Storable propellants for nuclear engine?

[KSK]

1 hour ago, YumonStudios said:

You can do that. It's called an Ion drive.

Also, @KSK anything on those alternate NTR propellants?

Boron based propellants? Lets take a look.

This article is a quick overview of so-called 'zip fuels' based on alkylboranes and explains why they never really took off. Toxicity, oxygen sensitivity and the tendency to coat the inside of the engine with boron carbide, made them high powered but impracticable fuels.

Ignition! by John D Clarke has a chapter on boron rocket fuels. Incidentally, if you like your humour extra-dry, I highly recommend Ignition!, either as a reference work or a comedy. On boranes:

"The results, not to put too fine a point on it, did not encourage euphoria. The performance was dismally bad —far below theoretical — and solid glassy deposits appeared in the throat (changing its size and shape) and in the diverging (downstream) section of the nozzle. These consisted, apparently, mostly of B2O3, but appeared to contain some elemental boron as well. This was a sure indication of poor combustion, and was not encouraging."

There's more but in general it seems that boron based fuels and chemical rockets don't really go together. Nuclear-thermal rockets though?

The boranes have relatively low (certainly compared to expected operating temperatures for an NTR) boiling points of anywhere between -93 degrees Celsius to 200ish degrees Celsius. So flowing them through the engine isn't a problem. However at the temperatures involved they'll decompose pretty rapidly to boron and assorted other gunk. Boron has a melting point of 2349K (2076 degrees Celsius) and a boiling point of 4200K (3926 degrees Celsius). A solid-core NTR isn't going to get hot enough to vaporise boron. A gas-core design might work but at the moment they're largely theoretical.

I couldn't find any information on how elemental boron might react with other materials.

Assuming that it either doesnt react with the inside of your engine, or you can find a suitable coating to mitigate that, my best guess is still that boron compounds probably wouldn't be that great in a solid-core NTR. If the operating temperature of your engine was over 2076 Celsius, any decomposed boron in your propellant stream would be liquid (likely fairly viscous liquid) and you'd be back to coating problems, especially on your engine nozzle. Running the engine below 2076 Celsius would avoid that but then (depending on the decomposition kinetics) you'd end up with a stream of gas (say hydrogen if you were using neat boranes as propellant) and boron particulates. Those particulates would be relatively heavy and would probably give you a worse ISP than you'd get from hydrogen alone.

However, that is all largely guesswork. If anyone else would care to chip in with a more informed answer or better source then please feel free!

 

 

 

Edited April 10, 2016 by KSK

[fredinno]

On 4/10/2016 at 1:08 AM, KSK said:

Boron based propellants? Lets take a look.

This article is a quick overview of so-called 'zip fuels' based on alkylboranes and explains why they never really took off. Toxicity, oxygen sensitivity and the tendency to coat the inside of the engine with boron carbide, made them high powered but impracticable fuels.

Ignition! by John D Clarke has a chapter on boron rocket fuels. Incidentally, if you like your humour extra-dry, I highly recommend Ignition!, either as a reference work or a comedy. On boranes:

"The results, not to put too fine a point on it, did not encourage euphoria. The performance was dismally bad —far below theoretical — and solid glassy deposits appeared in the throat (changing its size and shape) and in the diverging (downstream) section of the nozzle. These consisted, apparently, mostly of B2O3, but appeared to contain some elemental boron as well. This was a sure indication of poor combustion, and was not encouraging."

There's more but in general it seems that boron based fuels and chemical rockets don't really go together. Nuclear-thermal rockets though?

The boranes have relatively low (certainly compared to expected operating temperatures for an NTR) boiling points of anywhere between -93 degrees Celsius to 200ish degrees Celsius. So flowing them through the engine isn't a problem. However at the temperatures involved they'll decompose pretty rapidly to boron and assorted other gunk. Boron has a melting point of 2349K (2076 degrees Celsius) and a boiling point of 4200K (3926 degrees Celsius). A solid-core NTR isn't going to get hot enough to vaporise boron. A gas-core design might work but at the moment they're largely theoretical.

I couldn't find any information on how elemental boron might react with other materials.

Assuming that it either doesnt react with the inside of your engine, or you can find a suitable coating to mitigate that, my best guess is still that boron compounds probably wouldn't be that great in a solid-core NTR. If the operating temperature of your engine was over 2076 Celsius, any decomposed boron in your propellant stream would be liquid (likely fairly viscous liquid) and you'd be back to coating problems, especially on your engine nozzle. Running the engine below 2076 Celsius would avoid that but then (depending on the decomposition kinetics) you'd end up with a stream of gas (say hydrogen if you were using neat boranes as propellant) and boron particulates. Those particulates would be relatively heavy and would probably give you a worse ISP than you'd get from hydrogen alone.

