ai-aerospace/ams_data_full_2000-2020 huggingface dataset:
440 As a worthy note, through this race condition analysis of CDPR deployment fault case, the kickoff spring  made the deployment time approximately 33% faster than if there was only the lower torsion spring  available to drive the deployment. Such a finding reaffirmed that incorporating an initial motion spring (the  kickoff leaf spring) is a great sound design principle.  

This loaded deployment test was repeated in January 2020 as the last part of the DTM rover chassis and  differential design qualification static testing campaign. The CDPR was loaded to 1.2x the limit loads in  tension. The flexure was deployed in this configuration using a flight -like pin -puller. No jamming occurred  and the same expected dynamics response of the flexure from the strain energy release was observed,  resulting in a successful test.  

Summary  

For the Mars 2020 mission, a new CDPR achieved similar functionality as the MSL design, while using only  half the mass for the structural mechanism by using a new mechanism architecture. The MSL crank -slider  mechanism,  which was sensitive to misalignment and structural concerns , was replaced with a two -force  member spring -like flexure  that doubled as providing compliance for the system and as the deployable.  This flexure proved to have verified robust design. It was repeatable at temperature and did not require any  conditioning.  

However, this larger deployable required the torsion springs and other hardware of the mechanism to  deploy the flexure to be increased in size and strength in order  to actuate this larger deployable  for the  Mars 2020 CDPR . Having  the flexure  double as the spring dampener and the deployable had the benefit  of eliminating the required alignment precision from the MSL CDPR, but also introduced new close  clearances and race conditions. The biggest lesson learned was to not have race condition pot ential in a  deployable mechanism. It was a large amount of engineering effort to properly show that the mechanism  would win this race condition. The analysis on the race condition reaffirmed an important design principle,  to always have a simple initial motion spring integrated into a deployable mechanism. Future mechanism  could utilize the design of the CDPR f lexure to create a tunable linear spring while ensuring the design does  not have the same race condition concerns that were learned from.  

Acknowled gements  

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a  contract with the National Aeronautics and Space Administration.  © 2020 . California Institute of Technology.  Government sponsorship acknowledged.   References   1. Jordan, E. 2012, Mars Science Laboratory Differential Restraint: The Devil is in the Details,  Proceedings of the 41st Aerospace Mechanisms Symposium,  Jet Propulsion Laboratory, May 1618, 2012
441 Astrobee Free- Flyer Nozzle Mechanism  

Earl Daley* 

Abstract  

This paper describes  the development and design of the Astrobee Free- Flyer propulsion nozzle  assemblies .  As will be illustrated in this paper, the Free- Flyer nozzles are thrust  control  devices used to propel the  Astrobee Free-Flyer robot autonomously inside the International Space Station (ISS). The development  process, design evolution,  and prototyping methods , are described . Key design features  are discussed in  greater detail to highlight how  a seemingly simple design can present surprisingly large challenges . Several  lessons learned are given . 

Introduction  

The Astrobee project provides free- flying robots  (referred to as “ Bees ”), charging station, software, and  ground control operations to the ISS for crew support and guest science research. The Free-Flyers are  31.8- cm (12.5- inch) cubes , battery -powered,  air-propelled free- flying robot s designed for use within the ISS   (Fig 1). Two Free-Flyers (Bees Bumble and Honey) were launched in 2019 along with the charging station  (called the “Dock”) and are installed in the Japanese Experiment Module Kibo. A third Bee, Queen, is also  on station in storage. Working autonomously or via remote control by astronauts, flight controllers , or  researchers on the ground, the robots are designed to complete tasks such as taking inventory,  documenting  onboard experiments with their built -in cameras or working together to move cargo throughout  the station. In addition, the system serves as a research platform that can be outfitted and programmed to  carry out experiments in microgravity  [1]. Currently, Bumble is going through its commissioning phase and  has successfully completed mapping the Japanese Experiment Module in preparation for autonomous  operation.  

