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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

#121

Post by askvictor »

TheSpiceIsLife wrote: post #115
Retric wrote: post #91

Earlier quoted context omitted.

Nuclear testing got to carefully select locations to minimize harm, uncontrolled reentry of nuclear rockets has significantly less control. It’s not the 99 rockets falling into the ocean shortly after launch that are problematic, it’s the 1 that almost works but then lands in the city we should be concerned with.

How often do failed rockets, or any space debris, fall on cities?

Not falling on a city, but debris from Russia's launch last week was seen and heard in Melbourne, Australia: https://www.abc.net.au/news/2023-08-08/space-junk-leaves-tra...

That's pretty close for what would be high level nuclear material.

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

#123

Post by p1mrx »

dgoldstein0 wrote: post #79
p1mrx wrote: post #63

Earlier quoted context omitted.

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.

Wouldn't we care more about the momentum of the exhaust rather than the mass? I'm not quite following why lighter particles are better

Assume that total propellant mass is a constant (limited by the launch vehicle) and temperature is a constant (because melting the rocket would be bad.)

With constant mass, you increase the total momentum by increasing the exhaust velocity. With constant temperature, you increase the exhaust velocity by making the molecules smaller.

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

#124

Post by askvictor »

sandworm101 wrote: post #107
avar wrote: post #47

Earlier quoted context omitted.

> Or even if it broke up > in the atmosphere. The atmosphere we've already detonated over 500 nuclear weapons in?

Nuclear weapons testing was mostly designed to maximize power output by burning up as much fissile material as possible in one big explosion. Conversely, it was meant to minimize fallout. A broken nuclear reactor (no nuclear explosion) would be the opposite: lots and lots of fallout (broken reactor bits) scattered over a wide area. The radioactivity from a broken nuclear rocket could be more dangerous than that from…

One solution would be have a kill-switch that detonates the reactor fuel as a nuclear bomb (assuming it could be configured in such a way; this is probably more difficult than I imagine). There would be major political problems with this approach though, as you're then shooting nuclear missiles.

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

#125

Post by speed_spread »

fallingknife wrote: post #117
credit_guy wrote: post #108

For anyone who wants a refresher on the UNS nuclear thermal rocket propulsion program from the '60s, here's the final report. It's a very good read: https://ntrs.nasa.gov/api/citations/19920005899/downloads/19... For this new initiative, I can see 2 big differences. On one hand they will use medium enriched uranium (up to 20%), while the original program used highly enriched one (85%), so it's going to be more challe…

Why are they nerfing the uranium like that?

Is probably easier to control, reaching a sweet spot of complexity, ruggedness and lower weight of the overall propulsion system.

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

#127

Post by panick21_ »

eslaught wrote: post #126

Doesn't hydrogen have issues with embrittlement [1]? I'm no expert, but why are they choosing that as the propellant instead of something else? [1]: https://en.wikipedia.org/wiki/Hydrogen_embrittlement

Hydrogen embattlement is a long term issue, it isn't really a big problem for this application.

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

#129

Post by flagrant_taco »

TheSpiceIsLife wrote: post #115
Retric wrote: post #91

Earlier quoted context omitted.

Nuclear testing got to carefully select locations to minimize harm, uncontrolled reentry of nuclear rockets has significantly less control. It’s not the 99 rockets falling into the ocean shortly after launch that are problematic, it’s the 1 that almost works but then lands in the city we should be concerned with.

How often do failed rockets, or any space debris, fall on cities?

We might see more obvious direct human impact from debris landed in cities, but nuclear debris falling into the ocean or land that isn't densely populated by humans still has plenty of bad side effects.

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

#130

Post by flagrant_taco »

aziaziazi wrote: post #34
somenameforme wrote: post #7

Earlier quoted context omitted.

I think the obvious concern would be failure scenarios. There will always be failures in space technology. As space expands exponentially (as it is likely to do so in the future), failures will become relatively regular. So the obvious question would be what would happen if a nuclear rocket, with reaction in process, crashed to Earth. Or even if it broke up in the atmosphere.

It already happened with Kosmos 954 and will probably happen again soon: ~20 other satellites had been sent in space with the same tech. Kosmos crashed away from (human) life centers but that was a huge stroke of luck. Research area was 100k+ km2. Launches are less to be feared because they are oriented toward sea and the most critical moment are the first minutes. The device discussed is intended to travel away from…

It's crazy to me that people largely consider it safe as long as the unspent fuel or nuclear debris is dumped into the ocean.

(Not a direct comment on your opinion specifically, just building of the general idea)

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