Why is information on nuclear batteries (and other space batteries) so limited?

[Philippe Lemay]

I was reading up on various advanced space power sources such as solar cells and radio-isotope-generators (RTG); as well as their more modern counterparts, Advanced Stirling Radioisotope Generators (A-SRG). And yea, while I was reading I decided to compare a typical A-SRG to a 12 V car battery, because they seem to weigh about the same (20 kg).

Here's where things got complicated. I realized that every day electrical power sources (like chemical batteries) have a LOT of units used to describe them. There's volts, amps, ohms, and coulombs. Very few batteries offer up power (watts) or energy (joule) values.

I'm just wondering, why do space programs not give out any of these values for their space power sources? They seem to be more sophisticated, being all nuclear and stuff, yet they give us far less info. I'd be very curious to know the voltage or amperage of a typical A-SRG. (Or the coulomb even, but I only half understand that one...)

http://en.wikipedia.org/wiki/Advance...tope_Generator

[Philippe Lemay]

I was able to find that a typical 12 V battery holds approximately 1728 kilojoules.

And based on the ASRG’s lifetime of 14 years, and power of 140 watts, it should hold about 6.18 gigajoules. This is cool, since it shows how much stronger an ASRG is over the long run. But I still wonder what the ASRG’s volts and amps and stuff amount to.


EDIT: Re-did the numbers, ASRG actualy holds ~58 gigajoules.

[01101001]

I'm just wondering, why do space programs not give out any of these values for their space power sources?]

Google: nasa a-srg

First hit: Development of Advanced Stirling Radioisotope Generator for Space Exploration (PDF)

With a 140 W_e power output and a generator mass of 20.84 kg, the EU ASRG as currently being
designed and built will achieve a specific power of 6.7 W_e/kg, as compared to 3.4 W_e/kg for SRG110
design and 5.1 W_e/kg for GPHS-RTG. While the value of 6.7 W_e/kg for the EU ASRG is a substantial
improvement in specific power, it reflects the value for the EU program phase.

Another: Advanced Stirling Radioisotope Generator for NASA Space Science and Exploration Missions

Comparison of 650 and 850 °C ASRGs
ASRG specific power, W_e/kg
7.0 ~8.4

Such documents don't seem to match the facts assumed by your question.

Is there some specific A-SRG model for which no power data is available?

[Philippe Lemay]

Well most ASRG's give the power value, what I'm wondering about is the values you normally see with electrical batteries. Like voltage, amps and coulombs. If the final result of the SRG is electrical power, surely it must have these values?

Also, I was mistaken, the ASRG shown in the wiki article would hold 61.8 gigajoules total. But this doesn't take into account the plutonium decay, which I think has a half-life of 85 years. This wouldn't do much over 14 years, but it's probably significant to cut off a few gigajoules from the end result. I'll re-run the numbers.


EDIT: Calculating half-lives is hard... I came to approximately 58 gigajoules total.

[Philippe Lemay]

In case others are interested, I will post the reason why I started musing about space batteries tonight.

http://en.wikipedia.org/wiki/File:VASIMR_spacecraft.jpg

While looking at this pictures I realized those girders that stick out perpendicularly to the axis of the spacecraft look a lot like those RTGs I've seen on older probes (like the Voyagers and the Pioneers), with the radioisotope sources being placed away from the spacecraft to minimize radiation exposure. So I got to thinking... if these are RTGs, they sure seem like big ones. How powerful would they be?

And that's how I got to into wondering about the possible volts and amps of RTGs and SRGs. If anyone wants to help me figure out the possible values of a VASIMR power source as well, feel free. Sorry if I seem to be skipping from one question to the next like this, it's past 2 am here and I'm in a very hazy state right now, lol. My questions are in earnest though, and I would love some opinions and thoughts on the matter.

[Philippe Lemay]

While I'm at it I'll post my numbers on solar power. This isn't really a question, it's just me thinking out loud. Hope no-one minds me using this thread as a kind of work station, it helps me think. Note that all values were taken from Atomic Rockets or Wikipedia, so they aren't 100% reliable but should make good approximate guidelines.

