The International Space Station (ISS) suffered a failure of an external computer known as a Multiplexer/Demultiplexer (MDM) on Friday night (11 April), which will require a spacewalk on April 22 to replace in order to restore vital redundancy to the station. Despite the challenges, SpaceX’s Dragon CRS-3 mission – which could have been impacted by the MDM failure – was still on for Monday, prior to its scrub

MDM overview:

A Multiplexer/Demultiplexer (MDM) is essentially an electronics unit that sends and receives multiple streams of data, and thus all incoming and outgoing data to and from the ISS is routed to its correct destination via an MDM.

The ISS MDMs are part of the Command and Data Handling (CDH) system, which controls all major functions of the US segment of the ISS, including power generation and distribution, attitude control, environmental control, communications systems, and monitoring of scientific payloads. The MDMs contain all the software needed to control these systems, hence why they can essentially be thought of as computers.

ISS MDMs are organised in a hierarchical command structure known as Tier 1, Tier 2 and Tier 3, with each tier having its own specific purpose. Within these tiers, different types of MDMs exist, each designed to control a unique system.

Tier 1 MDMs, also known as Control Tier MDMs, are responsible for overall control of the entire US segment of the ISS. They consist of only one type of MDM, called Command and Control (C&C) MDMs, which are located inside the ISS. They are two-fault tolerant, meaning three are available, with one as primary, and two as back-ups.

Tier 2 MDMs, also known as Local MDMs, are responsible for system-specific control and data processing.

The different types of Tier 2 MDMs, known as External (EXT), Internal (INT), Payload (PL), Guidance, Navigation & Control (GNC), and Power Management Control Unit (PMCU) MDMs, all provide control for the individual systems to which they are assigned.

With the exception of the EXT MDMs, all are located inside the ISS, and all are single-fault tolerant, meaning two are available, with one as primary, and the other as back-up.

Tier 3 MDMs, also known as User MDMs, are responsible for data processing for the thousands of sensors aboard the ISS. The different types of Tier 3 MDMs are Node 1 (N1), Airlock (AL), Lab (LA), Photo Voltaic Control Unit (PVCU), and Starboard Zero (S0), with each providing sensor processing to their specific modules.

The N1, PVCU, and S0 MDMs are located outside the ISS, while the AL and LA MDMs are located internally. Fault-tolerances vary for these MDMs.

In terms of physical hardware, while all MDMs have common features, their construct varies depending on their type, which is governed by the fact that some MDMs need more circuit cards than others in order to operate, since some control more complex systems than others. Different types of cards inside the MDMs perform different functions.

There are three different sizes of MDMs – MDM-4, MDM-10, and MDM-16 – which simply means that some MDMs are sized for 4 circuit cards, some for 10 circuit cards, and some for 16 circuit cards, with each size having a wider enclosure to house their specific number of cards.

MDMs are installed in a relatively simple process, via two captive fasteners on their left and right sides, and a center jackscrew to hold the MDM enclosure in its location. All electrical and data connections to the MDM are made via blind mate connectors on the rear of the MDM, which are automatically made as the MDM is bolted into place.

While some MDMs are located inside the ISS and others are located outside, the only design difference between the two is the addition of spacewalk-friendly interfaces onto the external MDMs, such as tether rings. The MDM bolts are able to be driven via intra-vehicular power screwdrivers or extra-vehicular Pistol Grip Tools (PGTs).

EXT-2 MDM failure:

The specific MDM which failed on Friday night is the External-2 (EXT-2) MDM, which is a Tier 2 MDM responsible for controlling external US segment systems, and is one of two EXT MDMs located on the outside of the S0 Truss. It is not to be confused with the two S0 MDMs, which, while also located externally on the S0 Truss, are Tier 3 MDMs which provide control exclusively to S0 Truss systems.

The two EXT MDMs provide control to external equipment on the Truss segments, including Structures and Mechanisms (S&M), External Thermal Control System (ETCS), and Electrical Power System (EPS) equipment.

Specifically, this includes the Mobile Transporter (MT), Thermal Radiator Rotary Joint (TRRJ), Secondary Electrical Power System (SEPS), Passive Thermal Control System (PTCS), and the Solar Alpha Rotary Joint (SARJ).

As for the specific failure itself, exclusive failure impact notes available on L2 state that “the EXT-2 MDM did not boot fully when powered up today. Current draw is 33W vs. the usual 50W. It has not responded to mode code commands, despite several power cycles”. The notes go on to add that EXT-2 is now considered failed.

EXT-2, like EXT-1, is a size 16 MDM, meaning it has space inside its housing for 16 circuit cards, although in realty it only houses 5 cards. Its dimensions are 10.5 x 14.9 x 16.4 inches, and its weight is 50.8 pounds. It is 13 years old, having launched to the ISS with the S0 Truss on STS-110 in April 2002.

