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A_curious_fish t1_j6hyovl wrote

Now what about crazy stuff like assembling a nuclear engine to be only used in space and get it into space via a regular rocket? Don't we have nuclear powered boats floating around aka the carriers and subs with nuclear engines? Granted idk how in gods name they work but I feel like toss it in space it'll be fine

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GeorgeOlduvai t1_j6i7073 wrote

Nuclear powered and nuclear propulsion are two very different things.

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A_curious_fish t1_j6i7pti wrote

That's a good point and more so what I meant when I don't know how it works

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GeorgeOlduvai t1_j6i8j5m wrote

The subs and carriers have nuclear power plants aboard to generate electricity. While that power is used for propulsion, it's not the same thing as a nuclear engine. A nuclear rocket engine operates on the same principles but rather than using the heated water to turn a turbine, the water is directed through a nozzle to create thrust.

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danielravennest t1_j6ilp8k wrote

Nuclear rockets use pure hydrogen as propellant. Lighter molecules move faster, and H2 is much lighter than H2O.

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cjameshuff t1_j6j0557 wrote

And you can use heavier things as propellant, like ammonia (water and methane are both bad choices for various reasons), but anything but LH2 gives you only slightly more performance than chemical engines.

Meanwhile, instead of a pile of steel, copper, and nickel alloys carefully arranged to burn stuff really well, you need enriched uranium arranged to sustain a nuclear fission chain reaction. That's a huge step up in cost and regulatory complications, and nobody's going to do it for something barely better than a chemical engine, so LH2 it is.

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stanspaceman t1_j6jhkvt wrote

They don't use high enriched Uranium FYI, it's HALEU, mandated for all space systems currently being designed.

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cjameshuff t1_j6jk6m5 wrote

I didn't say HEU, I said enriched uranium. HALEU is enriched to a U-235 content of 5-20%, natural uranium is only 0.72% U-235.

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Pharisaeus t1_j6i84r2 wrote

> Don't we have nuclear powered boats floating around aka the carriers and subs with nuclear engines?

The issue with nuclear reactors in space is waste heat. Boats you're referring to have literally whole ocean around them to use as coolant. In space you don't have such luxury and you need massive radiators to dump the heat.

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hawaiianthunder t1_j6ivu6m wrote

Wouldn't the lack of our atmosphere make it easier to cool a radiator?

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carbonbasedlifeform t1_j6j5gfl wrote

Actually it makes it harder. Without air to use as a heat transfer medium you don't have anything to bleed heat into.

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MetallicDragon t1_j6j73zf wrote

On the contrary, it makes it much harder. On earth, you can radiate away heat by direct contact with air. No air in space means the only way to dump heat is by black body radiation.

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p-d-ball t1_j6jdjy3 wrote

So, now we have to bring an atmosphere with our ships???

​

(kidding, kidding)

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Pharisaeus t1_j6jgwal wrote

It's a common misconception that space is "cold". This is due to the definition of "cold" -> the kinetic energy of particles in certain volume is low. But while on Earth is means you have lots of particles, each with low energy, in space it's very different - you have very few particles, often with very high energy.

In order to cool something down, you need to transfer the energy. On Earth particles with low energy will steal some of the energy of your hot thing, cooling it down. The more particles, the better. In space this effect doesn't exist, there are no particles to steal the energy. You need to radiate the heat as infra-red.

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Polygnom t1_j6k65zb wrote

Heat can be transferred in three ways: Radiation, Conduction and Convection. Conduction is heat transfer by two solids touching. Or when you put your hand on your heater, you feel your hand getting warmer. Convection is heat transfer to fluids (gases). That is why the air in your room heats up when you put a heater in it. Radiation is the weakest form of heat transfer, by far. But if you aren't surrounded by an atmosphere or ocean to dump heat into, and instead are surrounded by a vacuum, radiation is the only way to get rid of heat. Conductive transfer will constantly heat up the spacecraft until parts start to melt if you cannot radiate the heat fast enough. Hence the need for large radiators on spacecraft that produce a lot of heat, e.g. the ISS or even the shuttle, whose whole payload bay doors were used as radiators and needed to be opened somewhat quickly once in orbit or the shuttle would overheat.

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DownvoteEvangelist t1_j6ig1d2 wrote

They produce electricity and electricity powers electric motor... This wouldn't work in space...

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Shrike99 t1_j6jofu2 wrote

It would work if you added some propellant to the mix. For example, add a big water tank and use that electric motor to drive a pump that sprays the water out the back at very high pressure.

Realistically you're not going to get a very good exhaust velocity with that method, so you'd instead use a different kind of electric engine to accelerate the propellant; electrostatic, electrothermal, or electromagnetic.

Indeed, the only example to date of nuclear propulsion actually being used in space was on SNAP-10A, which featured a nuclear reactor powering an electrostatic engine with cesium as the propellant.

Granted, it only worked for about an hour before it broke down, but it did work. It's a shame there hasn't been any followup in the 58 years since then.

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DownvoteEvangelist t1_j6jv4q5 wrote

Probably because you don't get much thrust with that type of engine.

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Shrike99 t1_j6jxdxg wrote

We've had plenty of solar powered electric propulsion since. The Dawn mission was a great example of what electric propulsion can do. Solar power just doesn't scale up well to larger vehicles, or work very well as you get furthur from the sun.

