Clean lithium fission rocket

[sevenperforce]

Lithium-6 decays exothermically into helium and tritium when irradiated with sufficiently energetic neutrons.

A layer of lithium-6 hydride deposited onto a beryllium bell nozzle and irradiated with high-energy neutrons from a standard neutron source ought to undergo fission and release hydrogen and helium at impressively high exhaust velocities. You'd need a bit of mass to give the neutron source a high enough energy, but wouldn't the clean fission give you a tremendous gain?

[aceshigh]

Interesting. Is the idea yours?

[sevenperforce]

Yeah, my idea.

I can crunch a few numbers to get an idea of the specific impulse it would offer, as well as thrust-specific power requirements. It should be a lot better in terms of thrust/power than ion thrusters because the power supply need only sustain the neutron source, not actually provide the energy to accelerate the propellant.

Higher thrust could be obtained by mixing in lithium-7 hydride, which has an endothermic fission reaction but spits out a neutron of its own, doubling the reaction mass for a given neutron flux but reducing exhaust velocity.

A huge advantage is that your propellant remains solid right up until the reaction. Having monatomic hydrogen as your reaction mass isn't as much of an advantage as you might think, though, because half of it is tritium rather than protium. Each fission event releases a tritium atom, a helium atom, and a protium atom. I suppose you could get more energy if the tritium and protium burned to form diatomic hydrogen, though this would result in the propellant all being basically helium by molecular weight.

[acsawdey]

Are you thinking of something like a focus fusion device as the neutron source?

[sevenperforce]

Are you thinking of something like a focus fusion device as the neutron source?

Yeah, this would be controlled fusion-initiated fission (as opposed to the uncontrolled fission-initiated fusion in a nuclear weapon). The neutron source could potentially be an inertial electrostatic confinement design or, more preferably, an accelerator-based design. An accelerator design could be beam-to-target (a beam of deuterium ions fired at a static tritium-enriched target) or beam-to-beam (a beam of deuterium ions fired at a beam of tritium ions).

If I recall correctly, the energy fraction in fission events is divided among the fission products roughly in proportion to mass so that momentum is conserved. I don't think lithium-6 fission produces gamma rays. And the chemical binding energy of lithium hydride is something like a million times lower than lithium-6's fission energy so that's negligible. You'll end up with a helium-4 atom, a tritium atom, and the protium atom previously attached to the molecule, with 4.78 MeV of energy to divide among them. If my recollection is right, the protium atom will get an eighth of this (0.60 MeV), the tritium atom will get 3/8 of this (1.79 MeV), and the helium-4 atom will get half of this (2.39 MeV). Exhaust velocity is 10,734 km/s. Not m/s; km/s.

[acsawdey]

Exhaust velocity is 10,734 km/s. Not m/s; km/s.

That is the lovely thing about nuclear reactions in this case. Chemical bond energy is generally negligible in comparison.

With an Isp of ~1.1e6 seconds, you wouldn't need much reactant mass. Given that the nuclear fragments are almost certainly going to be ionized, could you use a magnetic field for the nozzle?

[sevenperforce]

Exhaust velocity is 10,734 km/s. Not m/s; km/s.

That is the lovely thing about nuclear reactions in this case. Chemical bond energy is generally negligible in comparison.

With an Isp of ~1.1e6 seconds, you wouldn't need much reactant mass. Given that the nuclear fragments are almost certainly going to be ionized, could you use a magnetic field for the nozzle?

My thought was to cast/deposit the lithium-6 hydride in the shape of an exhaust bell and place the neutron source at roughly the focal point. But with particle energies that high you might need some sort of confinement....

The challenge would be getting neutron fluxes high enough for useful thrust. Back-of-the-envelope estimates suggest 10¹⁹ neutrons per second for a single Newton of thrust.

[acsawdey]

The challenge would be getting neutron fluxes high enough for useful thrust. Back-of-the-envelope estimates suggest 10¹⁹ neutrons per second for a single Newton of thrust.

So, compute the power output of a 1N thruster with 1.1e6 Isp. By comparison to ion thrusters, 1N is a huge thrust.

[sevenperforce]

The challenge would be getting neutron fluxes high enough for useful thrust. Back-of-the-envelope estimates suggest 10¹⁹ neutrons per second for a single Newton of thrust.

So, compute the power output of a 1N thruster with 1.1e6 Isp. By comparison to ion thrusters, 1N is a huge thrust.

Well, the power output at 1N is simply 5.5e6 watts. But that is the output, not the input. The input depends on how you are generating your neutron flux.

To match the thrust of a single SuperDraco you would need a neutron source capable of producing 5.91e23 neutrons per second.

[acsawdey]

Well, the power output at 1N is simply 5.5e6 watts. But that is the output, not the input. The input depends on how you are generating your neutron flux.

The quote that comes to mind is

a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive -- Larry Niven

So, you've got a drive that produces 5.5 megawatts of power expressed as a beam of relativistic ions. At close range this is a weapon. I don't think you need anything like the thrust of a SuperDraco because you'll not want to use this on a planet because you'll damage things. Also the effect in atmosphere could be interesting.

Probably the limiting factor is the neutron source. But even if you can only make micro-newtons of thrust, at 1.1e6 Isp you can thrust for a long time on one block of fuel. I wonder if you could build something like this into a cubesat?

[sevenperforce]

Well, the power output at 1N is simply 5.5e6 watts. But that is the output, not the input. The input depends on how you are generating your neutron flux.

The quote that comes to mind is

a reaction drive's efficiency as a weapon is in direct proportion to its efficiency as a drive -- Larry Niven

So, you've got a drive that produces 5.5 megawatts of power expressed as a beam of relativistic ions. At close range this is a weapon. I don't think you need anything like the thrust of a SuperDraco because you'll not want to use this on a planet because you'll damage things. Also the effect in atmosphere could be interesting.

Probably the limiting factor is the neutron source. But even if you can only make micro-newtons of thrust, at 1.1e6 Isp you can thrust for a long time on one block of fuel. I wonder if you could build something like this into a cubesat?

Damn, that would be quite a weapon.

You can mix in lower enrichments of lithium to bring up the thrust and bring down the velocity. The limit is around 30℅ enrichment though.

[Mark K]

To match the thrust of a single SuperDraco you would need a neutron source capable of producing 5.91e23 neutrons per second.

What kind of neutron source can generate a gram/sec of neutrons?What kind of power input would you need for that?

Even milligrams/sec is awesome. Micrograms/sec ...

[RotoSequence]

How would you avoid, or at least mitigate, chemical ignition of the lithium fuel? Lithium will chemically react with both of the fission products.

[Impaler]

Coating the fissile material on the inside of a nozzle dose not seem practical, the neutron bombardment will penetrate into the coating even if it is fairly thick and cause fission in the whole mass of fissile material.  The resultant fissile byproducts will be generated inside a solid fuel mass and will collide with neighboring atoms and cause standard thermal interactions as they slow down, the whole fuel mass will simply heat up rather then emit a surface sputtering that I think your looking for.

Some form of tiny-pellet feed system inside a magnetic nozzle seems to be the only way to get a well controlled reaction that would produce manageable thrust.  The pellets can also contain non-fissile materials preferably highly transparent to neutrons which will be vaporized and contribute the bulk of the reaction mass, this will lower the ISP down to usable ranges, different pellet mixtures could produce different thrust and ISP ranges for the engine allowing a kind of throttling which has been shown to improve overall trajectory planning.  The neutron source would need to be pulse based and would only need to produce a single standard pulse both of which should make it simpler.

[KristianAndresen]

Lithium-6 decays exothermically into helium and tritium when irradiated with sufficiently energetic neutrons.
[...]
You'd need a bit of mass to give the neutron source a high enough energy

According to what I could look up, this decay can happen at any neutron energy... and don't you need quite slow neutrons in order to have a high interaction cross-section and therefore a low penetration depth, considering what Impaler said?

[sevenperforce]

What kind of neutron source can generate a gram/sec of neutrons?What kind of power input would you need for that?

Even milligrams/sec is awesome. Micrograms/sec ...

It might work to use a "dirty" fissile primary as your neutron source, like a subcritical plutonium sphere surrounded by neutron reflectors. A little unsafe, but at least you don't have to worry about carrying a power source, and fissile material isn't released into the environment. I don't know what kind of neutron count is generated in controlled criticality...

