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The US government is taking a step toward space-based nuclear propulsion

arstechnica.com

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Re: The US government is taking a step toward space-based nuclear propulsion

#61

Post by vkou »

echelon wrote: post #27
jamesaurichs wrote: post #21

Earlier quoted context omitted.

US seems to be in a hardware/science renaissance, now that there is a new Cold War enemy, and manufacturing and research are being brought back to US.

Peace time = smart phone incrementalism, social media, ad tech, Bitcoin, NFTs, WeWork, low interest rate malinvestment, Twitter/Tumblr pessimism Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism I know most of this is happenstance and that peace time doesn't itself cause lackluster development, but it is interesting an…

Broad optimism? Science hype? Are we living in the same country?

Re: The US government is taking a step toward space-based nuclear propulsion

#62

Post by morkalork »

rich_sasha wrote: post #59
version_five wrote: post #55

The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…

The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit. In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides…

Would it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?

Re: The US government is taking a step toward space-based nuclear propulsion

#63

Post by p1mrx »

version_five wrote: post #55

The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…

The temperature of a gas is proportional to the average kinetic energy of its particles. So if you compare (e.g.) water vapor to hydrogen at the same temperature, the hydrogen molecules are moving faster.

A hydrogen+oxygen chemical rocket propels H₂O, while a nuclear thermal rocket can use the lightest propellant (H₂) because the heat comes from elsewhere.

Smaller molecule, faster exhaust, better rocket.

Re: The US government is taking a step toward space-based nuclear propulsion

#64

Post by ridgeguy »

version_five wrote: post #55

The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…

It's the high temperature and the lower exhaust molecular weight.

Specific impulse (Isp) is how much force you get from each mass unit of propellant you exhaust. So an engine with Isp = 300 gives 300lbs force for every pound of exhaust gas mass.

IIRC, Isp scales as the inverse square root of exhaust molecular weight. So a pure (molecular) hydrogen exhaust would have a molecular weight of 2. A pure H2/O2 engine would have an exhaust of H2O (assume it's running at stochiometric H2/O2 ratio to simplify things), or an average molecular weight of 18.

For everything else being equal (temperature, pressure, etc.), the H2 Isp would be sqrt(18/2), about 3x the H2/O2 Isp. That's mainly why rocketeers would like nuclear engines, more oomf from a given mass of propellant. It's also (conceptually) simpler because you only need to handle one propellant vs. two or more for conventional liquid fuel combustion.

It's more complex, of course. If the reactor is really hot, it can partially dissociate H2 into atomic hydrogen, lowering the exhaust molecular weight still more. But dissociation is an energetically expensive process, so there's bound to be a tradeoff between energy consumed by dissociation vs. increased Isp. I've no clue how that would work out.

Re: The US government is taking a step toward space-based nuclear propulsion

#65

Post by MPSimmons »

morkalork wrote: post #62
rich_sasha wrote: post #59

Earlier quoted context omitted.

The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit. In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides…

Would it be possible to skim the upper atmosphere while in orbit around a planet and top-up the gas reserves?

Almost definitely not. If there's enough material to collect in a realistically short timeframe, there's enough drag to bring the craft back into the atmosphere.

Re: The US government is taking a step toward space-based nuclear propulsion

#66

Post by morcheeba »

rich_sasha wrote: post #59
version_five wrote: post #55

The basic idea is straightforward: A nuclear reactor rapidly heats up a propellant, probably liquid hydrogen, and then this gas expands and is passed out a nozzle, creating thrust. But engineering all of this for in-space propulsion is challenging, and then there is the regulatory difficulty of building a nuclear reactor and safely launching it into space. Can anyone give a eli5 explanation of what makes this better…

The propellant is important for its bulk (some mass to shoot out the back) and it's energy. In a chemical rocket, the propellant provides both the bulk and the energy. As it happens, there isn't that much energy in chemical propellants per kg, but the rest of the engine is light enough that these things can fly to orbit. In a nuclear rocket, the energy comes from a nuclear reaction and the gaseous fuel only provides…

Good explanation! For completeness:

In an Ion jet, the energy comes from (usually) solar power and the gaseous fuel provides the bulk. These are even weaker because of the limited instantaneous power that can be generated to feed it, but they are very efficient in how much thrust they can provide per propellent weight. Some versions (like arcjet) are a combination chemical rocket/Ion jet.

Re: The US government is taking a step toward space-based nuclear propulsion

#67

Post by fastneutron »

dtagames wrote: post #13

This might be a good time to review the number of failed rocket launches we've seen in the last few years. Now, imagine them with nuclear materials aboard.

> Once it reaches a safe orbit, the reactor will be turned on.

Fission reactors aren't appreciably more radioactive than rocks you can dig out of the ground until you turn them on. This is something that wouldn't be brought online until the riskiest part of the launch has past.

Re: The US government is taking a step toward space-based nuclear propulsion

#68

Post by zgluck »

The reason communications with the Voyager 1/2 twin probes have lasted for like 45 years:

Voyager 2 is equipped with three Multihundred-Watt radioisotope thermoelectric generators (MHW RTG). Each RTG includes 24 pressed plutonium oxide spheres, and provided enough heat to generate approximately 157 W of electrical power at launch.

Re: The US government is taking a step toward space-based nuclear propulsion

#69

Post by wmf »

zgluck wrote: post #68

The reason communications with the Voyager 1/2 twin probes have lasted for like 45 years: Voyager 2 is equipped with three Multihundred-Watt radioisotope thermoelectric generators (MHW RTG). Each RTG includes 24 pressed plutonium oxide spheres, and provided enough heat to generate approximately 157 W of electrical power at launch.

Note that RTGs don't provide propulsion.

Re: The US government is taking a step toward space-based nuclear propulsion

#70

Post by rbanffy »

echelon wrote: post #27
jamesaurichs wrote: post #21

Earlier quoted context omitted.

US seems to be in a hardware/science renaissance, now that there is a new Cold War enemy, and manufacturing and research are being brought back to US.

Peace time = smart phone incrementalism, social media, ad tech, Bitcoin, NFTs, WeWork, low interest rate malinvestment, Twitter/Tumblr pessimism Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism I know most of this is happenstance and that peace time doesn't itself cause lackluster development, but it is interesting an…

> Cold war = rocket science, new fuels, materials science, major computing advancements, supply chain changes, innovation, science hype, broad optimism

Next time the cold war cools down, we need to invent something to replace it.

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