ATV-3 electrical power issue successfully resolved as first reboost is performed

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Following on from a system failure on Thursday night, electrical power has been successfully restored between Europe’s Automated Transfer Vehicle-3 (ATV-3) and the International Space Station (ISS) after a marathon weekend of replanning and troubleshooting work by control centers around the globe and crews on-orbit.

ATV electrical power system:

Although ATV spacecraft do possess solar arrays to provide electrical power to their systems during the free-flight phase prior to docking, ATVs require a maximum of 900 Watts of ISS electrical power during their Attached Phase Operations (APO), which is provided by the Russian Service Module (SM) “Zvezda” via the ATV’s Russian Docking System (RDS) interface.

This is because high solar beta angles, where the sun effectively shines on the ISS side-on, mean that solar arrays without beta rotation capability do not receive sufficient sunlight and thus cannot provide sufficient electrical power during periods of high solar beta angles.

This issue affects the ATV spacecraft since, unlike the solar arrays on the US Segment of the ISS, the ATV solar arrays do not have any beta rotation capability, which would allow them to rotate to face the sun when it shines on the ISS side-on. ATV launches are also constrained by solar beta angles, since free flights cannot occur during high solar beta angle periods.

Specifically, the SM provides power to the ATV’s Russian Equipment Control System (RECS), which features two power chains – a primary and a backup – and the RECS in turn distributes power via a power bus to the four Russian Systems Interface Units (RICUs), which interface the RECS with ATV’s 1553 bus, to which the ATV’s avionics system and electrical outlets are connected.

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ATV-3 ISS power loss:

Following ATV-3’s nominal docking to the SM Aft port on Wednesday (28th March) night, ATV-3 was successfully connected to the SM’s electrical power system, whereupon the hatches were opened between the SM and ATV-3 on Thursday, followed by the first crew ingress into ATV-3.

However, due to the fact that ATV-3’s hatch had been re-opened on the ground due to the need to correct a loose cargo strap, which caused the ATV-3 launch delay from 9th March to 23rd March, some concerns were noted about possible bacterial contamination of ATV-3’s atmosphere. As such, it was decided to take extra precautions following hatch opening, such as the use of the Russian POTOK air purifier.

Late on Thursday, following hatch opening, the command path to one of the four RICUs failed during the air scrubbing process, leading the RICU’s Fault Detection, Isolation & Recovery (FDIR) to shut down power chain 1 in the RECS, which is located in the Docking & Refuelling System (DRS) rack inside ATV-3. This caused ATV-3 to lose all electrical power supply from the ISS.

ATV-3 continued to receive adequate electrical power from its four solar arrays, however the need to recover the power supply from the ISS was time critical, since it was known that the ISS would head into a high solar beta period starting Monday, reducing the sunlight available to the ATV solar arrays and thus reducing their power output. This would have meant that ATV-3 would not have had adequate electrical power to keep all of its vital systems powered.

Thus, if the power supply from the ISS to ATV-3 was not recovered by Monday, ATV-3 would have been required to undock from the ISS – five months earlier than planned. As such, the ISS crew were immediately set to work on Saturday unloading a list of high priority cargo and loading trash in its place.

On Friday however, the RECS power chain 2 was successfully activated, and on Saturday was re-integrated into the RECS power bus, which powered the RICUs to which the ATV avionics and 1553 bus is connected. The integration was complete with a relays closed at 5:03 PM GMT on Saturday.

Although only three RICUs are currently working due to the failed fourth unit, which is not a problem due to the level of redundancy available, this means that the power supply between the ISS and ATV-3 has been restored, thus avoiding an undocking on Monday. An investigation is currently underway to determine whether RECS chain 1 is failed completely, and thus unusable, or just tripped, in which case it may be able to be recovered.

Since the chain 1 power loss occurred during the unplanned air scrubbing process, an investigation is underway as to whether some uncertified electrical equipment – namely the Russian POTOK air purifier – could have caused the power loss. This however is unconfirmed at this time. Due to the ongoing investigation, the power outlets inside ATV-3 will remain unpowered for now.

Following the recovery of the ATV-3 ISS power supply, the ISS crew were stood down from their cargo transfer activities, despite the fact that they managed to complete all priority transfers. The crew would have used Sunday to complete all planned cargo transfers, however as this was not required and since they worked all day Saturday, they will now get to have Sunday and Monday as their normal off duty days.

ATV-3 ISS reboost:

The mammoth task to restore the ISS power feed to ATV-3 – a multinational effort involving Mission Control Center-Houston (MCC-H), MCC-Moscow (MCC-M) and the ATV-Control Center (ATV-CC) in Toulouse, France – meant  that the first planned ATV-3 ISS reboost could be successfully performed, as it would have had to be cancelled should the ATV-3 ISS power feed not have been restored.

Performed on Saturday night at 9:54 PM GMT, the 4 minute 51 second reboost using ATV-3’s Aerojet-provided Orbit Correction System (OCS) thrusters, increased the ISS’ velocity by 1 meter per second, and increased its altitude by 1.7 kilometers.

The purpose of the reboost was to test ATV-3’s OCS for ISS reboost capabilities, and also set up ISS orbital phasing for the upcoming Progress M-15M/47P launch on 20th April, the Soyuz TMA-22/28S landing on 27th April, and the Soyuz TMA-04M/30S launch on 15th May.

(Images: L2, ESA)

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