was successful: the pivot performance was conserved and  no degradation was noticed. We did learn from the shock test that the torque of the M3 fixation screw  of the pivot had to be increased to avoid any slip.  Qualification on a StructuraVThermal Model of the SDhere (STM)  The STM is representative of the flight model except for the functional aspects. It includes the  structural parts and all the critical assemblies like the pivot, the getters, the sealing pipe, the radiative  screen and the feed-throughs. The STM cavity is not under vacuum but the covers are welded onto  the structural ring.  A complete qualification test of the seismometer design was performed on the STM. The STM was  opened (cut) after mechanical and thermal environment tests for visual inspection.  183
During the mechanical qualification, the STM had to undergo a 9g RMS random vibration test and a  200g-20ms shock on all axes (2 shocks per axis). The STM was instrumented to measure the  structural eigenmodes frequencies and the dynamic responses.  The random vibration test was performed in SODERN’s lab on a 40-kN (9000-lb) shaker. The low sine  tests performed before and after the random test did not show any discrepancy on the eigenmode  frequencies which gave a first clue that there was no damage, before sphere opening and visual  inspection. The main concern was the evolution of the eigenmode amplification factors after the test;  from -35% to +50% depending on the eigenmode frequency and measurement location. There is still  no reliable explanation of this phenomenon apart maybe from the use of non-metallic materials  (glueings and composite material of the insulating blocks or the presence of an unlocked mobile part)  and mechanisms.  The 6 shock tests were performed at CEA CESTA in France. The Low Sine tests performed after the  shock test did not show any frequency drift.  After the mechanical environment tests, the sphere was opened for visual inspection (cut by  machining). No damage was noticed.  r  Figure 26. STM Sphere opening Figure 27. STM Sphere ready for thermal balance  In addition to the mechanical tests, thermal cyclings in the extreme specified temperature range  [-120;+120°C] were successfully performed at the IPGP in the Martian environment simulator and in  Sodern facilities. No damage was observed.  Thermal balance test  A thermal test of the STM Sphere was performed in SODERN’s EV5 vacuum chamber in order to  update the mathematical thermal model and the thermal studies of the Sphere. Fifteen (15)  thermocouples were used in order to measure the temperature at various locations and correlate with  the mathematical model predictions in static and dynamic cases. Heaters were used to simulate the  power dissipated by the real electronics (70 mW).  The test showed that the thermal insulation of the sphere was lower than predicted. The  representativity of the mathematical model w.r.t the test conditions was analyzed and confirmed. In  particular, an additional test with only two thermocouples was performed and confirmed that the wires  of the 15 thermocouples were not doing an undesired thermal shunt. A sensitivity analysis was also  performed with the thermal mathematical model (I-DEAS) showing that a refinement of the model  assumptions (more accurate material data, more representative geometry, radiative exchanges  modeling improvements) and few corrections were able to suppress most of the discrepancies  185
between test and mathematical result. This correlated thermal model will be of course very useful for  any further analysis.  An improvement of the thermal insulation of the Sphere has still to be worked out in Phase C in order  to reduce the sensitivity of the seismometer to temperature variations between night and day. It is  however not really an issue and the use of MLI around the sphere should help a lot.  Getter activation tests  Functional tests were carried out on the getters. Firstly, preliminary tests were performed on a getter  mock-up to determine the best getter activation parameters (power supply, activation duration).  Secondly, a fully representative test was carried out with the STM. Both tests were performed under  vacuum of course.  Getters  Figure 28. Getter Breadboard Figure 29. Getters in Sphere STM  The getter Breadboard was made of the getter itself and the parts of the seismometer located close to  the getter in order to check they were not damaged during the getter heating. This test helped us to  determine the maximum acceptable temperature of the getter in order to prevent any damage of the  surrounding parts, actually 600°C. The activation duration needed to obtain the expected vacuum  level will be defined precisely in Phase C with a fully representative model of the Sphere. Indeed it is  very dependant of the material amount and outgassing properties.  