ISS relocates PMM in reconfiguration for future crew vehicles
Astronauts aboard the International Space Station (ISS) and their supporting ground teams have begun the process of reconfiguring the station from its current Space Shuttle-optimised configuration, to a new configuration optimised for future visiting commercial crew and cargo vehicles. The long-planned effort involved the relocation of the PMM storage module on Wednesday.
Station reconfiguration purpose:
*BACKGROUND* – *LIVE UPDATES*
Essentially, the ISS reconfiguration effort, which will involve relocating multiple modules and components from their current berthing ports, to new berthing ports, as well as adding new docking adapters, is aimed at just one objective – to create an additional docking port for future commercial crew vehicles.
This is because the ISS in its current configuration is still primarily optimised for visiting Space Shuttle orbiters, which used to dock at just one location on the ISS, thus only one docking port is presently available.
Future commercial crew vehicles however will require two docking ports, hence an additional docking port needs to be created.
At this time, no future commercial crew vehicles are actually planned to utilize two docking ports, since NASA has elected to use the “rental car” model of commercial crew transportation, whereby ISS astronauts will fly themselves to the ISS, with their vehicles remaining at the station for the duration of their stay, following which they will return to Earth aboard the same vehicle.
This means that only one commercial crew vehicle will ever be present at the ISS at any given time, since one vehicle will depart before another arrives in what is known as an indirect handover.
This also means that one Russian crewmember will always be required to fly on commercial crew vehicles, so that one NASA crewmember can continue to fly on Soyuz, in order to maintain a constant US presence on the ISS during periods when no commercial crew vehicle is present at the station.
The “taxi” model, whereby dedicated crew vehicle pilots fly the ISS crew to the station, prior to returning to Earth with the outgoing crew aboard a vehicle which arrived six months earlier, in what is called a direct handover, is not currently planned to be used.
As such, the requirement for two docking ports stems from the desire for one port to serve as a back-up to the primary port, should it ever fail, while also preserving the option to use the “taxi” model should that option ever be desired.
While creating an additional docking port may sound like a simple objective in theory, in practice it requires an extensive re-shuffle of various modules and components, not only to create the additional docking port, but also to preserve the present capability to have two unoccupied berthing ports available for cargo vehicles.
This is due to the differences between how crew and cargo vehicles mate to the ISS. Crewed vehicles attach to the ISS via the process of docking, whereby they essentially fly themselves all the way into the docking port.
At this stage, a capture ring on the vehicle impacts a corresponding mechanism on the ISS, following which capture occurs. The capture ring is then retracted, and all power and data connections are made automatically.
Docking is used for crewed vehicles since, due to the automatic connector mating/demating, it allows crews to conduct a fast escape from the ISS, should the vehicle ever be needed for its “lifeboat” role.
Cargo vehicles however attach to the ISS via the process of berthing, whereby they fly themselves to a point below the ISS, whereupon they are captured by the station’s robotic arm, which in turn positions the vehicle very close to the desired berthing port, known as a Common Berthing Mechanism (CBM) port.
Hooks then extend from the ISS side to pull the cargo vehicle into place, with bolts then driving to secure the connection.
Following hatch opening, all power and data connections are made manually by the crew, along with numerous other tasks being performed such as the removal of thermal protection covers and berthing control computers from the hatchway.
Berthing allows for the use of the station’s large-diameter hatches, which is ideal for cargo transfer operations, but the amount of manual installation work, along with the requirement to use the robotic arm for vehicle unberthing, makes the process totally unsuitable for crewed vehicles, since it would simply take far too long for the crew to escape in the event of an emergency.
As with docking ports, two berthing ports are required in order for one to serve as a back-up to a primary port, and also to allow for two cargo vehicles to visit the station simultaneously should the need ever arise.
The reconfiguration plan:
While the ISS in its present configuration does have two separate berthing ports, it only has one usable docking port, whereas two docking ports are required for future commercial crew vehicles.
Since, as described above, docking and berthing ports have differing constructions, a docking adapter is needed in order to convert a berthing port into a docking port. The current usable docking adapter is Pressurized Mating Adapter-2 (PMA-2) located at the forward end of the Node 2 module. This is the port to which visiting Space Shuttles used to dock.
