Summary

• No failure, degradation, or aging mechanisms attributable to the space environment.
• No significant change in design or materials in response to observed failures.
• Observed failures mitigated by process controls.
• Lube patch formation a random event (personal opinion).
• Flexlead corrosion an aging mechanism.

Description

• Six rate gyros
  • Rate gyro channels functionally separated into electronics control unit (ECU) and rate sensor unit (RSU).
  • Rate gyro channels packaged two per ECU and two per RSU. No cross strapping.
• Three channels required for normal vehicle control.
• Four channels typically used for fail operational capability.

Background

• Three channels failed prior to first servicing mission; fourth showed anomalous behavior.
• Post servicing mission evaluation of returned hardware isolated failure sites.

Failure Summary

• Rate gyro channel 1
  • Signature: Motor current to zero (failure).
    • Source: Shorted capacitor in ECU.
• Rate gyro channel 4
  • Signature: Gyro output to saturation (failure).
    • Source: Failed hybrid circuit card in RSU.
• Rate gyro channel 5
  • Signature: High mode scale factor transient (anomaly).
    • Source: Foreign particle in flotation fluid in RSU.
  • Signature: Motor current increase (anomaly).
    • Source: "Lube patch" on gas bearing in RSU.
• Rate gyro channel 6
  • Signature: Gyro output to saturation (failure).
    • Source: Failed hybrid circuit card in RSU.
  • Gyro motor spindown (failure).
    • Source: Open flexlead in RSU.

Lube Patch

• Lubricant is applied to reduce bearing stiction at wheel startup.
• Lubricant can accumulate on bearing surface resulting in increased stiction.
• Mechanism not well understood but appears to be related to run time.
• Gyro 5 lube patch accumulated to the point where bearing dynamic friction affected; requiring increased motor torque (e.g., motor current) to maintain wheel speed.
• Previously observed problem:
  • HEAO 1, 2, 3: 5 of 16 gyros fail to restart (4 of these recovered after repeated restart attempts).
  • IUE: 2 of 6 gyros have failure signatures consistent with lube patch accumulation.
  • Lube patch accumulation is major source of rejection during gyro screening.
• Processes already in place to mitigate lube patch problem.
  • Change lubricant from sodium stearate to tri-ethyl amine stearate (TEAS).
  • Controlled micro-pitting of bearing surface.
  • Increased motor startup voltage.
  • Screening: Measure variations in required startup voltage after wheel run-in.
• Current status
  • Lube patch problem is still with us.

  • Corrective actions seem to have mitigated its consequences if not its occurrence.
    • Gyro 5 continued to operate on-orbit.
    • Gyro 5 successfully restarted on the ground after return.
  • Further corrective action (all procedural).

  • Don't operate gyro for extended periods until you're ready to use it.
    • Lube patch accumulation appears to be function of operating time.

  • Once you've started using it, don't stop.
    • Can't guarantee motor will restart.

Flexlead Failure

• Output axis rotation facilitated by encapsulating the wheel assembly plus necessary electronics (e.g., float) and immersing it in a viscous "flotation" fluid.
• Motor current and output axis rebalance torque signals delivered to the float by thin conductive ribbons (e.g., flexleads).
  • Four flexleads carry motor current (two motor phases; two leads per phase).
  • Three flexleads carry output axis torque commands (plus and minus and a common return).
• Flexleads are immersed in the flotation fluid.

• Flotation fluid
  • Bromo tri-fluoro ethylene (BTFE); contains covalently bonded halogens (bromine, fluorine, chlorine).
    • Selected to satisfy viscosity and density requirements: float must be neutrally buoyant.
• Flexleads
  • 85% silver 15% copper aggregate.
    • Selected for conductivity/mechanical characteristics (flexibility, tensile strength).
  • Copper and (to a lesser extent) silver corrode in the presence of BTFE.
    • Extensive corrosion observed on flexleads of all returned gyros.

  • Corrosion/failure mechanism
    • Residual halides react with copper phase of the flexlead aggregate.
      • With copper (15% of material) gone, material is embrittled and tensile strength reduced 98%.
    • Corrosion continues at slower rate on silver phase.
    • Ultimate failure due to either fracture of embrittled silver lead or complete corrosion of silver.

  • Residual halides an inevitable byproduct of fluid manufacture.
  • Exacerbated by:
    • Moisture (forms acids).
    • Oxygen (replaces halogens in the polymer).
    • Light and heat (accelerate polymer decomposition).
    • Residual chlorides from solder flux and solvent.
    .
  • Indirect evidence that fluid lot used for all four returned gyros was a "bad" batch.

  • Corrective action
    • Specifications/screening for halide and moisture content of floation fluid.
    • Modify fill procedures to eliminate exposure to air (oxygen) during fill process.
    • Manage gyro operating time as a consumable.
    • Gyro operates at increased temperature.
    • Flexlead corrosion rate increases with temperature.

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