The Nuclear Salt Water Rocket - Possibly the Craziest Rocket Engine Ever Imagined.

hello it's scott manley here i've talked about the rocket science of the expanse in the past in that show the fusion drives are a rare thing for science fiction in that they are actually sort of within the laws of physics albeit likely beyond any technology that could be developed soon or possibly ever in the tv show the spacecraft have engines that are able to provide powerful thrust comparable to the chemical rockets that we use and to launch rockets into space but they're able to operate for days or even weeks at a time like the ultra high efficiency ion propulsion systems used on deep space probes today it would be really nice if there were something that we could imagine in real life that could do both right and you know there are smart people that have come up with a few ideas over the years so i want to talk about one particular thing known as the nuclear saltwater rocket and it's the brainchild of robert zubrin and you might have heard of him because he is a sort of guy behind the mars direct concept which has sort of become the basis for most of the plans to send humans to the red planet so rocket engines they obviously work by sending out exhaust your reaction mass at high velocities in one direction and the conservation of momentum means that you get thrust in the opposite direction and they need a lot of energy to throw out the material and conventional chemical engines the combustion of the fuel provides the bulk of the energy whereas in ion thrusters that energy comes from solar panels that are soaking up the sun so the power from those solar panels is actually pretty wimpy compared to the fury of the chemical reaction but both of them are much lower energy density than what's available from nuclear reactions now there are actually nuclear thermal engines and that's where you have a nuclear reactor and you blow propellant over it to cool it down um and so you can heat the propellant to a relatively high temperature and use the reactor sort of continuously with a lot of propellant now this may sound crazy but they've actually built and tested these in the past in fact they did a lot of testing in a place known as jackass flats the program to develop these was shut down probably because it was a key step to potentially extending the apollo program to mars and you know government really wasn't wanting to fund something as extravagant as that so anyway in your nuclear thermal thrusters they're about twice as efficient as the best chemical engines but there are limits on this because to make things better the reactor needs to be hotter and hotter but if it gets too hot then it'll melt i mean it won't melt down because it's in zero g but it having a you know liquid or even vaporized core is not good i mean it sounds catastrophic right but there are actually ideas for gas core nuclear reactors where the reactor is actually a superheated plasma containing you know fissile uranium or plutonium this is obviously a technical challenge but that's a whole other video no the nuclear salt water rocket works by having a sustained nuclear reaction in the propellant as it flows through the engine itself it's just like a chemical rocket which you know has a chemical reaction as the fuel flows through it but with it vastly more energy available to it the exhaust from a chemical rocket is thousands of degrees but it could be hundreds of thousands of degrees in a nuclear salt water rocket so the propellant is water with a uranium or plutonium salt dissolved in it the original proposal had a two percent uranium tetrabromide mixture with regular water the uranium itself would also be enriched to about 20 uranium-235 that's reactor grade uranium so in that mixture it contains everything it needs to sustain a chain reaction if you get enough of it in the right place you will have a critical mass and the neutron driven reaction will run away exponentially and the water will explode into hot plasma so the first problem is making a fuel tank that can store huge quantities of propellant and you do this by having a lot of neutron absorbing materials spread throughout the interior of the tank things like boron which will soak up those excess neutrons before they can drive a runaway reaction and ruin your day if you have a fuel leak by the way then you'd better hope that it doesn't accumulate in any one place otherwise it will eventually react when it collects a critical mass so the neutron suppression uh is a key part of the fuel handling and that has to be maintained right up to the point where you actually want it to react so in simple terms you pump the salt water down boron lined pipes into the chamber and if you do it at the right speed you can get a highly energetic self-sustaining nuclear reaction which generates incredibly energetic plasma and it's this plasma which generates the thrust this isn't like the slow chain reaction you get in a regular nuclear reactor where it's only barely critical so they can control it this is a reaction where the criticality is maximized to burn as much fuel as possible before it expands into space it's kind of like trying to replicate the prompt criticality conditions in the excursion that destroyed the chernobyl reactor but then you keep it running at that level continuously sounds fun doesn't it right so yeah the water is of course key to this whole process uranium at 20 enrichment doesn't undergo a chain reaction on its own because the highly energetic neutrons that come out of those fission events have a very low probability of triggering another fission event but slower neutrons are much more likely to make further fissions possible so the water basically acts as a moderator those highly energetic neutrons they freshly liberated from a fission event bounce around off the water molecules until they're slowing they're moving around slowly enough that they can make more reactions happen and sustain your reaction and this bouncing around leads to another trick that's in play in the design so you want to keep this highly energetic reaction under control because of course the neutrons are bouncing around in the water they actually get dragged along by the water flow right they're bouncing off the water molecules and that takes time and so they end up getting pushed along so if the water flow is fast enough you can actually stop the neutrons running upstream into the rest of your engine which is probably a good thing because you don't want a nuclear reaction running up inside your engine plumbing where you don't want nuclear reactions to occur depending upon how you do the math you might you need to get the water moving maybe at around 66 meters per second it could be higher depending upon how you do your math but this is something that's sort of doable it's not like supersonic uh water flows um of course you know that's about 240 kilometers per hour for those that are keeping track um but yeah do that and the majority of the reaction will stay happening in the place where you want it to so there's another way to think about this and that is as a continuous thrust version of the orion nuclear pulse drive that's where you take nuclear bombs and you set them off close to a pusher plate and that pushes the spaceship along but instead you've got a continuous nuclear explosion happening so to generate thrust effectively you need to direct all this fire and fury all these exploding gasoline gases need to be directed using a nozzle but with the amount of hue heat and radiation any unprotected physical nozzle would vaporize in short order so the design has a layer of regular clean water running around the outside and that acts as an insulating air you know layer protecting both the nozzle from the heat and the radiation so some numbers the paper estimates that perhaps you'd only have 0.1 percent of the uranium 235 undergoing fission which at your two percent concentration salt 20 enrichment gives you 3.4 gigajoules per kilogram that's about 900 times the energy density of tnt and that translates to an exhaust velocity of about 60 to 70 kilometers per second depending upon how good your nozzle is that's about the same as the exhaust velocity from ion thrusters but in this case you're no longer limited by the tiny amount of power that you can soak up through those solar panels so the sample design that was in the paper it used an inlet pipe which was six centimeters in diameter you know like that size with a propellant flow of 200 kilograms per second through that and this would generate a thrust of about 1300 tons that works out to a steady state power of 700 gigawatts which by the way when chernobyl reached pkx power during its explosion it was about 350 gigawatts for a fraction of a second this is 700 gigawatts continuously right it's a non-stop chernobyl going on now that thrust is roughly equivalent to two f1 engines that they used on the saturn v those of course to do that each required two and a half tons of propellant so if you only need 200 kilograms versus five tons that's about 25 25 times more efficient which is a nice step up to be clear you really wouldn't want to use this as the first stage to a rocket on the ground it would spread nasty radioactive fission products all over the place it would make the rocky flats nuclear facility cleanup look well easy in comparison but in deep space that radioactive exhaust is going so fast that it rapidly spreads throughout space and goes to almost zero concentrations in fact some of it is going so fast it will leave the solar system all together now if if you were to say look at spacex's starship and then build a version with a nuclear saltwater rocket same amount of propellant mass starship right now in theory gets about eight to nine kilometers per second of delta v you switch it up with a nuclear salt water rocket you could get 150 kilometers per second you could travel to jupiter in a couple of months and come back in a couple of months using your single fuel load it's quite amazing to people thinking about the future of space you know futurists and science fiction writers the nuclear salt water rocket is actually pretty attractive because there's nothing fundamentally about it that breaks the laws of physics there's no obvious show stoppers that a bright student can point out sure the simplistic sketch in the paper would likely to have a lot more complexity if it was turned into a real engine you'd probably need neutron sources and neutron absorbers near the injection point you could reconfigure to control the reaction and you know while pumping fluids at this rate isn't too hard there's a question of maybe how you would power those pumps without adding lots of excess mass but these are easy problems that we can think of indeed it's entirely likely that there's fundamental parts of material science that you know simply can't deliver the kind of properties that we would need to turn a theoretical possibility like this into an engineering reality but just say it could be made to work then the same paper did the math for a more powerful more efficient version instead of using reactor grade uranium it used weapons grade uranium enriched in 90 uranium-235 and then assumed more of the uranium undergoes fission and in that case something like this would have an exhaust velocity possibly over one percent of the speed of light and then you'd be in the position to send spacecraft to other stars in decades rather than centuries so you know even if science delivers the ability to fly around the solar system in a few months by fl riding a continuous nuclear explosion we'll still be able to dream of a sci-fi future where the ships could go faster still i'm scott manley fly safe [Music] you

