Atlantis ready to support ISS troubleshooting

by Chris Bergin

Engineers have successfully managed to re-plan Atlantis’ power consumption, in order to support an extra day docked to the International Space Station (ISS) if required.

Atlantis is aiding the station – mainly through occasional attitude control support – as troubleshooting continues with the ISS’ Russian computers, which have failed due to what is currently believed to be caused by a form of power related interference from the newly added S3/S4 truss.

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The computers, which are located in the Russian elements of the ISS and control the space station’s navigation and command and control system, first failed on Wednesday. A solution is being worked by the Russians, aided by NASA.

‘The Russian TVM (Terminal Computer) and TsVM (Central Computer) systems in the SM, each triple-redundant, have not yet been successfully restarted,’ noted the latest information from Thursday night’s On Orbit status report. ‘The situation is dynamic, with restarts and subsequent failures throughout the night and morning.

‘Troubleshooting telemetry is hampered by limited comm windows over RGS (Russian ground sites). Russian specialists, working closely with U.S. teams, have been concentrating on troubleshooting and restoring computer capability which is critical for attitude control, Elektron and Vozdukh operation, state vector updates for U.S. GNC MDMs, external SM thermal loops, and power transfers from the USOS to the RS.

‘NASA teams at JSC are focusing on four issue areas: (1) TVM/TsVM computer root cause analysis, (2) docked mission impacts, (3) attitude control modelling and Orbiter prop usage, and (4) attitude control without Russian computers.’

A theory – although not conclusive at this stage – is being worked in relation to a problem with the German made computers in the Russian segments, and the newly added power generating arrays was first mentioned early this morning.

‘There is lots of talk on the loop regarding power and computer issues with the ISS,’ noted one NASA memo Thursday morning. ‘One of the computers on the FGB (Functional Cargo Block/Zarya module) is completely down, and another is functioning, but not communicating with the ground. The Russian Segment Control System is consequently having issues, and the ISS CMGs keep getting saturated.

‘The Orbiter (Atlantis) has had to maintain attitude for the stack on occasion, and fuel consumption is becoming a concern. There has been talk about the FGB potentially losing power within the next couple of hours.

‘They woke up one of the Russian astronauts on the station because they want to do some testing to troubleshoot. They are not sure what is causing the issue, but one potential theory is that the problem could be caused by the addition of the new solar arrays!’

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Control with one ‘lane’ was established for only a matter of minutes, but enough time for Russian controllers in Moscow to secure requirements in the FGB away from the decreasing battery power.

‘Last night, discovered Russian computers also control power converters charging FGB, which charges Soyuz. Took Soyuz to internal power last night. Were able to get computer up this morning and latch on U.S.-to-Russian converter units,’ reviewed Thursday’s integration report. ‘Orbiter (was) charging the FGB and Soyuz, but this computer failed again. Haven’t yet understood root cause and put system back into proper configuration, but bought time.’

With the station relying solely on the US gyros for attitude control, Atlantis has been using her Reaction Control System (RCS) jets to aid the ISS. This is actually a benefit of having the orbiter docked to the ISS during this problem, allowing station to conserve the other method open to them – the use of the docked Progress cargo ship’s thrusters in the same fashion.

This benefit of a docked orbiter lead to evaluations on the possibility of having Atlantis with the ISS for an additional day, as a contingency.

While this may not be required, the option is now available, following some modifications to Atlantis’ power consumptions – based around the limited consumables of O2 for the fuel cells that generate electricity for the orbiter. Power saving options include – as an example – the powering down of Atlantis’ robotic arm when not required.

‘Worked a modified Group C procedure to be able to gain an extra day on orbit; all disciplines have looked at and produced a final version,’ noted NASA’s Electrical Generation and Illumination Engineering department. ‘Implementing the procedure would buy us 650 watts over the docked duration which comes out to ~1 hour margin per day – EPS is confident they will be able to gain this extra docked day (assumes that FD09 O2 transfer is done).

‘Orbiter is currently running on less than 12 kW to try to squeeze an extra docked day while ISS reviews attitude control problems. Orbiter can assume control for sometime longer without becoming prop critical. May go to alt-DAP (Digital Autopilot) mode and use aft primary thrusters for stack control if necessary. This takes the FRCS (Forward Reaction Control System) out of the equation, which is where we are prop limited.

‘ECCOM having discussion (with the flight) surgeon to remove exercise for shuttle crew except the commander and pilot from tomorrow (Friday) till undock to limit the Lioh use.’

So far, the longer than scheduled mission has very few negatives attached to it, with only the battery life of the Wing Leading Edge detection system being looked at, given they will expire before the end of the mission. However, those sensors are mainly used for the ascent stage, to detect debris impacts.

This extra day docked to the ISS may simply become a moot point, should the troubleshooting by the Russians prove to be successful, but NASA are building their options in case the Russians’ troubleshooting efforts take longer than hoped.

‘Had better days,’ opened the noted on the subject from Thursday’s Mission Management Team (MMT) meeting. ‘Working with Russian counterparts to assess how to fix the problem. Working to try to get into good posture for undocking when shuttle needs to leave.’

Troubleshooting is being carried out at time of publication – the first attempt to reboot the Russian computers occurring at 9pm Central – as the issue that caused the computers to fail continues to point towards when the new truss was attached to the station.

‘It appears the computers are sensitive to noise. Problems occurred when the truss was attached,’ added another note. ‘It’s possible that the ground path changed with the addition of the truss, which increased the noise level. There will be some troubleshooting later today. The crew will use a scope meter to check for noise.’

Meanwhile, STS-117 continues unabated, with Flight Day 7’s highlight being the continued efforts to retract the P6/2B solar array wing. The array is now retracted over half way, following a series of one bay retractions, along with the now customary wiggle of the array.

The P6 solar array wing (SAW) 2B remains partially retracted, awaiting manual assistance during one or both of the next two spacewalks,’ added the On Orbit Status Report. ‘Responsibility for retract/abort and MDA (Motor Drive Assembly) ‘On’ commands (when the BGA (Beta Gimbal Assembly) ‘wiggle’ technique is not being used) is with the station crew.’

That next spacewalk (EVA-3) will be split between the OMS Pod blanket repair – which will take no longer than two hours – before using the opportunity to aid the retraction of the P6/2B.

While some of the mass media use the ISS issues as an opportunity to gain ‘crisis’ headline exposure, the mission is proving to be highly successful, and not just on orbit, as NASA and their contractor teams – the often unsung heroes of space flight – continue to work the mission in tandem with creating optional support via Shuttle during Russian troubleshooting.

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