Discussion of metallic hydrogen propulsion split from another thread.

[Dirkidirk]

6 minutes ago, Lewie said:

It doesn’t hurt you. Don’t worry...the mean metallic hydrogen won’t bully you. 

You underestimate the power of the shiny hydrogen.

[DStaal]

14 minutes ago, Lewie said:

Like I said, it’s a game. And it’s not even a ‘science based game’. (Jool’s moons are impossible, we all know that). A game. Ksp is a much more realistic then other space games out there, so what if there are a couple engines that are speculative? A game. A GAME. It doesn’t hurt you. Don’t worry...the mean metallic hydrogen won’t bully you. 

I'm not as dogmatic as some, but I've generally come around to the against side.

Yes, it's a game.  And if it provided some needed mechanic for the game that nothing else even semi-realistic would provide: sure, go for  it.  Making the game work for the players is more important than being strictly realistic.

However - it doesn't.  There's half a dozen other techs which would provide similar performance and would be more realistic, so any needed mechanic in the game could be covered with a more realistic alternative.  So instead of using some tech which is highly dubious that it is even possible (I do admit @Master39's point that evidence is fairly slim either way at this point), why not use one where we know the physics works even if we can't build one yet?

[Guest]

9 minutes ago, Lewie said:

And it’s not even a ‘science based game’.

KSP2: Episode 1. Watch it. The devs themselves said otherwise. This is a science-based, educational game.

Jool's moons are a developer blunder, one which requires some knowledge to even spot. Yes, they're wrong, but that's not even simplification, or a gameplay tradeoff, or whatever. The whole Kerbal system is obviously fictional, but that can be excused, because the game is honest about it being fictional. It's not honest about metallic hydrogen not being a real technology. If it was called "magic pixie dust", it would be honest.

18 minutes ago, Master39 said:

And that's exactly why it isn't a big deal, the educational portion of the game, teaching orbital mechanics, is not touched.

Get this "educational portion" nonsense out of your brain already. The worst form of lies are half-truths, because sorting them out is harder than when the whole thing is bunk from start to finish. Or could you, perhaps, point me to a disclaimer that says "everything but orbital mechanics in this game is fantasy"? No, you couldn't. Because KSP2 doesn't only teach orbital mechanics. That was KSP1, which has very little content aside from orbital mechanics, anyway. And even then, it also teaches you a few basic facts about rocket engines.

KSP2 is going to teach more that, the devs mentioned colonies, ISRU and some form of life support. As long as the devs, in their videos, will pretend that the technology could be real, I will expect it to actually be so. 

[Lewie]

13 minutes ago, Dragon01 said:

KSP2: Episode 1. Watch it. The devs themselves said otherwise. This is a science-based, educational game.

Jool's moons are a developer blunder, one which requires some knowledge to even spot. Yes, they're wrong, but that's not even simplification, or a gameplay tradeoff, or whatever. The whole Kerbal system is obviously fictional, but that can be excused, because the game is honest about it being fictional. It's not honest about metallic hydrogen not being a real technology. If it was called "magic pixie dust", it would be honest.

Get this "educational portion" nonsense out of your brain already. The worst form of lies are half-truths, because sorting them out is harder than when the whole thing is bunk from start to finish. Or could you, perhaps, point me to a disclaimer that says "everything but orbital mechanics in this game is fantasy"? No, you couldn't. Because KSP2 doesn't only teach orbital mechanics. That was KSP1, which has very little content aside from orbital mechanics, anyway. And even then, it also teaches you a few basic facts about rocket engines.

*sighs*

I'm done with this bs. 

[Guest]

Well, if that means you stay out of future discussion on the subject and quietly stand aside while knowledgeable people talk the devs out of teaching the next generation of aerospace engineers about a technology that cannot exist, go right ahead. You can watch their videos while at it, so you don't accidentally spend money on an educational game when KSP2 comes out.

[Lewie]

8 minutes ago, Dragon01 said:

Well, if that means you stay out of future discussion on the subject and quietly stand aside while knowledgeable people talk the devs out of teaching the next generation of aerospace engineers about a technology that cannot exist, go right ahead. You can watch their videos while at it, so you don't accidentally spend money on an educational game when KSP2 comes out.

Ok, I’m back.

Do you really think that every single person that plays ksp will become an Aerospace engineer? Sure, it may spike people’s interest in space, but it won’t churn out a generation of aerospace engineers. Just so you are aware, it’s quite hypocritical of you to say “oh, metallic hydrogen is pixie farts” and then you go and say that ksp2 will create a new generation of aerospace engineers, and ‘spend money on an educational game’ huh....wow. That’s ironic, isn’t it?

[Guest]

10 minutes ago, Lewie said:

Do you really think that every single person that plays ksp will become an Aerospace engineer? 

Of course I don't. If I believe that, I would have said so, which I didn't. 