Figure 1. Astrobee Free Flyer s and Dock  in the ISS  

* NASA Ames Research Center, Mountain View, CA  

Proceedings of the 45th Aerospace Mechanisms Symposium, NASA Johnson Space Center, 2020
442 Free -Flyer Propulsion System  

The nozzles are part of the Free- Flyer propulsion system which  is comprised of two modules  located on   opposite sides of the Free -Flyer (Fig. 2). Air is drawn into the propulsion module by a centralized centrifugal  fan which pressurizes a plenum (~1. 5 in-H2O (0.37 kPa) ). Thrust is produced when the pressurized air is  exhausted through one of its six nozzles that are located around the plenum. For a constant impeller speed,   the thrust from each of the twelve nozzles  has fixed direction with magnitude controlled by adjusting the  nozzle open area.  

Figure 2. Free-Flyer Propulsion Module  

The key propulsion requirements are:   1.  provide holonomic control in 6- Degrees -Of-Freedom, i.e., the ability to produce instantaneous thrust  in  any direction and torque about any axis,   2.  produce 0.6- N max thrust on at least one motion axis, and   3.  keep noise below 65 dBA at max thrust.  

The two propulsion modules can be replaced by astronauts and are designed to be interchangeable. Their  impeller s rotate at the same speed and, because they are on opposite sides,  their gyroscopic moment and  drag torques are cancelled. The impeller speed is adjustable to trade thrust  performance vs. reduced power  and noise. The propulsion layout  (dual impellers feeding pressurized air to multiple thrust nozzles ) is  conceptually  similar to what was  employed by the NASA Ames Personal Satellite Assistant project in 2003.  

Novel features of the Astrobee Free- Flyer propulsion system include  low acoustic noise with high thrust ,  asymmetric nozzle layout which  reduc es the total nozzle count while maintaining 6- DOF maneuverability,  modular design, and variable thrust control.
443 Propulsion Development Process   During the early phases of the Astrobee project, several propulsion options were considered. The four  leading options were: 1.  Onboard compressor : Essentially replicate the Ames SPHERES cold -gas  propulsion system ( ref. [2])  except replace the liquid CO 2 tanks with compressed air tanks filled by an  onboard compressor. 2. Distributed fans : Use a system of six axial fans with reversible fan rotation, like a  multi -roto drone adapted for zero- gee, 3. Similar to option 2, but with Variable- Pitch Propellers (VPP ) to  increase fan thrust responsiveness without changing the rotor RPM, and 4. Centralized fans : Use two  impellers to drive airflow to many independently controllable nozzles. Option 3, multi -axial VPP fans, was  initially chosen for the Free -Flyer propulsi on system. Testing of Prototype 2 is shown in Figure 3.   

Figure 3. Astrobee Prototype 2 Multi -VPP fan Testing  

After encountering difficulties with packaging,  the VPP system (including limited future payload carrying  capabilities) and growing concerns over fan performance and reliability, a new propulsion trade study was  started. This time, the ducted nozzles and reaction wheels of the original  Personal Satelli te Assistant  centralized fan system were redesigned to a single pressurized plenum which greatly simplified packaging.  A propulsion system proof -of-concept was quickly made using 3D printed and off -the-shelf parts  (Fig. 4) .  The system immediately proved capable and was chosen to be developed for flight.  

Figure 4. Centralized fan Proof -of-Concept Testing on Force Plate
444 The basic elements of the centralized fan design are the centrifugal fan, plenum, and nozzles. The  propulsion system development  was iterative because no single element was independent of , or linearly  related to,  the other elements and they all needed to work together to meet the final requirements. This  meant that iteration occurred at different levels: Component Level,  Module Level, and Free- Flyer Level.  The challenge was to obtain the  data needed, then  iterate and then test at the next higher level, then iterate  again, and so on. This required significant testing, modeling, and analyses to narrow down the design  space. U nfortunately, this started late in the project  due to the earlier propulsion system  being chosen .  Some of the more challenging propulsion system requirements are listed in Table 1.  

