Clean lithium fission rocket

[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.