However, that is all largely guesswork. If anyone else would care to chip in with a more informed answer or better source then please feel free!

 

 

 

Sorry, one last question. It involves Beryllium in a NTR.

 

"Beryllium Hydride is another decent idea. It needs to be transported in a granulated powder, as it decomposes at its melting point of 250C, however, the hydrogen and Beryllium is very light, and storable. Pure Beryllium is toxic, but it doesn't seem to be toxic in its hydrogen compound.

Unfortunately, Beryllium only boils at 2469C, meaning you might have to deal with droplets of Beryllium on the walls of the nozzle. No idea if this is enough of a problem, or if Beryllium will be hot enough to boil regardless in an NTR."

[todofwar]

I believe these lighter metallic elements have issues of diffusing into other metals (I know this is a big problem in some battery designs) so Be and Li will probably give almost as many issues as hydrogen as far as storage is concerned. And anything that produces chemicals that will be liquid are probably a no go. If you take BH3 for example, it has a mass of 13.8 with 10.8 of that coming from B. So, if the B is a liquid it's not contributing to your thrust, and though hydrogen has a great ISP only 21% of your fuel is actually getting used. So all you've done is found a very heavy way to store hydrogen.

Chemically storing hydrogen is actually an active area of research for fuel cells, but no matter what you do your mass ratio will always be terrible. That's forgivable in ground based applications but less so in rockets.

Edit: Even as a solid you get some diffusion, but the real problem will be liquid metals forming amalgams and weakening components, so not so much a storage issue as an operational one. 

Edited April 18, 2016 by todofwar

[FreeThinker]

11 hours ago, fredinno said:

Sorry, one last question. It involves Beryllium in a NTR.

 

"Beryllium Hydride is another decent idea. It needs to be transported in a granulated powder, as it decomposes at its melting point of 250C, however, the hydrogen and Beryllium is very light, and storable. Pure Beryllium is toxic, but it doesn't seem to be toxic in its hydrogen compound.

Unfortunately, Beryllium only boils at 2469C, meaning you might have to deal with droplets of Beryllium on the walls of the nozzle. No idea if this is enough of a problem, or if Beryllium will be hot enough to boil regardless in an NTR."

Well I could limit BDiborane and Berelium Hydrate to (open/Closed) Gas core reactors only, their temperatures are high enough to prevent the so called droplets. It think it the Berylium Hydrate ability to capture neutron makes it also effective in seeding hydrogen in high neutron reactors

Edited April 18, 2016 by FreeThinker

[FreeThinker]

On 8-4-2016 at 7:18 AM, fredinno said:

How about the Diborane suggestion the other guy came up with (or Boron in general- despite the insane heats it melts at, it looks fairly decent at 10 mol- I know Beryllium won't work due to high toxicity and Helium is worse than H2 due to being heavier and more cryogenic)

I have a feeling the heating requirements for Boron makes it unviable.

Or how about https://en.wikipedia.org/wiki/Lithium_hydride

It decomposes at 900C, is that too low for use in an NTR?

It decomposes into Ammonia, which is lighter, while water does not decompose.

Lithium Hydrate would indeed be very intresting in a Solid Core NTR, as it would completely decompose and turn into a Gas at 2500K, which is exactly what we want

THe 3 big advantages would be an high effective Isp of 65% of Hydrogen and a density of 78% of liquid water and no cryo geneic storage needed!

Yes, it is poison , but Hydrazine is worse. Only disadavantage I can think of is that it's endothermic and quite expansive when used in large quantities.

A possible bonus is that the stuff can actualy be used to for shielding agains nuclear radiation, which is a plus in a NTR propelled vessel

Also in a high neutron environment, the Lithium would actualy turn into Tritium

Edited April 18, 2016 by FreeThinker

[sevenperforce]

58 minutes ago, FreeThinker said:

Also in a high neutron environment, the Lithium would actualy turn into Tritium

Lithium-7 has a really low fast neutron cross-section and needs ultra-fast D-T fusion neutrons to actually fission into tritium and helium. Not enough neutrons to do it, even in an NTR.

[KSK]

18 hours ago, fredinno said:

Sorry, one last question. It involves Beryllium in a NTR.

