Two spacewalkers – NASA astronauts Peggy Whitson and Jack Fischer – performed a short EVA on Tuesday to replace a recently failed piece of critical command and control hardware on the International Space Station (ISS). The short-notice unplanned spacewalk got underway at 11:20 AM UTC and finished two hours and 46 minutes later.





EXT-1 MDM failure:

On 20 May, the External-1 (EXT-1) Multiplexer/Demultiplexer (MDM) suddenly failed. EXT-1 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, the other being EXT-2.

The two EXT MDMs provide control to external equipment on the Truss segments, specifically the Mobile Transporter (MT), Thermal Radiator Rotary Joint (TRRJ), Secondary Electrical Power System (SEPS), Passive Thermal Control System (PTCS), and Solar Alpha Rotary Joint (SARJ).

EXT-1 is a size 16 MDM, meaning it has space inside its housing for 16 circuit cards, although in realty it houses fewer cards. Its dimensions are 10.5 x 14.9 x 16.4 inches, and its weight is around 50 pounds.

The EXT MDMs, like all Tier 2 MDMs, have a primary and a back-up. In this case, EXT-1 is the primary and EXT-2 is the back-up, which means that EXT-2 took over as soon as EXT-1 failed, resulting in no loss of operational capability to the ISS.

The concern, however, is the loss of redundancy – i.e. what would happen if the EXT-2 MDM were to fail. In such a scenario, all aforementioned equipment that the EXT MDMs control would become un-commandable. 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.

A big concern in the event of an EXT-2 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, command of which would be lost with an EXT-2 failure.

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 be lost with an EXT-2 failure. Beta rotation via the Beta Gimbal Assemblies (BGAs) would still be possible, however, BGA auto-track mode would not.

Control of other hardware which would be lost in the event of an EXT-2 failure would be Rate Gyro Assembly-2 (RGA-2), the Floating Point Measurement Unit (FPMU) and Wireless External Transceiver Assemblies (WETAs), and Control Moment Gyroscopes (CMGs) -1 and -2.

Due to these critical next-worst-failure impacts, a decision was made to perform a short, unplanned spacewalk as soon as possible to replace the failed EXT-1 MDM with a new unit. EXT-1, as with EXT-2, was only recently replaced with a new Enhanced Processor & Integrated Communications (EPIC) unit during a previous EVA.

A spare EXT MDM was assembled inside the ISS, with hardware such as the MDM On-Orbit Tester (MOOT) being available to test the functionality of the new MDM before sending it outside. It is not known whether the replacement MDM will be of EPIC standard.

This failure is very similar to the April 2014 EXT-2 MDM failure, which ultimately required an EVA to replace the failed unit.

MDM overview:

An 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 organized 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 the back-up.

The EXT MDMs are 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.

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. Locations and 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).

EVA procedures:

EVA-43 to replace the EXT-1 MDM was performed by astronauts Peggy Whitson as EV-1 and Jack Fischer as EV-2. In terms of procedures, EVA-43 was very similar to EVA-26 in April 2014 to replace the failed EXT-2 MDM.

On that occasion, the approximately 2 hour EVA began with the crewmembers egressing the Quest airlock, whereupon both crewmembers began 40 minutes of setup work that included translating to the MDM worksite at the center of the S0 Truss.

Once set-up at the worksite, the planned hour-long task of replacing the EXT-2 MDM began, with the first step being for EV-1 to use the PGT to release two secondary bolts on the failed MDM, with 7 to 14 turns expected on each. The center jacking bolt was released, which required nearly 32 turns.

A handling aid known as a “scoop” was then attached to the failed MDM, whereupon it was removed from its coldplate on the S0 Truss.

A visual inspection of the coldplate and blind-mate connectors was then performed, in order to check for any Micro Meteoroid Orbital Debris (MMOD) strikes, debris, or residual Chotherm insulation material. Some small debris was noticed, requiring some cleaning ahead of the installation of the replacement MDM.

The replacement was then installed onto the coldplate on the S0 Truss, using the same procedure used to remove the failed MDM – that was, driving the center primary bolt, and then the two secondary bolts.

At this point, the primary objective of the EVA was complete, and the two spacewalkers conducted cleanup tasks, prior to translating back to the airlock to conclude the EVA, bringing with them the failed MDM.

In addition, for EVA-43, another task was performed in the form of installing a pair of wireless communications antennas on the underside of the Destiny laboratory, a task that was deferred from EVA-42 earlier this month. This task was the only other task performed as it was straightforward and very near to the S0 Truss worksite.

(Images via NASA and L2 ISS)