I hope for this to be a purely engineering based discussion that could shed some light on this question.
Please only respond from an engineering perspective in this thread.
Data:
According to the "AMSAT Fox-1 Systems Engineering Documentation" the satellites were expected to last at least five years. [1]
For each satellite, before there were any issues with the batteries, they were able to power the transponder continuously without going into safe mode. However, once battery issues were encountered, the transponder would turn off unexpectedly while the satellite went into safe mode. Using that cutoff my back of the hand calculations show the following [2]:
AO-92: batteries lasted 35% of their expected lifetime AO-91: batteries lasted 60% of their expected lifetime. AO-85: batteries lasted 87% of their expected lifetime.
I know the batteries used were Nicad. I know they were matched. I believe the issue so far has been that one cell in the set has failed such that the minimum voltage required to run the transponder is not met. Is that last point correct?
So that is the data that I am currently aware of.
I think the next data to procure would be to understand what information was used to project a five year lifetime. Then you could look at the available data about the real satellites (from telemetry for example) to find out if there are any discrepancies that would explain the reduced lifetime.
From my scanning of the "AMSAT Fox-1 Systems Engineering Documentation" I didn't see the battery chemistry mentioned.
However, in addition to the "AMSAT Fox-1 Systems Engineering Documentation" there is a lot of information in the Fox engineering material from past Symposium Proceedings. These are available as PDFs to AMSAT members.
Do people agree that the next steps would be to understand what information was used to project a lifetime of five years?
If so, can anyone tell me, or can anyone go through the past Symposium Proceedings, and lay out what information was used to project a five year lifetime?
If not, what are the better next steps in understanding the reduced battery lifetime?
73, John Brier KG4AKV
1) 2.3 Reliability Requirements 2.3.1 The satellite shall be designed for a minimum 5-year, on-orbit lifetime. " https://www.amsat.org/wordpress/wp-content/uploads/2018/06/AMSAT-Fox-Documen...
2) AO-92 was launched/commissioned in January 2018. AO-92 has had battery issues since this fall such that it can't be used. AO-91 was launched/commissioned in November 2017 and now it too seems to be having the same battery issues. AO-85 was launched October 2015, declared EOM February 2020.