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Comment by cheschire | Hacker News Reader

> using a plutonium-powered radioisotope thermoeletric generator (RTG) this time that I think is rated to provide something like 100 watts for something like 15 years.

Or provide enough heat to keep a stranded botanist warm for a brief cross country drive.

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Just a note on this: you can’t “increase” the power output of these things. Basically you can imagine these power sources as a “heat pack” that lasts a very long time. So, while you can “store” the energy, and release it later... you can not “use it faster”.

As it produces 100W of electricity, the RTG also produces way more energy as heat. According to Wikipedia, the efficiency of an RTG is usually around 3% to 7%, so you get kilowatts of heat.

https://en.m.wikipedia.org/wiki/Radioisotope_thermoelectric_...

So, it's same order of magnitude to my electric space-heater, if it was stuck to "on". I trust Andy Weir's calculations on whether it is about right to keep a botanist warm on Mars.

Couldn't you use something like a Stirling engine to create more electricity from the excess heat? You might have to have an external radiator to create a heat gradient with a closed loop of liquid but it certainly seems possible.

Yes, you could, and you could do better with more moving parts and more stages, but they're optimizing for reliability not for power.

In theory, the best you could do is (T_hot - T_cold) / T_hot, or (430 - 210) / 430 ~= 50% , using some approximate values for fin root temperature and Martian climate. To exceed 10% of the theoretical optimum with something that contains no moving parts is pretty impressive, IMO.

Couldn't they use the heat to to drive the wheels directly, maybe they could go to more of a hybrid drive? Just seems like such a waste to produce that much heat for 15 years.

I'm not sure how heat would turn wheels directly. You need to convert the heat into either mechanical or electrical energy somehow. Any such system involves tradeoffs in efficiency (in terms of useful energy captured) against weight and reliability.

Specifically you wind up needing grease/oil more moving parts. The failure rates of reaction wheels on orbital craft speak well to the relative difficulty of keeping moving parts moving in space-environments without ongoing maintenance.

AFAIK NASA is already experimenting with Stirling engines + Plutonium for future missions. It’s called ASRG https://en.m.wikipedia.org/wiki/Advanced_Stirling_radioisoto...

well, unless you bring a number of the inner contents of several such packs into very close proximity, at which point the reaction is going to be very much accelerated.

Warranty probably not valid if you get anywhere near criticality ;-)

Just don't recreate the Demon Core... they're both plutonium, right?

Wrong isotope. The Pu238 used in RTGs is not fissile (does not undergo a chain reaction) - it decays by alpha decay. The fissile isotope used in weapons is Pu239.

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