ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
321   Figure 11.  Point of deployment where 'blooming' causes failure   As seen in an early prototype in Figure 11, ‘blooming’ can cause a failure in primarily two modes :  1) the  boom wraps expand radially into an oblong shape; eventually this shape can become large enough to bind  up between boom arms 2) near  to the end of deployment the gap at the center can become large enough  that the boom root can possibly yield and ben d backwards at the clamp. This second method of failure did  not occur during lab tests  as the deployment was halted before the root could yield, but if allowed to continue  would have certainly occurred.   Several approaches  have been developed in order to ei ther eliminate or mitigate ‘blooming’ during  deployment :   1) Adjustment of boom arm force on the boom wrap. By changing torsion springs, the contact force of the boom arms on the wrap can be adjusted to fit the necessary  force. It was noted that as the boom  length in the deployer increased, the required force also increased. A spring arm contact simulator was developed with compression springs and can be viewed in Figure 12. The compression springs allowed nearly instanta neous adjustment of the boom arm contact force. Once a force was found  which eliminated bl ooming using the simulator, the compression spring force was then exchanged for  a properly sized torsion spring creating the same force at the point of contact. It is  to be noted that the  greater the amount of force placed upon the boom wraps the more friction is introduced into the system and the greater the chance of locally yielding the boom. The contact force on the boom wraps should not be needlessly oversized.  

Figure 12.  Compression Spring Arm Contact Simulator   ‘Blooming’
323 Despite t he advantages of a split tape composite boom , including a large weight savings, it s significantly   greater height  made it  unable to package within the allotted volume. The composite boom required a height  of 6.5 cm compared to 3.5 cm for the TRAC boom . 

Figure 14.  Split Tape Composite Boom Deployer   Stepper Motor   A stepper motor with a planetary gearhead is used to rotate the boom spools. It is important to note that  given a well -balanced system the strain energy in the four booms will act to self -deploy the booms;  therefore, ideally the stepper motor is used solely to hold back and step out the booms slowly. In practice,  the motor is needed both to hold the booms back  as well as push them out . NanoSail -D chose not to utilize  a motor, and simply allowed the booms to self -deploy after activating the burn wire mechanism. This boom  deployment took only a few seconds and could be considered too violent for a larger sail. Furthermore as  the boom length increases, the necessity for a motor becomes more evident. LightSail -A chose to  implement a DC motor with and encoder and a worm gear transmission into their single spool.    The lim itation of the NEA Scout volume led to the use of a stepper motor with a planetary gearhead. The  detent torque of a non- energized stepper motor i s also seen as a benefit of a stepper motor and h as proven  to be enough force when combined with the gearhead t o hold the boom in place. In place of an encoder,  two infrared sensors are used to monitor deployment and provide feedback (shown in Figure 15; the brackets for each sensor are goldenrod). The first is an infrared gate sensor measuring a hole pattern machi ned into one boom spool’s top flange  (Figure 15, shown in orange, circled) . This sensor provides 1.8degree resolution at the boom spool. The second infrared sensor  is attached to one of the boom arms and  watches the boom as it exits the deployer. The sensor is positioned to read marks along the boom ’s welded  edge . By measuring both the rotation of the spool and the deployment of the boom  directly , it can be  determined in real -time if and when ‘blooming’ occurs. The ability to measure possible ‘blooming’ allows for  it to be mitigated by re versing the deployment as discussed earlier . 

Figure 15.  Boom Deployer with Two Infrared Sensor s Visible
324 TRAC Boom Thermal Deflection   The solar s ail design for NEA Scout  produced many design challenges. The original baseline for NEA Scout   was a four quadrant sail in order to benefit from the heritage designs of NanoSail -D and Light Sail-A.  However, after examining the thermal environment experienced by the TRA C booms, it became evident  that thermal deformation would prove too gr eat for an effective, quadrant designed solar sail. Initial results  for an unloaded 7.3- m TRAC boom at a 30o angle of incidence to the sun  indicated 1.48 m of tip  displacement  (Figure 16). This result is one to one orders of magnitude greater than what would be  considered acceptable from Guidance, Navigation, & Control . This is  caused by the low thermal conductivity  along the thin profile of the boom, the self -shading one half of the boom’s profile by the sunward half, and  the suboptimal optical properties of the uncoated TRAC boom (solar absorptivity and infrared emissivity).  

