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Comment by Knufen | Hacker News Reader

The idea was considered before and showed great promise: https://en.m.wikipedia.org/wiki/Project_Orion_(nuclear_propu... Of course there is always the backside, imagine a rocket exploding in the upper atmosphere leaving nuclear waste and rocket parts over vast areas of land.

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NERVA https://en.wikipedia.org/wiki/NERVA was way more practical. They actually built these things and experimentally confirmed specific impulse, they just never launched them.

It's so sad to listen to the documentaries and hear how NERVA "is on track to propel mankind to mars by the late 1970s or early 1980s." Ah well. At least we're starting to care about space again!

Even more promising was the nuclear lightbulb reactor [1], which was developed by United Aircraft Corporation (now UTC) for NASA under the Mars program in the 70s. They got just short of testing the engine with nuclear fuel and all of the remaining obstacles were with material science and computation which would have been very solvable given the tech developed independently in the 90s. The design could also have changed the trajectory of the entire nuclear power industry because it uses compressed uranium hexafluoride plasma to reach criticality so it has the benefit of using tens of kilograms of fuel instead of tens of thousands and it's an actively maintained magnetohydrodynamic system so if anything breaks or power is lost, the reactor slows down and fission stops. Large centrifuges are needed to recycle fuel from the buffer gas (when it's used as a terrestrial power plant instead of a rocket engine) so this reactor can also be self-breeding and recycle nuclear waste.

Many of the papers generated by UTC on this project were readily available a decade ago, though it seems many were reclassified since then.

[1] https://en.wikipedia.org/wiki/Nuclear_lightbulb

That reaches temperatures of 25,000C. AFAICT we don't have any materials that can withstand that, so that reactor remains theoretical rather than practical.

Rocket engines have a long history of obtaining combustion temperatures far in excess of what their combustion chamber would tolerate under static conditions, but yeah, 25kK vs 2kK seems excessive. Also, chemical rockets have an easier time with this trick because the heat is generated inside the propellant, so you just have to stop it from getting out quickly, and the fuel itself is cryogenic and makes for a very convenient coolant.

Still, the fact that the article makes a big deal out of quartz being transparent in the UV makes me wonder if there isn't a clever trick up someone's sleeve. Ideas?

The genius of the nuclear lightbulb design is the irrotational vortex of neon gas that contains the plasma - this design separates the hot stuff from any solid materials that might melt. This vortex provides the pressure required to reach criticality and separates the plasma from the single crystal beryllium oxide walls, which in turn separates the neon vortex from the reaction mass (hydrogen gas seeded with tungsten nanoparticles when used as a rocket engine and water when used as a power plant). At that temperature, the plasma emits most of its energy as a black body radiator and the SC BeO walls are designed to pass through all of that radiation. The constant flow of neon cools down the entire system.

Like I said, UAC actually built functional prototypes that went just short of using fissile material. The non-nuclear parts of the design were validated experimentally. The only remaining theoretical parts are precise control of the fission reaction, which they didn't have the computational power for back then, and long term operation/maintenance, since imperfections in the SC beryllium oxide degrade the container and cause it to melt down eventually. There were plans to demonstrate a slow neutron plasma, which would significantly reduce material degradation, but they never got to it before Nixon canceled the Mars program.

> There were plans to demonstrate a slow neutron plasma, which would significantly reduce material degradation, but they never got to it before Nixon canceled the Mars program.

Nuetron damage to container seemed to be the deal breaker to me, thinks for the additional information.

What is a slow neutron plasma?

An open cycle analogue: RD-600

https://forum.kerbalspaceprogram.com/index.php?/topic/151286...

Is this the same project? https://books.google.fi/books?id=HRs0kuGfTe0C

Nuclear pulse propulsion (Project Orion) is a different concept to nuclear thermal. Pulse works by using bombs to generate your energetic reaction mass in bursts, whereas thermal uses a more normal fission reactor to superheat some reaction mass that then passes through an ordinary rocket nozzle.

You're right that both are worrying until they're out of atmosphere.

> and showed great promise

As per the Wikipedia article there were numerous unresolved technical issues and really exciting failure modes. But a big 'no' was the massive nuclear fallout resulting from each (successful) launch, let alone the ones that failed.

Larry Niven's novel Footfall [0] (spoiler...) involves an interesting treatment of an Orion-style spaceship. The massive problems are tolerated in the face of an existential threat to Humanity unless a massive spacecraft can be launched into space. And very conveniently, it works perfectly first time, with no testing of the bomb engine mechanism, which seems a tad fanciful.

[0] https://en.wikipedia.org/wiki/Footfall

This is different, in that it uses heat from a nuclear reaction to push out a propellant. No explosion takes place.

However, nuclear thermal was also not just considered, but actively developed, under the NERVA program which ran from the fifties until 1973.

It's continued development afterwards in different forms. Project TIMBER WIND was a pebble-bed reactor upper stage for SDI boost phase intercepts. The reactor wouldn't be spun up until after the missile had cleared the atmosphere, so there's no fallout to worry about.

Yeah so let’s build an uranium enrichment center just outside Von Braun city on the Moon...

I always wondered how this was supposed to work. The 'pusher plate' is going to have to be made of strong stuff. A nuclear explosion launched a steel cap to incredible speeds [1]

How would a spaceship handle effectively being shot with a similar device. It would be ripped apart (I'm not a physicist)

[1] https://en.wikipedia.org/wiki/Operation_Plumbbob#Missing_ste...

This gist is that by controlling the size of the explosion you control the energy, and thus can keep it within a range that the materials you're using can handle. I realize that's a hand-wavy statement, but a lot of actual research has gone into this, and it seems like it should work without exotic materials.

Dilution is the solution

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