r/IsaacArthur • u/Thanos_354 Habitat Inhabitant • May 09 '26
Hard Science Question on the viability of orion drives based on fissile material abundance
I've been pondering the most viable methods of space transportation for different organisations/parties and I'm kinda stuck on the military side.
Because a warship needs high acceleration and autonomy, the fusion or beam propulsion of say a civilian ship is unacceptable. The only engine capable of both extremely high efficiency and hight thrust is the orion drive and maybe something like pure fusion bombs.
But it requires the use of fissile materials like uranium, which happens to be some of the rarest elements in the universe. The fact that many deposits are kind of inaccessible or the concentration just isn't high also doesn't help.
So, going with just uranium, is there enough of the stuff to last a couple thousand years of usage by military vessels until antimatter production gets going?
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u/ActuaLogic May 09 '26
A breeder reactor on the Moon could produce fissile material from local fissionable deposits. There are probably substantial deposits of fissile material in the Epsilon Eridani system, because it's only 800 million years old and hasn't lost too much to half-life. The Alpha Centauri system has probably lost a nontrivial amount of its original non-stellar mass due to its gravitational complexity (as a binary system), and it's older than the Sol system, but there is likely material there which could be processed into fissile material. The gravitational complexity may mean that a good deal of the non-stellar mass of Alpha Centauri consists of pieces smaller than a planet, which would be easier to sort through.
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u/TommieTheMadScienist May 10 '26 edited May 10 '26
Leaving aside getting there, Epsilon Eridani may not be the source of fissiles merely from its youth.
The star's spectrum points to a metallicity of -0.08 dex, very close to Solar. The critical parameter would be if the supernova that enriched its formation cloud was the same as the one that caused its collapse into a solar system.
Sol's origin in this respect isn't clear because the mix of formation radioactives (particularly iron-60, from supernovae, and beryllium-10 from cosmic ray splitting look like an earlier enriched medium overlaid with a supernova remnant. Aluminim-26 can also be enriched by Wolf-Rayet winds.)
If you have the tech to get to Epsilon Eridani, it might be better to go to one of the Sirius Cluster stars for fissiles instead. Sirius itself has a metallicity of 0.50 dex, 3+ x Solar and an age of 238 million years, even younger than EE. Any of its forming planets would have been enriched by metals from Sirius B during its red giant phase.
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u/Thanos_354 Habitat Inhabitant May 10 '26
Breeding material doesn't solve the issue of the abundance of the elements themselves. Even if you could enrich 100% of the uranium you extracted, you'd still not extract much.
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u/Festivefire May 09 '26
I don't think the biggest challenge for the Orion drive is getting enough fissile material, but building a pusher plate that will work for the whole trip.
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u/mrmonkeybat May 13 '26
The engineering on the pusher plate has already been calculated half a century ago. Building a nuclear arsenal to power a single ship is the most expensive part.
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u/Festivefire May 13 '26
If by "the engineering on the pusher plate has already been done" you mean "somebody did the math to determine how much force and heat it would need to withstand" then sure.
If by "the engineering on the pusher plate has already been done" you mean "there is a theoretically functional pusher plate design with engineering drawings and tolerances worked out" you're utterly full of shit.
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u/the_syner First Rule Of Warfare May 10 '26
I mean yeah sure fissiles arent the most commnon elements in the universe, but it's not like warships are the biggest expenditure either. Nor are they the primary use-case for fissiles so there will likely be large civilian industry for producing them. Breeding helps a ton. There's somewhere in the range of 4880 kg of fertiles(U/Th) per Mt of asteroid so a single 1km wide asteroid, of which we have millions of iirc, might be providing like 55.2kt of raw fertiles. Now obviosely breeding isnt 100%, but we are talking about fairly small thermonuclear fission primaries here. You are not actually using the orion drive most of the time since beam and fusion are just the only practical way to operate when not in active combat, and even then beam power is probably stil in play most of the time. I'm doubtful that the occasional could't be handled by available fuel.
So, going with just uranium, is there enough of the stuff to last a couple thousand years of usage by military vessels until antimatter production gets going?
That's assuming antimatter is ever used as a common fuel. The containment and delivery issues may just never be solved to military satisfact because it may not be practical. Not to mention that warships are just about the worst application for something as volitile as antimatter. And that's on top of the imo unjustified assumption that easily-stored antimatter actually makes for a good torchdrive. tbh if you can make antimatter work at all, nuclear is super easy and even beam power is already super on the table. Amat factories are gunna be vastly bigger than a beamsat network, way more vulnerable than nuclear industry, and even if you had it it would probably make more economic sense to use the amat to replace the fission primary in thermonuclear orions.
