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Public Lessons Learned Entry: 1113

Lesson Info:

  • Lesson Number: 1113
  • Lesson Date: 1999-02-01
  • Submitting Organization: HQ
  • Submitted by: David M. Lengyel


International Space Station (ISS) Program/Extravehicular Activity (EVA)/Simplified Aid for EVA Rescue (SAFER)

Description of Driving Event:

Lack of Sufficient Operational Assets to Procure Necessary Safety Hardware

Lesson(s) Learned:

The EVA project lacks sufficient operational assets to meet unplanned contingencies. There are no spare Extravehicular Mobility Units (EMU's). Only five U.S. Simplified Aid for EVA Rescue (SAFER) flight units will be available to meet a requirement to maintain three units on orbit. In addition, only four Russian SAFER units are planned.


To meet contingencies that are almost certain to arise, additional EMU's and SAFER units or their critical long lead components should be procured as soon as possible.

Evidence of Recurrence Control Effectiveness:

NASA concurs with the ASAP recommendation. With respect to the EMU, the inventory of life support system (LSS) hardware will be 14 (13 Class I and 1 Class II) by October 1999.Exceedences to our supply begin in 2000. In order to achieve a 90 percent probability of sufficiency, NASA must increase its inventory by two LSS's. NASA plans on addressing this issue within the Program Operating Plan (POP) 99. We plan to upgrade the current Class II LSS to Class I and upgrade the certification unit to Class II. This will increase our inventory to 15. NASA also plans to go forward with the recommendation to procure an additional LSS to achieve 16 LSS's. Additionally, the current space suit assembly (SSA) flight hardware models predict SSA demand beyond the current inventory of 15. The demand peaks at 23 for one month, but there are 15 months where it is at 18 through 2004. The current plan is to procure hardware to 18 through the POP 99.SS. A hardware shortages can be determined once crewmembers are selected. The lead times for SSA hardware are such that once shortages are determined, specific hardware shortages can be procured. The current training model for the EMU predicts demand not to exceed the procured inventory of 10; therefore, sufficient inventory exists for training.

With respect to the USA SAFER, NASA concurs with the ASAP recommendation on obtaining critical long lead components. In fact, the majority of the long lead components have already been procured. These components are expected to support the USA SAFER flight units for their 7-year life.

NASA can normally support the requirement to maintain three USA SAFER flight units on orbit with five flight units in service. The current rotation plan utilizes two of the flight units to accommodate rotation of back-to-back missions where the turn-around time is approximately 1 month. With one flight unit out of service, four USA SAFER flight units can be rotated to maintain three units on orbit for 92 percent of the flights per the International Space Station (ISS) assembly sequence dated February 22, 1999.The remaining 8 percent of the flights can also be supported with contingent coordination ahead of time to reduce the turnaround time from approximately 1.5 months to approximately 1 month.

Another option was already planned to deal with the margin in the rotation of five USA SAFER flight units. In order to increase the USA SAFER logistics margin, an extension of the 1-year certification will be assessed based on flight performance. Being able to leave the units on orbit longer will allow the rotation rate to decrease sufficiently to eliminate any problems with having one unit out of service. Data will be collected for analysis immediately after the flight units are declared fully operational. At the present, additional USA SAFER flight units are not needed for the following reasons:

  1. The rotation of five flight units can fully support the flight requirement
  2. The rotation of four flight units can support the flight requirement for at least 92 percent of the current ISS assembly flights
  3. The turnaround time can be reduced for special cases
  4. The on-orbit certification is expected to be extended with additional flight data.

However, in the event a USA SAFER flight unit is not available for any reason, the EVA crew is trained to use the two-fault tolerant tethering scheme to meet the safety requirement. This tethering scheme is fully certified and has been used successfully during several EVA's, including those on the recent STS-88 mission. Lastly, with respect to the Russian SAFER, NASA has revised its plan and will now produce five flight units, rather than four, in order to support the logistics model consistent with the USA SAFER plan.

Documents Related to Lesson:


Mission Directorate(s):

  • Space Operations
  • Exploration Systems

Additional Key Phrase(s):

  • Aerospace Safety Advisory Panel
  • Extra-Vehicular Activity
  • Flight Equipment
  • Procurement Small Business & Industrial Relations
  • Research & Development

Additional Info:

    Approval Info:

    • Approval Date: 2002-02-12
    • Approval Name: Bill Loewy
    • Approval Organization: HQ
    • Approval Phone Number: 202-358-0528

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