Figure 16.  7.3-m Uncoated, unshaded, unloaded TRAC Boom during thermal analysis simulations   Extensive analysis and testing were performed to determine the best method for mitigating boom thermal  deflection,  including an aluminum coating for  the TRAC boom and the use of a ‘sock’ to keep the boom  from direct sunlight. The final determination was to  change the configuration to a single sail design, which  would inherently shade that majority of the boom from the root to ~16 cm from the tip. An integrated model  analysis shows that max out -of-plane boom tip displacement reduced from ~100 cm in the four quadrant  case to ~ 4 cm in the shaded boom case (Figure 17). Figure 17 also shows a large amount of in- plane  displacement that further convinced designers  to move to a single sail.  Additionally, t he single sail increases  the flatness of the sa il, reducing the sail connection points from 12 to 4 interfaces . 

Sail Spool Design   Designing a deployment scheme for a single sail entailed further complexities for  the solar sail deployment  mechanism.  Due to the placement of the solar sail deployment mechanism in the center of the spacecraft  bus, the single square sail is packaged onto  a single oblong spool  (Figure 18) in order to maximize the  available volume. When spooled, the sail fits onto the spool in the shape of a racetrack. To protect the sail  from pinch points during deployment, foam will pad the structure supports within the sail spool.   • Most extreme gradient (30 -degree angle)   • 1.48 m tip motion   • Tip-to-tip distance shortened 4 cm   • Image shows deformed boom at 1:1 scale   3.5-cm TRAC   Quadrant  Sail  Uncoate d 3.5-cm TRAC   Square sail   Shaded root   Uncoated   Figure 17.  Thermal deformation results for the four quadrant and single quadrant sail
325   Figure 18.  Sail Spool Generation 1   The sail spool assembly mounts directly to the boom deployer at four points. A center post is utilized to  connect the two halves of the spacecraft structurally and pass -through a relatively large cable bundle.  Due  to the single sail design and the location of the solar sail module within the spacecraft, both cable harnessing  and the primary load path must go through the center of the spool. The center post act s both as a systems  tunnel and as the primary load path from the avionics portion of the spacecraft  to the cold gas portion. The  spool rotates independently from the rest of the system around the center post with the aid of two flanged  sleeve PTFE bushings located on the top and bottom of the spool -to-post interface. These two bushings   can be viewed in Figure 19. Only a small force from the booms are required to unspool the sail from the  spool. This has been demonstrated in half-scale testing.  

Figure 19.  Sail Spool Cross -Section Engineering Development Unit   The center post (grey) remains hollow to allow for the bus cable harness to pass through.      Sail Connection to Booms   In order to optimize the load going into the sail , the connection of the boom tip to the sail corner w ill advance  from a linear tension spring, as used by NanoSail -D and Light Sail-A, to a constant force spring. The sail  membrane is expected to thermally expand by ~2.9 cm more than the booms  at each corner . In order to  account for this , a long linear spring was designed  with a low spring coefficient . Otherwise,  a large force  range would have to be accepted in the sail membrane and boom. By using a constant force spring, the  force range should be constrained within a  range  of ±5% and the si ze of the spring can be reduce d, thus  reducing the total boom length.  For half-scale testing, 3 tension springs in series were used, similar to what  is shown in Figure 20 .  Bushings
326   Figure 20.  Boom Tip to Sail Connection:  Linear Tension Spring   Sail Deployment Tests  In preparation for the full -scale deployment tests to be conducted during the spring of 2016, scaled  deployment tests were planned to gain better understanding of the fully integrated system and test  functionality. Evaluating ground support equipment, opti onal test locations, and observations of rips and  potential dynamic behaviors caused by the deployment were primary goals of the half-scale  deployment  tests. Previous analysis and component tests that focused on blooming, thermal deformation, and boom  buck ling fed into the test results.   The scaled deployment test utilized a 36- m2, 2.5-micron -thick Mylar material as a representative sail  and  four 4-m Elgiloy TRAC booms. For the first deployment  (Figure 21) , the team used two booms from AFRL  and two produced  by NeXolve.  The second deployment utilized four 4- m TRAC booms manufactured by  NeXolve. The sail spool and most of the deployer mechanism were fabricated from ABS material via a fuse  deposition 3D printer. Metal fasteners, steel springs, ceramic rollers, and a stepper motor completed the  deployer assembly.  