Another PMA, designated PMA-3, is also currently attached to the ISS, however in its present location on the Port side of the Node 3 module, it is unusable as a docking port due to clearance issues with other parts of the station structure.
Thus, it has been decided that PMA-3 will be relocated from its current home on Node 3 Port, which is essentially in the middle of the ISS right below the Truss structure, to the Zenith port of Node 2, on the top-side of the front end of the ISS, away from any major structure. This will ensure future crew vehicles have adequate clearance to dock to PMA-3.
Although this plan does create two docking ports, thus satisfying the requirement for such, it also creates an issue by taking away a berthing port, two of which are required. This is because PMA-3’s new location on Node 2 Zenith is the current designated back-up berthing port.
As a result, a new back-up berthing port needs to be “opened up” elsewhere on the station. While there are unoccupied berthing ports currently on the station, located on Node 3, these ports are unsuitable for use as back-up berthing ports due to clearance issues with the station structure.
Whilst these clearance issues are not always an issue for permanent station modules, they do present issues for cargo vehicles, since cargo vehicles generally have protruding solar arrays, as well as requiring good robotics access for unpressurised cargo extractions.
The plan therefore was to relocate the Permanent Multipurpose Module (PMM) from its current home on the Nadir side of Node 1, to a previously unoccupied port on the Forward side of Node 3. This freed up the PMM’s former home of Node 1 Nadir, a port with no clearance issues and good robotics access, to serve as the back-up berthing port.
Interestingly, this move provides increased capabilities for what is known as Dual Berthed Visiting Vehicle (DBVV) operations, which essentially is where two cargo vehicles are berthed to the ISS simultaneously.
With the previous ISS configuration, DBVV operations would be very difficult, due to a unique complexity associated with the location of the back-up berthing port on Node 2 Zenith – namely, the fact that vehicles cannot be directly berthed to Node 2 Zenith.
This is because the Space Station Remote Manipulator System (SSRMS) must be based on the Node 2 Power & Data Grapple Fixture (PDGF) in order to capture cargo vehicles from the capture point 30 meters below the station. Once captured, the vehicles are normally berthed to the Node 2 Nadir port, right next to where the SSRMS is based.
However, due to SSRMS clearance issues associated with reaching around to the top of Node 2, the SSRMS cannot directly berth a newly captured vehicle to Node 2 Zenith.
Instead, the vehicle must first be berthed to Node 2 Nadir, following which the SSRMS must perform a “walk-off” (i.e. change its base point) from the Node 2 PDGF, to a PDGF on the Mobile Base System (MBS), in order to give the arm the required clearance to install the vehicle on Node 2 Zenith.
The arm, for obvious reasons, cannot perform a walk-off if one end of the arm is still attached to the cargo vehicle, hence the need to berth the vehicle to Node 2 Nadir first, so that the SSRMS can ungrapple the vehicle in order to perform the walk-off.
Once the SSRMS walk-off has been completed, the cargo vehicle can then be relocated from Node 2 Nadir to Node 2 Zenith.
It is not possible to have the SSRMS based on the MBS during the vehicle’s capture, since the SSRMS cannot reach the 30m capture point when based on the MBS – for this, it must be based on the Node 2 PDGF, which is the located on the underside of Node 2 closest to the 30m capture point.
Thus, the fact that, in order to allow for the SSRMS to change its base point, all cargo vehicles must first be berthed to Node 2 Nadir even if they are to be later relocated to Node 2 Zenith, adds complexity to the current DBVV process.
More complexity is added by the fact that, in reverse of the above procedure, a vehicle must be relocated from Node 2 Zenith back to Node 2 Nadir prior to its departure from the ISS, so that the SSRMS can once again perform a walk-off.
This means that if a second cargo vehicle were to arrive at the ISS (to be berthed at Node 2 Nadir) after the first vehicle had already arrived and been placed on Node 2 Zenith, that second vehicle must first depart Node 2 Nadir, in order for the vehicle on Node 2 Zenith to be moved back to the Nadir port for release.
This essentially means that the departure date of the vehicle on Node 2 Zenith limits the stay of the vehicle on Node 2 Nadir, despite that fact that the Nadir vehicle will have arrived at the ISS more recently than the Zenith vehicle.
