Russian assembly complete: Roscosmos attaches new docking node to station

After launching a new docking node module to the International Space Station (ISS) on Wednesday, November 24 at 13:06 UTC / 8:06 am EST, Roscosmos has successfully docked the node to the station’s Nauka module.

The module will add additional docking ports to the Russian Segment and is the final Russian model planned for the outpost.

Background

The original design for the Russian Segment of the ISS called for a Universal Docking Module (UDM) to expand the Russian Segment’s available docking ports for the addition of future modules. This module was canceled early in the ISS program due to budget issues.

However, out of the UDM concept grew a new proposal for a Nodal Module (NM), which would provide the Russian Segment with expansion options since all of its docking ports were either in use or reserved for visiting spacecraft.

The node, named Prichal (“pier”), marks a departure from the previous concept of Russian station design, which typically included a core module (which, in the case of ISS, is Zvezda) with an attached spherical docking compartment to which all other modules are docked.

The problem with this design is that it makes the core module an irreplaceable part of the station, as all the other modules would have to be undocked from the core module for the core to be replaced — something which is not technically feasible once all modules have been integrated.

This means that as the core module ages and its systems begin to fail, there is no option but to build a brand-new station, even though the other modules may be newer and perfectly functioning.

The Node Module concept essentially separates the spherical docking compartment from the core module and makes it into a standalone element. All station modules would then dock to the Node Module, the idea being that each module is then replaceable without having to undock them all.

In this sense, Prichal is essentially the Russian equivalent of the Node modules found on the US segment of the station. 

It was originally planned that two Science & Power Modules (NEMs) would be docked to Prichal as part of an expansion of the Russian Segment, with a view to then one day separate from the ISS into a free-flying station.

However, in April 2021, Roscosmos announced that the NEMs are no longer planned for the ISS and will instead form part of a new independent Russian space station for which a new Node Module would be built.

Prichal undergoing pre-launch testing at Baikonur. (Credit: Roscosmos)

While Roscosmos have since somewhat walked back on those statements given the budgetary realities of the Russian space program and the remaining lifetime of the ISS, it is unlikely that any additional modules will ever be docked to Prichal.

Therefore, while Prichal is an interesting module in many ways, it is already essentially a module without a purpose as it does not, in itself, add any additional capabilities to the ISS over what the station already has.

Design

Prichal is a spherical module featuring six docking ports in total – two axial and four radial.

The docking ports are of the hybrid type, essentially a combination of the Androgynous Peripheral Attachment System (APAS) and the SSVP probe & drogue system. 

The docking collar itself is the same as the APAS design, but the initial capture is performed using a probe & drogue rather than an extending capture ring. The purpose of the hybrid system is to give a wider hatch passageway for larger, permanent modules.

The zenith docking port of Prichal is an active hybrid port featuring a probe to allow it to dock to the ISS. The five remaining ports are passive hybrid ports, featuring drogues to allow other modules to dock to them.

The nadir docking port of Prichal features a special adapter called SSPA-GM to convert the hybrid docking system into one compatible with the SSVP probe & drogue system. This adapter converts the APAS docking collar of the nadir hybrid port into an SSVP docking collar but still uses the same docking drogue for initial capture.

This will allow Soyuz and Progress vehicles to dock to the nadir port of Prichal, thus maintaining the Russian Segment’s four docking ports for visiting vehicles.

In the now unlikely event that a new module was to dock to Prichal, this adapter would first have to be removed via a departing Progress spacecraft to convert the nadir port back to a hybrid configuration (with an APAS docking collar) to allow the new module to dock.

However, this would then render the nadir port of Prichal unusable for Soyuz and Progress dockings, reducing the number of available visiting vehicle ports on the Russian side to three.

Prichal also includes sockets for Lyappa arms. These are mini robotic arms used to relocate modules from one docking port to another, as future modules arriving at Prichal would dock to the nadir port as docking to any of the axial ports would present issues for approach corridors and rotational loads.

The newly arrived module would then relocate itself to an axial port via the Lyappa arm, which would extend from the module and connect to a corresponding socket on Prichal. The Lyappa arm would then rotate and swing the module from the nadir port to the axial port.

Lyappa arms were used on the Mir space station, and the new Chinese station also features a similar concept. The last time a Lyappa arm was used on-orbit was in April 1996 for the post-docking relocation of Mir’s Priroda module.

For launch, Prichal was attached to a modified propulsion segment of the Progress spacecraft, which will perform all necessary rendezvous and docking maneuvers. The specially modified propulsion segment, named Progress M-UM, is essentially a Progress spacecraft with the pressurized cargo compartment removed and Prichal installed in its place.

Once Progress M-UM has performed its duty of delivering Prichal to the ISS, it will separate from Prichal and perform a destructive re-entry. This same concept was used to deliver the Pirs and Poisk modules to the ISS in 2001 and 2009, respectively.

Launch and docking

Prichal launched atop a Soyuz 2.1b booster from the Baikonur Cosmodrome in Kazakhstan at 13:06 UTC / 8:06 am EST. Following separation from the Blok-I stage once in orbit, Prichal communicated down that all was well, and the module began a two-day rendezvous with the ISS. 

With Prichal successfully in orbit and healthy, the Progress MS-17 spacecraft undocked from the Multipurpose Laboratory Module (MLM) Nauka’s nadir docking port on Thursday, November 25. 

A Soyuz 2.1b rocket, with Prichal safely inside the payload fairing, is rolled to Site 31/6 at Baikonur for launch. (Credit: Roscosmos)

The departing Progress took with it an SSPA-GM Hybrid-to-SSVP docking collar adapter, which was of the same type featured on Prichal.

This adapter was present on Nauka’s nadir port, which is of the hybrid type, to allow Soyuz and Progress vehicles (which use the SSVP system) to dock there. This was done as an “insurance policy” in case Prichal fails to achieve orbit.

Only once Prichal was safely in orbit and on its way to the ISS was the adapter removed by the departing Progress MS-17, which converted Nauka’s nadir port back to hybrid configuration and made it ready to receive Prichal.

Prichal then docked to the Nauka nadir port on Friday, November 26 at 15:19 UTC / 10:19 EST, using the automated Kurs rendezvous system.

Following leak checks, hatch opening will be performed in the following days. The Progress M-UM propulsion segment is planned to be detached from Prichal on December 21. On January 19, a Russian spacewalk will be performed to connect cables between Nauka and Prichal. 

The first docking to Prichal is planned to take place on March 18, 2022, with the Soyuz MS-21 mission.

(Lead image: Prichal docks to the ISS. Credit: NASA)

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