Figure 21.  First Deployed Half Scale Sail  

Anomalies   To fold the sail, the team performed a z -fold pattern from one end of the sail to the center and z -fold pattern  from the other end of the sail to the center. With both sides of the sail mee ting in the center, the sail is then   manually spooled onto the sail spool assembly. The sail folding is  performed with minimal damage to the  sail. All holes and  rips caused by handling were patched with Kapton tape. After both deployments,  approximately  30 holes and rips were accounted for throughout the acreage of the sail with the largest rip  being the diameter of a nickel  (21 mm) .
327 The first deployment util ized four TRAC booms available at the time:  two manufactured by AFRL and two  manufactured by NeXolve. The AFRL booms had been through numerous component testing in  development of the boom deployer assembly. Therefore, the AFRL booms incurred various cracks, weld  delaminations , and deformations  along the length of the booms. These defects prevented the booms from  tightly  spooling within the boom assembly  (Fig. 22). Upon visual inspection prior to the first deployment, the  NeXolve booms spooled noticeab ly tighter than the AFRL booms , improving the assumed packing  efficiency calculated from previous components tests with the AFRL booms.  

Figure 22. NeXolve (left) and AFRL (right) 4m TRAC booms spooled prior to first deployment   To overcome blooming, the team decided to deploy the sail in increments. The fir st 5 minutes of deployment   extend ed the booms outward. Next, the deployer would be commanded to stop and reverse for 20 seconds.  The reverse motion pu lled the booms back into t he deployer constricting the boom  wraps  around the center  hub. This motion reduced the impact of ‘ blooming’  while the booms continued to deploy. However, after the  booms deployed approximately 3.5 m, the stepper motor stalled . The first deployment ended wi th the team  manually deploying the final meter of boom and sail area.  

The second deployment implemented lessons learned from the first deployment. The AFRL booms were  replaced with newly manufactured booms provided by NeXolve. The sail material was refolded and spooled with Kapton patches for small knicks and rips. The stepper motor was replaced with a h igher continuous  torque outpu t. Even though the 3D printed plastic gears  were beginning to  show wear, it was  decided not  to replace them  at the time. This decision did not impact the second deployment. The full deployment went  successfully with minimum blooming observed and without the need to mitigate ‘blooming’ by revers ing the  motor. The t otal deployment lasted 16 minutes for 36  m 2 of sail. Th e anticipated deployment time for the  full sail is estimated to be approximately 30 minutes.
328   Figure 23.  Deployed Solar Sail After Second Deployment  

Conclusion   The challenges inherent in development of such technology with the unusually rigorous constraints of a 6U  cubesat require a thorough development program. The resulting lessons are enlightening to the  complexities of a successful  solar sail mission. As the project continues towards the manufacturing and test  of the 86- m2 sail with 6.8 -m Elgiloy TRAC booms , these lessons will prove instrumental in advancing solar  sail capability  and expanding the use of the technology . Solar sails will continue to advance and enable  future missions similar to NEA Scout to perform science objectives, which would not have been possible give similar design and launch constraints.  

References  

1. McNutt , L.; Johnson,  L.; Clardy , D.; Castillo -Rogez , J.; Frick , A.; and L. Jones. “ Near-Earth  Asteroid Scout.” AIAA  Space 2014 Conference; 4- 7 Aug 2014; San Diego, CA; United States.    2. Whorton, M.; Heaton, A.; Pinson, R.; Laue, G.; and C. Adams. “NanoSail -D: The First  Flight  Demonstration of Solar Sails for Nanosatellites.” Proceedings of the 22nd Annual AIAA/USU Conference on Small Satellites, Logan, Utah, USA, August 11- 14, 2007.   3. Biddy, C .; and T. Svitek . “LightSail -1 Solar Sail Design and Qualification. ” Proceedings  of the 41st  Aerospace Mechanism Symposium, Jet Propulsion Laboratory, May 16- 18, 2012.   4. Johnson, L.; Sobey, A.; Sykes, K.; “Solar Sail Propulsion for Interplanetary Cubesats.” AIAA Propul sion and Energy 2015; 27- 29 Jul  2015; Orlando, FL; United States.   5. Murphey, T . W.; and J . Banik. “Triangular Rollable and Collapsible Boom. ” The United States of   America as Represented by The Secretary of The Air Force, assignee. Patent US 7895795 B1.  22 Oct  2007.
330 Introduction  

