Dragon Resilience performs port relocation to clear way for future vehicles

by Pete Harding

The SpaceX Crew-1 Dragon Resilience performed a relocation to a new docking port aboard the International Space Station on Monday in order to make way for future crew and cargo vehicles on the U.S. side (which includes Canada, Japan, and Europe) of the orbital complex. 

This type of move from one docking port to another, whilst common for the Russian Soyuz vehicles, is a first for a U.S. crew vehicle — as the Space Shuttle previously has no need to relocate ports as it was the sole U.S. human and cargo vehicle requiring docking ports until the Crew Dragon system came online in 2019.

Dragon Resilience’s relocation began at 06:30 EDT/10:30 UTC, whereupon the craft undocked from International Docking Adapter-2 (IDA-2) / Pressurised Mating Adapter-2 (PMA-2) on the forward port of the Harmony module.

The 45-minute procedure saw Resilience back away from the Station to 60 meters before commanding its Draco thrusters to perform an automated flyaround, which involved maneuvering through a 90-degree arc to align with PMA-3/IDA-3 on the zenith (space-facing) side of Harmony.

Dragon Resilience then initiated the final approach, using its rendezvous systems to guide itself and its four occupants back toward the Station for a re-docking at 07:15 EDT/11:15 UTC.

All four of Resilience’s crewmembers – NASA astronauts Victor Glover, Mike Hopkins and Shannon Walker and JAXA astronaut Soichi Noguchi – were onboard the spacecraft during the relocation to protect against the unlikely scenario of a failed redocking.

If the crew were not onboard their spacecraft during a relocation, and the craft failed to re-dock, it could leave them stranded on the Station awaiting rescue. 

Therefore, crews are always placed aboard their spacecraft during port relocation operations as the craft they launched with is usually their emergency evacuation vehicle as well as their ride back to Earth.

For today’s relocation, the Dragon crew also wore their tailor-made SpaceX pressurize suits to protect them from a highly unlikely depressurisation of the capsule in the event of an off-nominal approach and a contact between Dragon and the Station.

Relocations are usually timed, as well, so that if an off-nominal situation occurs and the need to come home somewhat immediately arises, the craft and crew are in a good position — both physically from an orbital mechanics standpoint and temporally from a timeline perspective — to return home within a few orbits if needed.

Upcoming U.S. crew vehicle schedule:

The immediate purpose of the relocation was to clear the PMA-2/IDA-2 port for the arrival of the Crew-2 Dragon Endeavour capsule, which is set to launch to the ISS on April 22 with its crew of NASA astronauts Shane Kimbrough and Megan McArthur, ESA astronaut Thomas Pesquet, and JAXA astronaut Aki Hoshide.

This will be followed on April 28 by the undocking and return to Earth of Crew-1 Dragon Resilience, which will vacate the PMA-3/IDA-3 port and clear the way for the docking of the CRS-22 Cargo Dragon in June – which is the ultimate purpose of Resilience’s relocation.

CRS-22 will be carrying a pallet in its trunk containing the first two of Boeing’s ISS Roll Out Solar Array (IROSA) panels, which will need to be robotically extracted by the Space Station Remote Manipulator System, or Canadarm2.

This requirement for robotic access is the driving force behind relocating Crew Dragon Resilience, as Canadarm2 cannot reach into the CRS-22 Dragon’s trunk if it is docked to the PMA-2/IDA-2 port on the forward part of Harmony.

In that location, the trunk would be located too far forward of the Station for the arm. Therefore, for Canadarm2 trunk access, Dragons must be docked to the PMA-3/IDA-3 port on the zenith side of Harmony.

The logical question then arises: Why does Crew-1 have to move their Dragon just before they come home?  Why can’t the Crew-2 mission dock to PMA-3 and then relocate to PMA-2 a few weeks after arrival?

The technical answer is that nothing would prevent that. Crew-2’s Dragon Endeavour is perfectly capable of performing a relocation a few weeks after arrival.

However, it is preferential to relocate the Crew-1 Dragon ahead of the docking of Crew-2 rather than dock Crew-2 to PMA-3 and later relocate to PMA-2 to protect against a failed redocking scenario, as there would be less impacts to the ISS if Crew-1 had to return to Earth only a few weeks early than if Crew-2 failed to re-dock and had to return to Earth several months early.

The CRS-22 mission is currently planned to launch in June and remain at the ISS until early-July, as the IROSA panels it will carry will need to be installed onto the ISS via spacewalks, following which the empty IROSA pallet will need to be re-installed into the trunk for disposal at the end of the mission.

Once the cargo Dragon CRS-22 mission has departed the PMA-3/IDA-3 port, there will need to be a relocation of the Crew-2 Dragon vehicle from PMA-2 to PMA-3.

A pending future. Boeing’s Starliner approaches PMA-2/IDA-2 on its OFT-2 mission while a SpaceX Dragon is docked to PMA-3/IDA-3 . (Credit: Mack Crawford for NSF/L2)

This is to clear the PMA-2 port for the arrival of the Boeing Starliner spacecraft on its uncrewed OFT-2 (Orbital Flight Test 2) mission, with PMA-2 being the preferred port for that test flight.

This means that the OFT-2 mission is unlikely to fly until at least mid-July. However, on July 15, Russia’s much-delayed Multipurpose Laboratory Module Nauka is set to launch to the ISS for a docking on July 23.

This is likely to delay the OFT-2 mission into late-July, where only a small window exists before Northrop Grumman’s Cygnus NG-16 mission is slated to launch in early-August.

This kind of vehicle schedule complexity is set to become more common on the ISS in future, as are relocations of U.S. crew vehicles — with three types of crafts (Crew Dragon, Cargo Dragon and Starliner) competing for just two available docking ports.

(Lead image: A Crew Dragon docked to PMA-2/IDA-2 on the forward port of Harmony. Credit: NASA)

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