Lithium+Fluorine+Hydrogen, why not?

[RuBisCO]

While the price and difficulty of handling liquid fluorine (highly corrosive, will combust with glass and cement, etc) and molten lithium (>180.5°C) (burst into flames if exposed to air or water) I did some calculations showing that a Centaur sized orbital stage would gain considerable performance if it burned this instead of oxygen and hydrogen.

A Centaur III has only 20,830 kg of propellant and a dry weight of 2247 kg, now lets look at what the same size upper stage could do with other fuel options:

If we replace oxygen with fluorine we might get the ISP up to 480 s, but if we burn lithium instead of hydrogen and cool that with hydrogen we could get an ISP of up to 540 s, even if we assume 20% extra mass in structure for the lithium tank and size changes in the other tanks, insulation, we still don't loss the performance benefits. For example a delta-v of at least 6300 m/s is needed to go from earth orbit to Jupiter transfer orbit. With a regular centaur only 4300 kg can be thrown towards Jupiter, but with even a heavier Li+F2+H2 burning centaur 6300 kg can be thrown towards jupiter, that is an extra 2 tons. Again a normal Centaur can only do 1550 kg to Neptune, a heavier Li+F2+H2 burning centaur can do 2850 kg to Neptune.     

[Spacescifi]

53 minutes ago, RuBisCO said:

While the price and difficulty of handling liquid fluorine (highly corrosive, will combust with glass and cement, etc) and molten lithium (>180.5°C) (burst into flames if exposed to air or water) I did some calculations showing that a Centaur sized orbital stage would gain considerable performance if it burned this instead of oxygen and hydrogen.

A Centaur III has only 20,830 kg of propellant and a dry weight of 2247 kg, now lets look at what the same size upper stage could do with other fuel options:

If we replace oxygen with fluorine we might get the ISP up to 480 s, but if we burn lithium instead of hydrogen and cool that with hydrogen we could get an ISP of up to 540 s, even if we assume 20% extra mass in structure for the lithium tank and size changes in the other tanks, insulation, we still don't loss the performance benefits. For example a delta-v of at least 6300 m/s is needed to go from earth orbit to Jupiter transfer orbit. With a regular centaur only 4300 kg can be thrown towards Jupiter, but with even a heavier Li+F2+H2 burning centaur 6300 kg can be thrown towards jupiter, that is an extra 2 tons. Again a normal Centaur can only do 1550 kg to Neptune, a heavier Li+F2+H2 burning centaur can do 2850 kg to Neptune.     

 

Boom?

 

 

[steve_v]

1 hour ago, RuBisCO said:

why not?

Because it'd be ludicrously dangerous, that's why.
Any one of those on it's own is frighteningly volatile and difficult to handle, let alone all three at once. Hydrogen is at least manageable, but Fluorine? Seriously? A substance that will burn ice and corrode glass? Imagine the paperwork.

[mikegarrison]

It's kind of a waste of fuel usually to directly launch to the outer planets anyway. Gravity assists are free delta-V as long as you can stand the flight time and schedule restrictions necessary.

[AngrybobH]

1 hour ago, mikegarrison said:

free delta-V

I wouldn't promote flying large amounts of liquid fluorine on a rocket and I would not ever be on a crew that had to handle that stuff. But I am curious, for a comparison, how much dV on average can you get for free going to Jupiter with gravity assists?

[kerbiloid]

If use often - the explosion is a question of time the explosions will be daily statistics.

If use rarely - will stay experimental, so the explosions will still be daily statistics. Better spend a bigger rocket.

Edited March 28, 2020 by kerbiloid

[DDE]

20 hours ago, RuBisCO said:

even if we assume 20% extra mass in structure for the lithium tank

That's rather unreasonable. Proposals for operational use featured lithium dust suspended in hydrogen.

20 hours ago, RuBisCO said:

With a regular centaur only 4300 kg can be thrown towards Jupiter, but with even a heavier Li+F2+H2 burning centaur 6300 kg can be thrown towards jupiter, that is an extra 2 tons.

Seems rather low. I remember being able to push a full 18 t to LEO out of a Soyuz with FLOX-70.

[Shpaget]

The title alone gave me heebie-jeebies.

 

[Guest]

Fluorine is ridiculously expensive, aluminium is not. Making a bigger rocket is vastly cheaper than any exotic fuel designs. People tend to act like Isp is everything, and that smaller rocket=better. This is wrong. Cost is everything. Chemical rocketry does has not yet reached the point where diminishing returns make increasing the size of a rocket pointless. Even a nuclear rocket with conventional propellant (hydrogen or methane) would be cheaper than a fluorine-based stage, and the main problem with those is cost, too (more so than politics). Not to mention an NTR is less dangerous and polluting of the two, and it has a higher Isp with most sensible propellants.