How would you avoid, or at least mitigate, chemical ignition of the lithium fuel? Lithium will chemically react with both of the fission products.

Well, I had proposed using lithium hydride rather than pure lithium metal, as this increases the particle count and also prevents the lithium from reacting (since it has already reacted). Of course the helium won't react with anything. The monatomic hydrogen might react with itself but that's not necessarily a bad thing. Specific impulse is less important here so an increase in molecular weight is not really a bad thing.

See below for more discussion of storage.

Coating the fissile material on the inside of a nozzle dose not seem practical, the neutron bombardment will penetrate into the coating even if it is fairly thick and cause fission in the whole mass of fissile material.  The resultant fissile byproducts will be generated inside a solid fuel mass and will collide with neighboring atoms and cause standard thermal interactions as they slow down, the whole fuel mass will simply heat up rather then emit a surface sputtering that I think your looking for.

Some form of tiny-pellet feed system inside a magnetic nozzle seems to be the only way to get a well controlled reaction that would produce manageable thrust.  The pellets can also contain non-fissile materials preferably highly transparent to neutrons which will be vaporized and contribute the bulk of the reaction mass, this will lower the ISP down to usable ranges, different pellet mixtures could produce different thrust and ISP ranges for the engine allowing a kind of throttling which has been shown to improve overall trajectory planning.  The neutron source would need to be pulse based and would only need to produce a single standard pulse both of which should make it simpler.

I was actually thinking of a very thin coating with a beryllium neutron reflector on the other side to produce more of a sputtering reaction. You could imagine stacking a series of thin "fuel layers" alternating between lithium-6 hydride and beryllium, with each pair of layers being discarded once most of the lithium hydride had been used up.

A magnetic nozzle is a possibility; lithium hydride is itself highly diamagnetic, but it's also an ionic conductor, so there are quite a few ways that could be used.

Since reaction mass is a much bigger concern than specific impulse (seriously, weakly relativistic ion spray is NOT nice), what about using something like enriched lithium-6 hydroxide saltwater? Lithium hydroxide dissolves in water at 129 g/L; a single liter of such saltwater therefore has 5.4 moles of lithium-6, with a total potential energy of 2.5e3 gigajoules. For reference, that's about 4% the yield of the Little Boy bomb dropped on Hiroshima...absolutely insane. Lithium-6 has the highest specific energy of fission of any fissile material.

If you used heavy water rather than light water, then you could use (cheaper) natural uranium as the neutron source. You'd want to cast natural uranium into a cylinder with a converging end and wrap it in a beryllium neutron reflector; put a tungsten diverging nozzle on other side. Give the inside some sort of neutron-transparent but very heat-resistant coating (because, for the love of all that is good, you don't want fissioning uranium spraying out with your exhaust stream); not sure what material might work for that. When you pump the enriched lithium-6 hydroxide heavy saltwater into the exhaust stream, the heavy water will slow the fast neutrons from natural decay down into thermal neutrons, causing the natural uranium to go critical and release a spray of neutrons. These, in turn, collide with lithium-6 nuclei and trigger the lithium fission.

A liter of enriched lithium-6 hydroxide heavy saltwater masses 1.229 kg, so the lower bound for exhaust velocity is going to be on the order of 2e6 m/s. Of course, you can decrease this and increase thrust by simply decreasing the enrichment of the lithium hydroxide or decreasing the amount of lithium hydroxide dissolved in the heavy water, though doing the latter tends to decrease the density of your propellant which is less ideal. Another issue is that not all the lithium-6 will fission; I don't really know how to estimate the percentage though.

The only power requirement is the turbopump to push the enriched heavy saltwater into the "combustion" chamber, and you can probably use a fuel coolant loop for that. I'd need to dig pretty deep into nuclear reactor design and criticality to see what the minimum possible size would be. Thrust to weight ratio is going to be respectable; if you were using full enrichment and full fission, matching the thrust of a SuperDraco would require a mass flow of just 36 grams per second.

If this design isn't clear let me know and I will do a graphic mockup.

Does anyone know of a material with very high heat resistance that is neutron-transparent?

[sevenperforce]

Did a little digging and it looks like someone else already had a similar idea:

Nuclear Salt Water Rockets Revisited

This proposal suggests the use of Lithium-6 deuteride, with the expectation that tritium-deuterium fusion will take place via Jetter Cycle. Such a cycle is highly unlikely in a rocket engine; the conditions for any fusion are extremely demanding. However, the use of a shielded high flux reactor is definitely a better plan than using a coated cylinder of raw uranium. Using lithium fission as the power source rather than imagining D-T fusion as the power source also allows for a much more manageable/realistic impulse and thrust. Finally, using dissolved lithium hydroxide rather than a "lithium hydride suspension" makes the propellant much more stable, inexpensive to manufacture, and easier to handle.

[aceshigh]

I had posted a thread about nuclear salt water rocket a few months ago.

quite interesting that the proposal of your link is basically the NSWR without the hiper radioactive exhaust!

[sevenperforce]

I had posted a thread about nuclear salt water rocket a few months ago.

quite interesting that the proposal of your link is basically the NSWR without the hiper radioactive exhaust!

The proposal at the link is a Thermonuclear Saltwater Rocket (TNSWR), which simply won't work. But dialing it back to a Lithium Fission NSWR would be simpler, more realizeable, and a lot cheaper to build/refuel/operate.

In the simplest form, you have something that looks like the attached image. You have natural uranium surrounded by a tungsten neutron reflector, but it cannot sustain a chain reaction without a neutron moderator. Once the heavy saltwater is pumped in, however, the neutrons are immediately moderated and a chain reaction is triggered, irradiating the lithium-6 and triggering fission, which expels everything out the bottom end at tremendous speed.

[Impaler]

This is a significant improvement over the traditional NSWR concept which would have relied on a fuel so enriched that it will spontaneously detonate in the absence of a neutron absorbing materials.  That would require a large and heavy storage tank to prevent the entire fuel mass from becoming the worlds largest bomb.  Needless to say this would have made fuel handling monstrously expensive and difficult, the only advantage is that the engine was devoid of moving parts and was just a nozzle without neutron absorbent.

But by using a stable non explosive fuel like lithium the fuel handling process is massively simpler and in fact you can have a JIT mixing system (presumably inside the turbo-pump itself) to combine water with a lithium rich 'syrup' (much like how you get Soda from a fountain) just before the flow enters the engine.  This would again allow for throttling the engine in thrust and ISP (which isn't possible with traditional NSWR), as well as smoothly shutting down or starting up the engine by transitioning from and too a pure water flow thus avoiding the 'hard start' issue with most rocket engines.

Lastly and perhaps most significantly it would allow such a vessel to carry a surplus 'syrup' which can presumably be sourced only on Earth, while refueling with more easily obtained water from in-situ sources in space (though probably not the desirable heavy water).  This would allow nearly unlimited range and proportionally higher payload fractions.

I'm going to term this the 'Nuclear Soda Fountain Rocket' concept.

[QuantumG]

This is a significant improvement over the traditional NSWR concept which would have relied on a fuel so enriched that it will spontaneously detonate in the absence of a neutron absorbing materials.  That would require a large and heavy storage tank to prevent the entire fuel mass from becoming the worlds largest bomb.

Where did you get this silly idea from? Ask for a refund.

[Impaler]

The original Zubrin NSWR paper specifies that the propellant needs to be stored in Boron-carbide tubes and that in the engine plenum where the propellant will undergo 'prompt superficiality' by simply allowing the fluid to flow at high speed through a tube 6 cm in diameter and 65 cm long without any Boron-carbide present which will and I quote 'creating a condition of virtually instantaneous detonation of the fluid'.

So yes this fluid is highly explosive and only the specially designed tank and the neutron absorbing properties their of prevents it from spontaneously detonating.  Zubrin throws out a tank mass fraction of just 4% with no supporting evidence at all and which I find laughably small considering that the tank must consist of countless small tubes with high surface area to volume ratios and the Boron-carbide is too brittle to serve any structural role and would just be an additional coating in the tubes.

[QuantumG]

It's forced supercriticality.. just like every other nuclear device. It's nonsense to suggest that somehow the tank can explode.

