For Questions That Don't Merit Their Own Thread

[ZetaX]

Like those https://en.wikipedia.org/wiki/Thermoelectric_generator ¿

[MinimumSky5]

Like those https://en.wikipedia.org/wiki/Thermoelectric_generator ¿

tl:dr, Yes, it's perfectly possible so long as a sufficient temperature gradient is maintained.

[magnemoe]

tl:dr, Yes, it's perfectly possible so long as a sufficient temperature gradient is maintained.

I think all current ones are solid state, it was plans for one using an stirling engine, this would be more powerful but have the downside of moving parts but I think it was canceled.

[Skyler4856]

Like those https://en.wikipedia.org/wiki/Thermoelectric_generator ¿

Ive always wanted to build skyscrapers that reach into the upper stratosphere, then use the temperature gradient to generate free electricity.

[K^2]

Is it possible to devise a solid-state thermoelectric generator? As in, one that does not require mechanical parts or circulating fluids to generate electricity from temperature gradients?

Not only do they exist, they are the principal way of generating electricity in RTGs. Unfortunately, they are very inefficient. So while they might be preferable in applications where weight and reliability are biggest concerns, if you are actually interested in generating a lot of power efficiently, you want to go with conventional heat engines.

[shynung]

Shame. I was expecting something like KSP Interstellar's solid-state generators, something that's more efficient than conventional heat engines.

[Kilmeister]

Well sticking with RTGs. Is the amount of Plutonium in the generator the primary factor in the electric output. IE To raise the output of an RTG of say 250W to 300W mostly a matter of adding more plutonium? I ask this because as mentioned in one of the New Horizon's press conferences he mentioned that NASA/DOE would be allowed to start producing Pu-238 again.

[Skyler4856]

Hey, in the Heisenberg uncertainty equation, what does a measure of uncertainty mean?

Does it mean a margin of error?

[GeneralVeers]

Well sticking with RTGs. Is the amount of Plutonium in the generator the primary factor in the electric output.

Not the amount, per se--it's the density. The phrase "critical mass" is misleading. As you increase the density of the plutonium core, the fission rate increases, and the temperature of the mass increases. And the thermocouples in the RTG produce more electricity. The problem being that, with a faster fission rate, the fuel source will decay faster.

Hey, in the Heisenberg uncertainty equation, what does a measure of uncertainty mean?

Does it mean a margin of error?

Kind of. "Margin of accuracy" would be a better term. Basically it says we can only know the position and/or velocity of a particle up to--but not above--a certain sum total accuracy. If you know its position more accurately, your measurement of its velocity must be less accurate, and vice versa. We can never know both with 100% certainty. Primarily because, in order to view something, we have to bounce radiation off it (same way our eyes work) and at that scale, the radiation we're shining on the particles moves them significantly.

Edited August 5, 2015 by WedgeAntilles

[ZetaX]

Not the amount, per se--it's the density. The phrase "critical mass" is misleading. As you increase the density of the plutonium core, the fission rate increases, and the temperature of the mass increases. And the thermocouples in the RTG produce more electricity. The problem being that, with a faster fission rate, the fuel source will decay faster.

RTGs are not about fission. You surely get fission if you accumulate enough sufficiently pure plutonium, but that's not the intended use. RTGs work with nuclear decay.

In the end, more plutonium will mean more power. If you avoid storing it too "dense", then you will have almost only decay and thus the maximal life expectancy.

[K^2]

RTGs are not about fission. You surely get fission if you accumulate enough sufficiently pure plutonium, but that's not the intended use. RTGs work with nuclear decay.

²³⁸Pu primary decay mode is alpha. Which is, technically, fission.

But yeah, it's not about neutron capture, as it is with reactors or nukes. So you only care about total amount, not density.

Edited August 6, 2015 by K^2

[ZetaX]

Pu²³⁸ primary decay mode is alpha. Which is, technically, fission.

But yeah, it's not about neutron capture, as it is with reactors or nukes. So you only care about total amount, not density.

In my native language (well, at least in basics physics education and as I just checked wikipedia), "fission" means the splitting of a nucleus different from the usual decays. It's interesting that in english (and possibly actual physics) this seems to formally include alpha-decay. Does it include beta (whatever type) and/or releasing neutrons¿

[K^2]

In my native language (well, at least in basics physics education and as I just checked wikipedia), "fission" means the splitting of a nucleus different from the usual decays. It's interesting that in english (and possibly actual physics) this seems to formally include alpha-decay. Does it include beta (whatever type) and/or releasing neutrons¿

Fission is typically defined as nucleus becoming split into two or more nuclei. Alpha and proton emissions qualify. Neutron and betas do not. Though, it is not uncommon to use the word to exclude alphas. Hence presence of the qualifier "technically" in my previous post.

In either case, the important question is whether neutron capture is relevant or not. There are spontaneous fission processes with heavy products as well. Likewise, there are alpha/beta decays that are triggered by neutron capture. The difference is specifically that RTGs tend to rely on spontaneous decay, while reactors and nukes rely on capture.

[GeneralVeers]

RTGs are not about fission. You surely get fission if you accumulate enough sufficiently pure plutonium, but that's not the intended use. RTGs work with nuclear decay.

