ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
73 Rotational motion of the VIS is constrained with a SKF double row angular contact bearing that also uses  shields instead of seals and is lubricated with Rheolube 2000.    Low rate springs attenuate exercise loads in all axes. 17-7 PH stainless steel extension springs are preloaded against one another to maintain a constant spring rate across the motion envelope and keep ARED centered. Compression springs were not favored because the combination of length and low spring rate would have required a support to prevent buckling. It was felt that the resultant friction would have reduced spring life, increased load variability, and perhaps resulted in a noise problem.    The dashpots act to dissipate energy in the system by reacting to motion with a force proportional to velocity. Airpot dashpots were chosen for their simple design, use of an inert working fluid (air), adjustability, and advertised long life. A similar component is successfully used in the Active Rack Isolation System that pr otects ISS experiment racks from vibration. The dashp ots consist of a carbongraphite piston in a borosilicate glass cylinder with Rulon-lined ball joints at each end of the piston rod to allow for misalignment as shown in Figure 8. An adjustable screw orifice with a check valve adjusts the damping rate in the pull direction. The dashpots are wrapped with Kapton tape to contain debris in the event of breakage.    The snubbers that attenuate impact loads from reboost and docking events are located to engage at the end of the motion envelope. Oil-filled models from ACE Controls were chosen because of their compact size and adjustability. The rotation axis snubbers are shown in Figure 9.  Figure 8.  Airpot Dashpot Adjustable Orifice Graphite Piston Glass Cylinde r Figure 9.  Snubbers Mounted in the Rotation Housing
74 A lock-out pin inserted by the crew passes through the rotation arm and both plates, anchoring the VIS in  the centered position to protect it from damage when not in use. In addition to creating simple crew interfaces, significant effort was expended in designing the VIS to be easy to inspect, repair, or even upgrade if needed. All of the isolation components are accessible by removing covers and serviceable using standard ISS tools. Individual components are replaceable instead of requiring large subassemblies to be kept as spares. 

The VIS attachment to the seat track in ISS is designed to accommodate misalignment due to initial  tolerance variations or changes over time due to temperature or pressure fluctuations. This is accomplished by attaching at 3 points with sliding trunnions through spherical bearings as shown in Figure 10.    This statically determinant mounting method does not transfer torque to the relatively torque-intolerant  seat track. Installation was greatly simplified and crew can remove and replace the entire ARED and VIS system easily for relocation, emergencies, or storage access.   Test and Qualification Approach   Testing VIS performance is difficult in the presence of gravity. An earlier development program attempting to isolate the iRED tested the system lying sideways on an air bearing floor. The human subject was also supported sideways to perform exercises, which was very difficult and cast doubts on the validity of the results. A parabolic flight pattern on a reduced gravity aircraft provides a more realistic environment to perform exercises, but the window of weightlessness is too short and given the extremely low spring rates, there is significant risk of hardware damage during the pull-out phase of flight. Even testing on-orbit may not capture worst-case exercise scenarios due to natural human variability. While a test of system level performance was impractical, a test program was developed to:   1) Verify that components meet performance requirements and adjust those with variable settings 

2) Life cycle test the system to identify and resolve potential issues 

3) Gather component performance data for the system level qualification analysis model   The test stand consists of a Motion Science MS700 electromechanical actuator (EMA), Sensotec Model 31 load cell, and a string potentiometer all mounted on a t-slot table as illustrated in Figure 11.   Misaligned  X YEach spherical bearing is  attached to seat track and  independently constrains 2 linear DOF.  As a system they constrain all 6 DOF. If one spherical bearing  (attachment point) moves, a  combination of sliding  (trunnion) and rotation  (spherical bearing) at all 3  locations self-aligns the  system.  Figure 10.  VIS Attachment to ISS Seat Track No Misalignment
75 Component level tests primarily involved the rails, dashpots, and snubbers. Several styles of rails were  tested to find which provided the lowest friction. The rail bearings in the VIS are mounted on eccentric bushings to provide preload adjustment and tuned to provide rigidity while minimizing friction. The dashpot testing focused on finding the right setting to meet the target damping rate. Each model was cycled through 7 discrete constant velocities at each setting. The average force was recorded for each constant velocity, plotted, and fit to calculate the damping rate as shown in Figure 12. The dashpot behavior fit well to the expected characterization of force being linearly proportional to velocity.  Figure 11.  Vibration Isolation System Test Stand T-slot TableElectromechanical Actuato r  String Potentiomete r  Test Hardware Load Cell  Figure 12.  Component Test Results: Force vs. St roke at 27.9 cm/s (11 in/s) and Average Force  vs. Velocity with Linear Fit for Y Dashpot Push Stroke   (Relieved by Check Valve) Average Force at 7 Constant Velocities  Pull Stroke  Average Load = 5.1 N (1.11 lbf)  Force vs. Stroke for 10 cycles
76 Snubber testing revealed a shortcoming of the EMA test stand in that it wasn’t able to supply a sufficient  force at high velocities to characterize the snubbers. A simple pendulum-based test stand was developed as shown in Figure 13. It was easy to design, manufacture, and adjust to deliver the correct mass and impact velocity. Testing revealed that the snubbers are capable of dissipating the resultant energy from ISS reboost and docking events without overloading the ISS seat track interface.     “Lead the fleet” life cycle testing is still underway with the EMA test setup. Each axis of the VIS is cycled  through 300,000 cycles per year. Several observations have been noted to date:   1) The linear rails (of 420 stainless steel) have developed surface corrosion during life cycle testing  after grease wore away. This may be an issue of test conditions being worse than service conditions as the test cycles start and stop in the same place, pushing the grease away at the cycle peaks. Regardless, the planned rail maintenance intervals were changed from once per year to 6 times per year as a measure of caution.   2) Graphite deposits were found on the cylinder walls after 100,000 cycles on the rotation axis  dashpots. The manufacturer had seen this in other applications and there was no change in performance, so testing continued. 