[RotoSequence]

How would you avoid, or at least mitigate, chemical ignition of the lithium fuel? Lithium will chemically react with both of the fission products.

Well, I had proposed using lithium hydride rather than pure lithium metal, as this increases the particle count and also prevents the lithium from reacting (since it has already reacted). Of course the helium won't react with anything. The monatomic hydrogen might react with itself but that's not necessarily a bad thing. Specific impulse is less important here so an increase in molecular weight is not really a bad thing.

This is not correct; Lithium will bond with helium in sufficiently energetic conditions, and lithium fires are not pretty things. We're still talking about some violently exothermic conditions that would be very difficult to contain and chemically volatile; I can't foresee a lithium driven vehicle being anything better than a nuclear equivalent of a contemporary solid fuel rocket.

[sevenperforce]

It's forced supercriticality.. just like every other nuclear device. It's nonsense to suggest that somehow the tank can explode.

A breach or leak would cause a tank explosion just like a breach between two hypergolic. Only this is a nuclear explosion.

It is the rocket equivalent of assembling multiple critical masses of plutonium in a supercritical state with boron control rods jammed in to prevent instant nuclear detonation and then slowly removing the control rods to generate heat, hoping you don't pull them out too far and cause a multiple-kiloton detonation.

How would you avoid, or at least mitigate, chemical ignition of the lithium fuel? Lithium will chemically react with both of the fission products.

Well, I had proposed using lithium hydride rather than pure lithium metal, as this increases the particle count and also prevents the lithium from reacting (since it has already reacted). Of course the helium won't react with anything. The monatomic hydrogen might react with itself but that's not necessarily a bad thing. Specific impulse is less important here so an increase in molecular weight is not really a bad thing.

This is not correct; Lithium will bond with helium in sufficiently energetic conditions, and lithium fires are not pretty things. We're still talking about some violently exothermic conditions that would be very difficult to contain and chemically volatile; I can't foresee a lithium driven vehicle being anything better than a nuclear equivalent of a contemporary solid fuel rocket.

Any combustion would take place downstream in the heart of a continuously burning nuclear flame so  I can't imagine that is going to prove problematic. If there is any place where it is OK to have a lithium fire it would be in the exhaust stream of a NSWR.

The advantages over conventional rockets are stupidly high specific impulse and thrust. The advantages over a Zubrin NSWR are nonradioactive exhaust, nonradioactive propellant, throttleability, storable non-pressurized non-cryogenic propellant, low tank mass ratio, lower-cost propellant, and a reasonable chance at partial ISRU.

Granted, the exhaust is going to melt the launch pad every time, but that's not too big of a deal.

[aceshigh]

Launch pads are expensive. How can we make spaceflight cheaper if we melt a launch pad at each launch?


but more seriously, can we get high thrust in this system with current tech?


lower thrust systems, making it possible to reach Mars fast, but without launching from Earth... feasible with current tech?



there is probably a catch somewhere in there. If there are only pros, but no cons, it would already be in use, since as you showed yourself, another guy FRIEND with Zubrin even already came with a very similar concept.

[sevenperforce]

Launch pads are expensive. How can we make spaceflight cheaper if we melt a launch pad at each launch?

but more seriously, can we get high thrust in this system with current tech?

lower thrust systems, making it possible to reach Mars fast, but without launching from Earth... feasible with current tech?

"Melt the launch pad every time" was a bit of hyperbole; I was merely pointing out that the exhaust stream is very, very energetic. For a launch from Earth you would just need to upgrade your water suppressant system, perhaps placing an Olympic-sized pool under the launch pad (no, seriously).

The ideal arrangement would use a tank of light water, a tank of heavy water, and a tank of saturated enriched lithium-6 hydroxide heavy saltwater. These would be blended at the turbopump before injection as shown in the attached schematic. The specific blend ratios would be able to control neutron flux, total thrust, and specific impulse.

You could also add a turbocompressor to mix atmosphere air in with the exhaust at launch in order to reduce the exhaust velocity and mitigate harm to the launch pad while significantly increasing thrust and specific impulse. The T/W ratio of this is likely going to be so stupidly high that adding more dry mass isn't going to be a problem, and the peak specific impulse is also ridiculous. Remember that lithium-6 has the most nuclear potential energy per unit mass of ANY fissionable isotope.

The attached diagram also provides for full tungsten containment; having a thin combustion chamber wall will allow sufficient neutron flux through it without exposing the uranium to the fissioning propellant. The uranium fuel rods can be replaced when they are spent. The fissile mass would last much longer than it does with a nuclear thermal rocket, though, because the "reactor" is only providing neutrons, not heating up hot enough to significantly heat the propellant.

there is probably a catch somewhere in there. If there are only pros, but no cons, it would already be in use, since as you showed yourself, another guy FRIEND with Zubrin even already came with a very similar concept.

There are cons. Although this doesn't spray deadly nuclear waste like Zubrin's NSWR, it does release a good deal of tritium. Even so, the chances of being able to use this for launch are likely far, far better than even a nuclear thermal rocket because the reactor is much cooler than the one in an NTR. I have done a moderate amount of searching and I have not found a single post, patent, or thread anywhere suggesting the use of lithium fission for powering a rocket engine.

[Robert827]

Hey guys, I'm kind of new here. I was just curious. Wouldn't such system produce an enormous pressure inside combustion chamber?

[sevenperforce]

Hey guys, I'm kind of new here. I was just curious. Wouldn't such system produce an enormous pressure inside combustion chamber?

Yes.

Hence tungsten everything and low area turbopump inlets.

[QuantumG]

It's forced supercriticality.. just like every other nuclear device. It's nonsense to suggest that somehow the tank can explode.

A breach or leak would cause a tank explosion just like a breach between two hypergolic. Only this is a nuclear explosion.

Completely wrong.

It is the rocket equivalent of assembling multiple critical masses of plutonium in a supercritical state with boron control rods jammed in to prevent instant nuclear detonation and then slowly removing the control rods to generate heat, hoping you don't pull them out too far and cause a multiple-kiloton detonation.

Except it isn't. Did ya even read the NSWR paper?

[Impaler]

We have both read the paper, the salt water Zubrin describes would be at critical mass if just a liter or so pools in an area without neutron absorption.  Their is nothing 'forced' about the super-criticality other then simply removing neutorn absorbers.

The most likely scenario for that to happen is a crack in a storage pipe that allows salt water to pool in the space between pipes.  The resulting explosion can then fracture more pipes, if your lucky the tanks contents move away fast enough and in small enough droplets that they their are no secondary explosions, but if their are they will be even larger.  Obviously you can't get anywhere near full burn-up but if that is your hang-up your being pedantic.

[QuantumG]

Their is nothing 'forced' about the super-criticality other then simply removing neutorn absorbers.

So, basically, you're saying you don't understand how shape matters in criticality. I'm just gunna stop talking to you now.

[sevenperforce]

Upon review, you would probably want to end up with something more like this:



You need the convex-section diverging chamber/nozzle because trying to choke the furious plasma firestorm in this combustion chamber will be pointless. This also allows the neutron flux to be focused to a point just within the nozzle but outside of the main pressure bearing region.

I added a separate coolant loop for the reactor; it can use a blend of heavy and light water to fine-tune neutron moderation. Pure heavy water and pure light water can also be added to the propellant stream as desired. The coolant loop exits around the central flow of propellant to protect the inside of the chamber and nozzle and also decreases specific impulse in exchange for increased thrust.

You can't expect full expansion so this will be a pressure rocket.

[acsawdey]

I added a separate coolant loop for the reactor; it can use a blend of heavy and light water to fine-tune neutron moderation. Pure heavy water and pure light water can also be added to the propellant stream as desired. The coolant loop exits around the central flow of propellant to protect the inside of the chamber and nozzle and also decreases specific impulse in exchange for increased thrust.

You can't expect full expansion so this will be a pressure rocket.

Wrap a set of coils around the outside of this and you have a magnetic nozzle:
http://alfven.princeton.edu/projects/MagneticNozzle.htm

This would allow you to get additional expansion since the fission products are going to be charged particles.

I really think you want a closed loop coolant system. It could provide power for the magnetic nozzle among other things. This seems to have the most potential for use as a deep space propulsion system, where you want to be able to operate in the highest Isp mode most of the time.

