You would think that the design could contain a small analog charge controller that could act as a failsafe for the digital system. I understand that there is a size and weight limit to these little beasties but my yard lights have analog charge controllers that can fit on a US Quarter. I know that the very small size limits the current that they can handle, especially in a vacuum but who cares if it charges at the rate of a few mA and takes several hours or a day to get the batteries up to power so that the digital system wakes up and takes over.
Just an idea since I don’t have any experience building satellites but I have built several (about 50) small yard lamps that run from a lithium polymer battery. The little solar panels produce 6 volts at 50mA in full sunlight and, as I said, the charge controller is one surface mount five-pin chip, a couple of capacitors and a couple of resistors. I think it’s just a fancy voltage and current regulator that prevents overcharging. You can get them from Digikey for about 50 US cents of you buy one. If I remember right, it’s a MCP73831T-2ACI/OT chip. The chip can handle up to 500mA of charge current without a heat sink (in air) so I’m sure that it could handle 50mA in a vacuum. Is it RADHARD? I doubt it.
Kevin, WB5RUE
From: Ashhar Farhan [mailto:farhanbox@gmail.com] Sent: Sunday, January 31, 2021 12:59 PM To: Dimitrios Simitas va3dsz@gmail.com Cc: AMSAT BB amsat-bb@amsat.org Subject: [AMSAT-BB] Re: Cubesats
Having built four missions and launched two, I can say that the main challenge of cubesats is their really small power budget. The rest of the risks just multiply because of that.
Very often, the time from insertion into the ppod to launch is more than six weeks. The batteries drain off, as the batteries drain off, the entire system is browned out. it is a miracle to put up back again. very often, the analog circuitry that would have charged the batteries through solar panels is replaced by software on OBC. But the OBC is unavailable, the analog state is undetermined. There is a colossal fight among schrodinger's cats inside the box.
On the other hand, why does the AO-7 continue to work? because it has an all analog design that will turn on the transponder as soon as power is applied and the oscillators can oscillate. If they fail to boot up on one orbit, there is always another orbit to bet on.
This is also compounded by the cubesat/IARU specification that insists the satellite should have a way to switch itself off under ground control. This means that unless the CPU decides to wake up the radio, it will remain silent. If the CPU has browned out, then bye, bye, bluebird!
- f
On Sun, Jan 31, 2021 at 10:06 PM Dimitrios Simitas <va3dsz@gmail.com mailto:va3dsz@gmail.com > wrote:
CubeSats enjoy a up to a 39% industry average failure rate. From most common to least, CubeSat failure (21%), launch failure (12%), and not ejected and not specified are tied at 2% each.
https://www.hindawi.com/journals/ijae/2019/5063145/
Successful deployment made up 61% of the 848 CubeSat launches between 2005 and 2018. So the Fox line has met industry standards (3 of 5 deployed successfully).
The hardware, even if purchased to be space tolerant, is still assembled and ground based testing in a educational or not for profit environment is limited compared to the big commercial companies. It's the reason why Es'hail 2's amateur component was built commercially using AMSAT specifications.
There is also the issue that as secondary payload CubeSats have been developed to not use active means of deployment. Such as hoping their solar cells, and antennas release. Which is suspected to have caused many failures as has been pointed out.
Even so, SpaceX is seeing a 5% failure rate on their $2.5 million Starlink Satellites and even NASA's $4 billion Hubble had issues. Space can be unforgiving. And due to size and weight constraints of CubeSats, unlike larger commercial satellites, it's nearly impossible to build redundant systems to keep the satellite functional after any system failures.
The term acceptable failure rate in engineering comes to mind. And expecting perfection out of a CubeSat just isn't going to happen.
Dimitrios
VA3DSZ
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