SSRMS Forward Plan 7A Launch Recommendation

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SSRMS Forward Plan7A Launch Recommendation
OM/Program Integration
6/18/2001
C.A. (Skip) Hatfield 281-244-7766
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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
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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

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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
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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
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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
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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

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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

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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)

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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.

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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/workaround
  s in place
• EVA keep alive option in place as last resort

Recommendation Proceed with 7A Launch in July
2001
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Backup Charts
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Team Organization/Responsibility

• The Team consisted of a steering committee and 4
  subteams.
• SSRMS ART
• Investigate the root cause of the SSRMS anomalies
• Develop appropriate corrective action
• Develop risk mitigation
• Operations Plans
• Develop operations plans to protect mission
  success
• Assure operations plans are safe
• 7A and 7A.1 Flight sequence impacts
• Evaluate impacts to reversing the flight order of
  missions 7A and 7A.1
• Provide flight sequence recommendation
• 7A.1 Manifest
• Prepare to fly spare SSRMS joint if required
• Evaluate options to decouple the 7A.1 manifest
  from the 7A manifest, in order to provide maximum
  flexibility in launch sequence