For the very highest Isp, store your Li6 as a solid block. Use a laser to vaporize some off the surface, then another laser to ionize it, then feed the resulting puff of plasma down the center of the magnetic nozzle into your neutron source.

[sevenperforce]

I added a separate coolant loop for the reactor; it can use a blend of heavy and light water to fine-tune neutron moderation. Pure heavy water and pure light water can also be added to the propellant stream as desired. The coolant loop exits around the central flow of propellant to protect the inside of the chamber and nozzle and also decreases specific impulse in exchange for increased thrust.

You can't expect full expansion so this will be a pressure rocket.

Wrap a set of coils around the outside of this and you have a magnetic nozzle:
http://alfven.princeton.edu/projects/MagneticNozzle.htm

This would allow you to get additional expansion since the fission products are going to be charged particles.

I really think you want a closed loop coolant system. It could provide power for the magnetic nozzle among other things. This seems to have the most potential for use as a deep space propulsion system, where you want to be able to operate in the highest Isp mode most of the time.

For the very highest Isp, store your Li6 as a solid block. Use a laser to vaporize some off the surface, then another laser to ionize it, then feed the resulting puff of plasma down the center of the magnetic nozzle into your neutron source.

That would be ideal for a deep space propulsion system that can accelerate indefinitely. However, for something I can actually use to get off the ground, I need high thrust.

[acsawdey]

That would be ideal for a deep space propulsion system that can accelerate indefinitely. However, for something I can actually use to get off the ground, I need high thrust.

I think the problem with that is the tritium -- 1 gram of that is 9650 Ci ... good luck trying to write an environmental impact report for the amount of tritium a launch would create.

[Lar]

trimmed off some sniping. Don't snipe.

[Impaler]

I'm curious what kind of ratios of heavy water, light water and lithium will be needed.  In fact we don't necessarily need light water, any fluid that can act as a reaction mass would likely suffice so long as it is will shield the engine from it's own heat production and can be pumped at sufficient speed/volume.  The atmosphere of nearly any planet should be sufficient if compressed or liquefied.  This is another argument in favor of a separate coolant loop for the engine as this would eliminate the need for the reaction mass flow to act as a heat sink which would make the engine more tolerant of variation in said reaction mass.

What kind of upper bound to lithium content might the 'syrup' have, acsawdey describes pure vaporized lithium from a solid block for highest possible ISP.  While I think that's far more lithium richness then is actually ultimately needed or is desirable he is on the right track that we want to maximize the richness and solubility of lithium-salts in water might not be the upper bound.

But lets look at what that upper bound might reasonably be, from what I can find Lithium chloride is noted for being hyper-soluable in water at 88.7 g/100 mL (40 °C), Lithium chloride is itself 16.3% Li by mass so final saturated solution would be 7.7% Lithium by mass, this is more then double the concentration of the Lithium Hydroxide solution sevenperforce described earlier which would come out too 3.3% lithium.

Unfortunately Chlorine is a poor neutron moderator, Florine is a much better moderator but Lithium Floride is comparatively insoluble in water at only around 1 gram per L.  If it were to be employed the engine might run on a pure molten salt which would boost the lithium percentage all the way to 26% but would require a system to melt the salt (presumably stored in a granular form) to 874 C, but by mixing Berilium Floride in the melting point is lowered to 459 C and the neutron moderation is improved, lithium richness drops to a still very nice 14% in the 2:1 ratio mixture typically employed in a molten salt nuclear reactor.  Normally a molten salt reactor uses pure lithium-7 because they want to avoid the neutron absorption of lithium-6 but here we want to do just the opposite, all lithium-6 so we get a detonation.

If at all possible we would like to avoid having to expel the neutron moderator in the propellant stream beyond the minimum amount needed to efficiently carry the lithium in a liquid which can be injected into the reaction mass stream.  This means we would want to maximize the neutron moderation occurring within solid engine components.  This would yield the highest ratio of dumb ISRU reaction mass to expensive Earth-only sourced lithium-salt.  I suspect an engine consisting of a large honeycombed beryllium/plutonium block wrapped in a neutron reflecting tungston as sevenperforce describes would work well.

[sevenperforce]

I'm not sure what can be done about the release of tritium. This would be orders of magnitude less dangerous than a uranium NSWR or even Project Orion, but it is still releasing full kilograms of tritium with each launch. Tritium is not terribly nasty but it is not exactly safe either.

I landed on heavy water as the combined fuel carrier + reaction mass + neutron moderator because it really simplifies the design and would allow for a lot of fine-tuning of engine performance without changing the essential configuration. Good for adjusting specific impulse and thrust and so forth. It is also intrinsically safe if your moderator is also your reaction mass, because its presence is what expels it.

But a more neutron rich fissile fuel might give more options. You could also use a different comment, like liquid ammonia, with a salt carrier like ethanol.

[aceshigh]

I'm not sure what can be done about the release of tritium. This would be orders of magnitude less dangerous than a uranium NSWR or even Project Orion, but it is still releasing full kilograms of tritium with each launch. Tritium is not terribly nasty but it is not exactly safe either.

I landed on heavy water as the combined fuel carrier + reaction mass + neutron moderator because it really simplifies the design and would allow for a lot of fine-tuning of engine performance without changing the essential configuration. Good for adjusting specific impulse and thrust and so forth. It is also intrinsically safe if your moderator is also your reaction mass, because its presence is what expels it.

But a more neutron rich fissile fuel might give more options. You could also use a different comment, like liquid ammonia, with a salt carrier like ethanol.

how about in a first moment, you think only of space propulsion and possibly re-launch from other planets, instead of launching from Earth?


How much fuel (of every kind) would be needed to get to Mars or Jupiter in a decent short time? And so, how big would be the spaceships with this engine? Considering the possibilities talked about in this thread, that is, high ISP and decent thrust, it would still be feasible to launch from a few Falcon Heavies. Never land it again. Every time it comes back to Earth, keep it in orbit and refuel it.

[sevenperforce]

How much fuel (of every kind) would be needed to get to Mars or Jupiter in a decent short time? And so, how big would be the spaceships with this engine? Considering the possibilities talked about in this thread, that is, high ISP and decent thrust, it would still be feasible to launch from a few Falcon Heavies. Never land it again. Every time it comes back to Earth, keep it in orbit and refuel it.

Getting an estimate of actual performance would depend on how small a high-flux, low-heat reactor can reasonably be made. Unfortunately I really have no idea what that would be, or how long it would last. The reactor will eventually run low on fissile material to produce the desired neutron flux and need to have its fuel rods replaced.

This isn't quite torchship level; I doubt it could manage a 1g brachistochrone to Mars (though it could probably do a 1g brachistochrone to the moon without much difficulty). It could potentially manage a partial brachistochrone, or an 0.3g brachistochrone to Mars. That's kind of the sweet spot because it reduces transit time dramatically and also prepare the crew for Martian gravity.

The need for fuel rods replacement means that it would be really nice to be able to return to Earth under its own power, flood, and remotely service.

To reduce tritium release, what if an element was included (either in the propellant or in the salt) which preferentially binds to tritium and would keep it out of the water supply long enough for it to decay? Chlorine might work. My nuclear chemistry is somewhat lacking though.

[Impaler]

I think Tritium is just going to be a showstopper for Earth surface launch unless the rockets production of tritium is really really tiny and the amount released would not significantly raise background levels that exist naturally and are released from conventional nuclear industry activities.  Can we try to estimate the quantity that might be released per unit of rocket impulse.

I'm thinking that the main challenge of the engine is going to be keeping a high enough neutron flux in the combustion chamber to get an acceptable burn-up percentage of the lithium while the lithium is removing neutrons from the chain-reaction that is producing these neutrons.  If you take too many neutrons and the chain reaction is quenched and the propellant will stop burning and likely can't be lit again until the lithium flow is halted so the neutron flux can recover.  If the reactor is over producing neutrons then you have a potential super-critical state and explosion particularly when their is no propellant flow in the reactor.  You want to have a way of shutting down the reactor and dropping the neutron flux, perhaps you really do want to propellant flow to contain most of the neutron moderators as their absence will naturally quench the reactor as fast neutrons don't absorb well and don't sustain a chain reaction as easily.  Also the proposed coolant loop could contain Helium which is an efficient moderator, losing the coolant either in a breach or by intentionally changing it to a different gas such as one with boron would rapidly quench any chain-reaction.

