scope:

Description of Driving Event:

Background:

In October 2001 the Payload Safety Review Panel (PSRP) conducted the phase III flight safety review for the Microgravity Science Glovebox (MSG) payload. During the safety review, the PSRP discovered that one of the MSG client payloads, a vibration attenuation device, did not address touch temperature hazards in the event of degradation or loss of an ISS critical service - cooling by the ISS Moderate Temperature Loop (MTL). The client payload was mounted inside the MSG work volume and utilized the ISS Moderate Temperature Loop (MTL) for cooling. The payload organization's (PO) thermal analysis did not cover the MTL failure scenario.

The PSRP directed the PO to perform additional thermal analysis for the MTL failure case. The new analysis revealed that the client payload's baseplate could reach a maximum temperature of 68 deg Celsius (154 deg Fahrenheit) which exceeded the NSTS/ISS 18798B Interpretation Letter (MA2-95- 048) maximum allowable temperature (49 deg C) requirement for intentional crew contact. The client payload was therefore not "safe without services" as required per the NSTS 1700.7B ISS Addendum.

In order to protect the crew, the PO added a temperature strip and caution-warning sticker to its payload to serve as the second control of the touch temperature hazard. The MTL was the first control. With the addition of the temperature strip, the client payload now satisfied the NSTS 1700.7B ISS Addendum fault tolerance requirement for a critical hazard. The PO updated its standard payload hazard report to reflect the updated thermal analysis and new second control. In February 2002, the PSRP approved the client payload for flight (STS-111/UF-2).

Root Cause: An integrated approach to the analysis, which would have included the potential for failure of critical services from outside the payload, was not thoroughly performed. The client PO did not include loss of services (MTL degradation or failure) in their original thermal analysis. This omission left a potential touch temperature hazard uncontrolled after a single point failure.