ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
Nose Landina Gear Udock Mechanism  The Space Shuttle Orbiter’s nose landing gear, nose landing gear door, and nose landing gear uplock  mechanism, shown in Figure 1, are interconnected, and must be rigged and operated together.  Following replacement of the door environmental seal, rigging was performed to achieve proper seal  compression. During nose landing gear cycling, the gear uplock indication did not illuminate because the  mechanism did not reach the full uplock condition. Binding in the rotational fitting between the uplock  fitting and the bellcrank prevented the uplock mechanism from going to the full over-center position for  gear uplock. Measurements of the width of the bellcrank and the internal width of the fitting showed an  interference fit between the two assemblies. Rework on the bushings per specification requirements  removed the interference condition, allowing the bellcrank to move freely.  Lesson Learned: Proper tolerancing and inspection are critical to preventing interferences in  mechanical systems.  Bungee location,  not shown  Environmental door seal,  both doors Shock strut  Fr/  Figure 1. Nose Landing Gear Mechanisms  During subsequent nose landing gear retract operations, there was an early indication that the gear uplock  mechanism was in the gear-up position. As the shock strut was entering the wheel well and bringing the  doors closed, the gear stalled prior to being fully up and locked. After an immediate halt to operations the  gear fell freely to the down position. It was observed that the uplock hook was in the gear-up position, thus  preventing the uplock roller from engaging. Upon investigation, it was discovered that when hydraulic  pressure was applied to retract the gear, the uplock actuator immediately drove the uplock hook closed to  the gear-up position. Normally, the mechanism is in a gear-down over-center condition and cannot move  prior to gear uplock roller engagement. When the gear uplock roller enters the hook, the roller pushes the  mechanism out of its over-center position and allows the uplock hook to engage with the strut and bring  the gear to the up-and-locked over-center position. A new source of binding in the mechanism had  prevented the hook from being in the full down position. During the rework of the mechanism for the  binding described above, inadvertent damage was imparted on the bungee spring. Tooling used to assist  in the rework efforts described above is believed to have caused benthaised metal on the bungee end  cap. The resulting binding in the bungee prevented the mechanism from freely going to the full down  over-center position. A replacement bungee was installed, and rotational pins and linkages in both the  114
gear uplock and in the door uplock mechanisms were inspected with no signs of damage observed. The  mechanism now properly “snaps” into both the gear-up and gear-down over-center positions.  Lesson Learned: Repair and rework of mechanical assemblies can cause collateral damage.  Video Camera SUDDO~~ Assembly  The Video Camera Support Assembly (VCSA) is used to mount video cameras in any of 14 potential  positions on the truss segments, nodes, laboratory module, or habitation module on the exterior of the  ISS. As shown in Figure 2, the VCSA is mechanically attached to ISS primary structure with a threaded  bolt that is installed by an astronaut during an extra-vehicular activity (EVA).  Bolt Assembly  \ Lu  Figure 2. Video Camera Support Assembly  During testing, the VCSA bolt experienced extremely high running torques. The recorded running torque  values were higher than the Pistol Grip Tool, which was to be used to drive the bolt during an EVA, was  able to generate. Investigation revealed that no lubricant thickness was specified on the bolt drawing, and  that the dry film lubricant thickness on the bolt was 10 times thicker than the requirement by the  application specification, causing an interference with the female threads. In addition, the surface of the  bolt was not bead-blasted prior to lubricant application, as required by the application specification. To  resolve the issue, the dry film lubricant was removed and the bolts were returned to print by first bead-  blasting the surface, and then reapplying the lubricant.  Lesson Learned: Dry film lubricant thickness is important and the appropriate surface preparation  must not be overlooked.  Fasteners  Fasteners remain one of the most problematic areas on mechanical systems. The problems can generally  be grouped into two subcategories, the first related to secondary locking features (or lack thereof), and the  second related to fastener installation torque and the resulting fastener preload.  115