Table 1. Challenging  Propulsi on System Requirements   Requirement  Value   Thrust  Minimum of 0.6  N in X -direction   Acoustics  Maximize NC -40 operation   (lower noise allows longer operating time)   Control  Holonomic control   Power (electrical) consumption  Operate flying ~ 2 hours without recharge   Size and Mass  Physical envelope and mass limits   ISS Requirements  Safety, materials, flammability, loads   Other project requirements  - Replaceable components   - Industrial Design   - Expanded research options: LED signals, swappable skin  

Component Level  Development   Examples of component level tests are shown in Figure 5. Nozzle aerodynamic performance characteristics  (along with the impeller) were determined through test using the ANSI Fan Test Rig within the NASA Ames  Fluid Mechanics Lab [McLachlan, B., Private Comm unication, 2019]. The test rig, representative test  articles and nozzle performance charts based on analysis and test are shown. In each chart, there are  multiple curves representing different fan sizes. The left chart shows combinations of fan rotation rate (N)  and nozzle open are a (A) that achieve the required maximum thrust (0.3 N for the X -nozzle). The right chart  shows the estimated Sound Pressure Level  (SPL)  for each combination.  

Figure 5. Examples of Component Level Tests  and Analysis  

Module Level  Development   Example module level test results and analyses are shown in Figure 6. The photo shows sound and  pressure measurements during proof -of-concept testing. Several CFD analyses were completed to study  grill effects and parameters such as spacing and depth dimensions. T he chart provides measured SPL vs.  Force (Thrust) for two different nozzle areas and four impeller RPM’s. The target SPL and thrust are
445 highlighted to indicate operational regions (note, acceptable SPL levels for one propul sion module were  estimated to be - 3 dB of two modules in operation).  

Figure 6. Examples of Propulsion Level Test Results  

Free-Flyer Level  Development   Examples of Free-Flyer  level test are shown in Figure 7. The granite table test allowed near friction -free  3-DOF testing (2 translational axis and 1 rotation) and was the only “flying” test prior to commissioning on  the ISS. As shown, the P4 prototype is mounted on top of a  goniometer  to test off -axis thrust components.  Along with EMI tests, early acoustic measurements were made in the Ames EMI test chamber. Official EMI  and Acoustic measurements were made at Johnson Space Center .  

Nozzle Description  

Thrust is controlled by varying th e nozzle open area for a constant impeller speed. The nozzle area and  impeller are sized to provide the required thrust performance (Maximum 0.3 N for the x -nozzle and ≈ 11m/s  exit velocity) . The forward and aft (X -direction) nozzles are physically larger to meet operational thrust  requirements. A propulsion module cross -section illustrating airflow and identifying a nozzle is shown  (Fig. 8).
447 Nozzle open area is adjusted by two gear  synchronized flappers  that are driven by an off -the-shelf hobby  style RC servo (Fig.  9). The two flappers provid e the benefit of dimensional compactness relative to a single  flapper design. The Nozzle grill and divider  (made of one- piece  3D printed Ultem 9085) provides  flow  straightening. Nozzle open area is determined by the flapper angle as shown in Figure 10 (flapper rotation  from closed to full open is 64 degrees) . 

Figure 9. Free-Flyer Nozzle  

Figure 10. Nozzle Open Area  

Figure 11 shows  a few of the nozzle  design  iterations with left to right going from the earliest prototype to  final flight design respectively . The initial proof of concept nozzle incorporated a single flapper  which  provided the desired thrust performance but was physically too large in the flow direction to fit the available  space. To address that issue,  a geared dual flapper  design was developed to minimize thickness while  providing an acceptable level of flow efficiency . To reduce development time, the first two nozzles were  made from 3D printed ABS material. The middle nozzle was an attempt to make a 3D printed “Flight” noz zle  (shown is an X-Nozzle), which proved problematic (see Lessons Learned).
448   Figure 11. Nozzle Iterations  

Nozzle Development Issues  

Several nozzle development  issues are expanded on.  

Nozzle Issue:  Flapper Slipping   Once calibrated, th e flapper  drive elements (consisting of clamps, gears and shafts) must not slip over the  life of Astrobee. However, slipping was observed d uring early prototype test s. The required flapper shaft  torque is 10 in- lb (1.1 N -m) (which includes a 5X slip factor ). Slipping can occur at two joints (Fig. 12): 1.  the flapper to the shaft, and 2. the press fit gear to the shaft. In torque tests, the integrated flapper clamps  worked well and were not considered the problem. T he press -fit gear did slip a t low torques . 