A second getter activation test was performed on the STM. The goal temperature of 600°C was not  reached on the 2 getters and the insulating blocks were locally damaged because of the temperature  of getter power supply wires. The cause of this problem is clearly understood and it can easily be  fixed. It is due to the electrical resistance of the feed-through. A larger feed-through solves the  problem. This has been successfully confirmed by complementary tests.  Conclusion  At the end of this phase B, the most critical points of the design of the very core of the seismometer,  the detection sphere, have been successfully verified. The few last points to be improved or adjusted  are clearly identified. The expected high performance was confirmed on the Breadboard and the  global consistency of the design with regard to the requirements was also confirmed.  SODERN is now ready to start a C/D phase for any future seismological planetary mission and will be  a very serious candidate on the ESA EXOMARS 201 1 mission.  186
Commercial off the Shelf Components in Reaction Wheels  Andrew Haslehurst' and Guy Richardson*  Abstract  This paper presents the approach adopted by Surrey Satellite Technology Ltd (SSTL) for development of  reaction wheels incorporating commercial components and the accelerated life test / qualification  philosophy. The paper focuses on the mechanical development and re-qualification of reaction wheel  which has flown successfully on many missions having conducted years of in orbit operation. The scope  of the requalification was to verify the wheels performance with ground testing for new missions where an  increased life was required, with some re-design where applicable.  Introduction  The Microsat Reaction Wheel (MRW) or Micro Wheel was developed at SSTL in the mid-nineties and is  unique in its design from the simple drive electronics which are enclosed in the base of the wheel to the  commercial brushless DC motor using dry lubrication, to the wheel volume at approximately 100*100*100  mm3 all weighing in at under 1 kg, this wheel was succeeded by the Enhanced Microsat Reaction Wheel  (EMRW) in the year 2000.  The aim of the new Superior Microsat Reaction Wheel (SMRW) was to build on the heritage and success  of the MRW and EMRW increasing the performance and extending the life in terms of revolutions of the  wheel. Figure 1 shows the MRW, EMRW and SMRW wheels. At the time of writing this paper the SMRW  is integrated onto the protoflight model of 5 imaging micro satellites currently being built at SSTL and due  for launch towards the end of 2006.  The Microsat wheels have led the way forward for the development of the next generation of larger more  powerful wheels which use similar electronics and lubrication systems. Although this wheel is not  discussed here in the paper, four smallsat reaction wheels also incorporating dry lubricated bearings have  been integrated onto the GIOVE A spacecraft ready for launch at the end of December 2005.  Figure 1. SSTL Microsat reaction wheels  Background  The small satellite market (400 kg) is particularly demanding not only for the small budgets involved or  the mass and power requirements but also for the fact that a typical program for SSTL from contract to in  orbit operation is normally from 18 - 24 months which leaves a very short period of time for development  and qualification.  ' Surrey Satellite Technology Ltd., Surrey Space Centre, Guildford, Surrey, U.K.  Proceedings of the 3d' Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  187
At this time ESTL performed a life test on a MRW motor miss aligning and loading the bearings to their  maximum capacity in an attempt to purposefully cause an early failure. The wheel conducted 0.6-billion  revolutions (proving mission life taking into account an acceleration factor) showing no signs of failure at  which point the test was stopped and the bearings inspected. Some pitting was observed on some balls  and these were examined under a SEM, which can be seen in Figure 3, the damaged areas showing  delaminating are typical with high sub surface stress and although not desired the wheel was operating  nominally.  The cages were weighed pre / post test and seen to have negligible cage weight loss. Assuming cage  wear limits the life then it is predicted that lifetime in the order of tens of billions revs could be achieved.  Although these tests fundamentally proved the motor was fit to meet current mission life requirements it  was thought the life could be extended even further. Hence, it was determined that bearing misalignment  should be taken out by some small design changes, which are presented in the development the SMRW.  Figure 3. MRW (SR4) Bearing inspection after life test  Since their launch the EMRW wheels in these spacecraft have performed millions of revolutions without  anomaly. On orbit data can be seen from two wheels of the UK-DMC spacecraft, in this case of operation  mode operating around zero. Table 1 summarizes the wheel parameters for the MRW, EMRW and  SMRW.  t  . . , . ... a.. I  w Lt 3mi8 tm wll 11-  Figure 4. Snap shot of in orbit data from a UK-DMC spacecraft showing speed (RPM) against time  190