[sevenperforce]

I think Tritium is just going to be a showstopper for Earth surface launch unless the rockets production of tritium is really really tiny and the amount released would not significantly raise background levels that exist naturally and are released from conventional nuclear industry activities.  Can we try to estimate the quantity that might be released per unit of rocket impulse.

I'm thinking that the main challenge of the engine is going to be keeping a high enough neutron flux in the combustion chamber to get an acceptable burn-up percentage of the lithium while the lithium is removing neutrons from the chain-reaction that is producing these neutrons.  If you take too many neutrons and the chain reaction is quenched and the propellant will stop burning and likely can't be lit again until the lithium flow is halted so the neutron flux can recover.  If the reactor is over producing neutrons then you have a potential super-critical state and explosion particularly when their is no propellant flow in the reactor.  You want to have a way of shutting down the reactor and dropping the neutron flux, perhaps you really do want to propellant flow to contain most of the neutron moderators as their absence will naturally quench the reactor as fast neutrons don't absorb well and don't sustain a chain reaction as easily.  Also the proposed coolant loop could contain Helium which is an efficient moderator, losing the coolant either in a breach or by intentionally changing it to a different gas such as one with boron would rapidly quench any chain-reaction.

One atom of tritium would be released for every 4.8 MeV lithium-6 fission event. So for every liter of enriched saltwater which fully fissions we are looking at roughly 55 grams of the stuff.

The combination of very high thrust and very high impulse makes it hard not to want to pursue it anyway though. There are numerous mechanisms for scrubbing tritium from exhaust vents or enriching it, so I am sure there is some way of dealing with it. If the expelled boundary-layer coolant contained fluorine or chlorine or sulfur, I'm pretty sure all three of those bond preferentially to tritium over protium, so perhaps they would keep it in a biologically inactive molecule long enough for it to decay.

Using the fuel as a carrier and propellant and coolant and moderator definitely adds the highest safety margin on the surface. The problem of lithium essentially acting as a neutron poison is worth analysis. However, the configuration shown in the diagram is not a limited one; you can have the fuel flowing around the outside of the reactor instead of through it.

[aceshigh]

This isn't quite torchship level

is torchship only a sci-fi term or is there a more serious design that could be called a "torchship"?

I doubt it could manage a 1g brachistochrone to Mars

thanks , I learned a new word.

[sevenperforce]

This isn't quite torchship level

is torchship only a sci-fi term or is there a more serious design that could be called a "torchship"?

The term originates in scifi, but it's not anything that couldn't exist in principle. It's basically any ship with a drive capable of accelerating at high levels of thrust (1g or greater) for a long, long time, long enough to take off vertically and not worry about pesky problems like gravity drag. No existing engine is that powerful, of course, but we can easily figure out what it would take. Antimatter-catalyzed continuous fusion drives would have that kind of performance.

Because a torchship uses a real engine rather than something purely scifi like "gravity displacement drive" or similar nonsense, we can demonstrate trivially that the exhaust of such a drive would need to be a furiously roaring fountain of relativistic death. Which is problematic. You kind of want an engine which mixes air in as reaction mass while in the atmosphere to cool things down a bit.

[Hanelyp]

To summarize the engine I see under discussion:

- A mass of nuclear fission fuel, subcritical absent the propellant.
- Introduce the propellant through coolant passages and down the reactor core, neutrons are moderated sufficiently to shift the reactor critical.
- Some of the neutrons produced react to fission lithium in the propellant, producing more heat.

Observations / questions:
- heating the propellant will reduce density, reducing neutron moderation effect.  Get the design right and the reactor will be largely self regulating.
- How much heat would be produced by the fission fuel producing the neutrons relative to heat from lithium fission?  The original post implicitly assumed a majority of heat being from the lithium fission, not the neutron source.

[sevenperforce]

To summarize the engine I see under discussion:

- A mass of nuclear fission fuel, subcritical absent the propellant.
- Introduce the propellant through coolant passages and down the reactor core, neutrons are moderated sufficiently to shift the reactor critical.
- Some of the neutrons produced react to fission lithium in the propellant, producing more heat.

Observations / questions:
- heating the propellant will reduce density, reducing neutron moderation effect.  Get the design right and the reactor will be largely self regulating.
- How much heat would be produced by the fission fuel producing the neutrons relative to heat from lithium fission?  The original post implicitly assumed a majority of heat being from the lithium fission, not the neutron source.

Yeah, you've got it about right. Note that there may be separate coolant/moderator channels so that the neutron moderation level can be adjusted independent of the lithium-salted fuel flow. Note also that the lithium saltwater may run around the reactor rather than through it, with a pure moderator/coolant channel in the center, so that the lithium doesn't poison the criticality. These are all things that will need to be specifically studied to get the ideal configuration.

Another thing that makes the whole arrangement self-regulating is that the moderator is being expelled as it is heated, so the higher the neutron flux rises, the faster the moderator will leave. A pump failure will thus automatically scram the reactor.

The majority of the energy will come from lithium fission. This is by necessity and by design. A reactor can be designed to produce a high level of neutron flux for very low levels of heat (at least compared to the typical reactor). You want the fissile neutron source to last as long as possible because it is hard to handle/refuel. The lithium saltwater propellant is very easy to refuel.

[gospacex]

You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.

[Crispy]

You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.

The idea is to induce fission in the propellant by bombarding it with neutrons from a secondary source.

[sevenperforce]

You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.

Hey, good questions!

Lithium-6 can't be used in nuclear reactors, nor as the fissile primary for a nuclear weapon, because its decay produces no neutrons. No neutrons means that it cannot participate in a chain reaction; rather, the presence of lithium-6 will rapidly terminate a chain reaction by sucking up all the loose neutrons. Thus, it is exactly the opposite of what you would want in a nuclear reactor for sustained power generation. Fissile material like enriched uranium or plutonium will automatically begin a chain reaction once enough accumulates in one place because their naturally ongoing radioactive decay produces the initial neutrons needed to start the chain reaction, but lithium-6 is completely stable and requires an external neutron source to fission at all.

Adding lithium to existing reactors to increase yield wouldn't work well either. Lithium-6 fission has a much higher energy yield than an equivalent mass of enriched uranium or plutonium, but those fuels are so much denser that they pack more energy into a smaller space, which is what you need for a reactor. Plus, it's not good to use a neutron poison as part of your reaction fuel; as it is used up, it will slowly increase criticality and could lead to a runaway chain reaction and meltdown. Adding lithium to try and increase the yield would require MORE fissile uranium or plutonium.

However, lithium's fissionable properties ARE used in weapons...in ever thermonuclear weapon after Ivy Mike, in fact. Fusion weapons are packed with lithium-6 deuteride so that the burst of x-rays from the smaller fission bomb will compress it, followed by the burst of neutrons which fissions the lithium. This extraordinarily high energy release in a very small area is enough to trigger fusion between the deuterium and the tritium produced in the decay of the lithium-6. Pretty powerful stuff.

For a rocket engine, you don't really want a chain reaction in your propellant. That's why lithium triggered with external neutrons would work so well.

[gospacex]

You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.

Hey, good questions!

...Plus, it's not good to use a neutron poison as part of your reaction fuel; as it is used up, it will slowly increase criticality and could lead to a runaway chain reaction and meltdown. Adding lithium to try and increase the yield would require MORE fissile uranium or plutonium.

Actually, this is not true that adding nuclear poisons to the fuel is a bad idea. For years, nuclear power industry is working towards making fuel campaigns longer, since each reload is a 20-30 day long reactor shutdown. 12 month power campaigns are generally replaced by 18-month ones now, and 24-month ones are in works.

To that end, more enriched fuels were necessary, and by now the 5% enriched fuel is standard. But reactors were not designed to use such reactive fuel, and nuclear poisons are deliberately added (I heard about Europium), to make fuel reactivity-versus-time curve flatter, and lower.

If Lithium is a nuclear poison which also exotermically fissions (unlike Europium), why is it not added to the fuel? Such fuel would be more energetic.

[gospacex]

I think I see the flaw with the proposal. To fission Lithium, you need a neutron source to generate *one neutron for every Lithium atom*:

Li-6 + n -> T + He-4 + 4.7829 MeV

Then you need to avoid thermalization of fission fragments and you need to collimate them into a directional beam. How far away form 100% efficient that will be?