"Spiking people's interest in space" is what it's all about. Quite a few people were inspired to seek a job at NASA by KSP1, actually, they posted on the forum about it. Putting random fantasies into an educational game is wrong, because it gets people to have unrealistic expectations. 

There is no hypocrisy in what I said, but I suspect you meant "inconsistent". I am consistent, however, in saying "this is an educational game, and it should not have misleading elements in it". Did you actually watch KSP2: Episode 1 or not? Or are you saying you're a higher authority on what the devs are trying to do with KSP2 than the devs themseleves? 

If you think I'm wrong on it being an educational game, tell me where I can find a statement that "KSP2 is not going to be an educational game" from a developer or a Take2 executive. I have already told you where you can find proof to the contrary.

Edited October 7, 2020 by Guest

[Lewie]

1 minute ago, Dragon01 said:

 There is no hypocrisy in what I said. I'm consistent in saying "this is an educational game, and it should not have misleading elements in it". Did you actually watch KSP2: Episode 1 or not? Or are you saying you're a higher authority on what the devs are trying to do with KSP2 than the devs themseleves? 

Hm...

Funnily enough, I have not said anything along the lines of ‘oh, the devs should do this, do that, make this stock. Remove this cus I don’t like it! Remove this because it doesn’t exist! No fairy dust engines!!’

nope, I haven’t said anything along those lines. 

If you did watch the near future tech dev diary, you will would have seen the reasoning behind the engines.

And yes...when you say that this is an educational game, but you go and complain and ‘unicorn fart engines’ that is hypocrisy.

[Guest]

54 minutes ago, Dragon01 said:

Get this "educational portion" nonsense out of your brain already. The worst form of lies are half-truths, because sorting them out is harder than when the whole thing is bunk from start to finish.

First, calm down.

Second, again, calm down and, if you can't, avoid replying to me.

 

54 minutes ago, Dragon01 said:

Or could you, perhaps, point me to a disclaimer that says "everything but orbital mechanics in this game is fantasy"?

Point me to that disclaimer on KSP1, since I'm basing my statements of them saying multiple times that "this is still Kerbal" and that they want to keep the feeling of the original.

 

54 minutes ago, Dragon01 said:

And even then, it also teaches you a few basic facts about rocket engines.

And a lot of wrong ones, like the vacuum optimized engines having the smaller nozzles.

 

54 minutes ago, Dragon01 said:

KSP2 is going to teach more that, the devs mentioned colonies, ISRU and some form of life support.

Basically some very obvious gameplay expansions when you want to have a game with something more to do on its celestial bodies than planting a flag and taking a screenshot of Jeb next to it.

And not even that hardcore at that, the game engine can't have underground structures and likely a colony will work exactly the same with the same modules and building either on Mun or Eve and let's not even start talking about what would it mean if they ever try to add a realistic life support that kills Kerbals.

Get mad all you want but the game isn't going to be the hardcore professional training software for the next generation of aerospace engineers you're expecting it to be.

It's going to be a game and one heavily based on the funny side of Kerbals messing with exploding rockets.

 

54 minutes ago, Dragon01 said:

Jool's moons are a developer blunder, one which requires some knowledge to even spot.

And somehow some obscure speculative engine technology called for years "the Graal of rocket propulsion" and only very very recently put at doubt by some research that is still very difficult to find unless you already know what you're searching (really, when searching a space stack exchange question about this very topic comes up firsts that the sources you linked) is different.

And you expect Devs that didn't know about SRB gimbaling to know that.

Curb your expectations, this game isn't going to be an engineer training software and certainly not because of metallic hydrogen.

[Guest]

Just now, Lewie said:

And yes...when you say that this is an educational game, but you go and complain and ‘unicorn fart engines’ that is hypocrisy.

Ah, I get it. You're an early victim of what I've been talking about all the time. You think metallic hydrogen could be a thing, don't you? That's the only way I can explain what you're saying. 

Well, check out the links in my signature, then. The rationale the developers gave was based on a theory that had been disproven by experimental evidence, and it was disproven even before they made the video. It's likely that neither them nor their scientific consultants were aware of those works at the time of making the video.

Unless you can produce experimental (not theoretical!) evidence that the experiments on which the papers in my signature are incorrect, then I'll stop arguing against metallic hydrogen in KSP2. My arguments are based on evidence, which I conveniently linked to in my sig. 

[Lewie]

Just now, Dragon01 said:

Ah, I get it. You're an early victim of what I've been talking about all the time. You think metallic hydrogen could be a thing, don't you? That's the only way I can explain what you're saying. 

And you take this into a whole different direction...

Not once did I say that humanity will be able to harness metallic hydrogen. Maybe, and this is a really big maybe, in a few hundred years as our technology expands, and we male leaps and bounds, we’ll be able to use mh. Is it feasible right now? Of course not. In 200 years? Maybe. 