Figure 12. Flapper slip locations
449   Option s to increase the torque limits of the titanium shaft to bronze gear included:   1. Increase press -fit interference  The nominal fit is  ANSI FN2  providing 0001- in (0.025- mm) interference. Torque values as low as  3.68 in- lb (0.416 N-m) were measured in tests . The i nterference was increased to the point that  bronze shavings were observed but  with little additional torque benefit.   2. Add adhesives   Loctite 7649  primer with Loctite 609 retaining compound were tried. Even though the measured  torque values were higher than those without adhesive, the decision was made to include option 3  for consistently higher results.   3. Knurl the shaft   Knurls are commonly used for  small  plastic gears on metal shafts ( The cold flow of plastic material  into various topographical feature can result in a more constant or increase in torsional strength  over time [3]). Also, referencing a common home remedy for slipping where a center punch is  used  to provide mechanical interlock, a straight knurl  on the shaft was considered as an option. The  bronze gear would not plastically flow into the knurl, but it could essentially be broached while  press -fit to provide an additional mechanical interlock . Several different knurls were tested along  with and without Loctite adhesives. The measured torque values were all above 12 in- lb (1.4 N -m)  (highest being 21 in- lb (2.4 N -m)). Because of the consistent  results, the knurl and Loctite  adhesives are used on the flight nozzles. Figure 13 shows the two shafts and Boston gears prior  to assembly. The gear in the right image was first press -fit onto the flapper shaft and then removed  to show how the bronze gear  is cold worked by the shaft knurl.  

Figure 13. Knurled shafts for improved torque  

Nozzle Issue: Acoustic Noise  and Vibrations   Initial nozzle prototypes generated noise that was noticeably loud and annoying. The Free-Flyer’s  operating  time on the ISS  is limited by the maximum noise produced,  so it was critical to minimize all acoustic noise  sources. The following tasks were completed to reduce nozzle generated noise :  1. Select the best servo   The servo was quickly discovered to be the primary noise source. The noise occurred as the servo   feedback continuously adjusted to hold a position, with  the noise  being louder under load. It was  observed the noise was different for different servos. The following test  was used to select the best  servo: With the servo stalled (by commanding it past its limits) measure a ) housing temperature,  b)  current draw , c) time before failure, and d) a qualitative noise judgement . Seven different servos
450 were considered with the MKS DS95 being the clear winner. The MKS DS95 had the lowest noise  and was still functional after being stalled for 15 minutes.   2. Remove nozzle  backlash springs   The initial nozzle prototypes used backlash springs to apply a small closing torque t o eliminat e gear  backlash (Fig. 14). Unfortunately , this had an unintended consequence in that the  servo overcom es  the backlash spring torque produced additional noise. See lessons learned for comments on  removing the backlash springs.  

Figure 14. Backlash Springs  

3. Isolate and dampen nozzle vibrations   Qualitatively , the servo noise was  amplified when mounted to a propulsion module. Also, t he small  vibrations could excite surrounding structure and be a motion jitter source for the Free- Flyer.  Vibration measurement s of the propulsion system plenum structure, nozzle frame, and servo body  were made for an operating nozzle (Fig. 15). Test s with the nozzle hard mounted and isolated,  along with the servo mounted with aluminum and nylon standoffs were made. Vibration levels were  lowest for the isolated nozzle using aluminum standoffs. The final isolated design is shown in  Figure 16.  

Figur e 15. Nozzle Vibration Test
451   Figure 16. Nozzle Isolation Design  

Nozzle Issue: Bearing Install ation  Aligning  and pressing the 6- mm and 12- mm O.D. nozzle bearings  by hand (and with other simple tools)   was time  consuming and often resulted in damaged bearings. Including all the flight and ground unit   nozzles , over 800 bearings  are installed.  To improve the bearing installation process , two custom bearing  tools were designed and fabricated that proved reliable and capable of rework when necessary. The  installation steps are given in Table 2 and illustrated in Figure 17.  

Table 2. Bearing Installation Steps   Step Description   1 Locate Bearing and Nozzle Frame   The frame and bearing are radially located by the guide pin and sliding locator (the locator  slides on the guide pin and sits on top of the spring) .  2 Press Bearing   The guide pin centers the Ram such that only the bearing outer race is contacted as  shown in the closeup. The bearing and frame are now conc entrically located and can be  pressed together using an arbor press and overcoming the soft spring. The bearing axial  position is determined when the Ram bottoms against the frame.   3 Remove   Remove the Ram and nozzle frame. Inspect bearing movement.  