Table 1. Micro wheel spec overview  Power  Momentum  Torque  Life Parameter MRW EMRW/SMRW  Mass ~0.75 kg 4.1 kg  Volume 1 OO*I OO*I 00 mm' 105*105*105 mm'  Voltage 1 1-1 4 V (1 2 nominal) 24-32 V (28 nominal)  0.4 W (constant speed)  3.0 W (peak torque)  0.21 Nms 0.42 Nms  3 mNm lOmNm  >I XI 0' revs 1.2 W (constant speed)  5.0 W (peak torque)  >2.2x1oy revs  SMRW  Backqround / Development  There were two main design changes to the SMRW from the EMRW:  0  0 The introduction 0; a cove;, deemed necessary due to increased harness packing around the  spacecraft which lead to concerns of harness clashing with the unprotected inertia disk.  The re-housing of the motor magnetics to increase the size and alignment of the bearings thus  striving to increase the life.  For reasons explained later in the paper with regards to ground testing the motor was partly reengineered  to provide:  a) a very low but repeatable bearing preload, thus low contact stress  b) a supplementary angular snubber for launch vibrations  c) a very high degree of bearing alignment  Figure 5 shows a cross section of the SMRW. The motor is supported on a cone that attaches to the  housing, which supports the electronics including optical encoder below. The motor houses the bearings  above the magnetics, which was a change from the EMRW to increase the alignment accuracy as the  EMRW bearings were supported at either end of the shaft in two different housings.  Figure 5. Cross section of the SMRW  The bearings are arranged in a back-to-back configuration using a deep grove type made from 440C  stainless steel. The size was increased size from the EMRW SR4 type to an SR6 size, both of which are  standard catalogue parts with relatively short lead times from European suppliers costing about 50 GBP  each ($100). This is very affordable when considering that some bespoke bearings and cages can be  orders of magnitude more expensive.  191
As previously mentioned the lubrication system is dry by means of a transfer cage. The cages are a  standard snap on design using PGM-HT, which is a PTFE base using chopped glass fibers for cage  strength and it is impregnated with MoS2. As small partials of the cage transfer to the balls and raceways  they get squashed out leaving a thin film that provides the lubrication.  The increase in bearing size was also necessary to increase the load capability as the launch  environments on some new missions are increasing. There was also a change to preload the bearings  (the EMRW bearings were shimmed to 50 microns with no preload) using a (standard catalogue part)  crest-crest spring to apply a low preload, the spring also simplifies assembly due to the high deflection to  low force characteristics of the spring thus designing in simple assembly and keeping the cost down.  The motor magnetics were primarily kept the same but a magnetic overlap that provides some preload on  the EMRW was removed thus removing the axial preload from the design so a lower preload can  achieved to maximize the life.  The electronics are nominally unchanged from the MRW consisting of a simple power stage utilizing  commercial components and a main PCB which houses the optical encoder and can be seen in Figure 6.  All the electrical components are readily available commercial parts, which do go through some increased  screening at SSTL upon inspection on critical and delicate components such as the DC-DC converters,  which have glass-isolating beads around their legs. Further detailed information on electrical commercial  components can be found in [3], [4].  MOTOR  E-PROM MOTOR -------  E-PROM HPWER STAGE  OPTICAL  ~~ ~~  Figure 6. Micro wheel electronics  Qualification testinq  The testing mentioned above and documented by ESTL clearly shows two effects. Firstly that using this  class of self-lubricating material above a critical peak Hertzian contact stress of around 1200MPa results  in very poor lubrication and should be avoided, Secondly, that below this stress there is a clear  relationship between ball raceway peak Hertzian contact stress and lifetime which suggests that  acceptably high lifetimes CAN be achieved with self-lubricating materials IF the contact stress between  balls and raceways are maintained at a sufficiently low level. Furthermore, because of the well defined  relationship between contact stress and wear rate (i.e. lifetime) it is possible to define a highly accelerated  test program in which not only the speed is increased (say by a factor 5), but also an additional  acceleration factor is obtained by increasing the bearing preload to some higher value than nominal for  the application. In this way life test acceleration factors of order 20-30 can be justifiably achieved.  192
