International Space Station (ISS) program managers and engineers are working on long-term plans to support future crew and cargo vehicles, including the installation of a new docking system onto the station for use by future commercial crew vehicles, along with the relocation of some Station modules, in order to free up ports for use by future crew and cargo spacecraft.

New ISS docking system:

In preparation for the arrival of new commercial crew vehicles to the ISS in late 2017 – which will use a brand new docking system – the existing docking ports on the ISS need to be converted to the new standard, before any commercial crew vehicles can dock to the outpost.

Docking is a process where a port on an arriving spacecraft impacts a corresponding port on the ISS, whereupon capture is made, with all necessary power/data connections later being made automatically upon hard-dock.

Docking is required for crewed vehicles since it allows for rapid departure from the ISS in the event of an emergency, without first having to manually disconnect multiple cables and perform lengthy unberthing procedures.

By contrast, berthing is a process where a vehicle is flown to a point below the ISS, whereupon the vehicle is captured by the station’s robotic arm and positioned close to a Common Berthing Mechanism (CBM) port, followed by the extension of hooks from the ISS Active CBM (ACBM) port to latch onto a corresponding Passive CBM (PCBM) port on the arriving vehicle, and pull the two ports together.

At this point, 16 bolts begin to drive to tightly lock the two CBM rings together, and following this, the hatches are opened and all power/data connections and ventilation ducting are made manually around the CBM hatchway by the ISS crew.

Thus, berthing is not used for crewed vehicles, since the time required to depart the ISS via a berthed vehicle in an emergency would be far too long, considering that it would require a lot of time to disconnect cabling/ducting, connect CBM control boxes to command the unberthing process, and then for the CBMs to be un-bolted and released by the station’s robotic arm.

Berthing, however, is greatly desired for cargo vehicles, since the CBM ports feature a large 50-inch diameter hatch, which allows for the transfer of large items to the ISS, and makes cargo transfer operations easier since crewmembers do not need to struggle to pass cargo though narrow hatchways.

Currently, the station has one usable port for docking – Pressurized Mating Adapter-2 (PMA-2) located on the Forward end of the Node 2 module. Another PMA (PMA-3) is also on the ISS, however in its current location (the Port side of Node 3) it is inaccessible for docking.

Both PMA-2 and PMA-3 feature a Russian-designed Androgynous Peripheral Attachment System (APAS) docking interface as was used by the now retired Space Shuttle fleet.

However, the new fleet of commercial crew vehicles currently planned to begin arriving at the ISS in late 2017 will use a new docking interface, known as the NASA Docking System (NDS).

The NDS has already had a storied history, undergoing many design changes over the years of its development. The initial system designed as the follow-on to the Shuttle era APAS was the NASA-designed Low Impact Docking System (LIDS).

Designed for the Orion capsule under the now cancelled Constellation Program, LIDS would have used magnets to allow for lower impacts between the capture rings of two docking vehicles, reducing the stresses imparted during dockings and thus extending the service lives of the spacecraft.

Two adapters to convert the current APAS system on the ISS PMAs to the LIDS standard – called the APAS To LIDS Adapter System (ATLAS) – would have been flown up on the initial Orion flights to the ISS via the Ares I booster.

This plan changed, however, when NASA managers began looking at building hardware to convert two CBM ports on ISS to LIDS ports, called Common Docking Adapters (CDAs), eliminating the need to use the Shuttle era PMAs.

In October 2010, the ISS international partners met to agree on a new International Docking System Standard (IDSS), which rather than being a specific docking system, was simply a standard to which any country or company could design their own docking system to. Hence, NASA’s implementation of the IDSS became known as the NASA Docking System (NDS).

Regardless, the plan to use LIDS essentially remained, with some design changes being made (namely using the proven APAS docking collar with the new LIDS capture ring) in order to make LIDS compatible with the IDSS, leading to LIDS becoming international LIDS (iLIDS).

However, due to clearance issues between winged vehicles such as Sierra Nevada’s Dream Chaser and the ISS, the plan to use the CDAs was dropped in favour of building two ISS Docking Adapters (IDAs) to convert the APAS mechanism on the PMAs into NDS/iLIDS ports.