 

"Beryllium Hydride is another decent idea. It needs to be transported in a granulated powder, as it decomposes at its melting point of 250C, however, the hydrogen and Beryllium is very light, and storable. Pure Beryllium is toxic, but it doesn't seem to be toxic in its hydrogen compound.

Unfortunately, Beryllium only boils at 2469C, meaning you might have to deal with droplets of Beryllium on the walls of the nozzle. No idea if this is enough of a problem, or if Beryllium will be hot enough to boil regardless in an NTR."

No worries.

OK, according to Project Rho, a solid core NTR operates at 2750K or 2476 degrees Celsius. The equivalent Wikipedia article suggests up to 3000K. Either way, that's plenty hot enough to boil beryllium and we can probably assume it's also plenty hot enough to decompose beryllium hydride to beryllium and hydrogen.

So the question once again becomes one of materials. What will beryllium vapour do to the inside of your engine? 

The answer is - I'm not sure and couldn't find much on the internet. Beryllium alloys are prized for their strength, light weight and thermal properties, so beryllium infiltration into the engine walls might actually improve them. On the other hand, if that causes a phase change (the new beryllium alloy having a different crystal structure to the underlying metal), then that would probably be bad and lead to spallation and erosion of the engine surfaces. On the other hand, beryllium itself might not make a bad engine material. Beryllium has also been selected as the blanket (plasma facing) material for ITER. So perhaps an actively cooled beryllium lining for your NTR would be the way forward?

The main problem with using beryllium compounds as propellant is that global production of the stuff is only a few hundred tonnes per annum and it's useful in so many applications that using it for rocketry seems unlikely.

Edited April 18, 2016 by KSK

[fredinno]

11 hours ago, todofwar said:

I believe these lighter metallic elements have issues of diffusing into other metals (I know this is a big problem in some battery designs) so Be and Li will probably give almost as many issues as hydrogen as far as storage is concerned. And anything that produces chemicals that will be liquid are probably a no go. If you take BH3 for example, it has a mass of 13.8 with 10.8 of that coming from B. So, if the B is a liquid it's not contributing to your thrust, and though hydrogen has a great ISP only 21% of your fuel is actually getting used. So all you've done is found a very heavy way to store hydrogen.

Chemically storing hydrogen is actually an active area of research for fuel cells, but no matter what you do your mass ratio will always be terrible. That's forgivable in ground based applications but less so in rockets.

Edit: Even as a solid you get some diffusion, but the real problem will be liquid metals forming amalgams and weakening components, so not so much a storage issue as an operational one. 

Lithum, Boron, and Beryllium can be stored in tanks much more densely, and are storable. We're bascially trying the highest ISP fuel that is storable. And solids probably don't seep into metals (I think?)

[todofwar]

11 minutes ago, fredinno said:

Lithum, Boron, and Beryllium can be stored in tanks much more densely, and are storable. We're bascially trying the highest ISP fuel that is storable. And solids probably don't seep into metals (I think?)

You'd be surprised. Copper diffusion into silicon is a problem in circuits below a certain size. Not saying it can't be done, but it is something to keep in mind. And again, you need to get these things hot and keep them hot until they leave your rocket. I'm not terribly familiar with NTR designs, how much does you fuel cool down before it gets expelled? And solids have the issue of not being mobile. So you need to liquefy them, and that means heating your whole tank. Not saying it's impossible, but there is ease of storage, ease of use, and efficiency. Probably nothing will max out all three, hence were having this debate

[fredinno]

1 hour ago, KSK said:

The answer is - I'm not sure and couldn't find much on the internet. Beryllium alloys are prized for their strength, light weight and thermal properties, so beryllium infiltration into the engine walls might actually improve them. On the other hand, if that causes a phase change (the new beryllium alloy having a different crystal structure to the underlying metal), then that would probably be bad and lead to spallation and erosion of the engine surfaces. On the other hand, beryllium itself might not make a bad engine material. Beryllium has also been selected as the blanket (plasma facing) material for ITER. So perhaps an actively cooled beryllium lining for your NTR would be the way forward?

It's bad to use in a rocket nozzle due to toxicity. It's actually a goal to remove as much as it as possible, like for abestos (also due to cost). That's why I wanted Beryllium Hydride.

A thin layer of Tugsten might be fine, with a 3,400C melting point, but is heavy.

