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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

#131

Post by ragebol »

askvictor wrote: post #124
sandworm101 wrote: post #107

Earlier quoted context omitted.

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.

Even as a layman in these things, I know that a nuclear reactor is far, far from a nuclear bomb and isn't easily made one. You can't 'configure' a reactor to become a bomb.

A better way would just be shielding, make sure it stays in 1 piece through reentry and impact. And still doesn't leak radiation then.

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

#132

Post by flagrant_taco »

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…

2000-2020 want exactly peacetime.

The US declared war on the concept of terrorism in 2001, directly invaded two nations as part of that effort, and overthrew multiple governments before ultimately abandoning the war and handing a broken Afghanistan back to the Taliban in 2021.

We did an excellent job of making sure the war was fought on foreign soil and largely stayed out of our daily life back home, but we definitely were at war.

(I'm focusing on the US here, but there were quite a few other major conflicts and military coups around the world during the same time)

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

#133

Post by seanmcdirmid »

ragebol wrote: post #131
askvictor wrote: post #124

Earlier quoted context omitted.

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.

Even as a layman in these things, I know that a nuclear reactor is far, far from a nuclear bomb and isn't easily made one. You can't 'configure' a reactor to become a bomb. A better way would just be shielding, make sure it stays in 1 piece through reentry and impact. And still doesn't leak radiation then.

Can’t we just send up the propulsion system and the fuel separately? So like, ya, either could fail, but one is just some fancy equipment being g destroyed and the other is…unert uranium or plutonium coming down, needing to be cleaned up but not in a particular dangerous mode. Then once in orbit and the risk of crashing into earth is much less, put them together, fire up the reactor and go?

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

#134

Post by WalterBright »

marcosdumay wrote: post #119
WalterBright wrote: post #99

Earlier quoted context omitted.

You'd also not have to worry about the propellant exploding.

Well, much less than with a traditional rocket.

How is H going to explode out in space?

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

#135

Post by rany_ »

flagrant_taco wrote: post #132
echelon wrote: post #27

Earlier quoted context omitted.

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…

2000-2020 want exactly peacetime. The US declared war on the concept of terrorism in 2001, directly invaded two nations as part of that effort, and overthrew multiple governments before ultimately abandoning the war and handing a broken Afghanistan back to the Taliban in 2021. We did an excellent job of making sure the war was fought on foreign soil and largely stayed out of our daily life back home, but we definitel…

Well, you could make the argument that during the 2000-2020s the US was at war with undeveloped countries that definitely had no chance of dethroning the US technologically; this is different because China is in a position to do just that.

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

#136

Post by dghlsakjg »

seanmcdirmid wrote: post #133
ragebol wrote: post #131

Earlier quoted context omitted.

Even as a layman in these things, I know that a nuclear reactor is far, far from a nuclear bomb and isn't easily made one. You can't 'configure' a reactor to become a bomb. A better way would just be shielding, make sure it stays in 1 piece through reentry and impact. And still doesn't leak radiation then.

Can’t we just send up the propulsion system and the fuel separately? So like, ya, either could fail, but one is just some fancy equipment being g destroyed and the other is…unert uranium or plutonium coming down, needing to be cleaned up but not in a particular dangerous mode. Then once in orbit and the risk of crashing into earth is much less, put them together, fire up the reactor and go?

I suspect that the fuel separate from the reactor is still a major risk.

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

#137

Post by AtlasBarfed »

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…

LFTR can be turned off pretty easily.

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

#138

Post by physicles »

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…

A rocket functions by ejecting mass out the back at high speed. By conservation of momentum, the rocket then accelerates.

You want to optimize for velocity gain (also known as delta-v) per unit mass of propellant. So the question is, should you eject something heavier at lower speed, or something lighter at high speed? Which is more efficient?

If you write out the equation for a rocket ejecting some propellant at some velocity, apply conservation of momentum, and solve for the ratio of delta-v to propellant mass (just a bit of high school physics), it turns out that a higher exhaust velocity makes a more efficient rocket.

As a sibling post points out, if you compare water vapor with hydrogen at the same temperature, hydrogen will have the higher velocity. Chemical rockets usually produce water and/or carbon dioxide, which are bigger molecules and therefore less efficient propellants than hydrogen. So if you have a way to heat hydrogen to the same kinds of temperatures you get in rocket exhaust — say, in a nuclear reactor — that’ll make a more efficient rocket.

There are of course considerations other than pure efficiency. Our most efficient rockets, ion engines, are indeed crazy efficient, but their thrust is measured in milli-newtons or newtons. They can’t be used to lift things off earth. They also use propellants like krypton and xenon, presumably because they’re easier to store than hydrogen.

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

#139

Post by dotnet00 »

flangola7 wrote: post #109
philipkglass wrote: post #26

Earlier quoted context omitted.

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 exper…

These aren't intended to be powerful complex designs like what the navy uses. They're going to be small, simple and low power with as few moving parts as possible. Probably passively cooled, and having just enough plumbing such that propellant is only heated up when the only way for it to go is out the nozzle.

After all, waste heat is much harder to get rid of in space than here on Earth.

For reference, other space nuclear reactor designs NASA has looked into have been in the lower kilowatts in terms of power output.

Space stuff doesn't yet need the power output relative to size of a modern military reactor, the bigger concern is being reliable and providing stable long term power.

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

#140

Post by safety1st »

numpad0 wrote: post #98
rbanffy wrote: post #70

Earlier quoted context omitted.

> 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.

Hot war in outer spaces is should do. War is fine so long 1.(selfishly) it won't kill me, 2. it has no chance of ending the humanity, and 3. the sites devastated can recover. Colonial wars in the asteroid belt and beyond. Interstellar wars. Maybe the first FTL drive could be on an autonomous space torpedo, and that could be lore explanation for the shape of warp nacelles in this timeline.

As wacky as this all sounds it's definitely not the worst possible outcome. Megalomaniacs ruling over tribes, kingdoms and nation states have been throwing their people into bloody wars since the dawn of time.

If America and China are dead set on having another dick measuring contest, we're all much better off if they do it with drones halfway between here and the moon, instead of near earthbound population centers. By all means let's make outer space the future of war, as far away from humans as possible.

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