Tripropellant Rockets

[shynung]

From John D. Clark's book Ignition! An Informal History of Liquid Rocket Propellants:

They worked at 500 psi chamber pressure, with a high expansion nozzle (exit area/throat area = 60) designed for space work. Their main problem stemmed from the high surface tension of liquid lithium, orders of magnitude higher than that of ordinary propellants, which made it difficult to design an injector that would produce droplets of lithium small enough to burn completely before going out the nozzle. Once this problem was overcome, their results were spectacular. Using lithium and fluorine alone (no hydrogen) their maximum specific impulse was 458 seconds. But when they proportioned the lithium and fluorine to burn stoichiometrically to LiF, and injected hydrogen to make up 30 percent of the mass flow, they measured 542 seconds â€â€probably the highest measured specific impulse ever attained by anything except a nuclear motor. And the chamber temperature was only 2200 K!

Why are these kinds of beasts were never developed today?

[DaveofDefeat]

Because you have to keep liquid hydrogen at cryogenic temperatures, Liquid lithium at 300 degrees, and then keep the fluorine from reacting with the tank they are putting it in, and just having to deal with fluorine in the first place.

[shynung]

The experimenters used gaseous hydrogen in the experiment, presumably to save themselves the trouble of handling them in concert to liquid lithium and fluorine.

[Kryten]

Liquid hydrogen is already bad enough, a rocket that would have to run on gaseous hydrogen would have an incredibly low mass ratio.

[shynung]

It was a static engine test, designed to analyze the performance of a specific propellant combination. Mass fraction is irrelevant.

Though, an actual rocket burning these propellants do need some mechanism to keep them liquid; gaseous hydrogen leaks off the tank it's put in, no matter what it is made of.

[Albert VDS]

An other problem would be that the exhaust would be ionized, which would interfere with electronics and radio communications.

[EpicRootHairCell]

Firstly toxicity, the exhaust gasses would be highly toxic.

Secondly the corrosiveness of fluorine.

Thirdly the safety of large quantities of lithium and fluorine in close proximity (makes LH2/LOX look like baking powder and vinegar).

Fourthly the engineering challenge of having 3 propellants instead of 2.

Fifthly the challenge of having hydrogen at -253 and the lithium at 180.

[KSK]

<snip>

Why are these kinds of beasts were never developed today?

Because fluorine. From Wikipedia:

Fluorine is highly toxic, effects starting at lower concentrations than hydrogen cyanide's 50 ppm:[34] significant irritation of the eyes and respiratory system as well as liver and kidney damage occur above 25 ppm. Human eyes and noses are seriously damaged at 100 ppm,[35] while inhalation of 1,000 ppm fluorine will cause death in minutes[36] as compared to 270 ppm for hydrogen cyanide.[37]

Not something you want to be keeping around in rocket-fueling quantities. Plus if anything goes wrong (and what could possibly go wrong with a rocket powered by liquid metal, fluorine and hydrogen), there's a high chance of large quantities of hydrogen fluoride being released.

Hydrofluoric acid, aqueous hydrogen fluoride, is a weak acid unlike the other strong hydrohalic acids,[107][note 10] but corrosive enough to attack glass.[109]

Hydrofluoric acid, the water solution of hydrogen fluoride, is a contact poison. Even though it is chemically only a weak acid, it is far more dangerous than the conventional strong mineral acids, such as nitric acid, sulfuric acid, or hydrochloric acid. Owing to its lesser chemical dissociation in water (thereby remaining a neutral molecule), hydrogen fluoride penetrates tissue more quickly than typical acids. Poisoning can occur readily through the skin or eyes or when inhaled or swallowed. From 1984 to 1994, at least nine U.S. workers died from accidents with HF.[275] Once in the blood, hydrogen fluoride reacts with calcium and magnesium, resulting in electrolyte imbalance (and potentially hypocalcemia). The consequent effect on the heart (cardiac arrhythmia) may be fatal.[275] Formation of insoluble calcium fluoride also causes strong pain.[277] Burns with areas larger than 160 cm2 (about the size of a person's hand) can cause serious systemic toxicity.[278]

Symptoms of exposure to hydrofluoric acid may not be immediately evident, with an 8-hour delay for 50% HF and up to 24 hours for lower concentrations. Hydrogen fluoride interferes with nerve function, meaning that burns may not initially be painful.

