SPP 4m Magnetometer Boom Proposal

Paul Turin10/17/12

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– 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 lies on SC centerline1m between mags, 2m to first FGM

Stowed on Bus

4m fits easily

Joints Stowed

Elbow/EndShoulder\Wrist

Stow Preload Braces

• 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)

Brace SC-Consil Cushions

Kickoff SpringsAll 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

Shoulder

Wrist

Elbow

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• 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

Cage spring

Stow cage

FC-4 Frangibolt

Wrist kickoff

plunger

Elbow kickoff

plunger Kickoff plungers extended

Stow tower

Ears support end

in cage

Uncaging of Wrist

Roller is caged in slot on

shoulderShoulder-

wrist kickoff

plunger

Shoulder kickoff

plunger

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

Pulley fixed to shoulder

Deployment rate of

shoulder controlled by

brake

Pulley fixed to end of

middle arm

Cord pays out as shoulder rotates

Rotation of middle tube controlled by cord-payout rate

from shoulder

Deployment Assist Cord

Cord pulls in as shoulder rotates

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.

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• 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 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

Deployed Modal Analysis

Stowed Modal Analysis

Includes same masses as deployed analysis

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 issuesCavity for

glue

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• 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