spp 4m magnetometer boom proposal n.
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SPP 4m Magnetometer Boom Proposal. Paul Turin 10/17/12. Design Drivers/Choices. Scientists want 4m length for acceptable mag separation Must stow within bus panel length Three segment boom only articulated choice that will fit on deck with imposed constraints Minimize mass

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design drivers choices
Design Drivers/Choices
  • Scientists want 4m length for acceptable mag separation
    • Must stow within bus panel length
      • Three segment boom only articulated choice that will fit on deck with imposed constraints
  • Minimize mass
    • CF tubes for light weight, stiffness, low thermal bending
    • Aluminum components for low mass, ease of fabrication
  • Two-point mounting on bus
    • Simplifies interface, low mass
  • Assume deployed 1st mode >.5Hz for ACS stability
  • Minimize SC resources: single point release, no heaters for simplicity, reliability
    • Couple hinge rotations to sequence deployment for safety
    • Provide coordinated segment sequencing – all arms deploy at proportional rates
    • Eliminate chance of bus strike with hung joint
  • Joint drive redundancy
    • Increase force on any hung joint
  • Zero play hinges for no deadband
  • Full-compliment duplex angular contact bearings for zero play, high load capacity
  • Torsion spring deployment springs
    • Reasonable T0-T1 ratio
    • Light weight, compact, simple
    • Nonmagnetic in Elgiloy
  • Spring load against stops at EOT – no latching for simplicity, reliability
  • Wide deployment and operational temp range
  • Flyweight brake speed control: no heaters req’d, low mass
boom deployed
Boom Deployed

Boom lies on SC centerline

1m between mags, 2m to first FGM

stowed on bus
Stowed on Bus

4m fits easily

joints stowed
Joints Stowed



stow preload braces
Stow Preload Braces


SC-Consil Cushions

  • Braces mounted to center tube are larger than spacing of stowed tubes
  • Outer tubes are bowed to provide preload to couple tubes for greater combined stiffness. Raises 1st mode 10Hz
  • Spreads FGM loads across tubes
  • Consil SC pads between tubes and brace for cushioning and damping (electrically conductive)
kickoff springs
Kickoff Springs

All joints have kickoff springs to provide high initial deployment forces until components start moving. Provides protection against sticking surfaces etc. while keeping joint drive torsions springs reasonably sized.

544 bronze plungers running in hard anodized bores, compression springs between pairs

joints deployed
Joints Deployed




hinge design
Hinge Design
  • Hinge designed for zero play, low friction
  • Zero play bearing cartridges
  • Aluminum axle, clamped to bearings and yokes for zero play
  • Vespel SP3 idler rollers for low torsion spring coil drag
  • Elgiloy springs – nonmagnetic, high yield (post-forming etching required to remove magnetic oxide layer)
bearing cartridge
Bearing Cartridge
  • Bushings don’t cut it -- .001” bushing radial clearance = .320” @ end of boom w/3 hinges
  • Utilize ZrO2 full ceramic, full-compliment, angular contact duplex pair bearings
  • Ground for precise preload, zero radial/axial play when inner and outer faces are pressed together
  • Ti housing with nut to provide preload – no shimming needed
  • ZrO2 and Ti have Very close CTE – zero play, constant preload across wide temp range (.0001” delta over 100C)
  • No lubrication required for low speed ceramic bearings – no added drag from grease at low temps
  • Post-deployment loads negligible
  • 600% margin on launch loads
  • 30% lower mass than steel
  • 50% stiffer than steel
stowing deployment initiation
Stowing, Deployment Initiation

Stow cage

Cage spring

Ears support end in cage

Wrist kickoff plunger

FC-4 Frangibolt

Elbow kickoff plunger

Kickoff plungers extended

Stow tower

uncaging of wrist
Uncaging of Wrist

Roller is caged in slot on shoulder

Shoulder-wrist kickoff plunger

Shoulder kickoff plunger

deployment control assist cords
Deployment Control/Assist Cords
  • Motion of arms needs to be coordinated well enough to prevent bus strikes
  • This can be done by “drafting arm” style control cords
  • Pulleys fixed to shoulder and arm ends, connected by one set of cords (red), synchronize arm movement
  • Second set of cords (green) are passive until a hinge slows down due to a problem, then applies torque of all hinge springs (3X) to that hinge. Prevents one arm from getting ahead of any other.
  • Allows one brake to control deployment rate of all segments – significant mass savings
  • A tech note is available with a detailed explanation of this operation and the forces involved
deployment control cord
Deployment Control Cord

Rotation of middle tube controlled by cord-payout rate from shoulder

Deployment rate of shoulder controlled by brake

Pulley fixed to end of middle arm

Cord pays out as shoulder rotates

Pulley fixed to shoulder

deployment assist cord
Deployment Assist Cord

Assist cords do nothing during normal deployment. If a hinge hangs up, torque of all hinge springs is transferred to stuck joint. Deployment is now governed by stuck joint. This puts triple the torque on that joint.

Cord pulls in as shoulder rotates

control cords
Control cords
  • Several fiber types will do, currently looking at braided Spectra (Honeywell product, UHMW Polyethylene)
  • Low stretch, high stiffness, high strength
  • 500# test = 1.2g/m .053” dia. Sees 11lb static load. Much stronger than needed – choose size for ease of handling
  • Hollow braid allows loop splice, provides high percentage of line strength, works well with bobbins for termination
  • Meets outgassing with bakeout
  • Lines are slightly slack at EOT – do not affect positioning
flyweight brake
Flyweight Brake
  • Same basic design as used on FIELDS antennas
  • Lower gear ratio for faster speed – target 5 rpm
  • Planetary gearbox and weighted, sprung brake shoes for low starting torque, V2 speed control
  • Smaller, lighter 2nd generation version has been completed. Fewer stages, lower mass, lower starting torque, meets target speed.
  • “Watchmaker’s” version under development for further weight reduction
misc design c alcs
Misc. Design Calcs
  • 1.94kg mass (not including mags, harnesses)
  • 4.3kg with all (assume 3” harness loops at hinges)
  • Maneuvering torque on boom 2.5 inlb
  • Maneuvering unseat torque ratio 3.8
  • Maneuvering load on worst-case bearing 2.9lb
  • 100g launch load on bearing 133 lb
  • Bearing capacity 936 lb
  • Bearing SF 9.3
stowed modal analysis
Stowed Modal Analysis

Includes same masses as deployed analysis

glue joint design
Glue Joint Design
  • Hysol 9309 structural epoxy, MJ55 to alodined Al
  • Undercut sleeves, injected glue with bleed holes
    • for tube centering and bondline control, uniform fill
  • Same design as STEREO
  • Joint design qualified to 35K on 8” dia
  • 8 joints on STEREO, 30 on THEMIS no issues

Cavity for glue

longer boom
Longer Boom

There is room on the bus to easily increase the boom length to at least 4.5m.

Only 82g mass increase

Further analysis is needed to look at effect on modes, but probably not an issue.

to do
To Do
  • Look at kickoff plunger loads on flyweight brake
  • Look at stowed loads in more detail
  • Harness routing – how to hold loops
  • Harness stiffness – measure at cold predicts
  • Torsion spring adjustments as necessary
  • Torsion spring termination