Lesson Learned: Liquid locking compounds are very application process-sensitive and not  verifiable, and therefore are not recommended for use as a secondary locking feature.  An interesting observation during the testing was that there appeared to be a relationship between  fastener preload and LLC adhesion, which may warrant further study. For details, refer to JSC-62850 [3].  Resumlv Stowaqe Rack  The Resupply Stowage Rack (RSR) is used to carry pressurized cargo to the space station in the Multi-  Purpose Logistics Module (MPLM). The RSR consists of various sizes of locker compartments. These  compartments, which accommodate individual stowage trays or bags, are bolted into the rack structure.  The compartments have structural doors with latches, shown in Figure 3.  Figure 3. RSR Latch Mechanism  Only one month prior to the scheduled launch of STS-114, an issue was discovered with the locker door  mechanism on the racks installed into the MPLM. The locker doors were held closed by a 90-degree latch  that was held in the closed position by a single thumb screw. The thumb screws thread into locking  inserts, but all had zero running torque due to numerous installation cycles. In addition, the engineering  drawings required the thumb screws to be torqued “hand-tight”. Testing was quickly performed to  determine that a maximum of approximately 0.34 N-m could be achieved, far less than a fastener of this  size would be nominally torqued. The fasteners essentially had no secondary locking features, and very  low preloads. One of the lockers contained a 36.3 kg component with a high-pressure tank, which created  a safety concern if the fasteners backed out and allowed the component to fall out of the open locker door.  Any fix to the lockers required opening the Payload Canister and the MPLM hatch, which had already  been closed out for flight. Because of this, a decision was made to temporarily fix all of the lockers (about  40) with Permacel Tape. The tape worked because it did not have to prevent the loss of preload, only  locker latch rotation. While not very elegant, this was an acceptable fix for STS-114. Prior to any future  flights, a secondary locking feature is being added to this system. The proposed concept consists of a  slider, which fits over the existing handle and prevents the thumb nut from backing out, and can be slid  away to permit operation.  Lesson Learned: Measure running torque during each fastener installation cycle and keep a  record for verification. Replace locking features that do not exhibit running torques within design  specifications.  Lesson Learned: Fasteners and locking inserts are not the proper locking feature solution for  latch designs requiring many cycles.  117
Bearina Motor Roll Rina Module SDanner Nut  The Bearing Motor Roll Ring Module (BMRRM) is part of the space station Beta Gimbal Assembly (BGA),  which provides mounting and gimbaling for the space station solar arrays. The BGA and BMRRM are  shown in Figure 4.  Figure 4. BGA and BMRRM  The BGA BMRRM assembly was experiencing anomalous behavior after acceptance vibration testing: it  would rotate smoothly in one direction, but would rotate slightly then seize when rotated the opposite  direction. A layer of debris was also noted on the exterior of the assembly, as shown in Figure 5.  Disassembly and inspection revealed that a large spanner nut, which was used to capture and preload the  internal components of the roll ring and housed inside the BMRRM, had loosened and rotated out. As the  nut rotated out, it closed up the clearance in a labyrinth gap as shown in Figure 5, resulting in metal to  metal contact between the rotating and fixed parts of the housing components and generating the debris. I  r  A  Lt' I1  FiS-." ". .... .. .... ---. .- and Labyrinth Gap and Spanner Nut  118
Pre-delivery photographs and post-installation photographs taken with a borescope were evaluated to  determine if multiple units had experienced a similar loosening problem. No other units were found with  this problem.  A review of the drawings and build processes revealed that the nut design used a light preload and a small  amount of LLC on its threads as its primary and secondary locking features. Due to a general concern for  the locking design and the inability to define the exact cause of the failure, a mechanical lock was devised  to fix the problem. The lock will be installed on all seven serviceable units. The three on-orbit units cannot  be serviced.  While the vendor had what appeared to be a properly documented and controlled application process, the  LLC had failed, allowing the nut to loosen. When the situation was discussed with the LLC manufacturer,  they noted that the application fell outside of the recommended usage for their product because of the  large size of the spanner nut and low preload and therefore would not guarantee its function.  