3) During testing of the rotation axis, debris was noticed on the rotation arm at the ball joint  attachment. No changes in performance were apparent, so testing continued. 

The component test data was used to update the damping properties of the dashpots and friction properties of the rails within the analytical model. As the VIS design matured, its own mass properties were added to the model as well. A case matrix was developed to include runs with both sinusoidal and real stroke profiles, exercise frequencies at the minimums, maximums, averages, and ISS structural mode frequencies for all 4 of the major exercises for a total of 51 cases. The load and torque time histories from this for each case were sent to Boeing for a coupled loads analysis within the ISS structural dynamics finite element model.   Snubbe rV Pendulum  Weights Test Article Close-up of Test  Article  Load Cell Angular Encoder  Figure 13.  Pendulum Test Stand for VIS Snubbers
77 The results predicted successful performance of the VIS in both Node 1 and Node 3 of the ISS as shown  in Figure 14 and the VIS achieved flight qualification. 

Operational Performance   The ARED and VIS were flown to the ISS aboard STS-126 (ULF2) in November 2008. Assembly occurred during ISS Expedition 18 over 4 days in late 2009, with checkouts following. Figure 15 shows the ARED and VIS installed in Node 1 of the ISS with NASA Astronaut T.J. Creamer performing a dead lift exercise.  On-orbit accelerometers have shown that the VIS is meeting its micro-g and loads requirements, though no localized vibration surveys have been done to validate analytical predictions.    After 2 years of operational experience with no corrosion, the planned rail maintenance interval was changed from 6 times per year to 4 times per year. After additional evaluation the time interval between rail cleaning may be increased even more.  While the VIS has performed very well overall, it has not been free of problems. A major mechanical problem and two procedural issues have been identified:  Problem 1:  After just 6  months of operation, a rotation axis dashpot failed at the swaged connection between the  stainless steel connecting rod and aluminum rod end ball bearing. When the failure wasn’t immediately ARED at Node 3 vs Node 1 0.0010.010.1110100 0.01 0.1 1 10 100 Freq (Hz.)MicroGARED Req. Envelope ARED N1 Envelope ARED N3 Figure 14.  Results of Coupled Loads Analysis for 51 Exercise Load Cases [5]
78 noticed, subsequent ARED use resulted in breakage of the dashpot’s glass cylinder as shown in Figure  16. The Kapton tape overwrap contained most of the debris. ARED was inoperable for a month until spares arrived. Since then, the swaged connection of the rotation axis dashpot has failed on 3 separate instances. However with spares available, downtime has been no more than a day. The failure appears to be most likely caused by several deficiencies in the dashpot design combined with the misalignment between the connecting rod ends as the rotation arm sweeps through its range of motion. A root cause analysis was undertaken to identify weaknesses in the design [6]. While the design does not have the robustness desired it was concluded that the dashpot has not been used in a manner that exceeded its specifications. Several design improvements for the rotation axis dashpot have been developed and approved for implementation:   1) Piston height has been increased to reduce binding that can cause excessive wear, generate  debris, and increase friction. A groove around the piston has also been added to capture any debris that is generated.     2) The connecting rod ends have been redesigned to increase strength and wear resistance. Inhouse testing revealed several swages that tested at lower static loads than expected (but not below the design limit load.) Swaged connections are eliminated in favor of a stainless steel ball end threaded into the shaft. The ball ends will fit into a two-piece Vespel socket for durability.  This failure was not observed during life cycle testing although a clue in debris generation at the ball joint was noticed as previously discussed. The life cycle test profile  velocities and ranges are based on  Figure 15.  NASA Astronaut T.J. Creamer Exercises on ARED (Image Courtesy of NASA)
79 averages and do not capture peak velocities and hence the peak dashpot loads seen during operation on  the ISS.  Problem 2:  Procedural lapses have resulted in the VIS being locked during exercise which is clearly detected by ISS accelerometers and violates micro-g requirements. This seems to happen mostly with new crew unfamiliar with ARED operation. New labels and training procedures have so far addressed these issues.  Problem 3:  Incorrect installation of the lock-out pin dislodged one of the rotation axis springs, but this was easily fixed by the crew. New labels and training procedures have so far addressed these issues.     Lessons Learned 