You can save yourself a lot of trouble if you just use your neutron source as a rocket engine. Must be about the same ballpark wrt trust....

Which hints that the showstopper here is that it's hard to generate that many neutrons. What do you propose to be your neutron source? What's its luminosity?

[sevenperforce]

...it's not good to use a neutron poison as part of your reaction fuel; as it is used up, it will slowly increase criticality and could lead to a runaway chain reaction and meltdown. Adding lithium to try and increase the yield would require MORE fissile uranium or plutonium.

Actually, this is not true that adding nuclear poisons to the fuel is a bad idea. For years, nuclear power industry is working towards making fuel campaigns longer, since each reload is a 20-30 day long reactor shutdown. 12 month power campaigns are generally replaced by 18-month ones now, and 24-month ones are in works.

To that end, more enriched fuels were necessary, and by now the 5% enriched fuel is standard. But reactors were not designed to use such reactive fuel, and nuclear poisons are deliberately added (I heard about Europium), to make fuel reactivity-versus-time curve flatter, and lower.

If Lithium is a nuclear poison which also exotermically fissions (unlike Europium), why is it not added to the fuel? Such fuel would be more energetic.

Lithium-6 isn't a typical neutron poison. Europium can accept quite a few neutrons; thus, it doesn't really get "used up" right away. Lithium-6 is only a neutron poison in that it has a large neutron cross-section; once it is struck by a neutron, though, it instantly fissions. Adding lithium-6 to an active reactor would cause its criticality to increase as a function of time and thus lead to a meltdown.

Reactor design isn't about increasing energy as much as it is about controlling energy. You want to increase the stable power output period of your reactor, not add in sudden energy spikes followed by near-meltdown.

I think I see the flaw with the proposal. To fission Lithium, you need a neutron source to generate *one neutron for every Lithium atom*:

Li-6 + n -> T + He-4 + 4.7829 MeV

Then you need to avoid thermalization of fission fragments and you need to collimate them into a directional beam. How far away form 100% efficient that will be?

You can save yourself a lot of trouble if you just use your neutron source as a rocket engine. Must be about the same ballpark wrt trust....

Which hints that the showstopper here is that it's hard to generate that many neutrons. What do you propose to be your neutron source? What's its luminosity?

The two problems you cite—thermalizing fission products and collimating the fission beam—are in fact each other's solution. I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass.

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

And if you think that equivalent thrust could be produced by using a neutron source directly...no. Thrust is momentum exchange per unit time; momentum is mass times velocity. How do you imagine that a thermal neutron traveling at 2,200 m/s will produce greater thrust than that same neutron inside a helium atom traveling alongside a tritium atom (7 times more total mass) traveling at 4% of the speed of light?

Of course even that amount of thrust is dwarved by the amount of thrust produced when that 4.8 MeV is thermalize across ten or twenty or fifty times more mass in a heavy-water solution. The usual rule in rocket science is to reduce the reaction mass for a given amount of energy so that your specific impulse is as high as possible, but when we are dealing with these energy levels we are going to want to cram as much reaction mass through our engine as we possibly can.

As I have said above, the neutron since would need to be a high flux low heat nuclear reactor.

[Impaler]

This seems to be the kind of core you would want

https://en.wikipedia.org/wiki/High_Flux_Isotope_Reactor

As we speculated earlier a large jacket of neutron reflector (beryllium) surrounds the core which has a 5 inch across hollow center to allow samples to be exposed to the high thermal neutron flux which is 2.5 x 10^15 neutrons per cm^2/sec, given this value it should be possible to calculate the fission rate of lithium within the reactor based on the known cross section of lithium-6.

[sevenperforce]

This seems to be the kind of core you would want

https://en.wikipedia.org/wiki/High_Flux_Isotope_Reactor

As we speculated earlier a large jacket of neutron reflector (beryllium) surrounds the core which has a 5 inch across hollow center to allow samples to be exposed to the high thermal neutron flux which is 2.5 x 10^15 neutrons per cm^2/sec, given this value it should be possible to calculate the fission rate of lithium within the reactor based on the known cross section of lithium-6.

The HFIR core is 0.61 meters high. We don't know the flow rate of our lithium saltwater, but that's okay; we can use the size of the flux chamber to get a generalized upper bound on what kind of power output and thrust we would be able to get from a reactor of this type and size.

The inner surface area is 1217 square centimeters so the total internal neutron flux is 3e18 neutrons per second. This offers a potential maximum fission energy of 2.31 MJ per second, burning just 0.11 milligrams of lithium hydroxide, or about 1.1 milligrams of our saturated saltwater solution.

However, the HFIR uses a bunch of different reflectors and neutron poisons to try and make the neutron flux as constant with respect to location as possible. Using the wild guess that a maximum-flux version would have a total flux ten times greater, and assuming arbitrarily that we use a 1:1 ratio of saltwater fuel to freshwater coolant by mass, this comes to a power output of 23.1 MW and a mass flux of 22 milligrams per second. That is 33 N of thrust with a specific impulse of 1,449 km/s.

[Vultur]

Zubrin throws out a tank mass fraction of just 4% with no supporting evidence at all and which I find laughably small

However, the specific impulse is so good that it hardly matters.

At Isp 7000, even a mass ratio of 2 would have nearly 50 km/s delta-v, far more than any chemical rocket or NTR could have.

[Impaler]

The math looks to be a lot more complex then that sevenperforce, these links look like they provide the means to calculate it.  See if they help, I'll dive into them myself tomorrow and see if I can produce an independent calculation to see if we agree.



Lesson1

Lesson2

[sevenperforce]

The math looks to be a lot more complex then that sevenperforce, these links look like they provide the means to calculate it.  See if they help, I'll dive into them myself tomorrow and see if I can produce an independent calculation to see if we agree.

Yeah, the maths will be significantly more complicated than that. I was just doing a first order approximation to get an idea of what power levels the HFIR could generate.

If flux isn't high enough for a straight path through the reaction chamber, it would be possible to design the flow path with a helical arrangement of some other similar geometry to increase neutron exposure. If all else fails, it could be an augmentation to a traditional NTR to reduce shielding requirements and increase propellant energy above the melting point of the reactor core.

Or we could look into the possibility of using a D-T fusion primary with a deuterium ion accelerator, designed to induce fission in a radioactive secondary layer and thus exponentially multiply neutron count. This could allow very high neutron flux even if total power wasn't much more than the full reactor design.

[sevenperforce]

The math looks to be a lot more complex then that sevenperforce, these links look like they provide the means to calculate it.  See if they help, I'll dive into them myself tomorrow and see if I can produce an independent calculation to see if we agree.

It looks like the best fluxes attainable from continuous high flux reactors are on the order of 10¹⁶-10¹⁷ neutrons per square centimeter per second. Molten salt or pebble bed reactors might be able to get higher...perhaps as high as 10¹⁹ n/cm²*s, but that's pushing it.

Higher rates might be attainable using a synchroton proton beam directed at a mercury target to produce spallation; spallation can create pulses higher than 10¹⁷ n/cm²*s without too much trouble. With a depleted uranium, tantalum, or beryllium neutron multiplier, this could go very high. Attached an exemplary layout.

[Impaler]

That would look more like a pulse system which is a much more complex system that would require a massive capacitor and power source.  I'd suspect that a pellet system and a spherical combustion chamber similar to some of the fusion implosion setups.

[gospacex]

I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass.

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

If you thermalize neutrons and fission products, you get a NTR and/or NSWR. If you don't thermalize them, you get a fission fragment engine (which is a good engine... if you can overcome engineering/materials challenges).

[sevenperforce]

I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass.

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

If you thermalize neutrons and fission products, you get a NTR and/or NSWR. If you don't thermalize them, you get a fission fragment engine (which is a good engine... if you can overcome engineering/materials challenges).

It's a NSWR. Just a neutron-pumped one.

Still concerned about the low number of neutrons. Are there any fissionable isotopes which produce a chain reaction without nasty nucleotide products? Preferably it wouldn't even be radioactive on its own. If such an element could be found, then it could be mixed into the saltwater and used as an intermediary to fission the lithium.