The MOPITT instrument is a multi -channel  thermal infrared (TIR) and near infrared (NIR)  instrument  that  continuously measures the concentration of carbon monoxide (CO) on a global scale. The instrument was  funded by the Canadian Space Agency and built by the prime contractor COM DEV International . Prior to  the launch of MOPITT  on the NASA Terra  platform , the only global data available was for four short  periods of measurements by the MAPS instrument on the Space Shuttle. Instrument r etrievals have  helped to understand the relationship of CO to ozone, whic h is linked to climate change . With MOPITT’s  high sensitivity to surface CO from simultaneous NIR and TIR signals , this has also been useful  to  support  estimati on of  CO 2 sources . Data have also been used in combination with that of other  instruments to produce improved data products  as described by Gille [1]. The current baseline, given  existing fuel, is to continue operation of Terra  until the early 2020s .   The Terra  platform is in a sun- synchronous polar  orbit at an altitude of  705 km . From Terra , MOPITT has  a horizontal spatial resolution of around 22 km x  22 km and a swath width of  ∼640 km, which allows  global coverage every 3 days. MOPITT  uses gas -cell correlation radiometry to detect  atmospheric CO  absorption at 4.6 μm (TIR  channels) and 2.3 μm (NIR channels),  [2]. The instrument contains a set of  long life mechanisms shown in Figure 1, which provide critical functionality in the collection of CO data.   

Figure 1 .  MOPITT’s Continuously Operating Mechanisms:  a)  Pressure Modulator Cell &  Molecular Sieve ,  b) Rotating Vane Chopper , c) Scan Mirror Mechanism , d) Airbus Stirling Cooler   & Lockheed Cooler Drive Electronics,  e) Length Modulation Cells & Sieve  

MOPITT contains four optical chains which are each split into two independent channels (total of   8 channels) . A summary of the optical channels and the mechanisms involved in each channel is given in  Table 1.  The optical signals are acquired via  four stepper motor driven  scan mirrors, each initiating an  optical chain at the front end. The hybrid stepper motor s for the scan mirror s were manufactur ed by  Tecstar (now MOOG) and have achieved long  lifetimes on the order of 2.5· 10 9 steps (1.8° ) or travel  equivalent to 12 million rotations,  with bearings  lubricated with MAC grease  (Rheolube 2000) . 

The optical input signal is routed from the scan mirrors to four optical choppers, which turn continuously  at fixed speeds of 1500 and 1800 RPM, with 16 v anes per rotation. Designed and built by COM DEV,  three of the four chopper mechanisms have each achieved over 15 billion revolutions . Long life  achievements have been facilitated by the use of  MAC (multiply -alkylated cyclopentane) oil lubricated  bearings  (Nye 2001T with 1.0% tricresyl phosphate and 0.3% phenolic anti-oxidant) . The formulation is  a) b)  e) c)  d)
332 the longest in- orbit Stirling cooler heritage point to date, surpassing those on the ERS satellites and  leadi ng slightly ahead of another Airbus cooler (from the same batch) flying on ODIN . Specific device  behaviors , including updates on cooler vibration compensation,  decontamination cycles , and  accelerometer response to radiation environment  are discussed later in the paper . 

A summary of the mechanism life record is shown in Table 2, with some key tribological parameters and  notes included for reference. The instrument is taken offline once per year for decontamination  (approximately  one week  duration, usually in March), hot calibrations , and a PMC characterization sweep .    Two additional mechanisms on board MOPITT, the Earth and Space Port Covers , deployed successfully  on 28 Feb 2000, retaining the ability to be re- closed if necessary. The port covers, as well as the other  mechanisms, have been described previously [ 3,4]. Refer to these references for details of the  mechanism designs and operation in the MOPITT instrument, and for a previous review of in- orbit  mechanism performance after  5 years in orbit . The following section provides an update to that review,  with further discussion of observations and status for the continuously operating mechanisms.  