Mass: 20 kg
Efficiency: 15%
Power per m² : 205 watts
Total power: 44 400 watts
Square meter coverage: 215 m²

So, assuming we are in Earth orbit, it would be smarter to use solar panels on a VASIMR than to use ASRGs.

[Philippe Lemay]

One more thing before I log out for the night.

I know a lot of info on them is probably still classified, but I've heard that the reactors used in nuclear submarines would work great as super batteries in larger spacecrafts. Compact, very energetic, sophisticated shielding methods and control systems already in place, etc. If anyone has reliable specs on some modern models and that they'd be willing to share, that would be great.

[TrAI]

In case others are interested, I will post the reason why I started musing about space batteries tonight.

http://en.wikipedia.org/wiki/File:VASIMR_spacecraft.jpg

While looking at this pictures I realized those girders that stick out perpendicularly to the axis of the spacecraft look a lot like those RTGs I've seen on older probes (like the Voyagers and the Pioneers), with the radioisotope sources being placed away from the spacecraft to minimize radiation exposure. So I got to thinking... if these are RTGs, they sure seem like big ones. How powerful would they be?

And that's how I got to into wondering about the possible volts and amps of RTGs and SRGs. If anyone wants to help me figure out the possible values of a VASIMR power source as well, feel free. Sorry if I seem to be skipping from one question to the next like this, it's past 2 am here and I'm in a very hazy state right now, lol. My questions are in earnest though, and I would love some opinions and thoughts on the matter.

Those are nuclear-electric generators to power the three VASIMR units, not RTGs, the combined power output is 12MW. This is a Mars mission concept by the Ad Astra Rocket Company.

[TrAI]

I was reading up on various advanced space power sources such as solar cells and radio-isotope-generators (RTG); as well as their more modern counterparts, Advanced Stirling Radioisotope Generators (A-SRG). And yea, while I was reading I decided to compare a typical A-SRG to a 12 V car battery, because they seem to weigh about the same (20 kg).

Here's where things got complicated. I realized that every day electrical power sources (like chemical batteries) have a LOT of units used to describe them. There's volts, amps, ohms, and coulombs. Very few batteries offer up power (watts) or energy (joule) values.

I'm just wondering, why do space programs not give out any of these values for their space power sources? They seem to be more sophisticated, being all nuclear and stuff, yet they give us far less info. I'd be very curious to know the voltage or amperage of a typical A-SRG. (Or the coulomb even, but I only half understand that one...)

http://en.wikipedia.org/wiki/Advance...tope_Generator

This PDF seems to give a little more information on the General Purpouse Heat Source RTGs used in several probes, perhaps that will help?

For batteries, you will often see the charge in ampere-hours, but this value have no real use for an RTG, as they do not have a limited electric charge, but a constant output of heat that slowly falls off, so with RTGs it is convenient to use watts of power available, and possibly provide a estimate for power fall of over time, if that is needed for the spesific case.

[Ilya]

One more thing before I log out for the night.

I know a lot of info on them is probably still classified, but I've heard that the reactors used in nuclear submarines would work great as super batteries in larger spacecrafts. Compact, very energetic, sophisticated shielding methods and control systems already in place, etc. If anyone has reliable specs on some modern models and that they'd be willing to share, that would be great.

I don't really know anything about submarine reactors, but one aspect seems to make adapting them for space use diffcult. A submarine has unlimited amount of efficient coolant available. Spaceship does not.

[Philippe Lemay]

Those are nuclear-electric generators to power the three VASIMR units, not RTGs, the combined power output is 12MW. This is a Mars mission concept by the Ad Astra Rocket Company.

Really... has that ever been done? Putting a full nuclear reactor in space. I know the Russians might have flown a few back when they were Soviets.

What about those modern day test bans on space? Do they only cover explosions? Would we be permitted to use a nuclear electric generator? And if so, is the only thing holding us back the considerable extra weight involved.