In essence, this means it was impossible to have DBVV capability where both vehicles are truly independent of the other, and thus the complexity outlined above means that it was simply far easier to de-conflict cargo flights, ensuring no overlap between flights, rather than have two vehicles at the ISS simultaneously.
This process has drawbacks, since de-conflicting flights may have the effect of pushing one flight into a period of launch range conflict, or into a beta-angle cut-out period, which can create a gap between cargo flights, despite that fact that the vehicle may have been ready to launch months earlier.
Now the PMM relocation is complete however, freeing up Node 1 Nadir as the new back-up cargo port, independent DBVV capability will be possible, since the SSRMS will be able to capture cargo vehicles at the 30m point and berth them directly to Node 1 Nadir, without needing any base changes and thus vehicle relocations.
This removes all the aforementioned complexity associated with DBVV operations, and means that it will be possible to have cargo vehicles arrive and depart both Node2 Nadir and Node 1 Nadir completely independently of each other, meaning cargo vehicle flight de-confliction will no longer be necessary.
This will have benefits to the ISS program in the form of more timely delivery of cargo and logistics, since the program will no longer have to wait for one flight to “clear the manifest” before another can be launched.
Previous reconfiguration analysis:
While the plan detailed above was ultimately chosen, many alternative plans were also analysed to create the required amount of docking and berthing ports. As far back as 2010, NASA were evaluating numerous other configurations, as detailed in L2 documents at the time.
These included using Node 3 Forward or Node 3 Aft as the back-up cargo ports, however this was determined to be unsuitable due to clearance issues between the cargo vehicles and the station structure.
Using Node 3 Nadir as the back-up cargo port was also considered, however this would have required a relocation of the Cupola module which resides on Node 3 Nadir, to another location, for which Node 3 Forward and Node 1 Nadir were considered.
However, relocating the Cupola was determined to be too complex as it would have required extensive, time-consuming internal re-wiring work.
The relocation of Node 3 itself, to either Node 1 Nadir or Node 2 Forward was even considered, however this would also have required very extensive external re-wiring work, hence it was also dismissed.
For a time, the preferred plan was to relocate the PMM to Node 3 Aft, rather than Node 3 Forward as is the current plan.
Computer modelling however indicated that, in this configuration, the PMM would come very close to the partially folded solar arrays on the Port side of the Russian FGB module, the arrays having been stowed years earlier to prevent clearance issues with the rotating thermal radiators on the P1 Truss.
In order to better determine the clearances between the PMM and the FGB arrays, NASA conducted extensive photogrammetric analysis of the folded FGB arrays, which ultimately showed the unexpected result that the FGB arrays were in fact deployed around 41 inches further outboard than the computer modelling showed.
After more analysis, it was determined by Russian specialists that “when the limit switch that controlled the solar array retraction process was tripped and power was removed from the retraction drive motor, the solar array may have rebounded outward by some small amount”.
Following the review of video footage of the Port FGB array retraction in September 2007, it could be seen on the footage that “when the array reached the point of maximum retraction, it rebounded outboard and oscillated several times before finally stabilizing in a configuration that was significantly less retracted than the minimum point”.
The end result was the determination that, due to the FGB arrays being more extended than originally thought, the PMM could not be located on Node 3 Aft due to clearance issues between the PMM and the FGB arrays during installation on Node 3 Aft, hence the plan was abandoned.
Not having the PMM located on Node 3 Aft does however leave said port open as a much-desired “expansion port” for smaller modules – specifically, the inflatable Bigelow Expandable Activity Module (BEAM) which will be installed on Node 3 Aft later this year.
Analysis was also conducted into whether having visiting cargo vehicles located on Node 1 Nadir would interfere with Soyuz spacecraft arrivals and departures from the nearby Russian MRM-1 module, however this was determined to be a non-issue.
Ultimately, the present plan of relocating of PMA-3 from Node 3 Port to Node 2 Zenith, and relocating the PMM from Node 1 Nadir to Node 3 Forward, was chosen as the plan which delivered the required number of docking and berthing ports, while requiring the least amount of reconfiguration effort.
The PMM – or Permanent Multipurpose Module – started life as the Multi Purpose Logistics Module (MPLM) “Leonardo”, designed to ferry cargo between Earth and the ISS via the Space Shuttle. As such, it was designed to launch into space and return to Earth inside the Shuttle’s payload bay, spending a maximum of two weeks at the ISS in between.