Lesson Learned: Liquid locking compounds cannot be depended upon when used in applications  outside of manufacturer’s guidelines.  Pre-test, post-test, and post-installation photos were used as a non-destructive method to identify any  other instances of nut movement. Since the units are not visible after being fully assembled into their next  higher assembly, a borescope was used to obtain the photographs needed to clear the units.  Lesson Learned: Maintain photo documentation of mechanical systems during assembly and  subsequent testing and usage with similar viewing angles to allow for comparisons.  Lesson Learned: Provide access for inspection of mechanical systems.  Torclue/Preload  A significant number of issues have been worked during the past two years relating to fastener installation  torque and the resulting fastener preload. Many of these issues were the result of the improper installation  torque being applied to the fasteners. Others were the result of an improper torquing sequence applied to  a fastener pattern. A recurring theme has been a lack of understanding of the torque-tension relationship.  InsDection Boom Assemblv and Shuttle Remote ManiDulatincl System  The Inspection Boom Assembly (IBA) supports components of the Orbiter Boom Sensor System,  including sensors and video cameras used by the crew for situational awareness and inspection of the  Orbiter. The IBA and Shuttle Remote Manipulator System (SRMS) are mounted in the payload bay of the  Orbiter. During flight the IBA is removed from the payload bay and operated as an extension of the SRMS,  as shown in Figure 6. 3 Orbiter Payload Bay  ,  119 L  Figure6. IBAo.. _...-._  An installation drawing review of the IBA handrails revealed that unusually high torque values had been  specified for some of the IBA fasteners, for which the vendor was not able to supply any supporting test  data. An analysis assessment showed that several fastener groups within the IBA could potentially be
torqued above the yield strength of the fasteners. Similarly, a stress assessment for the SRMS bolts  revealed that several SRMS fasteners could potentially be torqued above their yield strength as well.  A series of tests was conducted at JSC to evaluate the suitability of this condition. Torque-tension tests  were performed on four different fastener groups, two from the IBA and two from the SRMS, to directly  measure the relationship between torque and preload. The tests were able to show that the IBA and  SRMS were acceptable to fly in their current condition. For more details refer to JSC 63083 [4].  Lesson Learned: Ensure torque tables are substantiated by relevant test data, accounting for the  materials, lubricants, and installation process. Lubricants or sealants can significantly alter  torque-tension relationships.  Fliaht Releasable Attachment Mechanism  The Flight Releasable Attachment Mechanism (FRAM) provides a generic structural and electrical  interface between spare ISS hardware components, called ORUs and either the Shuttle or ISS. The  FRAM consists of an active half, which is mounted on the ORUs, and a passive half, which is connected  to Shuttle payload bay carriers and on-orbit stowage locations. This provides interchangeability between  storage locations.  After acceptance vibration testing of the FRAM, a post-test functional test and inspection revealed that a  locking collar was loose and minor damage to an ACME thread was noted. The locking collar is shown in  Figure 7.  Figure 7. FRAM Locking Collar  . It was discovered that improper torquing of the locking collar clamping fasteners resulted in a loose collar  and that the ACME thread on the shaft was damaged during the testing. The fasteners had been torqued  to their full level individually and were not alternated or checked a second time around to ensure that the  tightening of one fastener had not reduced preload in another. Unfortunately in this case, the clamping  force of the collar was very sensitive to the order and manner of torquing the fasteners. To correct the  problem, the fasteners were incrementally torqued to their full torque, while alternating between the two  fasteners.  Lesson Learned: Use proper torque sequence in multi-fastener patterns.  120