Lesson 1:  The use of motion simulation software was not only technically sufficient, but the visualization capabilities  proved to be instrumental in bolstering confidence in a novel design.   Lesson 2:  Qualification by analysis can be a viable alternative if physical testing cannot provide a clear assessment of performance. However, careful attention must be given to risk mitigation.     Figure 16. Broken Rotation Axis Dashpot (Image Courtesy of NASA) Broken Glass Broken Rod End
80 Lesson 3:  Designing for ease of repair has been critical in addressing unforeseen issues. The rotation axis dashpots  have proven to be easy to replace on orbit allowing for quick recovery from failures. This not only results in less time spent by crews on orbit, but by ground support crews as well since fewer procedures need to be developed.  

Lesson 4:   Despite training and consideration of designing for humans, hardware may still be misused. Assume hardware will require on-orbit repair and design accordingly. 

Lesson 5:  “Lead the fleet” life cycle testing should only be considered when the design is flexible enough that problems can be easily diagnosed when they arise and can also be easily fixed on-orbit.  Lesson 6:  Carefully consider life cycle test design for hardware used by humans. The profile used in testing the VIS revealed a corrosion issue that hasn’t developed on-orbit, but didn’t predict a hardware failure that happened after only 6 months of use. The profile is conservative in the number of cycles but doesn’t capture peak dashpot loads.     Conclusions   Despite several on-orbit problems, there have been numerous successes. A comparatively simple passive isolation system relying on COTS components was qualified by analysis and has demonstrated effectiveness saving untold development and sustaining engineering costs. Thorough characterization of load cases and attention to design for repair significantly mitigated risk and this has paid its dividends in reducing down-time while recovering from on-orbit failures.   References   1. Lamoreaux, Christopher D., and Mark E. Landeck.  "Mechanism Development, Testing, and Lessons  Learned for the Advanced Resistive Exercise Device." Proceedings of the 38 th Aerospace  Mechanisms Symposium , (May 17-29, 2006), pp. 317-330 

2. Bentley, Jason R., et al. “Advanced Resistive Exercise Device (ARED) Man-In-The-Loop Test  (MILT).” NASA TP-2006-213717, May 2006.   3. Guilliams, Mark, Mike Rapley, and Nahom Beyene. “Resistive Exercise Description Document.” JSC  29558, January 2002.   4. National Aeronautics and Space Administration, International Space Station Flight Crew Integration  Standard, SSP 50005C, December 1999, pp. 3-45   5. Laible, Michael. “ARED Microgravity Assessment in Node 3 at Node 1 Port.” Boeing EID684-13762,  November 2009.   6. Zamaitis, J.A. and G. Szymczak. “ARED X-Dashpot Performance Report.” EM-ARED-066, November  2011.
81 Passive Thrust Oscillation Mitigation for the CEV Crew Pallet System 

Matthew Sammons*, Cory Powell**, Joe Pellicciotti+, Ralph Buehrle++ and Keith Johnson++  

Abstract   The Crew Exploration Vehicle (CEV) was intended to be the next-generation human spacecraft for the  Constellation Program. The CEV Isolator Strut mechanism was designed to mitigate loads imparted to the CEV crew caused by the Thrust Oscillation (TO) phenomenon of the proposed Ares I Launch Vehicle (LV). The Isolator Strut was also designed to be compatible with Launch Abort (LA) contingencies and landing scenarios. Prototype struts were designed, built, and tested in component, sub-system, and system-level testing. The design of the strut, the results of the tests, and the conclusions and lessons learned from the program will be explored in this paper.    Introduction 

