Expedition 55 Flight Engineers Drew Feustel and Ricky Arnold have stepped outside of the International Space Station for the first of two EVAs this month, with the primary objectives of upgrading cooling system hardware and installing new and updated communications equipment for future dockings of commercial crew spacecraft.

US EVA-50 began with NASA astronauts Drew Feustel and Ricky Arnold leaving the Quest airlock. Both astronauts have previous spacewalking experience.

These spacewalks are the eighth and ninth of Feustel’s career, and the fourth and fifth of Arnold’s,  for what is the 210th and 211th spacewalks in support of station assembly, maintenance and upgrades.

The major task of EVA-50 is the relocation of two Pump Flow Control Subassembly (PFCS) units. A PFCS is a component of the Photo Voltaic Thermal Control System (PVTCS), which provides ammonia cooling to the power generation equipment of the station’s solar arrays.

Specifically, the PFCS pumps the ammonia around each of the eight power channels, two of which are found on each of the station’s four solar arrays (making for eight operational PFCS units on the ISS in total).

Back in May 2013, the channel 2B PFCS failed following an ammonia leak, which required an EVA to replace the unit with a spare. The failed PFCS, known as “leaky”, has since been stowed on the P6 Truss.

However, flight controllers would now like to move that failed PFCS to a stowage location on External Stowage Platform-1 (ESP-1), and in turn move a spare PFCS (named “frosty”) that currently resides on ESP-1 to the P6 Truss, in order for it to be more accessible should it ever be needed for a future replacement operation.

Names of the Pump Flow Control Subassembly.

These PFCS relocations are not to be confused with the spare PFCS (named “motley”) which was recently delivered to the ISS on a SpaceX Dragon flight, and an electrically failed PFCS (named “trippy”) which was disposed of via that same flight.

The Special Purpose Dextrous Manipulator (SPDM) “Dextre” conducted preparations for the EVA last week by removing the failed “leaky” PFCS from the P6 Truss and stowing it on its Enhanced ORU Temporary Platform (EOTP).

As such, today’s spacewalkers will simply need to remove the spare “frosty” PFCS from ESP-1 and install it onto the EOTP, whilst also removing the “leaky” PFCS from the EOTP and installing it onto ESP-1. The SPDM will later install the “frosty” PFCS onto its new stowage location on the P6 Truss.

Dextre supporting the EVA

The next major task of the EVA will be to replace the Camera Port-13 (CP-13) External Television Camera Group (ETVCG) on the Destiny Laboratory. This piece of hardware consists of a stanchion with attached Camera Light Pan/tilt Assembly (CLPA).

However, whilst the current assembly also consists of an External High Definition Camera (EHDC) which is attached to the CLPA, the replacement assembly will not, as astronauts preparing the replacement assembly inside the ISS were unable to mate the EHDC to its attachment mount.

The last major task of the EVA will be the replacement of the Space to Ground Transmit/Receive Controller (SGTRC) on one of the station’s two Ku-band Space to Ground Antennas (SGANTs) atop the Z1 Truss. As its name implies, the SGTRC controls the transmit/receive functions of the SGANT.

The two spacewalkers will then re-enter the airlock to conclude the EVA.

(This article will be updated through the two EVAs)

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