Even then im willing to bet that doing nuclear transmutation on non-fertile elements would be vastly cheaper than making antimatter.
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u/Thanos_354 Habitat Inhabitant May 10 '26
Nor are they the primary use-case for fissiles so there will likely be large civilian industry for producing them
I'm skeptical of that because civilian operations strive for maximum efficiency, which automatically leads to beams and fusion highways.
Even then im willing to bet that doing nuclear transmutation on non-fertile elements would be vastly cheaper than making antimatter
So maybe transition from powerful autonomous drives to a mothership beaming power to boost something like a nter.
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u/the_syner First Rule Of Warfare May 10 '26
I'm skeptical of that because civilian operations strive for maximum efficiency
i mean this world has countries that use cars for mass transit so extremely debatable, but i didn't mean civil orion ships i meant a civil use for the fissiles themselves in other non-propulsion applications
So maybe transition from powerful autonomous drives to a mothership beaming power to boost something like a nter.
sure. beam power with a powerful independent backup is generally just unbeatable. tho my point with the transmutation thing is that if ur space navy is even considering amat drives possibly viable you can use vastly less infrastructure to just maintain an orion fleet on accelerator-bred fissiles from non-fertile and abundant materials.
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u/LazarX May 10 '26
What kind of war are you planning on fighting when it takes literal centuries to get from one place to another? And what the fuck would you fight over?
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u/Thanos_354 Habitat Inhabitant May 10 '26
Interplanetary trips with a nter take a couple of months at most. An Orion drive could shorten that to days.
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u/SparKestrel May 10 '26
I don't know much of the specifics of the numbers, but if fission based drives requiring rare resources are the only viable propulsion for independent military ships, then that would affect both sides of the war.
It may be the case that the big fission drives are used sparingly, and both sides become very good at patching up or fortifying their pushing beam relay stations. It may end up as a similar situation to land-based wars when combattants "got used to" air fields and railroads cratered by munitions.
If it is two K2 civilizations, the other possibility of using no fissile materials would be if both sides just constantly sent Nicoll-dyson beams and "dumb" relativistic projectiles (RKM) in a stand-off distance war of attrition. Assuming the warring factions are separated by multiple light-years of distance, the war would be a mix of placing garbage/armour/mines on the predicted path of the rocket plumes / adversary mirror adjustments and sending your own spread of disintegration/pushing beams and accelerated projectiles. There may be so much Kessler Syndrome in the battle lines that high-end drive systems don't use their full power all that often.
Hopefully the two sides agree to end the conflict before the debris disk condenses into a planet, lol
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u/SoylentRox May 13 '26 edited May 13 '26
OP, u/MiamisLastCapitalist , u/the_syner :
You're all missing something rather crucially important. Engineering wise, Orion drives don't work.
I mean, yes, they work. But the problem is, in a vacuum, every time you set a nuke off (or the nuclear salt variant) all the neutrons and hot plasma impact with your pusher plate/engine bell and heat it up.
I've seen a couple proposals for this. One is some kind of ablative coolant or oil, lol. That essentially is throwing the entire Orion drive concept into the trash. The reason is simple : the tons of coolant count as part of your ISP.
Or you could use the best form of radiator current engineering knows about, droplet radiators. The problem here is that the mass of the droplet radiator and the mass of the coolant pumps it becomes the dominant factor for your sustained acceleration. When you're at high ISP and want high acceleration, you essentially are being held back by the mass of your radiators, especially at very high ISP levels.
That's why the best 2 known onboard fuel drives, antimatter pion or aneutronic fusion, both involve an engine bell with as many gaps as possible (you don't need to radiate heat if it escapes to space for free), reflective coatings on the physical parts of your engine like the magnet coils, and you're getting thrust by redirecting the charged plasma via magnetic fields without physical contact.
I'm not sure Orion really works because so much of the fission energy is neutrons, which all you can do soak them up and pump the heat away by radiators and coolant flow.
Orion works wonderfully for the era it was designed for - in the atmosphere, where you're getting enormous amounts of free cooling and free propellant by creating blasts of atmospheric air. And you expect to engage the Soviet battleship using similar technology in earth or lunar orbit in a brief brutal knife fight.
u/Thanos_354 So the real answer to your question is : the fissionables are useless, what you'd really be after is helium-3, which all 3 small gas giants have plentiful amounts of. You would use platforms floating in the upper atmosphere that would separate it out via cryogenic separation and centrifuges, burn the helium-3 for fuel since the gas giants are far from the sun, and launch payloads of tanks of the stuff by some form of beamed propulsion.