The story took yet another turn earlier this year, when the plan to use iLIDS for NDS was cancelled in favour of using a new design of docking system by Boeing, called the Soft Impact Mating Attenuation Concept (SIMAC), which is understood to be a non-proprietary design essentially based on APAS but featuring a narrower soft-capture ring.

Thus, the current plan is now to fly two IDAs to the ISS, which will attach to the two PMAs in order to convert their APAS interfaces into SIMAC interfaces, which in turn will be used by all future commercial crew vehicles.

ISS reconfiguration:

As detailed in a new Flight Planning Integration Panel (FPIP) chart on L2 – a document that lays out all notional vehicle traffic to the ISS through its currently planned retirement in 2020 – 2015 will see a series of module relocations occur on the ISS in order to re-configure the station for the docking of future commercial crew vehicles.

While the schedule is extremely notional at this point, the current plan calls for the SpX-7 flight (the 7th SpaceX Dragon CRS flight) in April 2015 to deliver IDA-1 to the ISS, with a US spacewalk being conducted a short time later to install IDA-1 onto PMA-2 on the Forward end of Node 2, in order to convert its APAS port into a SIMAC port.

However, as two docking ports are required by NASA due to the desire to have a back-up docking port should one fail, in May 2015, PMA-3, currently inaccessible for docking due to its location on the Port side of Node 3, will be relocated by the station’s robot arm to the currently unoccupied Zenith port of Node 2, in order to make it accessible by future crew vehicles.

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Node 2 Zenith is currently designated as the back-up berthing port (with the primary port being Node 2 Nadir) for cargo vehicles, which use the CBM interface. As with docking ports, NASA requires two berthing ports – a primary and a back-up – to protect against a CBM port failure, or protect against a cargo vehicle becoming “stuck” on the ISS, due to, for example, a thruster system failure.

Thus, when PMA-3 moves to Node 2 Zenith, the back-up berthing port will no longer exist.

To solve this issue, in July 2015, NASA will “open up” a new CBM port to serve as the back-up cargo vehicle port, by relocating the Permanent Multipurpose Module (PMM) from its current location on the Nadir port of Node 1, to the currently unoccupied Forward port of Node 3, thus freeing up Node 1 Nadir for cargo vehicles.

Before the PMM can be relocated, a number of preparatory tasks need to be completed via spacewalks, including relocating a Camera/Light Pan/Tilt Unit [PTU] Assembly (CLPA) and its attached Video Stanchion Support Assembly (VSSA) from Camera Port-7 (CP-7) on the Nadir inboard side of the P1 Truss, where it would cause clearance issues with the PMM on Node 3 Forward.

The CLPA & VSSA will be relocated to CP-8 on the Zenith outboard side of the P1 Truss, affording the first ever external camera view of the Zenith side of the ISS.

Currently a Wireless External Transceiver Assembly (WETA) and attached VSSA is located at CP-8, and so it will be relocated to the currently unoccupied CP-11 on the Zenith side of Node 2, a task that had been planned for the cut-short US EVA-23 on July 16.

Additionally, an Articulating Portable Foot Restraint (APFR) and attached Tool Stanchion (TS) needs to be relocated by spacewalkers from a Worksite Interface (WIF) on the Nadir side of the P1 Truss, to a WIF on the S0 Truss, in order to avoid clearance issues with the PMM on Node 3 Forward.

Spacewalkers will also need to open the Centerline Berthing Camera System (CBCS) flap on the Center Disk Cover (CDC) of the Node 3 Forward CBM port, as this external protective flap is currently covering up the porthole window on the Node 3 Forward port that will need to be uncovered so that the PMM can be guided in and aligned via a video camera system mounted in the porthole.

Finally, before any cargo vehicles can be berthed at the Node 1 Nadir port, internal modifications will need to be conducted inside Node 1 by the ISS crew in order to route power and data cables needed to support cargo vehicles, which are expected to be completed in July 2015.