One additional benefit of using solid lubricants at low Hertzian contact stress is that the torque noise,  which is in pari related to lumpy and uneven transfer of the cage material onto the raceways, is reduced.  The disadvantages of using this approach are relatively modest; firstly some extra care must be taken  during ground testing (at low preload the rotor mass and 1-g effects can be significant) and reduced  angular stiffness of the wheel shaft during launch vibration.  Given the improvements to the SMRW over the original unit, and by extrapolation of data from the design  guide and other test programs, lifetimes of order 1 -2E10 revs were predicted to be achievable even in the  absence of the improved bearing alignment.  Two SMRW wheels have successfully passed the following qualification program:  0 Vibration (to 23.5 Grms)  0 Micro vibration  0 . Life test (to a factor of 2 on life in thermal vac performing hot / cold cycles)'  0 Micro vibration  Including two different spacecraft operational scenarios, QM#1 nominal four-wheel operation in a 1'  tetrahedral configuration biased at constant speed of 300 RPM with a higher preload (6.1 N) to  allow accelerated testing, and QM#2 degraded spacecraft mode with three wheels biasing around  zero and constantly crossing zero with a nominal flight preload of 2 N.  Over a typical mission life of seven years in nominal mode this equates to 2.2 billion revolutions including  a factor of 2. Taking into account acceleration philosophy mentioned above the life test only needs to run  for a few months.  The nominal operational mode also included operational scenarios from orbital maintenance maneuvers,  which increase the wear on the cage due to the acceleration of the wheel. During these maneuvers the  wheels are spun up from their nominal speed to around 4500 RPM to stabilize the spacecraft during  thruster firings. They are then driven slowly back to nominal speed using the spacecraft's magnetorquer  rods to dump the momentum. These maneuvers have been scaled to take into account the increased  speed of the life test covering a representative number of revolutions at the correct rate of acceleration.  This equated to performing approximately 1 slew cycle per minute at 3 RPM per second acceleration,  Figure 7 shows the predicted profile.  The degraded spacecraft mode with the wheels constantly crossing zero was performed nominally from  +/- 100 RPM at approximately 2 crossings per minute for over 156 thousand zero crossing. The wheel  was set up in a fully flight configuration including cover and the bearing preload was set at 2 N.  During the life test, both wheels also demonstrated hot and cold survival (-30°C / +60"C) / operational  20°C / +50°C) cycles including start ups at the beginning and end of test at the temperature extremes.  1LW  193
Figure 7. Typical flight like orbital maneuvers imposed over the accelerated life test profile  QM#1 (nominal oDeration)  During vibration it is not uncommon to see responses at the inertia disk in the order of 60g. This relates to  a peak Hertzian contact stress around 3500 MPa, which is pushing the bearing to its limits with the onset  of sub surface plastic deformation at 4000 MPa. No visible damage was evident on any bearing surfaces  ball or race as was seen with the MRW.  Figure 8 shows a snap shot from the life test of the high slew maneuvers. There is some difference  between the predicted acceleration rates, which is due to the way the wheel was controlled during the  test. This actually gave the bearings a slightly harder time, as the higher acceleration rates increase wear.  The wheel conducted over 1 13 thousand slew cycles at high speed with no anomalies.  Figure 8. SMRW QM#1 Snapshot of life test data  Micro-vibration was conducted on the wheels pre and post life test to investigate the size of the effect  cage degradation has on the micro-vibration, a plot from post life test can be seen in Figure 9. The  bearing fundamental frequencies such as cage / ball frequencies can be seen tracking as predicted up to  about 350 Hz and showed a small increase in amplitude. The main increase in noise was in two  frequency bands, 400-500 Hz and around 1000Hz bands where there is a slight increase in amplitude.  This ties in with modes of the inertia disk around 400Hz and 1000 Hz that can be seen from vibration sine  sweeps.  194