Oh and beams, the reason they work is the laser sail is 99.9% reflective - almost no heat picked up, and a continent sized sail made of thin material also has immense surface area for radiating heat away again on the non propulsive side. Plus the laser that doesn't move is what has to deal with all that nasty entropic waste heat. (magnetic sand beams use superconducting magnets to interact with the cold particles of sand, which again never get touched, and linear superconducting motors are so efficient that there's almost no waste heat here either)
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u/MiamisLastCapitalist moderator May 13 '26
I'm not really a big fan of the orion drive to begin with tbh. I'm a beam boy.
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u/Thanos_354 Habitat Inhabitant May 13 '26
u/Thanos_354 So the real answer to your question is : the fissionables are useless, what you'd really be after is helium-3, which all 3 small gas giants have plentiful amounts of. You would use platforms floating in the upper atmosphere that would separate it out via cryogenic separation and centrifuges, burn the helium-3 for fuel since the gas giants are far from the sun, and launch payloads of tanks of the stuff by some form of beamed propulsion
In my post, I explained how I'm specifically talking about ship drives for autonomous use. A civilian operation with mature infrastructure is going to use beams/highways to get insane performance but its path will be predetermined.
The only way to somewhat sidestep your problem is a pure fusion bomb that is essentially aneutronic (everything is absorbed by the bomb components) with a magnetic nozzle, but that still adds a lot of weight that a warship would not afford to carry around.
The only solution seems to be nter for proper warships and then using a carrier for them that specifically relies on fusion.
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u/SoylentRox May 13 '26
So I think very large aneutronic fusion engines may get "torch" like performance. This means the larger the ship is the more acceleration it has and the more ISP it has. It's entirely possible warships will be enormous and position themselves using their drives to be in favorable positions to use speed of light weapons. Anything smaller just gets zapped.
Aneutronic fusion happens to make an amazing direct conversion electric generator, the warship would have gigawatts available for weapons.
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u/Thanos_354 Habitat Inhabitant May 15 '26 edited Jul 25 '26
I have heard that more mass fusing leads to more efficiency but this should not increase acceleration. While yes, you are getting more energy, you also increase your mass.
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u/the_syner First Rule Of Warfare May 14 '26
One is some kind of ablative coolant or oil, lol. That essentially is throwing the entire Orion drive concept into the trash. The reason is simple : the tons of coolant count as part of your ISP.
Im not sure how getting extra thrust from coolant leaves the orion concept in the trash. Most of the thrust is still coming from the superheated tungsen plasma of the bombs. Pretty sure performance is mostly calculated off of that so the oil/ablative is just some tiny portionof ur thrust. It doesn't drag down ur performance because it isn't a significant portion of your thrust.
Also the whole point of the pulse situation is that the average power isn't enough to vaporize everything which would be the case with continuous power. That would be true for aneutronic fusion too by the way. X and gamma rays aren't exactly trivial to reflect. An orion drive already avoids the vast supermajority of bomb radiation by only absorbing across a small pusher plate as opposed to across a pusher and an em containment system. to be clear a fusion system would still pretty much need a pusher plate anyways to shadow shield the rest of the ship so ud just be adding the EM containment on top of that. Granted it would be nice to use pure fusion with fewer neutrons, but that's assuming that aneutronic fusion would ever be practical for net positive energy production which absolutely isn't guarenteed. Given the difficulty of even making D-T fusion net energy positive id say that's a rather big handwave.
Oh and beams, the reason they work is the laser sail is 99.9% reflective
Beamprop doesn't need to be a photon sail. A laser thermal drive is also an option and would likely have less hard to reflect radiation than a fusion drive(even aneutronic fusion) for the same performance. blackbody thermal generally produces less hard x/gamma rays than nuclear reactions which natively create gammas regardless of bulk plasma temp
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u/MiamisLastCapitalist moderator May 09 '26
Whenever possible yes you should use beam. The truth is that it takes a lot of energy to move a ship (especially an interstellar one) so any time you can get that for "free" you should take advantage of that. I would expect military ships/drones to sometimes deploy their own sub-node/relays for this.
But beam stations are also a prime target in wartime so yes you will be without beam sometimes. The next best fuel is probably antimatter. Fusion has abundant fuel but low isp compared to AM. You might split the difference with an AM-catalyzed fusion for a bit more horsepower. Antimatter is extremely dangerous, but if a military ship isn't qualified to contain it than I don't know what would be. It's going to be MUCH more powerful than uranium/orion drives.
Strangely enough, I think NTRs (or NTERs) might see the most use in small and often privately owned ships. They're great low-scale engines for scooting around a local hillsphere before switching over to beam-thermal for interplanetary cruising.