While Node 1 Nadir will serve as the cargo vehicle back-up port, it will also allow for two cargo vehicles to be berthed at the ISS simultaneously, known as Dual Berthed Visiting Vehicle (DBVV) capability. Presently, although two cargo vehicle ports are available on the ISS via the Node 2 Nadir and Node 2 Zenith ports, it is a lot of effort to have two cargo vehicles berthed to the ISS at the same time.

This is because the Space Station Remote Manipulator System (SSRMS) robotic arm – that captures the cargo vehicles – must be based on a Power and Data Grapple Fixture (PDGF) on the Node 2 module, in order to reach to capture the vehicles from a point 30 feet below the station. However, when based on the Node 2 PDGF, the SSRMS does not have the reach to directly install a cargo vehicle to the Node 2 Zenith port.

This means that a cargo vehicle must first be berthed to the Node 2 Nadir port, and then relocated to the Node 2 Zenith port only once the SSRMS has relocated itself via a “base change” from the Node 2 PDGF, to a PDGF on the Mobile Base System (MBS), from where it does have the reach to install a vehicle onto the Node 2 Zenith port.

Thus, at present, if two vehicles want to berth to the ISS simultaneously, one must first be relocated to Node 2 Zenith prior to the arrival of the other vehicle, since a vehicle cannot be present on Node 2 Nadir during the arrival of another vehicle, as it would preclude the newly arrived vehicle from being berthed to Node 2 Nadir while the SSRMS relocates itself in order to install the new vehicle on Node 2 Zenith.

Additionally, a newly arrived vehicle on Node 2 Nadir would have to leave the ISS before the older vehicle on Node 2 Zenith, as the older vehicle must be relocated to Node 2 Nadir so that the SSRMS can relocate itself before maneuvering the older vehicle from Node 2 Nadir to the departure point 30 feet below the ISS.

All this complexity leads the ISS planning teams to decide to simply “de-conflict” cargo flights at present, thus having only one cargo vehicle present at the ISS at any one time.

This means that the Japanese HTV, SpaceX’s Dragon, and Orbital’s Cygnus sometimes have to “fight” for the same cargo vehicle slot, as is currently the case with a December window currently being contested by both Dragon’s CRS-3 mission and Cygnus’ CRS-1 mission.

The Node 1 Nadir port however is within reach of the SSRMS when it is based on the Node 2 PDGF, as it needs to be for vehicle captures, and so having Node 1 Nadir as the second cargo vehicle port will allow a second cargo vehicle to be directly berthed to the ISS while another cargo vehicle is present on the Node 2 Nadir port, without the need for any SSRMS relocations.

This will make cargo vehicle planning much easier as two vehicles will no longer need to fight each other for one slot in the manifest.

While the PMM will be relocated to the Node 3 Forward port in July 2015, the L2 FPIP chart also shows that in June 2015, the Dragon SpX-8 flight will deliver the Bigelow Expandable Activity Module (BEAM) to the ISS, which will be installed opposite the PMM on the Node 3 Aft port, where it will remain for at least two years while being used as a technology testbed for inflatable structures in space.

BEAM’s hatch however will remain closed for the majority of that time, in order to reduce risk, and cut down on interaction between BEAM and the ISS, and thus reduce the integration costs of the module.

Finally, to round out the ISS reconfiguration effort, in November 2015 the SpX-9 flight will deliver IDA-2 to the ISS, which will be installed onto PMA-3 on Node 2 Zenith during a US spacewalk shortly after its arrival.

IDA-2 will, in addition to serving as a back-up to IDA-1 on PMA-2, enable two commercial crew vehicles to be docked to the ISS simultaneously, should it ever be required or desired.

However, the FPIP chart shows that, under current planning, only one commercial crew vehicle will ever be docked to the ISS at any one time, since one crew vehicle will return home at the end of its 6-month ISS stay prior to the launch of another crew vehicle, in what is known as an “indirect handover”.

Another interesting fact per the FPIP chart is that the US spacewalk to install IDA-2 will occur in December 2015, meaning that the spacewalk could very likely be conducted by Britain’s first ISS astronaut, Major Tim Peake, who will arrive at the ISS in November 2015.

(Images: L2 Content, NASA, SNC)

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