They work with heat. The thermocouples in the generator convert heat into electricity, and they don't care where the heat comes from. If you want more voltage, a denser and/or more spherical plutonium core will do it.

[ZetaX]

They work with heat. The thermocouples in the generator convert heat into electricity, and they don't care where the heat comes from. If you want more voltage, a denser and/or more spherical plutonium core will do it.

Sure, but then that's not an RTG. Thus this does not really answer the original question whether the amount of Pu is the determining factor for RTGs.

[K^2]

They work with heat. The thermocouples in the generator convert heat into electricity, and they don't care where the heat comes from. If you want more voltage, a denser and/or more spherical plutonium core will do it.

No, that's entirely false. ²³⁸Pu is not a source of neutrons. So density of fuel has absolutely no bearing on the reaction rate. 1g of fuel to 0.5W of power output. Regardless of shape. Regardless of density.

[GeneralVeers]

Sure, but then that's not an RTG.

Sure it is. Radiothermal. Simply that the heat is being produced by two different processes.

No, that's entirely false. ²³⁸Pu is not a source of neutrons. So density of fuel has absolutely no bearing on the reaction rate.

True. But only for that specific isotope. That isotope is used in RTG's a lot, but it's not the only one. The initial question didn't specify an isotope, and Kilmeister's reply about it only mentioned number 238 offhandedly.

Edited August 6, 2015 by WedgeAntilles

[K^2]

True. But only for that specific isotope. That isotope is used in RTG's a lot, but it's not the only one. The initial question didn't specify an isotope, and Kilmeister's reply about it only mentioned number 238 offhandedly.

It's the only isotop of Pu I'm aware of being used in RTGs. And since not being a neutron source is usually a selection criterion for RTGs, I doubt there are any that rely on neutron capture. If you know of any that do, I would appreciate a reference.

[GeneralVeers]

Wikipedia has some stuff on a type of RTG called a "subcritical multiplicator" RTG (wow, that name sounds like a five-year-old came up with it!) This type of RTG uses beryllium, and produces additional heat through subcritical fissions. As of 2013, NASA was doing feasibility studies on this, but I haven't found anything more recent. ²³⁸Pu would be preferable, but it's in short supply these days.

[K^2]

Wikipedia has some stuff on a type of RTG called a "subcritical multiplicator" RTG

Well sticking with RTGs. Is the amount of Plutonium in the generator the primary factor in the electric output.

Not the amount, per se--it's the density. The phrase "critical mass" is misleading. As you increase the density of the plutonium core, the fission rate increases, and the temperature of the mass increases. And the thermocouples in the RTG produce more electricity. The problem being that, with a faster fission rate, the fuel source will decay faster.

So there is one type of proposed RTG that exists only on paper to which your quote applies. Kilmeister's statement applies to every single RTG ever constructed. Are you sure this is the argument line you want to keep pushing?

[Mitchz95]

Would a red dwarf star actually look red from space?

[K^2]

Would a red dwarf star actually look red from space?

More orange than red, even for the coldest ones. Hottest ones would be closer to yellow.

If you want one that's actually red, you're probably looking for an L-class brown dwarf.

Edited August 6, 2015 by K^2

[More Boosters]

I have a question, what will a manned mission to Mars achieve that all the unmanned missions missed? I'm not saying it wouldn't achieve anything, I'm simply saying that I am not sure what. Would prove that we can, sure...

[RainDreamer]

I have a question, what will a manned mission to Mars achieve that all the unmanned missions missed? I'm not saying it wouldn't achieve anything, I'm simply saying that I am not sure what. Would prove that we can, sure...

Well, anything that require more precise control than the current ~20 minutes delay for signal going from earth to mars? I mean, picking up rocks and analyzing things can feasibly be done with that delay, but if we want to build things, that likely will need either human hand or more sophisticate technology in automation.

[PB666]

²³⁸Pu primary decay mode is alpha. Which is, technically,

fission.

^^^^ 5

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Fission is typically defined as nucleus becoming split into two or more nuclei. Alpha and proton emissions qualify. Neutron and betas do not. Though, it is not uncommon to use the word to exclude alphas. Hence presence of the qualifier "technically" in my previous post.

In either case, the important question is whether neutron capture is relevant or not. There are spontaneous fission processes with heavy products as well. Likewise, there are alpha/beta decays that are triggered by neutron capture. The difference is specifically that RTGs tend to rely on spontaneous decay, while reactors and nukes rely on capture.

I read an article about 10 years ago, maybe more in which you have an RTG in which the two radioactive materials were embedded in a rubbery plastic, when they roll into each other the generator core they exchange particles and react with each other producing heat and fueling the thermocouple. I imagine if the craft still had fuel to manuever the rubber would be cut and it would drift into space. This was an idea for fuel deep space spacecraft with lifespans of 50 to 200 years. I think the Voyager craft have another decade left in their RTGs before they do not have enough to power the antenna and they will go silent.

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Actaully denser will work better but not because of nuetron decay.

Denser works better because if you thermocouple is at the center of the mass, the absolute heat gradient increases from the interior to the exterior. So a bigger mass means hotter at the center.

But if you look at the RTGs, they modulate the heat the thermcouple runs at a certain position at the center and heat is radiated through the radial fins. The design is not spherical, but cylindrical, which means not looking for more volts, but more amps, which means wider or more thermocouples.