A nuclear electric system has the potential to be much faster than a chemical rocket over long distances, i.e to Mars or especially beyond.

The real issue has been the reluctance to put nuclear reactors into space. SNAP-10A remains the only example the US has ever launched, even though much better designs like the SAFE-400 and KRUSTY have since been developed.

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iheartbbq t1_j6i9j8s wrote

And then what? Nuclear power on earth just steam power. Propulsion in space requires Newtons 1st law - to go forward you gotta shit some stuff out the back. You can't just heat up water and shoot it out the back, I mean, you can, but that's a lot of squeezing for not a lot of juice.

Just permanently emitting a stream of decayed nuclear atoms would produce a tiny amount of thrust, but it could build up to tremendous speeds over time. But again, not really practical for transit during human life time scales.

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danielravennest t1_j6ilews wrote

> You can't just heat up water and shoot it out the back,

That's exactly what the third stage of the Artemis I rocket did on Nov 16th. Except the water was carried as separate hydrogen and oxygen tanks, and burning them is what produces the heat. What comes out the nozzle is superheated steam.

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iheartbbq t1_j6imc8u wrote

Groan. Worst kind of pedant.

It's also what the main boosters of the shuttle system did.

The combustion process adds significant velocity to the propellant when properly nozzled. What is the point of adding the danger of a nuclear energy source in space when the propellant is completely expended? Just use chemistry.

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danielravennest t1_j6imxym wrote

> What is the point of adding the danger of a nuclear energy source

Because a nuclear-thermal engine can use pure hydrogen rather than a hydrogen-oxygen mix. Lighter molecules go faster at a given temperature, and H2 is much lighter than H2O. So you get roughly twice the exhaust velocity/specific impulse.

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iheartbbq t1_j6ipagh wrote

Again, the juice is simply not worth the squeeze. You're adding nuclear complexity to every launch (nobody wants a dirty bomb going off in the sky) and you're just not getting significant benefits. You're still going to run out of propellant after an X minute burn. And now you're stuck with a super complex, hazardous, expensive boat anchor on your space craft that's VERY hard to cool because you only have radiation as conduction and convection don't exist in space.

Also

>Lighter molecules go faster at a given temperature, and H2 is much lighter than H2O. So you get roughly twice the exhaust velocity/specific impulse.

Oh, twice huh. H2 weighs 2 grams per mole, it will need to be ejected at nine times the velocity of a water molecule at 18 g/mol to have equal the force.

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danielravennest t1_j6ivn9h wrote

> (nobody wants a dirty bomb going off in the sky)

Before you start up a reactor for the first time, the core is low radiation. Reactors produce short-life fission products, which is what makes nuclear waste dangerous.

Rocket mass is in kg, not moles. Exhaust velocity is ~9 km/s for hydrogen, vs ~4.5 km for H2-O2 engines.

I'm a space systems engineer, who has worked on nuclear rocket designs. My opinion is the time for nuclear-thermal propulsion is past. Solar-thermal can get the same performance - both heat H2 to the limits of the materials. But solar doesn't have all the nuclear baggage to deal with.

Nuclear-electric has much higher performance (3-20 times), though like all electric systems it has longer burn times. The reactor can be much smaller (1 MW rather than 1 GW), making radiators and such easier to do.

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iheartbbq t1_j6iwpzo wrote

Right, and I'm a SUPER spaceman Thunderbirds engineer.

All that matters is mass and rate of the amount of shit that gets shot out the back, doesn't matter if it's in moles or kg, according to your claim 18x more H2 coming out the ass, is that true?

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danielravennest t1_j6j7zjo wrote

I'm writing a textbook on Space Systems Engineering. Check the "view history" tab on any page to see who wrote it.

>according to your claim 18x more H2 coming out the ass, is that true?

That's your number, not mine, and it is wrong.

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Shrike99 t1_j6js9tu wrote

You seem to be ignoring the minor fact that lower molar mass also means more moles, so it cancels out.

If you pump 1kg of water into the engine, that's 55.5 moles. If you pump 1kg of hydrogen into the engine, that's 500 moles.

So hydrogen produces 1/9th as much force per mole, but it also has 9 times as many moles per kg of fuel. The end result is that both produce the same total force when that kilogram is expelled from the engine.

Or at least, they would if they were both expelled at the same speed. Since hydrogen actually comes out twice as fast, it produces 1/4.5th as much force per mole, while still having 9 times as many moles, and hence produces twice as much total force.

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MetallicDragon t1_j6j7y4l wrote

>Oh, twice huh. H2 weighs 2 grams per mole, it will need to be ejected at nine times the velocity of a water molecule at 18 g/mol to have equal the force.

Or just with 9x the mass flow rate. And thrust doesn't matter too much in space, what matters is fuel efficiency. A weaker nuclear rocker might need to do longer burns, but for the same mass of fuel as a conventional engine, it will get you going a lot further.

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Shrike99 t1_j6jstqe wrote

>Or just with 9x the mass flow rate

You need 9x the molar flow rate, not the mass flow rate. And since hydrogen has 1/9th the molar mass of water, it ends up cancelling out.

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