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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

#101

Post by WalterBright »

aio2 wrote: post #18
ben_w wrote: post #16

Earlier quoted context omitted.

Depending on the design, failure during launch can be anywhere from "meh" to "basically high-altitude Chernobyl" — random actinide decay products (and plutonium fuel) are bad, unreacted uranium fuel is mostly a heavy metal hazard rather than a radiation hazard. (Could be worse, but the Orion drive is currently illegal by treaty and hopefully nobody is dumb enough to use that in Earth's ionosphere anyway).

I remember learning that when they started testing atomic bombs (I think?) the nuclear debris in the air spread basically throughout the whole united states and the government tried to keep quiet about it. Something similar will probably happen here if they mess up with the launch. Btw, the video I saw was on Veritasium.

There's a difference between a lump of uranium metal falling back to earth and a lump of uranium that's been turned into plasma.

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

#102

Post by avmich »

gibolt wrote: post #19
dzhiurgis wrote: post #17

Earlier quoted context omitted.

Falcon9 hadn’t had a failure for nearly a decade

Falcon 9 has had more than 200 consecutive successful flights in a row. That is more than double the next most successful, Delta II with 100 in a row. It also has well over 100 successful landings in a row, while the next closest rocket has 0. (Rocket, not capsule or shuttle, which are all still far lower)

https://en.wikipedia.org/wiki/Soyuz-U

"The rocket had a streak of 112 consecutive successful launches between 11 July 1990 and 5 May 1996"

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

#103

Post by numpad0 »

somenameforme wrote: post #7
mdorazio wrote: post #2

Finally! This is one of the few fission uses I'm genuinely excited about. Nuclear-thermal just makes so much sense for in-space propulsion of large vessels compared to chemical rockets.

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.

The solution to this is to build it in the moon from moon rocks and send it by maglev trains. Full scale spaceships can be 3D printed in parts, assembled on-orbit, and Dragon/Starliner class rowboats can be used to board it.

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

#104

Post by Consultant32452 »

dotnet00 wrote: post #33
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.

Well that's the most obvious way to put FUD out there. It might be a good time to review the number of failed launches on crew rated rockets (the safety requirements on which are roughly equivalent to the requirements for carrying nuclear material). Particularly ones which led to a loss of crew, as those would correspond most closely to a possible nuclear material release. It's far less scary, but alas not as conveni…

I would add it's much easier to protect a small amount of nuclear material from a rocket exploding than to protect a human from a rocket exploding.

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

#105

Post by [deleted] »

asu_thomas wrote: post #49
sircastor wrote: post #41

Earlier quoted context omitted.

"Meh" is a casual response that means "this does not matter much". I don't know that it's proper English by any means, but it is certainly a common colloquialism. A cursory search suggests that it's imported from yiddish. [1] https://en.wikipedia.org/wiki/Meh

I have never seen it used outside of Hacker News.

[deleted]

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

#106

Post by credit_guy »

devanatha wrote: post #12

https://en.wikipedia.org/wiki/TEM_(nuclear_propulsion)

This is not the same. It's nuclear electric propulsion, not nuclear thermal propulsion. A nuclear reactor generates electricity and that electricity drives an ion thruster. The specific impulse is astounding (7000 seconds vs 450 seconds for the best chemical rockets), but the thrust is absolutely wimpy (15 Newtons, vs 250 kilo-Newtons for the US NERVA nuclear thermal rocket engine).

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

#107

Post by sandworm101 »

avar wrote: post #47
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.

> 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 a great many bombs.

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

#108

Post by credit_guy »

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 challenging to extract as much power from a compact reactor. On the other hand, in 2023 we have computers and nuclear reactor simulation software that was not available in 1960, so that could provide a huge advantage.

The biggest challenge I see is how to power down the engine. You can't simply flip a switch a turn off a nuclear reactor. You can stick in all the control rods, and the fission stops in less than a second, but the fission products will continue to generate heat for a few hours, initially at about 10% of the full reactor power. During this cool-down period, you will use the same coolant (hydrogen) as during normal operation, but you won't get the same exhaust velocity. With a traditional chemical rocket you don't have this problem. If you get a specific impulse of 450 s, you always get that. If a nuclear thermal rocket can get 900 s running at full throttle, but it goes down to 100 seconds in cool down mode, the main advantage might not be so great after all.

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

#109

Post by flangola7 »

philipkglass wrote: post #26
flangola7 wrote: post #9

Earlier quoted context omitted.

RTGs do not use fission and do not create fission products. Nuclear fuels are harmless in comparison to Strontium-90 and Cesium-137.

There won't be any fission products in the fuel if the chemical launcher fails to take the nuclear payload to orbit. The reactor activates for the first time after the initial orbital launch. The virgin fission fuel is less radiotoxic than RTG fuel.

>The reactor activates for the first time after the initial orbital launch.

So they're going to launch an untested reactor into orbit without even turning it on first? Getting a reactor up and running is hard enough on the ground. I would expect any reactor to have had thousands of hours of testing and tweaking before being placed on the launch pad; that is the process for military boats and we have 60 years of experience in that environment.

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

#110

Post by leereeves »

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…

> During this cool-down period, you will use the same coolant (hydrogen) as during normal operation, but you won't get the same exhaust velocity.

That seems like a waste of propellant.

Once the reactor has cooled enough that you won't get the desired exhaust velocity, couldn't you allow it to finish cooling down by radiative cooling?

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