1、Best Practices Entry: Best Practice Info:a71 Committee Approval Date: 2000-03-09a71 Center Point of Contact: GSFCa71 Submitted by: Wil HarkinsSubject: Spacecraft Deployed Appendage Design Guidelines Practice: This guideline describes design practices for deployable appendages which can improve accur
2、acy of analyses, simplify and optimize designs, and minimize fabrication and test problems to produce reliable deployables. This guideline includes techniques used on successful missions and will help avoid past deployment problems.Programs that Certify Usage: N/ACenter to Contact for Information: G
3、SFCImplementation Method: This Lesson Learned is based on Reliability Guideline Number GD-ED-2209 from NASA Technical Memorandum 4322A, NASA Reliability Preferred Practices for Design and Test.Benefit:Increases confidence in designs and their operational reliability. Ensures accuracy of design analy
4、ses, completeness of requirements in procurement documents and thoroughness of test planning. Ensures functional compatibility of assembly and test fixtures.Implementation Method:Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-Designs must factor in
5、effects of the space environment. Relevant differences between ground test conditions and flight deployment should be identified for consideration at the design and test stages to assure operation with adequate margins. G-negation pickup points can be designed in. Historically, some deployables have
6、 passed ground tests but failed in space. Space imposed conditions include thermal gradients and thermally induced loads, outgassing, low moisture, zero gravity and ballooned thermal blankets. Handbook material values do not always apply for the space environment. Physical parameters vary such as so
7、lid film lubricant friction which varies with moisture content. The space environment is not easy to simulate in earthbound tests and allowance should be made for expected parameter variations.Deployed appendages should be designed using the following guidelines:1. Avoid complication; simple designs
8、 are more reliable.2. 3. Avoid single point failures. If unavoidable, assure generous margins. Where practical, design them out or employ redundancy unless redundant complication reduces reliability. Assure redundancy is truly independent, not coupled.4. 5. Margins - Provide minimum torque/force mar
9、gin: Tt= 1.25 Tf+ 4.0* Tvwhere: Tt= minimum total available torque/forceTf= fixed and non-variable loads such as Ia termsTv= worst case variable torques/forces such as coulomb friction and other loads which vary with environmental conditions and operating life.*Factor 4.0 is the Design Goal, reduced
10、 where excessive mass, power or volume is required, e.g., motor driven mechanisms. Maintain a minimum factor of 2.0 under worst case degraded conditions.6. Force analyses should use best available loads and forces and examine the full range of expected minimum and maximum parameter values, make adeq
11、uate allowance for uncertainty. Torque requirements are higher with high friction, deployment velocity and momentum higher with low friction. Use test verified values where available for such critical parameters as friction (if only handbook values available, multiply by 3 to cover uncertainties), c
12、able flexure torque, bearing drag torque. Include worst case thermal effects, wearout, friction changes and end-of-life conditions.7. 8. Designs should tolerate moderate increases in friction. Maintain calculated torque/force margins above 2 with m assumed up to 0.5 for typical material combinations
13、. Friction force Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-estimates are untrustworthy, handbook values represent controlled conditions. Use cautiously, test if possible. Consider case differences between published numbers and actual conditions
14、 such as effects of part surface treatments, differences between ground test conditions and space environment (humidity, pressure, loads). Use appropriate fits, alignment tolerances, surface treatments, deburring and hardness differentials to avoid galling which nullifies friction estimates.9. 10. T
15、emperature effects at deployment should be accounted for, e.g., torque/force changes at hingelines and separation points from temperature gradients within assemblies, bulk temperature effects and differential temperatures between deployables and mounting structure.11. 12. Humidity effects, e.g., mol
16、ybdenum disulfide (MoS2) friction varies and nonmetals change size with moisture content, can affect torque margins and fit clearances. Moly lube should not contain graphite.13. 14. Rolling element bearing loads for long life devices should be kept below 320,000 psi mean Hertzian stress for launch,
17、200,000 psi or lower for operation, less if lubrication is marginal.15. 16. Consider effects on bearing preload, torque and life due to thermal gradients and bulk temperature variations.17. 18. Analysis. Loads analyses and load sharing between deployed appendages and spacecraft structure can be grea
