What exactly is Voyager 2's backup Power Supply?

[NTL2009]

TL;DR Summary

I've read it's a voltage regulator, so how much and how does it store energy? Caps?

From: www.jpl.nasa.gov/news/nasas-voyager-will-do-more-science-with-new-power-strategy

Voyager is equipped with a voltage regulator that triggers a backup circuit in such an event. The circuit can access a small amount of power from the RTG that’s set aside for this purpose. Instead of reserving that power, the mission will now be using it to keep the science instruments operating.

For all my searching, this basic description just gets repeated. I'm just really curious about the specifics. I understand the basics of the main power supply, the RTG (en.wikipedia.org/wiki/Radioisotope_thermoelectric_generator), thermocouples generate current with a temperature difference from the heat of decaying radioactive material and cold space - simple (in concept), reliable, no moving parts, etc.

But if a backup source was tapping power from the RTG, wouldn't just shutting it down make more power available? And what sort of backup consumes much energy on-going. Once a cap or battery is charged, it doesn't take much to maintain it. And this stuff is 45 years old, so it has to be some very long term reliable technology. They describe it as providing voltage regulation for short term drops from the RTG, but they are looking at this extending the mission by several years. I don't think 'super-caps' were even available to the military at that time. I just don't see how energy stored in a voltage regulator could amount to 'years'. It doesn't really add up for me.

Anyone here know any of the details?

TIA - NTL2009

 

[.Scott]

@NTL2009 :

Without looking at anything other than what you quoted in your post, this is my interpretation:
For some critical operation - perhaps the radio receiver, perhaps attitude control - there is a main circuit and a backup. The voltage regulator powers the main circuit, but on a main circuit power failure it will trip a backup circuit that uses the RTG.

The satellite can be remotely configured to use the RTG to power several possible circuits - but normally, a certain part of its power budget is reserved for that critical backup circuit. With the new configuration, a power failure to the main circuit has the potential of overloading the RTG power, thus shutting down all devices powered by it including the backup circuit.

 

[256bits]

Isn't the RTG the only power supply power for the satellite?
And the power from the RTG goes through a voltage regulator to keep the voltage a a safe level for the instruments, in case the RTG has a blurb of energy
There is no passive energy storage device such as caps or a chemical battery.
Shutting off the voltage regulator, or rather by-passing it, gives 'raw' voltage to the instrument.
If you look at any voltage regulator circuit, they have a Vin higher than the Vout.
With the by-pass Vout == Vin. ( possible since the RTG is decreasing in strength as the radioactive isotopes are diminishing in amount )

I think the problem is with this :
Voyager is equipped with a voltage regulator that triggers a backup circuit in such an event. The circuit can access a small amount of power from the RTG that’s set aside for this purpose. Instead of reserving that power, the mission will now be using it to keep the science instruments operating.

The backup circuit isn't fully explained as to why it can only '.. access a small amount of power .. ", but it isn't reserving, or storing any energy. The energy comes from the one and only power source, the RTG. I imagine the small amount means only that amount necessary to power the instrument.

Would be nice to see how a signal from earth can trip the circuitry, but I imagine that is hard to come by.

 

[NTL2009]

Thanks for the replies. Yes, maybe their wording is overly-generic, not meant for engineering minds, and maybe that's causing the confusion for me. Really hard to say, but since they use the term "voltage regulator", I think @256bits may be (now I think you *are*!) on the right track, and the 'reserve' power is about how it regulates the RTG output (dipping into the voltage overhead, the Vin > Vout as you say), rather than anything to do with an actual separate 'reserve'. When they talked about 'reserve', and a 'voltage regulator", I was thinking about keeping the voltage *high* enough and a separate supply as 'backup'. But now it's coming together, it was regulating the initial *high* voltage down "designed to protect the instruments in case the spacecraft’s voltage – the flow of electricity – changes significantly." I kept reading "change significantly" as a dip, not the increase from being 'new', and 'protect' as to keep them operating if the voltage dips. So yes, if they can bypass (essentially "short out" the regulator), they could get Vin=Vout, and tap into that margin.

But I'm thinking, they know the RTG will degrade over time, and they must configure a lot of the thermo-couples in series to get a usable voltage, and maybe they have a way to switch them around to get more in series to boost the voltage? But then again, the mission was for 2 years, and while they obviously built it very robustly to last this long, they would be limited to what they could do in additional hardware to account for this long of a degradation period.

 

[Rive]

There is no passive energy storage device such as caps or a chemical battery.

I'm not so sure (anymore).

It's really a chore to dig through all the trash google can throw up, but finally I've found a relevant-looking document:

Supplement to Voyager RTG spacecraft design definition, Voyager Task C​

I cannot tell if it's about the design phase or about the final spacecraft, but there are some interesting details (page 39, as a PDF-page)

On page 42:

Ps.: I've found one more relevant document: 'Phase 1A Study Report - Voyager Spacecraft' from 1965

 

[256bits]

It's really a chore to dig through all the trash google can throw up, but finally I've found a relevant-looking document:

I feel your pain. It's redundancy ( with no credit given to the original author ), or unrelated.
anyways.
Launched in 1977 and here it is working away some odd 45 years later.
Would the battery have sustained viability after all these years is one question to ask,
Another is the heating system for electronics and the battery itself.
Operating temperatures would have to be reasonable - perhaps as low as 0C for these things to function properly.
If the RTG was sending power to a battery heater to keep the battery up and running in standby mode, then that is a continuous 'loss' that could be put to use for the instruments when the time comes for the low power considerations.
The battery can supply only x amount, and then it has to be re-charged from the RTG.