In September 2009 however it was decided that the MPLM would be left behind at the ISS on one of the final Shuttle flights to serve as a permanent storage module for the station crew. Hence it was adapted for long-duration spaceflight – including adding mode debris and thermal shielding – whereupon it became the PMM.
It was launched to the ISS on STS-133, the final flight of Space Shuttle Discovery, in February 2011, whereupon it was installed on its present port of Node 1 Nadir, where it has remained for the past four years and 3 months.
Preparation for the PMM’s relocation has been conducted on past US spacewalks, which have included relocating a camera stanchion from the P1 Truss in front of the Node 3 Forward port, along with other items of hardware, so that they do not cause clearance issues with the PMM.
Additionally, the Centerline Berthing Camera System (CBCS) flap on the Node 3 Forward port – essentially an external hatch window cover – was opened, so that the PMM can be guided into place by a camera vision system which will look through the Node 3 Forward hatch window.
The Advanced Resistive Exercise Device (ARED), located inside Node 3 around the radial hatchway area, was also rotated to ensure it does not block access to the Node 3 Forward hatch.
On Tuesday 26th May, the PMM was closed-out in preparation for the relocation, which included shutting down ventilation to and from the module, disconnecting ventilation ducting, along with power and data connections, and closing the PMM’s hatch.
Additionally, a thermal cover was installed over the PMM’s hatch, following which four Controller Panel Assemblies (CPAs) – computer boxes which control the berthing/unberthing process – were installed around the ISS side hatchway.
The Center Disc Cover (CDC) was then installed to protect the hatch on the ISS side from orbital debris, following which the Node 1 Nadir hatch was closed. The vestibule area between the ISS and PMM hatches was then depressurised and leak checked.
On Wednesday 27th May, the actual relocation began, when the SSRMS, controlled from the ground by robotics flight controllers in Houston, grappled one of the Flight Releasable Grapple Fixtures (FRGFs) on the PMM. The 16 bolts holding the PMM to the ISS then gradually released, four bolts at a time, whereupon the SSRMS began to pull the PMM away from the Node 1 Nadir port.
The PMM was then “flown” the short distance to the Node 3 Forward port by the SSRMS, and with the assistance of the aforementioned CBCS vision system, which comprises a camera mounted in the Node 3 Forward port window looking at a target on the PMM’s hatch, the PMM was positioned close enough to the Node 3 Forward port for the berthing process to begin.
The berthing process itself consisted of four hooks extending from the Node 3 Forward port and grasping onto the PMM, which then retracted and pulled the two CBM berthing collars together. In reverse of the unberthing process, 16 bolts then drove, four at a time, to secure the PMM to its final home.
The four “petals” covering the berthing collar on the Node 3 Forward port were opened ahead of time, while the vacated Node 1 Nadir port’s petals were closed after the PMM had been unberthed.
The PMM relocation was planned to be conducted on June 12, however due to the delayed Soyuz schedule associated with the Progress M-27M failure, it was brought forward, since on June 12 ISS will be down to only three crewmembers following the delayed departure of Soyuz TMA-15M, with Terry Virts, Samantha Cristoforetti, and Anton Shkaplerov on June 11.
Noteworthy is the fact that Virts helped install Node 3 on the ISS in February 2010 as a crewmember on the STS-130 mission, and so once again helped to configure Node 3.
Additionally, NASA astronaut Scott Kelly, who is currently flying aboard the ISS as a year-long crewmember, was aboard the ISS during the PMM’s installation onto Node 1 Nadir during STS-133 in February 2011, and now got to participate in the installation of the PMM for a second time.
Also noteworthy is the fact that the PMM was actually installed upside-down – i.e. 180 degrees out of sync with the rest of the horizontally oriented US modules. Normally, horizontal modules are oriented so that the lights are on the “ceiling”, in order to maintain a standardised module layout to aid crew orientation.
However, due to the location of the PMM’s FRGF grapple points near to the top-forward end of the module, it was actually impossible to install the PMM in the usual lights-up orientation, due to SSRMS clearance issues with the P1 Truss right above the PMM’s new home on Node 3 Forward.
As such, it was necessary to rotate the PMM 180 degrees, so that the FRGFs face downwards, in order to avoid any SSRMS clearance issues with the P1 Truss.