EXT-2 failure impacts:

As explained previously, all Tier 2 MDMs have a primary and a back-up. In the case of the EXT MDMs, EXT-1 is the primary, and EXT-2 is the back-up, meaning that only the back-up EXT MDM has failed, and thus there is no actual loss of capability to the ISS at this time. The concern however is loss of redundancy – i.e., what would happen if the EXT-1 MDM were to fail.

Should the EXT-1 MDM fail currently, the problem would be that all the aforementioned equipment that the EXT MDMs control would become un-commandable. The power would still be available to the equipment, but ground controllers and the crew aboard the ISS would be unable to send any commands to the hardware.

With regard to Monday’s planned launch of the SpX-3 mission, an EXT-1 MDM failure would not affect the Space Station Remote Manipulator System (SSRMS), since the arm is based on Node 2 during Dragon captures. NASA confirmed the launch of the Dragon is still on for Monday.

As the EXT MDMs only control Truss systems, the only way the SSRMS would be affected by an EXT-1 MDM failure is if it were based on the Mobile Base System (MBS), since command of the MT (to which the MBS is attached) would presently be lost in an EXT-1 MDM failure.

The MT is currently located at Worksite-4 (WS-4), which is right on top of the location of EXT-2 outside the ISS, and thus the MT needs to be moved prior to any spacewalk to replace EXT-2. Additionally, the MT will need to be moved to WS-2 for the capture of the SpX-3 Dragon, in order to provide the crew with adequate external camera views.

Both of these MT movements would not presently be possible in the event of an EXT-1 failure. Ground controllers could command the MT to move to WS-2 now in order to solve both of these issues and protect against an EXT-1 failure, however an issue with this plan is that doing so would expose the Trailing Umbilical System (TUS) cable to Micro Meteoroid Orbital Debris (MMOD) for a long period of time, which is undesirable.

Scientific experiments mounted on the Truss – including the Alpha Magnetic Spectrometer (AMS) and experiments mounted on the ExPrESS Logistics Carriers (ELCs) – would be unaffected by an EXT-1 MDM failure, as they are controlled by the Payload (PL) MDMs.

The real concern in the event of an EXT-1 MDM failure would be for hardware such as the two Thermal Radiator Rotary Joint (TRRJs), which rotate the radiators to face away from the Sun, and command of which would presently be lost with an MDM-1 failure. To protect against this scenario, ground controllers have already commanded the TRRJs to the locked position.

Additionally, command of the two Solar Alpha Rotary Joint (SARJs), which provide alpha rotation to the solar arrays in order to allow them to track the Sun, would presently be lost with an EXT-1 failure. Beta rotation via the Beta Gimbal Assemblies (BGAs) would still be possible in the event of an EXT-1 MDM failure, however BGA auto-track mode would not.

Control of other hardware which would be lost in the event of an EXT-1 MDM failure would be Rate Gyro Assembly-2 (RGA-2), which has already been powered up to protect against this scenario, the Floating Point Measurement Unit (FPMU) and Wireless External Transceiver Assemblies (WETAs), both of which would be relevant during a potential EVA and which may be powered up as a precaution, and Control Moment Gyroscopes (CMGs) 1 and 2.

SpX-3 outlook and potential EVA:

While a final decision now taken, L2 sources were already suggesting that NASA was looking to allow SpX-3 to launch on Monday and rendezvous with the ISS as planned despite the EXT-2 MDM failure, partly due to the preventative action already taken to protect against an EXT-1 failure.

An Extra Vehicular Activity (EVA) will also be conducted on April 22, in order to Remove and Replace (R&R) the EXT-2 MDM.

The EVA would likely be a short EVA (around 2.5 hours), owing to the relative simplicity of the task, although an EXT MDM R&R is listed as one of the “Big 12” most serious EVA tasks.

It will be conducted by a currently undecided combination of NASA astronauts Rick Mastracchio, Steve Swanson, and JAXA astronaut Koichi Wakata, all of whom have previous EVA experience.

Should SpX-3 launch on Monday as planned, the EVA could lead to the first ever photographs of Dragon berthed to the ISS taken from outside the station.

A spare EXT MDM is already available inside the ISS, since they are small enough to fit through the Quest airlock hatchway, although it would need to be checked out prior to use. The location of the EXT-2 MDM outside the ISS is the Starboard-Nadir corner of the S0 Truss, very close to the Quest airlock, meaning minimal translation would be needed.

According to L2 notes, the ISS is currently GO for EVA, with suit 3011 recently having had its Fan Pump Separator (FPS) replaced, however suits 3005 or 3010 currently aboard the ISS would also need an FPS change-out prior to any EVA. The plan currently calls for the use of 3011 and 3005.

However, should SpX-3 launch on time, a brand new spacesuit (3003) which is aboard Dragon would be able to be used for an EVA without requiring any FPS change-out, as a back up option.

The EXT-2 MDM, once removed, could be brought back inside the ISS, whereupon the individual circuit card inside it could be replaced either aboard the ISS or on the ground following return to Earth, meaning it could be returned to serviceable status as a spare EXT MDM.

(Images: via L2 Special Sections and NASA)

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