Channel: Scott Manley

Views: 608,611

Rating: 4.9684105 out of 5

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Id: cvZjhWE-3zM

Length: 13min 12sec (792 seconds)

Published: Sun Jan 10 2021

Reddit Comments

[u/atheistdoge]

Probably the closest to a torch drive I've seen. Zubrin has been and is a treasure.

[u/EpLiSoN]

It's an honest to god Torchship.

[u/NearABE]

SFIA has proposed quasar drives and supernova drives.

The salt water rocket proposed by Zubrin had 427 gigawatt power output. If one was flying by any other planet in the solar system you would not be able to see it from Earth.

427 gigawatt has to be sane or we cannot talk about interstellar travel with a colony ship that arrives within a sane timeframe.

[u/[deleted]]

I talked about a cleaner design for a NSWR on this subreddit:

https://www.reddit.com/r/IsaacArthur/comments/iibw3o/lithium_salt_water_rocket_a_cleaner_nswr/

[u/KnightFox]

I think the Craziest is still the Orion but this is still super cool. How do these compare to Solar Thermal Rockets?

[u/KhanneaSuntzu]

Manny Scotty eh?

[u/Wise_Bass]

It'd pretty wild, although the fuel cost would be a big barrier. That's a lot of uranium and plutonium to ditch out the back end without a chance at realistic recovery, and they're relatively scarce. Whereas NTR uses extremely abundant hydrogen, and you can recycle the core materials later.

[u/MiamisLastCapitalist]

So I have a question about this (and if I need I might make a new thread about it), but...

So they concentrate the mixed water in the engine bell where it can reach criticality via the uranium concentration. BUT with the infamous Demon Core experiments we learned it's horribly easy to use neutron reflectors to prompt criticality at lower concentrations.

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So couldn't we use those reflector materials in the throat of the engine bell to prompt the critical reaction safer and "burn" up more fuel?

[u/MiamisLastCapitalist]

How safe is life on Earth from the use of this in low orbit? Obviously not for lift off, but from a craft passing by Earth or perhaps at the second stage of a rocket launch.