Mass Simulator ConceptsMass Simulator Concepts

Robert Besuner12 October 2005

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Dynamic breakdown of observatoryDynamic breakdown of observatory• Telescope/optics/focal plane, apart from SM support

assembly are essentially monolithic (all the lowest modes are SM support modes).

• Spacecraft is monolithic.• SM support is compliant.• Connection between telescope and spacecraft exhibits some

compliance.• Baffle and baffle connection to spacecraft exhibits

significant compliance.

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Secondary Mirror StructureSecondary Mirror Structure• OTA fundamental modes are all in

SM structure deflections.• Flight design all CFRP:

— Legs 250mm wide, taper from 19mm to 38mm wide, 2mm thick, plus fat bases. Mass for three legs together is 23.5kg.

— ‘Lampshade’ plus SMA (including adjusters) ~15kg.

• Proposed simulator:— 2”x6”x1/8” aluminum rectangular

hollow tubing.— 575mm diameter x ¾” thick disks

for SMA/lampshade.• Comparison of flight to simulator:

— Total mass within 0.5kg.— cg position within 100mm— 1st modes SM structure mounted to

rigid base.• Flight: 46Hz rotation, 47Hz sway.• Simulator: 44Hz rotation, 53Hz

sway.— Some refinement can dial it right in.

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Telescope and focal planeTelescope and focal plane• Two basically triangular plates separated,

assembled essentially rigidly together.— Triangular rather than circular intended to

help keep Izz down.— Separation allows for convenient mounting

place for SM structure and bearing sphere (top plate), a convenient mounting position for telescope supports (bottom plate), and lets you put the cg where it ought to be.

— Hole in lower plate allows for air bearing support and tilt clearance.

• If separation were somewhat greater, could consider orienting observatory horizontally.

• Mass props:— Flight: m=442kg, Ixx=164kg-m^2 ,

Iyy=231, Izz=242— Simulator: m=436kg, Ixx=102kg-m^2,

Iyy=102, Izz=191 (should perhaps let it be more circular).

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OTA iso viewsOTA iso views

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SpacecraftSpacecraft• Big donut (82mm thick, 2.5m OD, 1.5m hole) to which telescope, baffle,

and some ACS components attach.— 3”x3”x1/4” aluminum box tube for telescope support.— 1-1/2”x3”x3/16” aluminum box tube for baffle support. — Telescope and baffle support tubes land near each other on spacecraft to avoid

big load paths through spacecraft plate.— Might still want a flange or something to stiffen spacecraft...need to examine

fea.• Big hole for bearing clearance makes moments of inertia bigger than

flight will likely be.• Mass props:

— Flight: m=707kg, Ixx=288kg-m^2, Iyy=288, Izz=537— Simulator: m=706kg, Ixx=376kg-m^2, Iyy=376, Izz=752.

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Observatory Free-Free ModesObservatory Free-Free Modes19hz 27hz 30hz

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BaffleBaffle• Height constraints limit size.• Desire simplicity.• How to match fundamental

modes? This concept is tunable (not yet tuned).—Column section props can be

varied to change compliance.—Masses can be added many

places to match flight mass props.

—Cosmetic skin could be applied, left off for easy access.

—Alternative might be to use sheet metal walls on this design, tie one panel to the next in varying degrees to tune stiffness.

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Some questionsSome questions• Would it be better to just build a relatively lightweight

frame and install mass (lead, brass, steel) separately, rather than beefy platestock everywhere?

• Connections...bolted? welded? bolted then tacked and welded?

• Handling features?