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iheartbbq t1_j6i9j8s wrote

Reply to comment by A_curious_fish in NASA tested new propulsion tech that could unlock new deep space travel possibilities by Creepy_Toe2680

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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