[sgt_flyer]

Hehe fluorine is really nasty stuff, very difficult to store. (Most storage capabilities occurs when the metal container forms

an fluorine oxid layer - if you scratch that layer, chances are that the oxid layer won't reform fast enough...

One of the fluorine compounds, CiF3 is hypergolic with a lot of things - including, concrete, gravel and even glass and sand !

(A 900kg CiF3 industrial spill once burned through 30cm of concrete and 90cm of gravel underneath).

add to that the price of the lithium (as it's used in batteries and solar panels)

[phoenix_ca]

Elemental fluorine. That alone is enough to run away, fast.

[Thiel]

â€ÂIt is, of course, extremely toxic, but that's the least of the problem. It is hypergolic with every known fuel, and so rapidly hypergolic that no ignition delay has ever been measured. It is also hypergolic with such things as cloth, wood, and test engineers, not to mention asbestos, sand, and water-with which it reacts explosively. It can be kept in some of the ordinary structural metals-steel, copper, aluminium, etc.-because of the formation of a thin film of insoluble metal fluoride which protects the bulk of the metal, just as the invisible coat of oxide on aluminium keeps it from burning up in the atmosphere. If, however, this coat is melted or scrubbed off, and has no chance to reform, the operator is confronted with the problem of coping with a metal-fluorine fire. For dealing with this situation, I have always recommended a good pair of running shoes.â€Â

That really says it all about fluorine chemistry and why you don't want rocket sized tanks of any of it.

[Elthy]

Cant we just stick to NERVA? Im not that confortable with radiactive stuff on top of thousand tons of explosives, but fluorine is worse. Also i dont want to think about the pollution such a rocket leaves in our athmosphere...

[Razor235]

Eh, it wouldn't really be that bad. Lithium fluoride salts and elemental hydrogen.

[bsalis]

Sounds more like an industrial accident waiting to happen rather than a propulsion solution.

[helaeon]

To give you an idea the danger of fluorine and hydrogen fluoride in particular:

When I was in organic chemistry in college hydrofluoric acid was something that we weren't really even allowed to talk about. If you were doing a hypothetical synthesis and busted out HF the solution was wrong. Stated reason was the absurd danger of it and carbon-fluorine reactions in particular. We did things with fuming (often north of 30 molar) hydrochloric, sulfuric, and nitric acid. We made gunpowders from scratch. But fluorine was too dangerous. I'm not even sure if there was any hydrofluoric acid on campus at all.

Which interestingly, we're around fluorine all the time. Mostly in ionic form but carbon-fluorine chemistry too. Teflon is polytetrafluoroethylene (PTFE), which if you make it wrong explodes.

Rocket fuels we know that's dangerous and there is some risk tolerance there for what we need it to do, but you're talking about reacting liquid lithium (also not really all that safe), with hydrogen (not safe, especially in gaseous form near oxygen), and fluorine... around things with carbon like steel, us, our plastics, and oils... yeah that's a bad idea.

Here's a video about fluorine

[William Mook]

Dr. Garvin Von Eschen worked on this back in the day. I learned propulsion chemistry from him at the Ohio State University.

It is my understanding a Cryogenic hydrogen liquid with nanoparticles of passivated fluorine and lithium suspended in them would form a type of mono propellant milkshake that explodes when warmed. This approach was used by Rocketlab high density mono propellant consisting of hydrogen peroxide and passivated carbon nanoparticles 1.7 g/cc that explodes when mixed with catalysts. 

 

[DDE]

7 hours ago, William Mook said:

Dr. Garvin Von Eschen worked on this back in the day. I learned propulsion chemistry from him at the Ohio State University.

It is my understanding a Cryogenic hydrogen liquid with nanoparticles of passivated fluorine and lithium suspended in them would form a type of mono propellant milkshake that explodes when warmed. This approach was used by Rocketlab high density mono propellant consisting of hydrogen peroxide and passivated carbon nanoparticles 1.7 g/cc that explodes when mixed with catalysts. 

 

Hello and welcome to the forums!

The problem is, as John Clarke pointed out, any monopropellant is just an explosive with the reducing and oxidizing ends of the molecule firmly separated by fingers crossed.

The practical solution to this was to separate fuel and oxidizer; much research bas indeed been done into metallized slurry fuels, but mostly abandoned as the fuel industry overall mostly froze in place.

[Nothalogh]

On 8/4/2019 at 10:08 PM, William Mook said:

would form a type of mono propellant milkshake that explodes when warmed

No possible way that could go wrong