2 minutes ago, todofwar said:

20 minutes ago, fredinno said:

 

You'd be surprised. Copper diffusion into silicon is a problem in circuits below a certain size. Not saying it can't be done, but it is something to keep in mind. And again, you need to get these things hot and keep them hot until they leave your rocket. I'm not terribly familiar with NTR designs, how much does you fuel cool down before it gets expelled? And solids have the issue of not being mobile. So you need to liquefy them, and that means heating your whole tank. Not saying it's impossible, but there is ease of storage, ease of use, and efficiency. Probably nothing will max out all three, hence were having this debate

You can heat the tank through the nuclear reactor. Beryllium hydride melts at only 250C, making it a great idea to put in an NTR... If it wasn't so expensive...https://en.wikipedia.org/wiki/Beryllium_hydride

[FreeThinker]

Beryllium hydride only makes sence in Gas Core Reactor or in Electricly Heated Engines like Vasmir, where it is heated to a few million degrees while confined in a magnetic field.  For Solid Core NTR, Ammonia would be a better choice, it has about the same Isp but more dense and thanks to it's decomposition, boosts both Isp and effective thrusts and can be made ISRU. Also if you got a leak or explosion, it won't kill your astronauds. and is dirt cheap and very easy to store.

[fredinno]

9 hours ago, FreeThinker said:

Beryllium hydride only makes sence in Gas Core Reactor or in Electricly Heated Engines like Vasmir, where it is heated to a few million degrees while confined in a magnetic field.  For Solid Core NTR, Ammonia would be a better choice, it has about the same Isp but more dense and thanks to it's decomposition, boosts both Isp and effective thrusts and can be made ISRU. Also if you got a leak or explosion, it won't kill your astronauds. and is dirt cheap and very easy to store.

But the above posts just showed a nuclear engine can vaporize Beryllium....

The problem is cost, not viability. If only Lithium worked....

[FreeThinker]

On 19-4-2016 at 4:57 PM, fredinno said:

But the above posts just showed a nuclear engine can vaporize Beryllium....

The problem is cost, not viability. If only Lithium worked....

The problem with lithium as a propellant in a NUclear Engine is that it poisons the neutronisicy of the reactor

https://www.researchgate.net/publication/236371473_THE_NUCLEAR_EFFECT_OF_USING_LITHIUM_HYDRIDE_AS_THE_PROPELLANT_IN_A_NUCLEAR_ROCKET_REACTOR_thesis

Only Lithium 7 has a change of working in a NTR as any high energy neutron obsorbed , will generate another neutron

Edited April 22, 2016 by FreeThinker

[sevenperforce]

13 minutes ago, FreeThinker said:

The problem with lithium as a propellant in a NUclear Engine is that it poisons the neutronisicy of the reactor

https://www.researchgate.net/publication/236371473_THE_NUCLEAR_EFFECT_OF_USING_LITHIUM_HYDRIDE_AS_THE_PROPELLANT_IN_A_NUCLEAR_ROCKET_REACTOR_thesis

Only Lithium 7 has a change of working in a NTR as any high energy neutron obsorbed , will generate another neutron

Lithium-7 has a vanishingly low probability of absorbing the high-energy neutrons in a nuclear reactor. It needs ultra-high-energy fusion neutrons for the absorption to have any reasonable probability.

[FreeThinker]

1 minute ago, sevenperforce said:

Lithium-7 has a vanishingly low probability of absorbing the high-energy neutrons in a nuclear reactor. It needs ultra-high-energy fusion neutrons for the absorption to have any reasonable probability.

Yes, but that's exactly wat we want in a NTR, no neutron poisoining, which does happen with Lithium 6 which absorbes our precious thermal neutrons, causing the reactor to get cold = not good

Edited April 22, 2016 by FreeThinker

[todofwar]

What about hydroxylamine? Solid, stable enough, happy to decompose at high temps. 

[passinglurker]

Out of curiosity how would aerozine50 (a hypergolic that ksp "liquid fuel" is often compared against) fair in a NTR?

[Thomassino]

CH4 as fuel is potentially very abundant, can be made pretty easily using ISRU, but it has an issue - nozzle carbon clogging. It would be only a minor issue in expendable ships, but in reusable, Hermes-like ships, it would be a serious issue.

Also, you can't use anything containing oxygen. Some compounds decompose during reactor exposure and oxygen at those temperatures can oxidise insides of the engine, potentially causing leaks, RUDs even.

 

[todofwar]

I really think the two best options are water and hydrogen. These Be and other light element ideas sound great in theory, but they suck to work with in practice cause of cost, reactivity towards your components, etc. Hydrogen has the best isp (helium is good but don't you dare waste a vanishing resource like that unless it's to get us more helium from somewhere) but it has storage problems. Water is abundant, light in terms of molecular weight but dense so easier to store. And it's not likely to decompose. 