[RuBisCO]

2 hours ago, Dragon01 said:

Fluorine is ridiculously expensive, aluminium is not. Making a bigger rocket is vastly cheaper than any exotic fuel designs. People tend to act like Isp is everything, and that smaller rocket=better. This is wrong. Cost is everything. Chemical rocketry does has not yet reached the point where diminishing returns make increasing the size of a rocket pointless. Even a nuclear rocket with conventional propellant (hydrogen or methane) would be cheaper than a fluorine-based stage, and the main problem with those is cost, too (more so than politics). Not to mention an NTR is less dangerous and polluting of the two, and it has a higher Isp with most sensible propellants.

 

[Guest]

I remember this one. Good point in the first post, then the rest was fawning over theoretical performance. Fluorine never went anywhere for a reason. Safety is not the only reason why fluorine is pricey, BTW. The main reason is also precisely why it makes such a great rocket fuel on paper: it's incredibly reactive. Due to this, once it binds, it binds very tightly, and this is why most non-halogen fluorine derivatives are safe and usually quite inert. To get it out of minerals that contain it, one must input a lot of energy, and the conditions for the reaction to occur are really severe. It's also not all that common, as far as elements go. All this means you have a process that uses relatively expensive feedstock, lots of electricity, expensive equipment, and has stringent safety regulations to deal with. Any industrial plant making the thing, even as a byproduct (though I can't think of a process that'd have pure F2 and not another fluorinated compound as a byproduct), will have very high operating costs.

Fun fact: oxygen does not fall into the same problem because literal plants are constantly supplying the atmosphere with it, using a solar-powered process the efficiency of which would give any industrial manager fits. Hydrogen actually does have the same issues, but it happens to be an ubiquitous byproduct of natural gas refining. It's easier to get off a molecule than fluorine, but if we had to, say, electrolyze it out of water, we likely wouldn't have had LH2-powered rockets at all. It's only because we use fossil fuels so much that hydrogen is affordable (though LH2 still isn't exactly cheap, because the liquifying process is another electricity hog).

Edited March 29, 2020 by Guest

[jost]

Some insight why Fluroine won't be part of any space program in the near future can be found in the blog of chemist Derek Lowe:
https://blogs.sciencemag.org/pipeline/archives/2008/10/21/things_i_wont_work_with_triazadienyl_fluoride
https://blogs.sciencemag.org/pipeline/archives/2010/02/23/things_i_wont_work_with_dioxygen_difluoride
https://blogs.sciencemag.org/pipeline/archives/2008/02/26/sand_wont_save_you_this_time

Edited March 29, 2020 by jost

[K^2]

We rarely even see hydrogen as a fuel these days. People are mostly trying to build better kerlox or methane rockets. Sure, you need less fuel with LH₂, but you need heavier tanks, so by the time you have a rocket, a lot of advantage is gone. And if you build your rocket reusable, it's just cheaper to refuel it with RP1 or some other cheap fuel. In order for us to consider a toxic, expensive, highly specialized, highly corrosive fuel that will require special plumbing from pumps to nozzle cooling, which will also reduce lifetime resource of the engines, the advantage on I_SP would have to be something like 2:1 or better, not 10%.

[Kerbart]

I think John Clark used to describe the rocket scientists playing with those kind of fuels as “considered insane, even by rocket scientists”

[kerbiloid]

Nothing ventured, nothing gained.

I would also recommend to use pentaborane+fluorine for turbopumps and ullage.

[RuBisCO]

Ok by my calculations Li+F2 is 23784 kj/kg enthalpy of combustion , nearly twice that of H2+O2 at 13419 kj/kg, there is only one reaction in existence that uses normal molecules (no metallic hydrogen or radical molecules elevated to obscure energy states that are unstable or even theoretical and not achievable in reality) and produces more energy per mass then Li+F2, and that is Be+O2 at -23949 kj/Kg.

Nano-particles of beryllium gelled in liquid hydrogen could greatly improve performance without requiring new tankage, in fact it could reduce the size and mass of a hydrogen tank by increasing density,  obviously the problems are the toxicity of beryllium and its cost. A RL-10 engine would need clean liquid hydrogen to drive the pumps thus nanoparticles of beryllium would need to be injected as a tri-propellant mixed with an ISP sucking gellant, either that or totally diffrent pump powering system. After combustion there is the possibility of BeO slag building up on the engine walls.

 There is a cheaper, less toxic alternative to beryllium, that is aluminium, though this would provide only 16435 kj/kg. Perhaps an electrically pumped engine using nano-aluminium in liquid hydrogen and oxygen could work.  

7 hours ago, kerbiloid said:

Nothing ventured, nothing gained.

I would also recommend to use pentaborane+fluorine for turbopumps and ullage.