Strength  Mechanical system components and linkages need to be designed with sufficient strength to tolerate an  actuation forcehorque stall condition at any point of travel and maintain a positive margin of safety with an  ultimate factor of safety applied. Mechanical systems that incorporate end of travel mechanical stops need  positive strength margins for worst case dynamic loading conditions, considering variables in inertia  properties, actuation force/torque, drive train resistance, and other environmental conditions. Exposed  mechanical system components, protective shrouds and covers, and mounting structure need to  accommodate inadvertent impact loads from manipulator systems, payload operations, and crew activity.  Main Landina Gear Door Retract Mechanism  The Space Shuttle Orbiter main landing gear (MLG) door retract mechanism is shown in Figure 8. The  door retract mechanism is a four bar over-center linkage, with the orbiter structure forming the fixed link. A  spring-loaded bungee is also part of the mechanism and helps to hold the mechanism over-center when  the door is open.  Door /I Hockey stick  Figure 8. MLG Door Retract Mechanism  The door retract link on the starboard main door retract mechanism of Atlantis (OV-104) was found to be  bent and cracked during a routine inspection. Figure 9 shows the damaged link in comparison with its  counterpart on the port side.  121
Figure 9. Comparison of Starboard and Port 452 Links  Extensive failure analysis including metallography of the failed part, historical data retrieval, dimensional  verification, loads analysis, and borescope inspection of Discovery, were conducted to understand the  scope of the problem. It was concluded that the damaged part had adequate design properties, and that  this part was damaged during replacement of the O-ring seals in the piston axle assembly of this main  landing gear strut. This ground operation involved using a hoist to support the lower part of the main gear  from overhead by attaching ground support equipment to the hockey stick. The over-center link was  overloaded during this procedure, causing the damage. The vehicle flew two flights in this condition.  Discovery was inspected to ensure that it did not have a similar problem, and the damaged over-center  link on Atlantis was replaced with a good link borrowed from Endeavour. Ground servicing procedures will  be changed to prevent this problem in the future.  Lesson Learned: Ensure that strength analyses are performed for planned ground operations of  mechanisms, and prior to any unplanned ground operations.  Positive Indication of Status  All movable mechanical systems should provide positive indication that the mechanism has achieved its  desired position (i.e., ready-to-latch, latched, open, closed, etc.) and end of travel stops should be  provided for all movable mechanical systems.  Limit Switches  Limit switches are often used to provide positive indication of status for mechanisms. Limit switches are  small electronic devices not capable of sustaining high mechanical loads, and are typically protected by an  actuating lever mechanism. Limit switches require ground rigging to set the actuation lever in the proper  location. Due to their size, there is usually a small adjustment window in which the switch will indicate the  proper status.  One example of an SSP mechanism that incorporates limit switches is the payload bay door drive  mechanism. This system has switches that sense the position of its rotary actuators, and also indicate the  location of the door. The rotary actuator limit switches are internal to the actuators, and are therefore  protected from extreme thermal gradients between the hardware that is being sensed. The limit switches  122
that indicate the position of the door are incorporated into the bulkhead switch module, which is installed  on the payload bay bulkheads, and are exposed to space.  There have been numerous failures during missions with the bulkhead limit switch module and other limit  switch applications. The switches do not accurately change status as the mechanism is operated, and  take a few seconds to hours to flip to the proper indication. These failures do not always repeat  themselves on the ground. The phenomenon is not currently understood, and despite tearing down the  limit switch assembly, rebuilding, reinstalling, and ground rigging, the failures tend to repeat on orbit, and  are attributed to thermal effects.  Extra effort should be made to locate limit switches so they are not subjected to extreme thermal  environments. In the event that the switches must be installed in these environments, redundancy should  be bui!t into the limit switch system and extensive testing should be done to understand the interaction  between the various components of the system under the applied thermal gradients.  Lesson Learned: Ensure that thermal effects have been considered in the design and analysis of  limit switches, and have been reproduced during environmental testing.  External Tank Door Mechanisms  Following the External Tank (ET) iettison, the ET doors are used to close out the aft areas where the  Space Shuttle Orbiter was attached to the ET. The doors, shown in Figure 10, are open for launch and  ascent and then must be closed while on-orbit and remained closed for entry.  