[Thomassino]

6 hours ago, todofwar said:

I really think the two best options are water and hydrogen. These Be and other light element ideas sound great in theory, but they suck to work with in practice cause of cost, reactivity towards your components, etc. Hydrogen has the best isp (helium is good but don't you dare waste a vanishing resource like that unless it's to get us more helium from somewhere) but it has storage problems. Water is abundant, light in terms of molecular weight but dense so easier to store. And it's not likely to decompose. 

Water is actually pretty terrible propellant. Its ISP is roughly half that of H2 with a slightly bigger thrust. In solid-core NERVAs, this gives you ISP of around 400s. Together with its high density and relative mass it is better to use chemical bipropellant engines.

And water decomposes at temperatures upwards from 2200°C, so you get quite a lot of molecular and atomic oxygen in the nozzle, which is not good.

Another thing is using it as propellant in hypothetical gas-core nuclear engines, where core temperature reaches 30000°C. You can get ISP of 1500s even with water. If you find a way to get around nozzle oxidation, (and of course other engineering difficuilties )you have a really powerful and useful engine.

Edited April 24, 2016 by Thomassino
typo

[todofwar]

1 hour ago, Thomassino said:

Water is actually pretty terrible propellant. Its ISP is roughly half that of H2 with a slightly bigger thrust. In gas-core NERVAs, this gives you ISP of around 400s. Together with its high density and relative mass it is better to use chemical bipropellant engines.

And water decomposes at temperatures upwards from 2200°C, so you get quite a lot of molecular and atomic oxygen in the nozzle, which is not good.

Another thing is using it as propellant in hypothetical gas-core nuclear engines, where core temperature reaches 30000°C. You can get ISP of 1500s even with water. If you find a way to get around nozzle oxidation, (and of course other engineering difficuilties )you have a really powerful and useful engine.

But if ISP tracks with molecular weight, you're not likely to do much better. Not allot of light compounds to choose from. Methane is lighter, but I would imagine it breaks down worse than water, and at least all your water break down products are gasses. Maybe supplement your stream with some hydrogen, so that the oxygen is more likely to go after it than your engines. From what I understand people have been talking about storage, and water is pretty unusual in its stability, high boiling point, and density for such a small moletcule. Really, I can't imagine a better small molecule if long term storage is your concern. 

[Thomassino]

18 minutes ago, todofwar said:

But if ISP tracks with molecular weight, you're not likely to do much better. Not allot of light compounds to choose from. Methane is lighter, but I would imagine it breaks down worse than water, and at least all your water break down products are gasses. Maybe supplement your stream with some hydrogen, so that the oxygen is more likely to go after it than your engines. From what I understand people have been talking about storage, and water is pretty unusual in its stability, high boiling point, and density for such a small moletcule. Really, I can't imagine a better small molecule if long term storage is your concern. 

Yeah, storage wise, it can really be the best molecule to choose. Only problem is that it is quite heavy.

IMO Ammonia is the "sweet spot" for NTR engines. You can get ISP of about 650s (in solid core NTRs), it stays liquid in temperature range from -77°C to -33°C and it has density of 681.9 kg/m^3. It can even be prepared using ISRU from nitrogen and hydrogen using iron as a catalyst (Haber process).

[todofwar]

Just now, Thomassino said:

Yeah, storage wise, it can really be the best molecule to choose. Only problem is that it is quite heavy.

IMO Ammonia is the "sweet spot" for NTR engines. You can get ISP of about 650s (in solid core NTRs), it stays liquid in temperature range from -77°C to -33°C and it has density of 681.9 kg/m^3. It can even be prepared using ISRU from nitrogen and hydrogen using iron as a catalyst (Haber process).

But ammonia is terribly corrosive. I'm surprised at that jump in ISP, then again water is weird for so many reasons. You will still need to keep it cold, but that is less of a problem in the vacuum of space. 

As for the weight of water, that is actually a benefit because it means your tanks can be smaller for the same amount of propellant mass. 

[kunok]

5 hours ago, Thomassino said:

In gas-core NERVAs, this gives you ISP of around 400s

That's an errata no? That's the isp for a solid core

 

2 hours ago, todofwar said:

But ammonia is terribly corrosive

Not that much, most of the propellants are corrosive, starting with the liquid oxygen.

[Thomassino]

1 minute ago, kunok said:

That's an errata no? That's the isp for a solid core

Oh, a wild typo appeared, thanks for pointing it out