Re-work was necessary when the frame was not held down to the bearing tool allowing a slight angular  press . When this occurred, the bearing was pressed through the frame and re- pressed. A better method to  securely clamp the frame to the tool would be an improvement.
452   Figure 17. Bearing Installation Tool  

Lessons Learned  

Several lessons learned are expanded on.  

Lessons Learned : Limitation of 3D printed plastic parts  Significant use was made of 3D printed plastic parts during nozzle development and Astrobee development  in general . 3D printed materials make up 32% of a flight Free- Flyer dry mass  (the materials include: Ultem  9085, Windform XT 2.0, SOMOS Watershed XC 11122). 3D printed test articles and early prototypes  reduced development time and hel ped catch errors in the design.  

Partly due to the success the team had using 3D printed materials and their lightweight properties, the  initial  flight nozzles were designed to incorporate 3D printed flappers and nozzle frames  (Fig. 11). However, when  significant issues with outside vendor part quality and assembly reliability issues were discovered, those  parts were changed to a more traditional machined aluminum. The  inlet and grill dividers are the only flight  nozzle parts made from 3D printed plastic . 

The Astrobee hardware is classified as non- critical. That meant the 3D printed material strength  requirements are driven by the project ’s goal to have a reliable and long- lasting robot , and not th e more  rigorous fracture critical material requirements. There are no released NASA standards for materials and  parts made of 3D printed plastic (A  committee is working on NASA -STD-6030 for Additive Manufacturing,  which included 3D printed plastic, but it is not yet released) . 

A significant effort was required to develop 3D printed plastic standard processes, analysis assumptions,  and best practices  to assure a level of strength and quality.  And in some cases, alternatives to 3D printed  plastic shou ld have been explored. A good example is the Free -Flyer painted bezel shown in Figure 1 and  Figure 18. A simple design change would have allow ed the part to be easily machined thereby eliminating  the additional labor cost necessary to fill and smooth the Ultem 9085 raw surface in preparation for painting.
453   Figure 18. Free-Flyer 3D printed plastic painting  

Lessons Learned: Backlash Testing   Gear backlash affects nozzle open area and associated thrust for a given command input . It’s worse for  small nozzle openings since the backlash error is proportionally greater for a smaller open area. It wasn’t  clear how accurate the open area needed to be during the early development phases . The aerodynamic  forces acting on the flappers should pre- loading  the gear train and reduce backlash. However, flapper  binding was observed on some early nozzle prototypes  (which could act in the opposite direction of the  aerodynamic forces)  and t he decision was made to add a nozzle backlash spring to eliminate all source s  of error. A custom 0.012- in (0.30- mm) diameter 302/304 CRES torsion spring with 0.04 in- lb (4.5 mN -m) of  torque was added as shown in Figure 14 . The spring torque is  applied to close the flappers  and help  minimize leakage when fully closed (minimizing leaks  was another early concern) . The torque direction  would minimize the required servo torque since the plenum and airflow pressure s are trying to open the  flappers . 

Two issues  developed that focused attention on the backlash springs. The first was the nozzle servo noise  as discussed earlier. The backlash spring made  the servo noise much worse.  The second was the discovery  of broken backlash  springs on the Prototype 4 (P4) Granite Table Testing shown in Figure 7 . Smal l 0.012- in  (0.30- mm) diameter spring pieces floating in the ISS cabin would be an obvious safety concern.  The  decision was made to try operating P4 without the nozzle springs. The P4 testing had been successful and  going on long enough that any adverse changes to the Free- Flyer could be measured. The  tests showed  no change  without the springs and the servo noise was noticeably lower . The backlash spring was removed  from the design (however, the flapper still has a notch for the spring due to fabrication schedule constraints ). 

The backlash springs were added because of uncertainty in the required nozzle opening accuracy and  nozzle function. As it turned out, the springs were added to solve a theoretical concern that wound up  causing a real pro blem. It was fortunate that the design was not impacted when they were removed.