[Impaler]

A neutron multiplier without a radioactive byproduct, if it exists it would certainly be desirable.  From what I can find Beryllium dose multiply neutrons by splitting into 2 alpha particles + 2 neutrons and by transmuting into carbon + 1 neutron upon absorbing alpha particles so it seems the High Flux reactor noted earlier is likely making significant use of such multiplication.

The only other elements I can identify that are good multipliers are Lead, Bismuth and at a much reduced rate Zirconium which has the advantage in a much higher melting point.  All of these can expel 2 or 3 neutrons when absorbing a thermal neutron WITHOUT fission though they do form radioactive isotopes like Polonium.  But all are considered inferior to Beryllium on Earth before mass has even become a concern as it would in an engine, only the toxicity of Beryllium and it's cost are an issue, and these are likely to be surmountable in a rocket engine.

In addition I'm reading that carbon is considered a good neutron reflector and it's low density would be desirable in keeping the engines power to weight ratio up.  I'm envisioning an engine with a conventional combustion chamber throat and funnel with one or more hollow central channel reactors injecting lithium salt into the combustion chamber.

[sevenperforce]

A neutron multiplier without a radioactive byproduct, if it exists it would certainly be desirable.  From what I can find Beryllium dose multiply neutrons by splitting into 2 alpha particles + 2 neutrons and by transmuting into carbon + 1 neutron upon absorbing alpha particles so it seems the High Flux reactor noted earlier is likely making significant use of such multiplication.

The only other elements I can identify that are good multipliers are Lead, Bismuth and at a much reduced rate Zirconium which has the advantage in a much higher melting point.  All of these can expel 2 or 3 neutrons when absorbing a thermal neutron WITHOUT fission though they do form radioactive isotopes like Polonium.  But all are considered inferior to Beryllium on Earth before mass has even become a concern as it would in an engine, only the toxicity of Beryllium and it's cost are an issue, and these are likely to be surmountable in a rocket engine.

In addition I'm reading that carbon is considered a good neutron reflector and it's low density would be desirable in keeping the engines power to weight ratio up.  I'm envisioning an engine with a conventional combustion chamber throat and funnel with one or more hollow central channel reactors injecting lithium salt into the combustion chamber.

https://www-nds.iaea.org/sgnucdat/a6.htmHere's a nice table of neutron yield for various isotopes...of course everything here is wildly radioactive.

If there's no way to get a high enough external neutron count, it still might be worth it to boost internal neutron count using fissile isotopes mixed with the lithium. It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium. This would make it hopeless for in-atmo use but could still be promising as a transfer engine. I guess that would make it a subcritical lithium-boosted NSWR? Wonder how much lithium-6 you could add before it would stop working, and what kind of specific impulse you could get. Could be feasible for a Mars Transfer Tug.

Another option with a pulsed accelerator design would be to use lithium-6 deuteride as the fuel, then chemically separate the two ions and smash them together in an accelerator, fusing into ⁸Be which decays to 2 Helium-4 atoms with 22 MeV of energy. That's even cleaner than lithium fission (and releases no harmful radiation whatsoever), but I'm not sure how much energy it would take to get them to fuse, nor whether high enough particle fluxes could be attained.

[gospacex]

Are there any fissionable isotopes which produce a chain reaction without nasty nucleotide products?

There are not. Otherwise military would be all over it: ideal bomb material!

[gospacex]

It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.

You are still fixated on lithium.

What "high energy of lithium"? It gives you 4.78 MeV from one Z=6 nucleus fissioning, a bit below 0.8 MeV per nucleon.
Fission of usual suspects U/Np/Pu gives ~200 MeV per Z~=235 nucleus, which is about the same power density, actually a bit better: 0.86 MeV per nucleon.

[Hanelyp]

It's getting away from the "clean" exhaust, but the reactor chamber under discussion seems promising with a fissile salt at a concentration which would not go critical in a simple bulk tank.

[sevenperforce]

It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.

You are still fixated on lithium.

What "high energy of lithium"? It gives you 4.78 MeV from one Z=6 nucleus fissioning, a bit below 0.8 MeV per nucleon.
Fission of usual suspects U/Np/Pu gives ~200 MeV per Z~=235 nucleus, which is about the same power density, actually a bit better: 0.86 MeV per nucleon.

Hmm, I was under the impression that lithium-6 had a markedly higher specific energy. Where did that come from?

Ah, here. I misread something -- they were factoring in fusion yields. No wonder I was overestimating.

Anyway, it would still be useful, as it would allow a subcritical fuel to still have roughly the same specific energy.

⁶Li + ²H fusion-fission would yield 2.75 MeV per nucleon, which would be much nicer, if it could be done. That's one of the major reactions in brown dwarf stars and requires significantly lower energies than hydrogen-burning processes...a bit lower than 3e6 K. This corresponds to a deuteron energy of around 300 eV, which is quite low. I'm not sure how large of a linac you'd need to get that, nor what kind of flux could be possible.

Is deuteron decay a thing? Alpha decay is, of course, but I'm wondering whether there are any decay reactions or fissions which knock a bound proton-neutron pair off of an existing molecule. If so, this could prove quite useful.

[mlorrey]

You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.

Uranium has a high atomic mass, therefore it is going to have much lower Isp as a propellant. This design uses low  enriched uranium as a neutron source to fission the lithium. Lithium IS used in nuclear weapons, lithium deuteride and tritium is bombarded with neutrons from a supercritical uranium or plutonium  fission reaction to fuse the deuterium and tritium, and lithium fission happens in there as well.  The only real question in this rocket design is whether you'll get deuterium and tritium fusion happening in the middle of this whole firestorm and I would  say probably not, since a thermonuclear weapon relies on the pressures of the implosion to drive the  deuterium and tritium to solar levels of pressure and temperature along with the neutron bombardment, to trigger fusion. This sort of thruster MIGHT be used in clusters to focus their exhaust streams at each other like particle accelerators to create fusion between exhaust streams, but that would take  a whole lotta calculations and work I'm obviously not gonna do here.
Anyways, yeah you are not gonna have a constricting neck on this engine. I'd have, say, three or six of these as cylindrical thrusters in an equilateral orientation pointed toward the focus of a parabolic nozzle. Any chance at fusion is  going to happen at the focus of the parabola, exhaust that doesn't fuse will reach the wall at the far side and be turned aft as it expands and cools like a normal bell nozzle should. You can introduce additional mass (LH2, water, or, potentially, ram-compressed and cooled air) axially into this nozzle to cool the exhaust and produce greater thrust for lower Isp.

[mlorrey]

I think I see the flaw with the proposal. To fission Lithium, you need a neutron source to generate *one neutron for every Lithium atom*:

Li-6 + n -> T + He-4 + 4.7829 MeV

Then you need to avoid thermalization of fission fragments and you need to collimate them into a directional beam. How far away form 100% efficient that will be?

You can save yourself a lot of trouble if you just use your neutron source as a rocket engine. Must be about the same ballpark wrt trust....

Which hints that the showstopper here is that it's hard to generate that many neutrons. What do you propose to be your neutron source? What's its luminosity?

