STS-123: Final EVA completed as OBSS takes up ISS stay
With the docked Late Inspections of Endeavour’s TPS behind them, the STS-123 crew then turned their attentions to the all important EVA-5 which saw – among other things – the stowing of the OBSS (Orbiter Boom Sensor System) to the ISS for use by Discovery (STS-124) later this year.
EVA-5 also saw the installation of the MISSE-6 experiment – following previous problems during EVA-3, and further inspections of the troublesome Starboard SARJ (Solar Alpha Rotary Joint).
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With the docked late inspection of Endeavour’s TPS complete, astronauts Bob Behnken and Mike Foreman are gearing up for a record setting fifth Shuttle/ISS mission EVA that will see the scheduled completion of three events.
The primary focus of EVA-5 is the attachment of the OBSS to the S1 truss of the ISS. The OBSS will be attached to the orbiting structuring by way of Orbital Support Equipment (OSE) that was installed on the Station back in August 2007 during STS-118.
In addition to providing physical attachment points for the OBSS, the OSE also provides attach points for the keep-alive attach device (KAD) which will provide electrical energy and heating for the OBSS sensor packages.
During the spacewalk, Behnken and Foreman will install Stanchion/avionics boxes, the KAD, and the P411 connector before manually connecting the OBSS to the OSE.
After stowage of the boom is complete, Mike Foreman will install a thermal cover over the critical sensor packages of the OBSS. This cover will provide not only additional thermal barriers for the sensor packages, but also MMOD (Micro-Meteoroid Orbital Debris) protection.
Following activities with the OBSS, Behnken and Foreman will split up to accomplish two additional tasks before the completion of the EVA. For Behnken, the remainder of the EVA will be dedicated to the installation of the MISSE-6 following a previously unsuccessful attempt to install the payload on EVA-3.
Following a big picture re-think by ground engineers, a new installation plan has been drawn up.
‘MISSE 6A (bent) install will be attempted first followed by 6B (straight). MISSE 6A was NOT attempted on EVA-3. As long as one 1/4” pip pin is installed, MISSE is considered secured. If only one is installed, must be wire tied,’ noted yesterday’s Mission Status Summary.
‘The following actions will be taken in the order listed to achieve MISSE installation. Install 1/4” pip pins (2) using nominal method. Crew member has a go to use as much force as they deem appropriate. Install 1/4” pip pins (2) using hammer and apply light tapping force.
‘Excessive impact force (hammering) not permitted. If one or both 1/4” pip pins are seated so that the edge of pin is past flush on opposite side of the probe socket but the ball detents are not engaged, wire tie the pip pin to secure it.’
Nevertheless, NASA is prepared for a possible issue with the 1/4” pip pins.
‘If can’t secure at least one 1/4” pip pin. Cut both 1/4” pip pin tethers to remove both pip pins. Install two 3/16” pip pins and wire tie them to the probe. If 3/16” pip pins are used, two pins are required and they must be wire tied as the ball detents will not retain the pins in a 1/4” hole,’ noted the Mission Status Summary.
Furthermore, if Behnken cannot secure any pip pins, he will tether MISSE-6 to the installation location. For this scenario, MISSE-6 would not be in a data collection mode. Additionally, if an EVA contingency occurs, MISSE-6 ‘will be placed in a structurally safe configuration with tethers.’
While Bob Behnken works with MISSE-6, Mike Foreman will translate out to the Starboard SARJ (Solar Alpha Rotary Joint) to continue inspection operations which began last year.
“The SARJ assessment team presented some early materials analysis based upon the tape samples returned. 99.9 percent of the material comes from the Nitrided hard surface on top of the outer surface race ring of the SARJ,” noted the Mission Status Summary. “Initial indications point to a classic spalling failure of the surface initiated by unexpectedly high friction loads. The team is working hard to characterize what the friction loads could have been and what might be causing them.”
To assist with this investigation, Foreman will conduct visual inspections of the SARJ under covers six, eight, nine, eleven, sixteen, and eighteen.
In addition to these inspections, the ISS Program has requested further testing of the SARJ’s Autotrack feature. “Program believes that moving the SARJ at the slightly higher rates provided by Directed Position mode may be leading to an increased level of overall system vibration compared to the slightly lower rates of the Autotrack mode. They have requested a one orbit test after 1JA/STS-123 to measure the difference.”
Endeavour continues to perform very well as the mission proceeds along the updated timeline. No decisions are required from the MMT at this time, a continuing sign of the smooth flow Endeavour and the mission are enjoying.
The SSPTS (Station to Shuttle Power Transfer System) continues to operate as designed providing three additional docked days and one additional undocked day should the need to extend the mission arise.
“Both margin numbers assume SSPTS power transfer for nominal mission duration. No longer protecting O2 transfer margin for ISS; second airlock fan now off. No concerns for middeck transfer at this time,” noted the FD-11 Mission Status Summary.
One item of interest relating to O2 Repress was noted in the Mission Status Summary. “Received warning for Major Constituent Analyzer (MCA) ppO2 out of limits during ISS stack repress from Orbiter. Raw value exceeded 24.1 percent limit, but adjusted value did not. Localized issue in airlock due to poor airflow mixing when both Oxygen Introduction Valves (Lab and Airlock) were in use. Successfully restarted the repress using only the Lab OIV.”
Overall, STS-123 is proceeding very well as Endeavour and her crew head into the final days of docked operations ahead of a one revolution earlier landing on Wednesday, March 26.