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.
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”.
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.
Notably it was Dragon’s CRS-2/SpX-2 flight that delivered two new CDRA beds to the ISS, featuring a redesigned heater core with significantly thicker Kapton insulation to reduce risk of short, and completely re-engineered attachment points to the wiring harness to reduce strain at the wiring interface.
However, the issues highlight how the ISS is providing lessons learned into the Environmental Control and Life Support System (ECLSS), a key requirement ahead of deep space missions to Beyond Earth Orbit (BEO) destinations.
A problem with the ECLSS on Orion – or an associated Deep Space Hab (DSH), would require hands on maintenance and solutions via the equipment already onboard, with no possibility of a resupply ship heading out to provide additional hardware.
Another option could be 3D-printing of replacement parts, with the ISS set to demonstrate the first in-space 3D printer next year.
The challenges, even at 250 miles above the surface of the Earth, are well-documented, with recent ISS status notes showing a problem had also occurred with the Sabatier system.
The Sabatier was installed in late 2010 by the Expedition 25 crew, after it was delivered to the ISS via Discovery’s STS-131/19A flight.
The system produces water and methane using excess hydrogen from the Oxygen Generation System (OGS) rack, and carbon dioxide from the CDRA – both of which are otherwise vented into space.
The water is processed through the Water Recovery System (WRS) Water Processing Assembly (WPA), and the unneeded methane is also vented into space.
Sabatier resides inside the OGS rack, which is located in Node 3 – along with the CDRA and WRS WPA. Having Sabatier, OGS, CDRA and WRS WPA all working in conjunction with each other enables Node 3 to become a highly sophisticated regenerative life-support module.
The long-term aim of Sabatier is to decrease the Station’s dependency on water in the post-Shuttle era, greatly aiding engineers in designing regenerative life support systems for future missions beyond Earth orbit. However, it too will require some fine-tuning during its tour of duty on the ISS.
“Sabatier Failed Update: The failure was due to delta pressure fault detected at the inlet to the reactor assembly. The second attempt was unsuccessful when liquid water was detected on both Liquid Sensors on the Sabatier vent line. After allowing the liquid sensors to dry out, the ground performed a third attempt which was successful,” L2 ISS notes added.
“Sabatier ran for an overnight period before faulting out and shutting down. This last shutdown was due to another delta pressure fault. The fault signatures are indicative of moisture in the CO2 accumulator which is partially blocking flow.”
It is understood the Sabatier team is concerned with moisture being received from CDRA, or condensation that has accumulated in the CO2 accumulator tanks. As a result, the Sabatier will remain shut down until this is anomaly is better understood, a matter that is currently under review.
While the ECLSS failures are a menace to the ISS crews and ground teams, they do have a value all of their own, in that they teach the next generations of engineers what to do, and what not to do, when designing systems for use on long-duration space missions where reliability will be essential.
This is, after all, what the ISS was always designed to do – prove out essential technologies before committing to venturing far beyond Earth with no hope of return for many years. Thus, the hard-worn experience ECLSS of the ISS will have a direct and tangible benefit to exploration missions for decades to come.
(Images via L2 and NASA).
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