Minor issues with elements of the International Space Station (ISS) Life Support hardware continue to show the need for highly robust systems on spacecraft bound for Beyond Earth Orbit (BEO) missions. A problem with a valve on the Carbon Dioxide Removal Assembly (CDRA) is continuing to require mitigation from ground crews.
ISS Life Support Issues:
The CDRA system is used to keep the ISS atmosphere scrubbed of carbon dioxide (CO2), and thus is a vital part of the ISS Environmental Control & Life Support System (ECLSS).
There are two rack-sized CDRAs in the US Operating Segment (USOS) of the ISS – one in the Node 3 module, and the other in the US Lab “Destiny”, although only one is operational at any one time – thus the second one provides vital redundancy.
The Russian Segment (RS) of the ISS also has its own CO2 scrubbing capability in the form of the Vozdukh CO2 scrubber, although this unit is based on older technologies than the more modern CDRAs on the US side.
In the past, during nominal ISS flight, the Lab CDRA was operational, with the Node 3 CDRA powered off due to ongoing issues with its Air Selector Valves (ASVs) sticking, a problem that has been present for a few years now.
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In early September however, one of the ASVs in the Node 3 CRA – called ASV-103 – was Removed & Replaced (R&Rd) by the ISS crew. “This R&R was deemed necessary after the valve failed to reach its commanded position on 2 separate occasions last month” noted L2’s rolling ISS Status section.
“This R&R will use the last remaining spare selector valve on-board; therefore the selector valve that is being removed will be retained on-board for use in a contingency”.
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Despite the R&R of ASV-103, problems with other valves in the Node 3 CDRA have persisted over the past month, this time with ASV-101, leading to the Node 3 CDRA having to be restarted several times as a result.
“Node 3 CDRA Shutdown: The Node 3 CDRA failed due to ASV- 101 not transitioning from position B to A in its allocated time,” noted L2 ISS Status information. “ASV-101 has been trending slower transitions over the past week. (However,) this is the first occurrence of the ASV-101 causing the CDRA to shutdown.”
“The valve was cycled and Node 3 CDRA is now operational. The LAB CDRA is available, if ASV-101 transition failures become frequent enough to where attempting to operate Node 3 CDRA is no longer a benefit to removing CO2.”
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While engineers are repairing additional valves scheduled to head uphill to the ISS on the SpaceX-3 flight next year, no spare units currently exist aboard the station. Only valves that have previously been used are available as “contingency” back-ups, however these do not class as spares since they are in a degraded state.
Other options however include cleaning the valves to help their performance, a process which may be required following another temporary failure of the CDRA over the weekend.
“The Node 3 CDRA experienced another failure of the ASV-101 not transitioning from position B to A in its allocated time,” added L2 ISS Status.
“Ground teams were successful in restarting CDRA, with the Lab CDRA available if Node 3 ASV-101 becomes operationally difficult.”
Sending spare valves to the Station won’t be a problem, with both Orbital’s Cygnus and SpaceX’s Dragon able to loft the hardware uphill. In the event of an expedited requirement, the Russian Progress would likely be the transport of choice.