Actually boron fluoride compounds suck a lot of energy, lower ISP. Also why not ClF3 or ClF5 instead of flourine? I calculated out that hydrazine-methanol + ClF5 should provide 20 s more ISP then MMH+N2O4, oxygen is needed to prevent carbon fluorine molecules from forming that will suck energy in higher heat capacity in intedr-molecular vibration states. Four fluorides bounding around a carbon center allows for a lot of energy storage and thus low kinetic energy of the whole molecule, so instead of bouncing away from the rocket faster and providing more impulse, it just vibrates more violently. Ideally we can produce carbon monoxide, N2, HCl and HF as exhaust, simple diatomic molecules with limited vibration states. Methanol is needed to reduce the melting temp of hydrazine and make the fuel more resistant to spontaneous decomposition while in the engine cooling channels     

Edited March 29, 2020 by RuBisCO

[DDE]

16 hours ago, RuBisCO said:

There is a cheaper, less toxic alternative to beryllium, that is aluminium, though this would provide only 16435 kj/kg.

That, and it's extremely clever at coating itself with alumina and not burning properly.

16 hours ago, RuBisCO said:

Also why not ClF3 or ClF5

Because of a degree of concern for the sanity of the other people in this thread.

[Hannu2]

Space tech was very special and classified military tech at 50s and 60s when they considered fluorine and other exotic and dangerous chemicals as a fuel. But now space tech is civil industry and have to obey strict environmental and human safety regulations. Which are going to develop to more strict direction on next decades.

Space companies have tendency to move to safer, cheaper and more common chemicals from all exotic and expensive stuff. I think the trend of next decades is to replace hydrogen and hydrazine derivatives with safer stuff instead of search the last digits of ISP with unobtainium fuel. Cheap reusing and next generation of private companies' huge rockets will make safety and reliability issues even more important and take away the advantage of dangerous superfuels.

 

[Guest]

Not to mention during 50s and 60s, only proto-hippies cared much about environment, and you didn't have many of them in the space program.  That, and nuclear technology was in its infancy, and so nuke propulsion, while also considered (and indeed, the craziest designs come from this period) wasn't as reliable as a conventional engine running on an exotic fuel. That was also a time when costs mattered less if it meant gaining the advantage over the other side (this kind of thinking eventually sank the USSR, in part due to costs of the Buran project).

[KSK]

19 hours ago, RuBisCO said:

Also why not ClF3 or ClF5 instead of flourine?

Because, if you so much as look at either of them cross-eyed,  they’ll send you on a one way trip to hell, only for the devil to kick you back out for breaching health and safety standards.

Seriously, I don’t even want to think about rocket sized tanks full of compounds that burn *everything* including concrete.

Edit. Also, any vehicle powered by hydrogen and fluorine is most probably going to throw out rather a lot of hydrogen fluoride in its exhaust. Now I’m not sure if hydrogen fluoride makes it to the top of my ‘chemicals I have no desire to be downwind of’ list but it’s definitely a contender for a solid top ten place.

Edited March 30, 2020 by KSK

[p1t1o]

Quote

Lithium+Fluorine+Hydrogen, why not?

Because lithium, fluorine and hydrogen?

Just....no.

Even if everything works perfectly, the launchpad is now a toxic deathtrap. 

Cheaper and [much] safer to simply build a larger hydrolox rocket - and why wouldnt you?

 

 

[Xd the great]

47 minutes ago, p1t1o said:

Cheaper and [much] safer to simply build a larger hydrolox rocket - and why wouldnt you?

MOAR BOOSTERS!!!!!!!!

4 hours ago, Hannu2 said:

Space companies have tendency to move to safer, cheaper and more common chemicals from all exotic and expensive stuff. I think the trend of next decades is to replace hydrogen and hydrazine derivatives with safer stuff instead of search the last digits of ISP with unobtainium fuel.

Is hydroxylammonium nitrate safer than hydrazine..?

[RuBisCO]

4 hours ago, DDE said:

That, and it's extremely clever at coating itself with alumina and not burning properly.

Because of a degree of concern for the sanity of the other people in this thread.

Well nano-particles are not going to do that... maybe

I hail Cthulhu. 

[Hannu2]

1 hour ago, p1t1o said:

Cheaper and [much] safer to simply build a larger hydrolox rocket - and why wouldnt you?

Because hydrogen is so impractical and dangerous too. It is good choice for upper stage of expendable rocket, but it seems that SpaceX and Blue Origin will use safer and simpler methane in their big reusable rockets.

51 minutes ago, Xd the great said:

Is hydroxylammonium nitrate safer than hydrazine..?

I have understood that it is main reason why it is investigated for propellant. It is not actually safe but not as dangerous for workers and environment than hydrazine compounds. It has other good properties too but I do not know why it is not used. Is there some unsolved problems or is space industry just so conservative that nothing changes if it is not absolutely necessary.

[p1t1o]

1 minute ago, Hannu2 said:

Because hydrogen is so impractical and dangerous too.

Its dangerous in that its a flammable rocketfuel yes, but what makes you say it is more dangerous - or more impractical - than a triple-fuelled rocket with fluorine and lithium?