Oh, and I just remembered, the environemto-fanatics might have something to do with it as well...


@ TrAI & 01101001
Thanks for those PDFs, they look quite interesting. If anything they should keep me entertained for a while. lol


EDIT: And, wait. If that Ad Astra spacecraft is run on nuclear generators, how come the heat rejection system are placed so far from the generators? They look like they're sticking out of the propellant tanks. I always figured that heat rejection systems work better if they are placed between the reactor and the rest of your spacecraft.

[PraedSt]

What about those modern day test bans on space? Do they only cover explosions? Would we be permitted to use a nuclear electric generator? And if so, is the only thing holding us back the considerable extra weight involved.

As in nuclear fission? I think you'd be allowed to. But there would be...paperwork.

EDIT: And, wait. If that Ad Astra spacecraft is run on nuclear generators, how come the heat rejection system are placed so far from the generators? They look like they're sticking out of the propellant tanks. I always figured that heat rejection systems work better if they are placed between the reactor and the rest of your spacecraft.

I'm not quite sure what you mean here. The coolant fluid will be circulated between whatever it is that you need cooling and the radiators. It travels in pipes so you can place your radiators more or less wherever you want.

[captain swoop]

Plus your pipes are radiators as well.

[neilzero]

53 years ago, one large missile/drone aircraft = Rascal ran on 28 volts dc, so the RTG can supply 7.32 amps assuming the 205 watts is conservative, and the impedance is matched. Other models can supply different voltages and current whose product is 205 watts. Batteries are available that can supply 205 watts for perhaps one day, thousands of watts for a minute or two. Look at specs for deep cycle batteries and Marine batteries, which usually give amp-hour ratings. Rechargeable lithium ion batteries are better = more watts per pound and they tolerate, fast charge and fast discharge without degrading. See the Tesla motor car web site for details on their 52,000 watt-hour battery. Neil

[TrAI]

Hmmm... I thought I had replied to this, I must have forgotten to post it or something...

Really... has that ever been done? Putting a full nuclear reactor in space. I know the Russians might have flown a few back when they were Soviets.

What about those modern day test bans on space? Do they only cover explosions? Would we be permitted to use a nuclear electric generator? And if so, is the only thing holding us back the considerable extra weight involved.

Oh, and I just remembered, the environemto-fanatics might have something to do with it as well...

Well, If I understand correctly, the treaty bans the deployment of any device that could be considered a weapon of mass destruction in orbit, on another body or in space in general, so I expect certain types of propulsion might be affected by this treaty, but these reactors would probably not be possible to use as wmd's in any practical way. Reactors like these really are needed, the RTG's just does not have the energy to weight ratio for a mission like this.

@ TrAI & 01101001
Thanks for those PDFs, they look quite interesting. If anything they should keep me entertained for a while. lol


EDIT: And, wait. If that Ad Astra spacecraft is run on nuclear generators, how come the heat rejection system are placed so far from the generators? They look like they're sticking out of the propellant tanks. I always figured that heat rejection systems work better if they are placed between the reactor and the rest of your spacecraft.

Well, I would suspect that there really is no way to practically radiate the amount of heat these things make on the surface of the reactors themselves, so therefore the large radiators on the ship. It is possible the vanes on the reactors too provide some radiator surface. Anyway, it is hard to get a feel for the scale from the picture...

[Philippe Lemay]

Well, I would suspect that there really is no way to practically radiate the amount of heat these things make on the surface of the reactors themselves, so therefore the large radiators on the ship. It is possible the vanes on the reactors too provide some radiator surface. Anyway, it is hard to get a feel for the scale from the picture...

I just got to thinking, while reading one of those "Trip to Mars in 39 days" articles that have been popping up I saw that the mission would require a 200 MWe reactor. Modern nuclear plants seem to run at around 33-35% efficiency. Even if we assumed 40% efficiency though, that still leaves us 300 megawatts of waste heat to contend with.

Will the length of piping and the small radiators we saw in the picture (the one I linked be above) be enough to dump all that heat?