This will however mean that the interior of the PMM is 180 degrees out from the rest of the horizontal US modules, with, from the crew’s perspective, the PMM’s interior lights being on the “floor”, rather than the “ceiling”.
On Thursday 28th May, the Node 3 Forward hatch will be opened – which in fact will be the first ever time this hatch has been opened on-orbit, since no module has ever been installed on this port before – following which the CDC will be removed.
Due to the fact that the Node 3 Forward port carries a higher MMOD risk than other ports, due to the fact that it faces forwards towards oncoming debris, the Node 3 Forward CDC is of thicker construction than standard CDCs, to provide extra debris protection.
Following CDC removal, the four CPAs will be removed, and the PMM hatch thermal cover will be removed, following which the PMM hatch will be opened. Power and data connectors will then be mated, as well as ventilation ducting, to make the PMM fully active on its new home.
Future reconfiguration plans:
Once the PMM relocation to Node 3 Forward was completed, Node 1 Nadir will not immediately be usable as a cargo vehicle berthing port, since an internal “berthing mod kit” must first be installed, which consists of modifications to the power and data connection available on Node 1 Nadir, in order to support cargo vehicles.
The berthing mod kit was originally scheduled to be installed around mid-August, however the recent ISS schedule upheaval as a result of the Progress M-27M failure may impact this date.
The Japanese HTV-5 was planned to be the first cargo vehicle to berth on Node 1 Nadir when it arrives in late August, however the timing of this flight may now change also.
Looking towards the end of this year, it was also planned that, for the first time ever, both an HTV and a Dragon, and, separately, a Dragon and a Cygnus, would be berthed to the ISS simultaneously via the new cargo port arrangement, however whether or not this will now happen will depend on the schedule realignment.
Undoubtedly, at some point in the future, the new port arrangement will allow different types of cargo vehicles to be able to greet each other on-orbit for the first time ever.
Other reconfiguration tasks planned for this year include the PMA-3 relocation from Node 3 Port to Node 2 Zenith, which had been planned for mid-October.
Additionally, the installation of Bigelow Aerospace’s inflatable BEAM module to the Aft port of Node 3 will take place, which was planned for early September with the arrival of the SpX-8 Dragon mission, however both of these dates may now change.
And in addition to that, two International Docking Adapters (IDAs) will be flown to the ISS later this year aboard separate Dragon vehicles, both of which will be installed via spacewalks onto the ends of the two PMAs.
This will be done in order to convert the PMA’s old Shuttle-era Androgynous Peripheral Attachment System (APAS) docking systems, to new Soft Impact Mating Attenuation Concept (SIMAC) systems which will be compatible with future commercial crew vehicles.
The change in docking systems is necessary to allow future crew vehicles to dock to the ISS more softly than the Space Shuttles did, which will minimise the docking impacts to the ISS structure, helping to prolong the service life of the station.
IDA-1, which will be installed onto PMA-2, will launch on SpX-7, which had been planned for mid-June, and IDA-2, which will be installed onto PMA-3, will launch on SpX-9, which had been planned for early December. However, both of these dates, and the associated IDA installation spacewalks, may now change.
At the conclusion of the reconfiguration effort, only one CBM port will be “unassigned” on the ISS – Node 3 Port, the former home of PMA-3. This port however has very tight clearance issues with the rotating P1 Truss radiator, so its use as a future expansion port is limited. The Node 3 Aft port however will become available again once the BEAM departs the ISS a few years after its installation.
The Node 3 Zenith port, while not in use, will never be available as a berthing location due to clearance issues with the P1 Truss, and Z1 Truss antenna. As such, it has been covered over with a grapple fixture to serve as a storage location for the station’s Dextre robotic hand, rendering it permanently unusable for berthing.
This year’s ISS reconfiguration effort is the most significant change of station configuration since the retirement of the Space Shuttles in 2011, and may well be the final time that we see a major change in the configuration of the US section of the station for the remainder of its operational lifetime.
However, the benefits of this reconfiguration will be immense, as for the first time it will allow for two simultaneous US segment cargo delivery flights to occur completely independently of each other, and also pave the way for future commercial crew flights to the station.
Images: NASA, L2 ISS and L2 artist Nathan Koga – full collection of L2 ISS renderings *available here*.)
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