In-Orbit Performance of Continuously Operating Mechanisms  

Scan Mirror Mechanism Perf ormance   The scan mirror sequence was described by Caldwell [3], covering a ±25.2° swath with periodic swings to  a space view and to an internal blackbody calibration. Figure 2 shows that the drive currents have been extremely stable over the mission durati on. The current itself proves that the device was commanded  reliably, while the response of the unit has been confirmed through consistency of data and encoder readings. The customized IR encoder readings, which provide 3 bits of information to indicate th e center   and edges of the swath, as well as calibration positions, have not generated any significant errors over the mission lifetime. An anomaly occurred with Scan mirror 1 on 06 Apr 2002, with current rising rapidly  and oscillating for about 90 seconds  before the voltage dropped to zero (see Figure 4a). The voltage drop  is consistent with either a blown fuse or power supply failure. The current increase which preceded the failure was brief, with no obvious warning signs noted previously, so it is thought that the failure is more likely to be an electrical fault than a mechanism failure. Because this side of the instrument was already non-operational due to the cooler failure, no further in- depth investigation was pursued.  

               Figure 2.  Stepper Motor Current vs Time for a) Scan Motors 1 and 2, b) Scan Motors 3 and 4  

   b) a) Scan Mirror 1&2 Motors     Scan Motors 3&4 Current [A]
334 Rotating Vane Chopper Performance   The rotating vane choppers  1, 2 and 4 were shown to have stabilized after 5 years , as reported previously  [3]. The stable performance has continued over the subsequent  11 years . This slow stabilizing behavior  and periodic trends are believed to represent effects  of the porous polyimide retainers, with small  changes in wear and related coupling to the bearing races  (refer to Figure 3) .  Differences in the average  current levels are partly driven by the relativel y large preload tolerances , speed of operation, and  manufacturing and assembly tolerances ( described in Table 2).  

Chopper 3 stopped working in 2001 after 1.3 years of operation. Current at the drive amplifier was  observed to increase over about 45 seconds  before going to zero (refer to Figure 4) , which could be  consistent with a short  in the harness or some other electrical component failure (not consistent with an  open circuit) . The chopper failure was attributed to a previous issue with a blown fuse, whi le a NASA  report cites the most likely cause of the blown fuse was  related to a Transistor Drive Circuit [10].  Fortunately, the chopper  stopped at a completely open position, which allow ed for  the reconfiguration of  data processing without the chopper . The failure actually had no impact on scientific data due to the  thermal stability  of the instrument .            Figure 3.  Chopper Brushless -DC Motor Currents of a) Chopper  2,  b) Chopper  4 

Figure 4.  Motor Current and V oltage at F ailure for a) Scan M irror 1 and b) Chopper 3         Chopper 4 Motor Current [A]   CCSDS Time (~7 minute  window)  CDSDS Time (~3 minute window)  Scan  Mirror 1  Motor Current [A]   Scan Mirror  1 Motor Voltage [V]  a) b) a) b) Chopper 2 Motor Current [A]   Chopper 3 Motor Current [A]   Chopper 3 Motor Voltage [V]
336 in LMC 3, but it had not been used for this purpose prior to this point in the missi on. The effect was for the  LMC 3 pressure to drop from ~68.5 kPa to ~65.3 kPa over 10 hours. The pressure then remained stable  around 65.25 kPa. One attempt to reactivate the heater of Sieve 3 was unsuccessful . The resulting action  was to modif y the MOPITT  forward model (for LMC new cell pressure) used for science data. This  recalculation was performed by the NCAR team with a good result , and deemed to have zero impact on  the quality of science data retrieved by MOPITT .                                                                                                                   Figure 6.  Length Modulated Cells - Pressure Sensor Signals  Over Life for  a) LMC 3, b) LMC 4   In 2011 the sieve was reactivated after an instrument reset during a calibration procedure, suggesting  that the previous anomaly may have been due to an SEU  effect . A later anomaly with Sieve  1 was  thought to be similar to the S ieve 3 anomaly , occurring about a year later (02 Dec 2010)  and resetting  under similar circumstances .  

Pressure Modulated Cell (PMC) Performance  The P MC devices have continued to operate as expected. Spikes in the data are believed to be due to  effects related to decontamination cycles of the coolers . Referring to Figure 7b, the gradual drop in drive  current for PMC 2 is consistent with a slow leak of gas approximately 1% per year,  which affects the  resonance of the mechanism . The s tep change in settings  in 2001 is related to a change of setting after  the cooler anomaly . 

       Figure 7.  Pressure Modulator C ell Drive Currents  for:  a) PMC  1, b) PMC 2      

b) a) PMC 1 Drive  Current [A]  a) b) LMC Pressure [kPa]   PMC 2 Drive  Current [A]