18、tly affected by compliance of typical appendage joints. Examples are bolt preloaded joints, separation joint clamping devices, bearing supported hinge lines, and movable mechanical joints such as gimbals and solar array drives. Accurate stiffness knowledge of these devices is crucial for valid loads
19、 analyses. Verification and validation should be addressed in design, and plans made to acquire sufficient data to verify models. Kinematic diagrams should be constructed including all degrees of freedom and constraints. For accurate loads and stress analyses, assure correct component parameters are
20、 used in the Finite Element Analysis (e.g., bearing stiffness, friction coupling, compliance of bonded joints, stiffness of movable joints - clearance fit or preloaded, transmissibility across joints). Perform loads analysis based on true compliance of individual members for accurate assessment of l
21、oad sharing. Cross-check with deflection analyses, compare with measured values. Incorporate proper deployment angles, velocity, acceleration and impacts to size components with adequate predicted margins. Include critical component parameters in analysis based on actual measured unit data where pos
22、sible, eg. bearing friction, compliance of assemblies. Account for fabrication and assembly variables which affect design Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-parameters used in analysis.Evaluate zero-g case to determine impact on design a
23、nd verification. Jerking (non-uniform acceleration) can be critical in deployment. Its effects should be evaluated. Stick-slip friction conditions can worsen the condition.Stowed and deployed frequencies should be evaluated. These affect launch loads and Flight Attitude Control.19. 20. Separation Pl
24、anes Employ kickoff springs where practical in the separation joints; especially important for long appendages. Use anti-seize, anti-weld coatings, dissimilar materials. Provide retention joint flexure capability to accommodate launch loads, e.g., spherical, vs conical cup-cones. Assure adequate dyn
25、amic envelope clearance. Use the following guidelines for wire and cable routing at hingelines. a72 Mechanical design of appendages should allow cabling across the hinges to be left fixed and undisturbed after component deployment testing throughout spacecraft assembly and integration. Provide field
26、 joints for connection of hinge and cable subassemblies.a72 Minimize flexure torque through entire travel, allow for temperature effects. Test new designs.a72 Use wire guides and clamps, braided and laced cables to maintain free loop control in 1g testing and launch environment and to avoid snags du
27、ring deployment.a72 Control chaffing during deploy and stow tests and launch vibration.a72 Use proper cable clamp size for no-slip position control and cushioned clamps for Coax cables.a72 Control Coax cable minimum bend radius.21. 22. Coax Cable Protection during Component Assembly and Integration
28、and Test Design-in armor protection where possible. Provide takeup allowance for connect/disconnect without cable kinking. Train personnel in correct handling procedures. Follow planned Coax cable handling procedures, use safe temporary support when free ends are not connected, provide for safe and
29、gentle tie-down, prevent crush.23. 24. Sharp Edges Eliminate sharp corners that cut through lube coatings, produce galling. Use specific drawing callout to control break and blend of sharp corners on moving elements. This is not always Provided by IHSNot for ResaleNo reproduction or networking permi
30、tted without license from IHS-,-,-covered by general drawing callouts or implemented under General Workmanship Standards. Double-check workmanship during assembly; visually inspect for sharp corners, handcheck running fits and clearances and verify clearance of chamfers to inside corners.25. 26. Con
31、tamination Control Design-in labyrinth or contact type seals to contain lubricants, minimize outgassing and limit external contamination.27. 28. Limit Switches for Position Monitoring Switches are prone to misadjustment and failure. Use only to indicate safe/stow/deploy positions, not to control dep
32、loyment sequence. Allow for overtravel and deadband (hysteresis) effects, provide adequate margins for actuation force and stroke. Provide simple, stable adjustability at assembly.29. 30. Avoid galling of close-fit parts in moving members during assembly or operation. Prefer non-galling metal-metal
33、couples. Provide adequate differential hardness of parts. Substitute anti-galling stainless steel for conventional SST. Provide anti-gall surface treatments to prevent damage in assembly/disassembly/adjustment operations. Provide anti-gall treatment for titanium parts and fasteners.31. 32. Provide a