If it was me,( and that is where my answer was coming from when I mentioned that there is no chemical battery or caps available since they would only be a typical ramp up-ramp down procedure - one might have to drop an instrument to re-charge ) I would shut off the heater to the battery and use that power for the instruments to keep them running a bit longer in duration. If it is going to fail it's going to fail and with luck on your side..... I think that is where the 'The circuit can access a small amount of power from the RTG that’s set aside for this purpose.' comes from.

Not sure but as an engineer, if the system has been quite reliable so far, with its orientation basically constant and with no more flybys of planets, electrical backup and voltage regulation from the battery circuit might not be needed anymore.

I am somewhat surprised that since quite a few satellite/probes have been using these systems, that there is not a more comprehensive description of the whole system available somewhere.

 

[Rive]

I am somewhat surprised that since quite a few satellite/probes have been using these systems, that there is not a more comprehensive description of the whole system available somewhere.

I've found a mention that Pioneer-10 had a possibly similar system:

Duration and Steering Logic (DSL) was turned off in February 1995 to conserve power, after which it was turned on again only for spacecraft maneuvers. RTG power levels are low enough that the spacecraft occasionally relies in part on battery power (accumulated during inactive periods) to run experiments and other systems.

BTW I've found one more relevant document: 'Phase 1A Study Report - Voyager Spacecraft' from 1965
It is an earlier but more comprehensive paper.

 

[256bits]

Thanks for the replies. Yes, maybe their wording is overly-generic, not meant for engineering minds, and maybe that's causing the confusion for me. Really hard to say, but since they use the term "voltage regulator", I think @256bits may be (now I think you *are*!) on the right track, and the 'reserve' power is about how it regulates the RTG output (dipping into the voltage overhead, the Vin > Vout as you say), rather than anything to do with an actual separate 'reserve'. When they talked about 'reserve', and a 'voltage regulator", I was thinking about keeping the voltage *high* enough and a separate supply as 'backup'. But now it's coming together, it was regulating the initial *high* voltage down "designed to protect the instruments in case the spacecraft’s voltage – the flow of electricity – changes significantly." I kept reading "change significantly" as a dip, not the increase from being 'new', and 'protect' as to keep them operating if the voltage dips. So yes, if they can bypass (essentially "short out" the regulator), they could get Vin=Vout, and tap into that margin.

But I'm thinking, they know the RTG will degrade over time, and they must configure a lot of the thermo-couples in series to get a usable voltage, and maybe they have a way to switch them around to get more in series to boost the voltage? But then again, the mission was for 2 years, and while they obviously built it very robustly to last this long, they would be limited to what they could do in additional hardware to account for this long of a degradation period.

Seems as if it was over-engineered to some extent.

don't take the Vin=Vout too seriously since there is a lot more going on than that utterly and definitely obvious over simplification. It was meant more as an analogy, and a bad one at that.

What I just thought of is the RTG.
If the radioactive substance has a half life of 87 years , then that would mean its heating value would fall to half value after 87 years due to the radioactive intensity also being cut in half.
The RTG hasn't reached its half life yet - its about 70%.
How that translates to the temperature of the hot side of the RTG module I will have to think about a bit as the temperature differential between hot/cold determines the voltage output. To keep the voltage up, the amperage drawn has to decrease with a resulting decrease in power output. If it follows the I squared law then that means only 1/4 power available for the instrumentation.
Just thinking out load here.

 

[NTL2009]

... don't take the Vin=Vout too seriously since there is a lot more going on than that utterly and definitely obvious over simplification. It was meant more as an analogy, and a bad one at that. ...

It might not be the right answer, but to me, it does make some good sense. At the start, the RTG is going to be putting out max voltage. They can afford to 'waste' it in a regulator, as the source is putting out what it will, you can't conserve it (production-wise) in any way, though you could (and apparently do) store excess in a battery. They could maybe use a shunt regulator for low drop-out, but that might require too much wattage to be practical in this application. But once the RTG has dropped low enough to not pose an over-voltage threat, it could make sense to bypass a series regulator.

I'll try to get through the documents that were posted later (might not be until tomorrow), but it does appear that type of silver-cadmium battery could last 45 years? I have not found much info on them, they aren't used much because they are so expensive, but cost isn't much of a factor for space missions.

 

[Tom.G]

Looks like shelf life, power/weight, and flat discharge curve were the magic combination.

The requisite Google search:
https://www.google.com/search?hl=en&q=SERVICE+life++of+SILVER+CADMIUM+BATTERY

Ag-Cd has about twice the power/weight ratio of Ni-Cd batteries
and "...a good shelf life."Dated March, 1963, the below is from:
https://apps.dtic.mil/sti/pdfs/AD0402981.pdf

SILVER-CADMIUM BATTERY PROGRAM
Technical Documentary Report No. WADD-TR-61-131, Pt. II
Pg. 44
"4. Cycle life up to 3000 cycles at 20% depth in 90 minute period"

As a result of the work on this contract, sufficient knowledge has
been gained to provide an insight into the continued R&D required
to bring Solver-Cadmium cells to a higher level of performance as
sealed cells.

Much more info available at:
https://www.science.gov/topicpages/s/silver-cadmium+batteries

Cheers,
Tom

 

[Rive]

I have not found much info on them

NASA has some of that too
... they are a treasure trove

No trash finds like with Google...

Might be a bit outdated, but a perfect fit for the age

Batteries for Space Power Systems

Sealed Silver-oxide Cadmium Batteries for Space Flight, 1960 - 1977