The problem is you can't use neutrons as a rocket engine propellant, because neutrons simply do not like to be shaped into an exhaust. They either go through things in all directions, or are absorbed by those things, heating them up and making them radioactive. Neutrons don't flow like a rocket propellant. Even a beryllium "neutron mirror" has limited reflecting abilities that are eliminated by the meltdown of the neutron source.
In this case the neutron source is subcritical, uranium low enriched fuel rods arranged in a cylinder embedded in the nozzle "combustion chamber" producing a focus of neutrons along  the axis in the center of the chamber, through which your LiOD / D2O fluid passes, absorbs neutrons, and has its own fission reaction, the results of which are largely non-neutron emitting, ionized particles you can shape as you wish with a nice magnetic nozzle bell for as big as you wish in a vacuum to maximize vacuum Isp.
As for the environmental impact of releasing tritium in the environment, the risks of tritium are largely overblown.
The quantity of tritium present in the biosphere considerably increased due to thermonuclear weapons testing in the atmosphere and with the development of the nuclear power industry. At global scale, a return to concentration levels similar to those that preceded weapons testing is underway. Thanks to radioactive decay, almost 90% of the tritium introduced into the environment between 1945 and the late 1960s has disappeared. Tritium has a half life of around 12.32 years. The nuclear  industry releases  tritium into the environment regularly. A single plant releases between 8-30 grams of the stuff each year, usually into the ocean. Inland reactors do the  same into rivers. Dilution is the point: tritium as a pure substance ingested can mess you up by slowing metabolic reactions, but diluted, as 0.000001% concentration, poses no health risks. You get more radioactive exposure by eating a banana or a cup of brazil nuts.
Tritium is naturally  produced in the upper atmosphere by impact of atmospheric nitrogen and oxygen with  incoming cosmic rays.
Current tritium levels in rainwater are only slightly above pre-atomic natural levels (natural levels are  less than 0.6 Bc/L of  water, during the age of nuclear testing, levels reached as high as 470 Bc/L of rainwater. Current levels are 1-4 Bc/L. Pure tritium is about 3.5x10^14 Bequerels of radiation per gram of T. Dilution of launch  exhaust products thus obviously mean mixing with quite a lot of water if done locally, but given the launch vehicle spreads its exhaust across thousands of miles of atmosphere, and which  oxidizes with atmospheric oxygen into T20, the launch self-dilutes the tritium in the exhaust into the environment where, given the volume of water in the environment, there is roughly 12,600,000 liters of water in the world for each Bc of tritium exhaust or rather, you produce approximately 0.00000126 Bc per liter of environmental water for each gram of tritium released into the environment, thus, each launch, producing 30G of tritium emissions, will raise global tritium radiation levels by 0.000037~ Bc. You would need thus 100,000  launches of this sort of rocket to raise global tritium radiation by 1 Bc/liter of rainwater. To reach the levels we saw at the height of atmospheric testing of thermoneuclear weapons, of 470 Bc per liter, we'd need 4,700,000 launches of this rocket, which would be a problem, I imagine, if these were rolling off the assembly line like automobiles, but I sorta doubt we're ever gonna reach that level of popularity.

[edzieba]

As well as the coated-nozzle layout and the liquid flow-through layout, one could also produce an 'augmented Fission sail' layout: Coat the inner surface of a large sphere-section sail in Lithium-6 Hydride, then mount the Neutron source at centre of radius of the sail (ideally at the nose of your craft also mounted there) 'hanging' from the sail by tethers. Activate the source, and the neutrons impact the inside of the sail producing fast reaction products that propel the sail forwards. The downside is the front your craft is dusted with Tritium, but the front already needs protection from the Neutron source anyway so that shielding can pull double-duty.

Another much older concept using Neutron-riggered Lithium fission is the far more exiting Fizzer.

[Cererean]

Neutron Induced Lithium Fission Reaction

The showstopper is, of course, the need to produce neutrons.

If you have a cheap source of neutrons, you could also consider Calcium-48. If turned into Calcium-49, I think the decay chain Calcium -> Scandium -> Titanium gives a total of 12 MeV, via beta minus decay (I looked it up years ago, this is from memory). Not so good for an engine, but good as a powerplant.

[mlorrey]

Neutron Induced Lithium Fission Reaction

The showstopper is, of course, the need to produce neutrons.

If you have a cheap source of neutrons, you could also consider Calcium-48. If turned into Calcium-49, I think the decay chain Calcium -> Scandium -> Titanium gives a total of 12 MeV, via beta minus decay (I looked it up years ago, this is from memory). Not so good for an engine, but good as a powerplant.

the OP suggested unrefined natural Uranium as a sufficient neutron source. I'd suggest going to low or mid-enriched uranium to improve T/W and operating  it  at high-sub-critical. I'm working up a possible 3d model using 12 such thrusters, each 15 cm dia, with 8 mid-enriched uranium rods surrounding the Lithium Deuteroxide injection chamber, all thrusters arranged radially, injecting into a 2 m dia parabolic nozzle chamber that has its own tritium injector in the center axis that the 12 lithium deuteroxide fission plasma streams impact against from all sides.

[mlorrey]

Ok so here's conceptually how this could work for both atmospheric and vacuum operation,with vacuum magnetic containment nozzle of the ionized fission (and hopefully fusion) products extending outward  upon reaching vacuum, looking sorta like the engines on a Y Wing in star wars, or the A Wing.

[mlorrey]

The problem of adequate neutron supply would be solved if you treated this LiDO fission reaction as an injector/accelerator to impact a stream of tritium flowing in the axis of a central core chamber to trigger DT fusion there. The fusion reaction would supply the neutrons needed to boost and maintain the Li6 fission reaction long term. You can then inject LH2 along the skin  of the chamber for cooling, to absorb more neurons and thermal energy, to boost thrust and even have LOX/Air injection in the exhaust nozzle to boost thrust even higher.

[Pete]

I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

[Bubbacub]

Hi,
Really interesting idea.
To my knowledge you would need fast neutrons to get lithium to undergo fission - unfortunately using heavy water as a moderator with u238 will create a neutron flux of thermal neutrons which won't be helpful in creating lithium fission.
Perhaps you could rejig your concept on the lines of a fast neutron reactor by using a cylinder of plutonium or highly enriched uranium to create a high energy neutron flux that would be able to induce fission in your lithium. I've attached a PDF which characterises the neutron flux of plutonium dioxide as used in a rtg. My knowledge of this subject isn't enough to do the maths but perhaps someone cleverer could tell us if plutonium dioxide, for example, as used in the paper could create enough of a neutron flux to power our rocket engine?

[Twark_Main]

I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#Limitations

It's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.

[Lemurion]

I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#Limitations

It's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.

It can be argued that even Orion is cleaner than NSWR. Talk about the very definition of a low bar.

[Michel Van]

i found this NASA paper, hope this helpful

FISSION NEUTRON ATTENUATION IN LITHIUM-6, NATURAL LITHIUM HYDRIDE AND TUNGSTEN

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680020529.pdf

[sanman]

Could you use this to augment thrust of a chemical rocket?

[Michel Van]

Could you use this to augment thrust of a chemical rocket?

Like in HydroLox engine ? No !

Even i you manage to mix Lithium-6 into liquid hydrogen
you need neutron source to trigger it and you get a engine that's heavy and i mean to heavy for any advantages.
and from proposals to use nuclear reactor to heat Chemical fuel
or use as Afterburner on Chemical engines failed like wise to heavy


so Lithium-6 in D20 is here best way for high thrust and exhaust speed.   
 

[sanman]

I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#Limitations

It's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.

How about some other neutron-rich isotope, besides tritium? Tritium is less stable, and thus less "clean". But there are other neutron-rich isotopes which are more stable, even while having a larger nuclear cross-section, thus allowing more chance of collision to liberate more neutrons.

Do we take the definition of "clean" to mean less chemical toxicity, as well as less radioactive toxicity?
You'd want a stable isotope with a rapid decay chain, while not being overly reactive or a heavy metal.

What are the candidates meeting that definition?

[tea monster]

I came up with a concept for the vehicle described in the Atomic Rockets post by William Mook.
I was shocked at how big this thing was that could (theoretically) get several people to Jupiter.
He described it with an Orion-style pusher plate (without the shock absorbers).

[tea monster]

I worked up a slightly larger ship design. The glowwy red bits are radiators. I'm not sure if this design would require them, or if the heat would be carried away by the exhaust. There is an aeroshell form to the hull. At the apex of the vehicle is debris armour. Landing legs are built into the fins, though there is some debate as to whether you could safely land this engine anywhere without polluting the pad.

I modelled a version of the engine design that mlorrey came up with.

With this amount of delta V with this little vehicle mass, you could go 'full Sci-Fi' with your ship design. You could even bring a variant of the 1930's Flash Gordon space bus to life!

[Pete]

A layer of lithium-6 hydride deposited onto a beryllium bell nozzle and irradiated ...

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

You really need to make it more clear that you are talking about two **COMPLETELY DIFFERENT** systems here.
May I humbly suggest you start a new thread about this alternate system, rather than mutating your existing thread and confusing the poor readers?

[vasimv]

Just wondering, why not use pulsed mode for this? Like in https://en.wikipedia.org/wiki/Pulsed_nuclear_thermal_rocket but with solid lithium hydroxide (sheets/powder/wire/etc) or a solution in water injected instead hydrogen. This will reduce thrust a lot but will increase ISP and solve many problems - cooling reactor (neutron source) will be easier, much less meltdown chances. Even with only tens of newton thrust but high ISP this engine will be very useful for cargo transportation and such.