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certain_people t1_j7h8mtw wrote

Reply to comment by GlockAF in Rolls-Royce Nuclear Engine Could Power Quick Trips to the Moon and Mars by darthatheos

Relevant xkcd

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[deleted] t1_j7hdf0z wrote

Left solar and wind off the chart

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certain_people t1_j7hdwzq wrote

Not left off so much as can't be shown. What's the energy density of wind?

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[deleted] t1_j7hg3vn wrote

They measured it in joules/kg. Solar is infinite. Wind I'm not so sure.

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certain_people t1_j7hgogl wrote

It's just not the right way to compare nuclear with solar and wind. And you can't put wind or sunlight in a fuel tank.

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[deleted] t1_j7hh9tb wrote

Seems reasonable to me? Light is zero kg, so infinite energy per kg. I can't put uranium in my fuel tank either. Just not practical for the everyday person.

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moral_luck t1_j7hxh0p wrote

Nuclear reactors use light as the energy for power.

Coal generators use light as the energy for power.

Natural gas generators use light as the energy for power.

Geothermal generators use light as the energy for power.

Light is the way energy is transferred from one atom or molecule to another atom or molecule.

Heat is light.

For example:

Coal is burned, the chemical reaction creates lots of photons. Those photons get absorbed by water molecules. The water molecules become excited. Excited water molecules move faster. Faster moving water molecules have more kinetic energy. Steam is the result. High velocity steam spins a turbine. The turbine spins a magnetic field inside an coil. This produces electricity.

Energy density is really a measure of the number and energy of the photons generated by a certain fuel. Light is not the fuel (hint: fusion).

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[deleted] t1_j7i6i75 wrote

Fusion is what happens on the sun

Not what we do on earth

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moral_luck t1_j7hwh78 wrote

Solar power is neither infinite in energy density, total amount of energy or duration of energy.

Not sure what you mean by "Solar is infinite."

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[deleted] t1_j7hwnqn wrote

How many megajoules per kg is it?

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moral_luck t1_j7i2c65 wrote

Einstein's most famous equation (E=mc^(2)) limits energy density of any fuel to 9 x 10^(10) MJ/kg

And that would be an antimatter reactor. Physics means no fuel can have infinite density.

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[deleted] t1_j7i5cy1 wrote

Light. Zero mass. Lots of energy.

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SowingSalt t1_j7jjchk wrote

Therefore in the ratio mJ/kg, light is undefined and cannot be on the chart; which was the original point.

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moral_luck t1_j7hz7ym wrote

Solar energy is hydrogen fusion, deuterium–tritium fusion is about 338,000,000 MJ/kg.

But we don't have the technology to create miniature suns in our rockets.

BTW, U-235 fission has an energy density of about 144,000,000 MJ/kg.

source

Another source puts the energy density of the sun's core around 6.43 x 10^(8) MJ/kg, or 643,000,000 MJ/kg.

The second source is titled The Source of Solar Energy.

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[deleted] t1_j7i5md1 wrote

That's fusion reactors not solar energy on the surface of the earth

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moral_luck t1_j7i7uhl wrote

Where does the solar energy on the surface of the earth come from?

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Taxus_Calyx t1_j7hxzqa wrote

"Wind powered travel to Mars." M'kay.

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John-the-cool-guy t1_j7i9xfp wrote

What about solar winds? And a sail to catch them? Feels like I've heard about this someplace.

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TheGacAttack t1_j7ku5zo wrote

Some of us have got this idea..... we want to land a craft. Deploy solar sails. It'll have a great, big canopy! Solar winds will be caught by the mylar sails....

C'MON, GUYS!

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[deleted] t1_j7i6mr4 wrote

That's not the sick burn you think it is

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moral_luck t1_j7icz4b wrote

It was 3rd degree.

I get the feeling you'd rather "win" an "argument" than be correct. Your whole premise is born of ignorance, but it's easier for some to remain ignorant than to recognize they are wrong.

Heat has infinite energy density, so we should just burn coal for rockets. <- your logic at work.

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