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Comment by sandworm101 | Hacker News Reader

I think they are going for something more than a simple RTG. I read a few years ago about Stirling engines in zero/low gravity. I think that is what they are going for, something with moving parts, rather than a relatively inefficient static RTG.

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Yeah, it’s called Kilopower. A small fission reactor using Stirling engines. Up to 10 kilowatts electric

I'm less familiar with Kilopower but I spent a couple years working on something very similar that ran off of PU-238. Looks like Kilopower is U-235.

Looking into this, it needs a simple control system. The PU-238 bricks just made heat continuously with no control. But, they were also much less power.

If there's a control system you can launch the reactor cold. With the Plutonium bricks you cannot, meaning your cooling system (engine in this case) has to keep running during launch.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/200900...

https://rps.nasa.gov/technology/

An rtg can be designed to handle cooling failure. It isn't like a water reactor. As the brick heats it expands, decreasing reactivity and heat production. So an rtg can live perfectly well without its cooling system. It won't be making useful power, but it isn't going into meltdown either.

This wasn't a typical RTG. It had stirling engine and was a lot more efficient (made a lot more power per unit weight of Pu) than RTG.

I don't think there was any way to save and restart anything if an engine failed as it would have likely destroyed the engine.

https://rps.nasa.gov/system/downloadable_items/36_APP_ASRG_F...

The RTG has to be kept cool continuously on the launch pad with a nitrogen purge. All cooped up in a fairing, it’d cook the spacecraft if the heat wasn’t rejected somehow

That would be nice down in the tunnels on Mars. The waste heat would be used to heat the habitat.

Can I have two for my house?

Half a century ago some companies actually planned for things like that - a small nuclear reactor that you would bury under your house and it would provide you with power for decades. It went the same way the nuclear car did - the concept would work but it's just too dangerous without constant supervision.

I had never heard of that, but the idea of it makes me think of all my neighbors growing up in the 90s who were spending tens of thousands of dollars cleaning up contamination from leaking underground heating oil tanks. It was a major issue for people buying and selling older houses...sellers hiding knowledge of leaking tanks, surprise discoveries etc. In the worst cases the soil decontamination could cost over $100k. I can only imagine what fun these end-of-life home nuclear reactors would be causing 50 years later!

I visited a home once that was built in the mid 20th century on the assumption that electricity would become too cheap to meter. The original heating system was described to me as just a bunch of resistive elements strung in the attic. I think the house actually got an award similar to LEED (or maybe it was LEED?) at its construction.

My parent's late 70s house has something like this. There are heating elements sandwiched between two layers of drywall in the ceiling. It actually works really well, because it heats objects rather than the air--sort of like the sun on a cool, cloudless day--but electricity is like 4x the cost of natural gas so it never gets used. Also, the attic has insufficient insulation, so half the heat would go the wrong way.

Oh yeah my (boarding) high school had a dorm built in the “electricity will soon be too cheap to meter” era and had heating elements set into the concrete floors. It housed like 50 students and used more electricity than all of the other buildings on the 500 student campus combined. The warm floors were very nice though!

What on earth do you need 20 kilowatts continuous for? That's a huge amount of power for domestic use.

Typical US domestic service is 24kW (120V/200A nominal at the main breaker).

Fair enough. Still, it's one thing to be specced for 24kW peak load (the breaker) and quite another to require 20kW continuous (two 10kW nuclear power plants). I still think 20kW is an awfully huge amount of power - the average US house only uses about 1 kilowatt when you smooth it out.

(1) Heat. Space is cold. (-100c)

(2) Cooling. Space is hot. (+400c)

(3) Running the computer.

(4) Running the communications gear.

(5) Cooling 4+5.

I'd be amazed if GP's house is in space. But hey, life's full of surprises!

How can you use electricity to cool things in space?

Same as in a car or any air conditioner. You pump a hot fluid to/from radiators. Look at the iss. See the big black structures always held perpendicular to the solar panels. Those are the radiators.

https://en.wikipedia.org/wiki/External_Active_Thermal_Contro...

You build your walls out of Peltier elements. On one side of capsule, hot side will be in, cold side out, the other side of the capsule, the reverse. And then you use the generated power to drive a laser to shoot the energy right back into the sun.

Piezo-electric? But then you need to cool the other side even more..

You run a pump on a coolant loop, just like any air conditioner.

What on earth do you need more than 640k of memory for? That's a huge amount of memory for domestic use.

Nobody knows what kind of innovations a huge readily available sustainable energy supply would allow, but I bet they'd be cool.

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