The Constellation Program aimed to send human explorers back to the Moon and beyond as part of the Vision for Space Exploration. The CEV, also called Orion, was the proposed human spacecraft capsule for the program. The launch environment of the proposed mission included the TO phenomenon described below. It had been determined that the effects of the TO event must be mitigated for crew safety and operational reasons.    CEV Background  Originally conceptualized as a six-man vehicle, the CEV was ultimately designed to support a four-man crew on trips to the Moon, Mars, and other destinations in the solar system as part of the Constellation Program. The interior of the Crew Module (CM) portion of the CEV contains a floating crew pallet structure supported by eight struts upon which the crew seats are installed. See Figure 1 for an image of CM interior design and crew seat pallet with struts. The goal of the NASA Engineering and Safety Center (NESC) team was to develop a Crew Impact Attenuation System (CIAS) to mitigate the TO effects on the crew.  TO Description  Like all launch vehicles, the launch vehicle dynamic environment is a significant input to the overall payload launch loads. The Ares I First Stage has a small oscillation in thrust at a frequency band centered at approximately 12 Hz ± 2.5 Hz, for about 10 seconds late in booster burn. Since the overall launch vehicle resonated with the input frequency of the boosters, the crew pallet struts had an additional requirement to mitigate the thrust oscillation resulting from the vehicle resonance.                                                           *  ATK Aerospace Systems, Beltsville, MD  ** NASA Goddard Space Flight Center, Greenbelt, MD  + NASA NESC (Goddard Space Flight Center), Greenbelt, MD  ++ NASA Langley Research Center, Hampton, VA 

Proceedings of the 41st Aerospace Mechanisms Symposium, Jet Propulsion Laboratory, May 16-18, 2012
82 

Figure 1. CEV CM with Crew (left); Crew Pallet with Struts (right)  

Requirements 

The TO isolation solution was driven by the requirements and constraints imposed on it derived from evaluations of a spring-based isolation system’s impact on the crew in a TO event, LA scenario, and landing scenario. In addition, volumetric constraints and project constraints (resources, time) led to the design solution. This section will detail the results of the TO event, landing scenario, and LA evaluations.   CM Interior Environment  There are eight struts total: four in the X direction, launch axis, and two each in Z and Y directions. The CM baseline strut was modeled and tested using wire bender struts that absorb landing loads by dissipating energy through plastic deformation of steel wires – they have a one-time stroke during landing.  As a pallet-based isolation system was selected (as opposed to seat-based), it was determined  that the struts would need to be in series with the wire bender struts. Analyses showed that TO Isolators only needed to be present in the four X struts.   Crew Impact from TO Event  A requirement resulting from an investigation by the TO Focus Team and imposed by the Crew Office was to reduce crew response acceleration levels during the TO event to 0.25 g maximum at 12 Hz. This became the primary design driver for the isolation system.   The NESC team used the Brinkley Dynamic Response model to assess the likelihood of injury to the crew in both the landing and LA scenarios. For more information regarding the Brinkley injury risk criteria, see Reference 1.   Isolation Frequency  Knowing the TO frequency band of 12 ± 2.5 Hz, a NASTRAN® coupled loads model was used to evaluate the effectiveness in g-reduction at different isolation frequencies. Following the theory of load transmissibility, if the pallet system frequency is greater than that of the input, dynamic amplification will occur. By dropping the pallet system frequency below that of the input, dynamic amplification can be eliminated and transmissibility can be less than 1.0.     X Struts (+Z side)  K = 600 lb/in spring  X Struts (-Z side) K = 1200 lb/in spring +X  +Y +Z 12  34  56 78
84         

Figure 2. TO Isolator Strut In-line with Wire Bender Strut    The principal details of the TO isolation design are:   Two 105 kN/m (600 lbf/in) springs (+Z) and two 210.2 kN/m (1200 lbf/in) springs (-Z) were  required to achieve a balanced 4.5 Hz pallet system frequency.   Springs are extended 4.123 cm (1.625 in), preloading the Center Rod in compression, so that the  Isolator does not unseat prior to the TO event. During launch, the rod will not unseat from the housing thereby keeping the springs out of the load path until just before the TO event’s quasi-static load is reached. The pallet then floats at 4.5 Hz for the range of 3.25 to 4.5 g’s.   During the TO event, cyclical motion of the Isolator will occur in the range of 0.5 cm – 1 cm (  0.2 in – 0.4 in). The design allows for motion up to 1.59 cm (0.625 in).   In the preloaded condition, the Isolator design does not affect the baseline strut stiffness.    Design Detail  The overall Isolator architecture is a large machined spring housed between two hubs. Threaded into and protruding from the exterior of the Front Hub is a threaded rod end with a spherical ball joint at its end. The interior of the Front Hub has a large counter-bore upon which a Delrin® pad sits, providing a contact surface for the Center Rod, which is preloaded against the pad. The Center Rod passes through the center of the spring and screws into the Back Hub which also provides a threaded interface to the wire bender strut. The concentric alignment of the entire Isolator assembly is controlled by a Flanged Sleeve which is pinned to the Front Hub at assembly and bolted between the front hub and the machined spring. Inside the flanged sleeve are some tight tolerance Delrin® tube bushings which provide a slip fit guide for the Center Rod. Refer to Figure 3 for a cross-section view of the Isolator assembly.     TO  Isolator Threaded  Interface Wire Bender  Strut Load Cell   (for testing)