Atomic hydrogen

[KerikBalm]

A slightly less ridiculous alternative to metastable-metallic hydrogen, is monatomic hydrogen.

Pure monatomic hydrogen is very reactive, and desperately wants to form H₂. As a result, it can be a very energetic chemical reaction with a very low mass reaction product H + H> H₂.  This reaction can, in theory, produce an I_sp > 1500s, which is comparable to that of decompression of metallic hydrogen. Like metallic hydrogen, the question becomes... how do you actually store the stuff.

From what I've found, aside from very low density storage and low density complex storage using strong magnetic fields, the best way is to mix it with H₂, and freeze it. I guess that in a matrix of solid H₂, the single H's don't encounter each other and react. I've seen discussion of concepts using solid pellets of H₂/H within liquid helium.

So far it seems they haven't exceeded 2% by mass storage of H within H₂, if that could be enhanced to 15%, an I_sp of 750 is attainable. 

 

How likely is that to be attainable?

Then the next question: how would you use it in a sci-fi setting such as a drop-ship and ISRU. Without ISRU from gas giants, you aren't going to be able to use much helium.

How do you design a system that can:

1) Operate without a working fluid/ operate with a fluid that remains a fluid at temperatures low enough to stop H₂/H from melting

2) Keep the H₂/H pellets from melting

3) Do 1 and 2 without helium/ be able to be resupplied in situ, without requiring somehow mining helium from gas giants

Liquid helium can act as a heat sink, with excess heat removed by boiling it off (the liquid will never be above about 4k). It also provides a working fluid - solving 1 and 2, but its virtually unobtainable from ISRU on rocky planets.

I had thought about supercooling hydrogen, but if its incontact with frozen pellets, they will act as nucleating agents, and everything will freeze. Could we simply put some solvent in the H₂ liquid to lower the freezing point, and be able to keep the "doped" H₂ liquid cool enough that it doesn't melt the solid H₂/H pellets?

What about our heat sink? sure you could cool mass to nearly 0K, but that doesn't give you a lot of leeway. Warming less than 14 K results in the H₂/H pellets melting, the H combining with H, and the whole thing exploding like a mini-nuke. I'd think that you'd really like to have some sort of phase change to keep temperature from rising. After helium and hydrogen, the next lowest freezing point is neon. Hydrogen melts at 14K, neon melts at 24.5K... that phase change won't help/ be much of a usable heat sink.

As near as I can tell, this propellant isn't feasible. I'm imagining a drop ship with a helium/H₂/H slush that descends, has a very limited time on the surface as the helium boils off, and ascends again before the whole thing blows up. It better have a mothership in orbit with cooling equipment for a quick rendezvous, or jettison all unused H₂/H once reaching orbit... and that mothership better have more helium, and when it runs out, no more going back to the surface until you gt more helium from a gas giant...

If we could use some doped H₂ with a depressed freezing point, with some frozen so that the phase hcange provides a heat sink... we could at least perform ISRU anwhere that there is water/ammonia/hydrocarbons/any other hydrogen containing compound.

 

Or we could just use nuclear power and deal with the radiation issue.... or beamed power (if you have a truly massive ground station or mothership), and drop the idea of some sort of advanced chemical-ish propellent.

Edited February 23, 2021 by KerikBalm

[kerbiloid]

Afair in 1980s in "Foreign Military Review", a CIA _{(who knows, why...)} study of perspective antimatter engines considered this a proper way to store antihydrogen atoms instead of magnetic fields.

[DDE]

On 2/23/2021 at 5:31 PM, kerbiloid said:

CIA

While we're on the topic of Alphabet, some years prior the same people comissioned monoatomic hydrogen research as part of prep work for Project SUNTAN. I last researched this almost a decade ago and it looks like time and the metallic hydrogen hype have buried the link.

But id definitely involved non-metallic ice.

On 2/23/2021 at 5:31 PM, kerbiloid said:

_{(who knows, why...)}

AURORA? Antimatter wouldn't be the least nutty thing those technofetishists would try to come up with.

[kerbiloid]

An antimatter-assisted turbojet would be a great idea for next several years of this forum discussions.

[AngrybobH]

How do you manufacture monatomic hydrogen? Is it possible to store H₂ then make it on demand? And, would that end up more or less efficient than just using that energy to throw heated H₂ out the back?

[KerikBalm]

17 hours ago, AngrybobH said:

How do you manufacture monatomic hydrogen?

Without going into detail, by adding a lot of energy to H₂, and causing it to split. Heating H₂ to 10,000K will do it. Cooling it down without it recombining is difficult though

17 hours ago, AngrybobH said:

Is it possible to store H₂ then make it on demand?

Sure, but you'd need to get the energy to do it first

17 hours ago, AngrybobH said:

And, would that end up more or less efficient than just using that energy to throw heated H₂ out the back?

Definitely, keep in mind, you get energy out of monatomic H by having it recombine, both ways, you spit heated H2 out of the back. How can you do anything but lose efficiency by adding steps?

Although you may be able to get a higher Isp, since stored monatomic H would be mixed with inert frozen H2 to keep it reacting, thus lowering Isp from 1500s or so down to the high 700s.

A hot enough NTR can split the H₂ and gain Isp simply by having a lower MW exhaust.

[sevenperforce]

One of my physics professors was one of the first to really analyze the ionization behavior of endohedral fullerenes -- atoms trapped inside buckminsterfullerene:

I wonder if it would be possible to trap monatomic hydrogen inside a fullerene-like molecular structure (it could be sphere, a toroid, or a continuous hollow web) in such a way that the hydrogen atoms could not react with each other, for whatever reason? Molecular hydrogen has an anomalously long bond length: about 1.8 angstroms, roughly twice as long as a covalent O-H or N-H bond. I believe molecular hydrogen is on the order of 7 angstroms from end to end. If you could place monatomic hydrogen inside a molecular substructure too dense to allow bonds to form, you could cram it very very full of the stuff.

[KerikBalm]

You would need to trap a lot of monatomic H in there, considering the molecular weight of the fullerene. I guess the energy output should be enough to crack open the bucky balls though.

[sevenperforce]

On 2/27/2021 at 3:56 AM, KerikBalm said:

You would need to trap a lot of monatomic H in there, considering the molecular weight of the fullerene. I guess the energy output should be enough to crack open the bucky balls though.

Yes, trying to trap one single hydrogen atom inside a buckyball certainly wouldn’t get you very far. It doesn’t matter how energetic your propellant is if you have an effective tankage mass ratio of 720:1.

I’m more thinking about what could be theoretically possible on the very edge, regardless of whether we have any clue about how to actually pull it off. For example, what if we messed around with ionization? An atom (or even molecule) trapped inside a fullerine remains in place due to the quantum degeneracy pressure of fermion exclusion; the electron lattice of the fullerine is dense enough that the molecule cannot simply float through. 

What if we were to ionize a bunch of hydrogen atoms and trap their electron inside a carbon nanotube? The carbon nanotube would become negatively charged, attracting the free protons, but the protons would repel each other which would prevent them from entering. Once decomposition starts; this gives you both the ~4.3 eV from each pair of hydrogen atoms, but also the whopping 13.6 eV of ionization energy from each individual atom. 

If you were able to do this in a carbon nanotube lattice at even 25% hydrogen by weight, you’d end up with a propellant specific energy of 380 kJ per gram. 

[kerbiloid]

To describe a solution with a single phrase, you should just provide the local physical conditions when a change of state of the monoatomic hydrogen is stistically impossible. 

If you ask, how, it would be a second phrase.