Status Report

SSRMS Forward Plan 7A Launch Recommendation

By SpaceRef Editor
June 18, 2001
Filed under , ,

SSRMS Forward Plan

7A Launch Recommendation

OM/Program Integration

6/18/2001

C.A. (Skip) Hatfield

281-244-7766


Purpose/Groundrules

  • The SSRMS Forward Plan Team was established to investigate the SSRMS anomalies, evaluate options for recovery (including flight sequence impacts), and recommend a forward plan for the program.
  • Constraints for this task included:

    • Assure crew rotation in August 2001 (7A.1 mission must fly prior to August 18 range closure)
    • Assure mission success (including preventing scenarios that are impossible to realistically recover)
    • Work within all Beta cutout and range constraints
    • Assure multiple launch attempts for both 7A and 7A.1 missions
    • Finally, minimize impact to flight sequence if feasible

  • Following extensive evaluation, a recommendation was achieved. The balance of this presentation provides the detailed rationale.

    Recommendation: Proceed with 7A Launch in July 2001


    Problems to Resolve

  • There are 3 significant intermittent faults with SSRMS – Explained
  • Elbow Pitch joint RDC BIT fault
    – Occurred once, and cleared

  • Brake Voltage Faults
    – Occurred several times, then cleared

  • Shoulder pitch joint RT comm error
    – Most serious of the faults

    – Persistent for several days, then cleared


    • 7A.1 mission is coupled to 7A – Resolved
  • If launched first, several cargo items need to be removed from 7A and relocated to 7A.1
  • If problem not resolved, must terminate 7A attempts NLT 6/22/01

    • Elbow Pitch RDC BIT

  • This fault occurred once on Day 137.

    • Appearance of this fault causes the MSS system to auto-safe
    • The fault immediately cleared, brakes were removed and operations continued
    • Malfunction procedures lead to the transient conclusion
    • Note: The elbow pitch RDC BIT flag is unrelated, and believed to be a result of preload on the SSRMS prior to ungrapple

  • Fault Tree Analysis has concluded there are only 2 probable causes.

    • Transient due to noise on SIN/COS or 3Khz excitation signal, or
    • Single Event Upset (station was at high latitude at time of occurrence)

  • For either of these causes, the fault is highly likely a transient condition.
  • Therefore, this event is considered to be a nuisance item that may reappear, but can be immediately cleared and operations continued.

    Not a constraint to flight

    Brake Voltage Mis-compare

  • Occurred on Day 137 following the wrist roll RDC BIT fault
    – Three different messages indicating loss of brake bus voltage, caused safing and application of brakes
  • Failure repeated on next 6 attempts to release brakes
    – 2 times immediately after fault, then again 5 hours later
    – 3 times when brake diagnostic tests were performed 9 days later
  • Failure has not returned after 50 successful brake releases over 28 days
    – Leaving the brakes off for long periods (>1.5 hours) did not trigger failure
  • Fault Tree Analysis has concluded there is one probable causes of the fault
    – Short on ACU Brake card causes erroneous current limiting, leading to loss of voltage under load.
  • Fault is clear with no evidence of recurrence.
  • If fault returns and in persistent, string functionality can be restored with spare ACU replacement (spare is available on orbit)

    Not a constraint to flight


    Shoulder Pitch RT Error

  • Faulty first occurred on Day 143 when operational on redundant string
    – Fail to set RT address during initialization

    – Fail to communicate in standby/operational (2/3 of occurrences)

  • Fault was persistent until day 158
  • Diagnostic patch indicates JEU transmits/receives data correctly
    – JEU sets RT error flag periodically during transmission received correctly by ACU, resulting in safing

  • Fault Tree Analysis has concluded there are 2 probable causes of the fault, both in the 1553 chipset in the JEU
    – Loop back test (most likely)

    – Watchdog timer

  • Investigation has revealed a known issue with 1553 chips from this vendor susceptible to intermittent errors on loop back function
  • Fault can be masked with “simple” patch with no secondary effects or loss of capability, available on or about 23 June.

    Not a constraint to flight

    Summary of Technical Evaluations


  • At the present time, both strings have been in operation for an equivalent of 2 years expected operation
    – Over 1200 hours in keep alive

    – Over 400 hours in operational

    – Over 5 hours of arm motion

  • The longest fault was the shoulder pitch problem, which persisted for 14 days
    – Approximately 28% of total powered (Keep alive) time

  • Why have these faults occurred only on the redundant string?
    – It appears that chance has caused this, with no common root cause identifiable that would explain any grouping of these events

  • Are these faults generic or confined to a single string
    – ACU/Brake fault is isolated

    – Intermittent RDC BIT errors may occur on either string

    – The 1553 RT error may not be isolated to a single string


    Mission Success

  • Presently, we have 2 functioning SSRMS strings
    – Technical Rationale in place to support probable root cause

    – SW Patch needed to protect for 1553 RT faults

    – Spare ACU in place for brake fault if required

  • EVA options in place to support driving the SSRMS to Airlock keep alive position if required
    – Can protect for loss of both 1553 strings

    – Can install 2 of 4 high pressure gas tanks

  • Missions 7A and 7A.1 near decoupling
    – LPM Sharon Castle to present any required decisions to support this


    Forward Work

  • Complete 1553 SW Patch
  • Conduct alternating string demonstrations of continued SSRMS performance
    – Opportunities exist every Tuesday (limited) and Thursday through launch

    – Develop plan for how much testing to do (probably conduct a mix of airlock dry runs for proficiency, L-48 type performance checkouts)


    Readiness

  • Teams have worked hard to complete this evaluation. The teams were polled for their readiness to proceed, given the recent high workload
    – CSA/MDR:

    – Engineering/MER:

    – Flight Control:

    – LPM:

    – Increment Manager:

  • Procedure Readiness
    – All procedures needed for 7A operations are in place.


    Conclusion

  • CSA/MDR certify SSRMS ready for 7A mission at this time
  • There are no safety constraints
  • Robust plan for mission success
    – Working Prime String

    – Anomalies on redundant string explained/workarounds in place

    – EVA keep alive option in place as last resort

    Recommendation: Proceed with 7A Launch in July 2001

    • SpaceRef Editor

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