Figure 10. ET Doors  A centerline latch mechanism holds the doors open. Once the latches have been released, the ET door  drive mechanism is used to drive the doors into a nearly closed, ready-to-latch position. The uplock  mechanism then operates to pull the doors into a closed position.  For the ET door drive and uplock mechanisms, mechanical limit switches are incorporated into the  actuators. This provides an indication that the actuator has rotated the proper amount, but does not  directly indicate the status of the doors. If, for example, debris became wedged between the door and the  frame, the actuator might be able to turn the correct number of degrees but the door opening might  actually not be fully sealed. Given the external tank foam-shedding problem, this was a concern during the  STS-114 return to flight mission. A better way to determine the true door status is being developed.  Lesson Learned: Provide true indication of status for all safety critical mechanisms.  123
for verification. Replace locking features that do not exhibit running torques within -  design specifications.  Fasteners and locking inserts are not the proper locking feature solution for latch  designs requiring many cycles.  Liquid locking compounds cannot be depended upon when used in applications  outside of manufacturer’s guidelines.  Maintain photo documentation of mechanical systems during assembly and  subsequent testing and usage with similar viewing angles to allow for  comparisons.  Provide access for inspection of mechanical systems.  Strength  Positive Indication  of Status Ensure that strength analyses are performed for planned ground operations of  mechanisms, and prior to any unplanned ground operations.  Provide true indication of status for all safety critical mechanisms.  Ensure that thermal effects have been considered in the design and analysis of  limit switches, and have been reproduced during environmental testing.  Design life of mechanisms should consider maintenance cycles of interfacing Testing  I I systems.  Acknowledgements  The authors wish to acknowledge the team members that contributed to the resolution of the problems  that have been described in this paper: Mechanical Systems Working Group, ISS Structures and  Mechanisms System Problem Resolution Team, and SSP Mechanical System PRTs.  References  1. Mechanical Systems Safety, MA2-00-057, NASA JSC, Sept. 28, 2000.  2. Materials Control Plan for JSC Flight Hardware, JSC-27301 D, NASA JSC, Feb. 2000.  3. Use of Liquid Locking Compound on ESP-2, JSC-62850, NASA JSC, Jan. 2005.  4. Inspection Boom Assembly and Shuttle Remote Manipulator System Fastener Test Report, JSC-  63083, NASA JSC, Aug. 2005.  125
Reliability and Fault Tolerance in ISS Thermofoil Spaceflight Heaters  Victor J. Bolton'  Abstract  Extra-vehicular avionics systems and mechanisms used on the International Space Station (ISS) typically  require redundant survival heaters due to cold thermal extremes. Two such survival heater systems have  completely failed to date. These designs were not truly fault tolerant because a failure within the heater  patch would bring down both the prime and redundant systems. This report is intended to make designers  and operators of mechanisms requiring survival or operational heaters for spaceflight applications aware  of common issues that contributed to in-flight failures.  Introduction  A dual element thermofoil heater is in essence set of two resistive elements laminated together. Failures  in either element can easily propagate to the other by damaging other element or the laminate structure.  Figure 1 illustrates a layered, dual element design.  El nentsM -I  r Kapton J  Teflon  Figure 1. Layered Design Flexible Heater Exploded View  Heaters are often treated exclusively as electronic components; however, standard electronics  qualification and acceptance testing is insufficient to certify a heater design. Flight history lessons learned  lead directly to design and testing recommendations for mechanism heaters. Survivability in particular  was not thoroughly examined in the design and testing process.  Failures are presented with a short description to provide data points that suggest areas to pursue  alternative designs. Recommendations are meant as solutions to these known problems and other  sensitivities of heater patch design and installation. History and descriptions presented here were  developed through in depth analysis of failure telemetry, and by direct interview of hardware designers,  manufacturers, and operators.  Boeing ISS Structures and Mechanisms, Houston, TX  Proceedings of the 3d' Aerospace Mechanisms Symposium, Langley Research Center, May 17- 19,2006  127