34、dequate clearance fit allowances in moving members to accommodate: Bulk temperature effects, thermal expansion (CTE) mismatch effects and temperature gradient effects.33. 34. Design slip fit interfaces for thermal expansion takeup. Assure friction control where intentional slip is required. Use surf
35、ace treatments, lubricants and fit clearances which prevent lockup or cocking where slip is intended.35. 36. Deployment and Temperature Effects. Determine temperature effects on stowed and deployed frequencies. If explosive deployment devices are used, determine near source shock levels and effects
36、on deployables (instruments, optical devices, etc.). Deploy latches must not rely on appendage momentum; latches should lockup under static (zero velocity) appendage force/torque. If friction dependent, control surface contamination for predictable friction. Avoid thermally induced misalignment (e.g
37、., Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-differential temperature of deployable boom legs). Consider post-deployment thermal contraction of deployable boom lanyards causing limit switch reactuation at low temperature. Control deployment end
38、-of-travel impact, use energy absorbing dampers where necessary. Use floating pin/flange arrangement for hinge joints where practical to provide redundant sliding surfaces. If system will be dormant for a long time, it should be designed so all deployable parts can be partially deployed and retracte
39、d periodically to prevent “lockup“ due to stiction, lubricant changes, or adhesion of plastics.37. 38. Handling - large items Design-in handling fixture attachment points. Locate attachment points to position c.g. for stable lifting. Where feasible, provide for 1g counterbalance attachment on hardwa
40、re for functional testing.39. 40. Design-in self-fixturing alignment and assembly guides. Self-aligning guides permit assembly without expensive fixtures, reduce need for highly skilled personnel. Provide self-aligning parts, with lead-in where practicable to protect precision surfaces, interfaces a
41、nd bearings and minimize assembly debris generation.41. 42. Provide disassembly features. Plan for unexpected disassembly, rework or reinspection. Design-in pry slots on close fit parts to permit easy separation without damage during typical assembly and disassembly for fit checking, shim selection
42、or end shake checks. Provide puller holes, slots, jacking screw threaded holes or punch holes for disassembly of press fit components.43. 44. Key electrical connections. Use differently keyed or dissimilar electrical connections for squibs and motors to prevent inadvertent interchange between prime
43、and redundant and prevent malfunction.45. 46. Use specific fastener hardware designs for appendages. Use plated self-locking nuts that reduce thread wear and reduce probability of galling and thread seizure but withstand few reuses. Fastener seizure and removal degrades critical hardware. Limit reus
44、e of self-locking designs. Threads can become damaged, generate wear particles. Choose head designs for driver engagement appropriate to application, torque requirement and accessibility for tools. Small hex drive screws strip easily, Torque-Set and coinslot drive types are difficult to remove. Torq
45、ue stripe where appropriate to ensure integrity, permit later verification. Avoid countersunk (flat head) fastener usage where Provided by IHSNot for ResaleNo reproduction or networking permitted without license from IHS-,-,-practical; driver engagement is easily damaged.47. 48. Provide clearance fo
46、r thermal blankets and tiedowns at moving interfaces. Provide a taping edge or draw string flange to control blanket edges at moving interfaces. Provide adequate clearance for launch environment dynamic deflections with no protrusions or sharp edges to catch blankets. Provide adequate clearance gap
47、with securely taped edges at moving interfaces; minimum one inch between hard surfaces which get blanketed.49. 50. Lubrication for deployed appendages. Lubrication is crucial for deployables, many of which are mission critical. Key lubricant functions include the separation of surfaces and friction
48、reduction. This is critical because often the deployment forces and kickoff spring forces are kept small to minimize dynamic forces. For sensitive science missions, extremely low outgassing is essential. Most deployables are single use and experience few cycles through test and flight so long wear l
49、ife issues are not paramount in the lube selection. However, movable joints must sustain ground and launch environments, Flight conditions of sometimes long duration, and then deployment, often at temperature extremes. The clamped separation joints may experience micromotion at interfaces during ground handling, test and launch environments which can disturb surfaces. Lubes must st