[Deddly]

It's interesting to see two polar opposite opinions from people who are all passionate about the game. Where there is more than one person, there will always be differing opinions on things. I myself have followed development of KSP 2 with interest. I do very much look forward to the game, but right from the beginning it has been clear from my point of view that KSP 2 would have certain things that are, shall we say, speculative. As for whether metallic hydrogen is actually metastable, well, be careful about that - a single study rarely, if ever, proves something. More studies will have to be conducted before it is conclusively disproved. But this is the tip of the iceberg isn't it? KSP 2 seems to have a lot of tech that may or may not be impossible, including FTL, it would seem.

So to me, KSP 1 leans more towards realism, but KSP 2 is more "hard scifi" or "possible future tech". That's for better or worse, but that's what I think they appear to be going for.

As interesting as the discussion is, please try to take it down a notch, because it looks to be getting a little heated at the moment. Address the argument rather than the person, that sort of thing - you know how it works. Never reply in anger, because then people stop listening to your arguments, no matter how powerful.

[Guest]

10 minutes ago, Deddly said:

So to me, KSP 1 leans more towards realism, but KSP 2 is more "hard scifi" or "possible future tech". That's for better or worse, but that's what I think they appear to be going for.

FYI, this is what seems under discussion here (emphasis mine). A quick (or not so quick, depending on how good you are with materials science) look at links in my sig will show that metallic hydrogen has, as of recently, moved out out of "possible future tech" category. Evidently, this is difficult to accept to those hyped up on the video I mentioned.

Oh, and FTL won't be a thing, it's already been asked about and answered. All the tech they showcased is actually quite firmly grounded in the "possible" side. I just want KSP2 to be actually true to the direction that you mentioned, and not plant a discredited tech among ones that we could actually work towards.

11 minutes ago, Lewie said:

Is it feasible right now? Of course not. In 200 years? Maybe. 

It's impossible, not infeasible. Not now, not in 200 years, not in 2000 years, not in 20 000 years. It never was, and never has been, it's just that we only recently found that out. I'm all for drives that are infeasible. This drive, however, requires an effect (metastability) that was predicted by a model that we know is no longer true, based on recent experiments. This theoretical basis was, BTW, as flimsy as Alcubierre drive is, and only one of a number of predictions about metallic hydrogen. It simply turned out not to be true. 

Edited October 7, 2020 by Guest

[DStaal]

2 minutes ago, Lewie said:

And you take this into a whole different direction...

Not once did I say that humanity will be able to harness metallic hydrogen. Maybe, and this is a really big maybe, in a few hundred years as our technology expands, and we male leaps and bounds, we’ll be able to use mh. Is it feasible right now? Of course not. In 200 years? Maybe. 

Metallic hydrogen is only useful if it's in fact metastable.  We would then need to make leaps and bounds to be able to reliably produce it - but we'd have a known goal to work towards.

At the moment the best evidence is that it is not metastable.  If it's not metastable, then we can't make it metastable - that's a property of some atomic arrangements, and if the arrangement you're working with isn't metastable then you're stuck.

That was the point of my 'metastable water' argument: That you could substitute any other material into your statement and make just as much sense.  The only reason we're discussing this is that one flawed theory from the 70's indicated that MH *might* be metastable.  While we have very flimsy evidence against metastable MH, we have stronger evidence against that theory, and overall the evidence is fairly convincing that this isn't going to work - ever.

Metallic Hydrogen is a thing that exists in this universe.  Metastable Metallic Hydrogen is 95+% sure not a thing that can exist in this universe.  If it's not a thing, no amount of technical advances will let us use it, because it doesn't exist.

[Deddly]

9 minutes ago, Dragon01 said:

FYI, this is what seems under discussion here (emphasis mine). A quick (or not so quick, depending on how good you are with materials science) look at links in my sig will show that metallic hydrogen has, as of recently, moved out out of "possible future tech" category. 

I did read the links - at least, the parts that were not behind paywalls. I agree that the evidence is against MH being metastable, but more studies will be needed to be sure of that. Beware of accepting a single study as conclusive proof of anything. They are not even 100% certain they created MH between those two toroidal diamonds. They appear to have done so, but, as I say, more studies will be needed. 

[Guest]

4 minutes ago, Deddly said:

They appear to have done so, but, as I say, more studies will be needed. 

Yeah, there is some room, but there had been a few other attempts that corroborated the evidence. Paywalls are a big problem with citing scientific evidence, especially "hot from the anvil", so to speak.  There's not much literature on the subject, and most of it is paywalled.

The big deal, though, is that the experiments showed results that widely diverged from the theory that predicted metastability. The only reason it ever came up was because of a specific theory from the 70s, which made certain predictions about the properties of metallic hydrogen. There is no theory that explains the properties that were actually observed, that would also predict metastability. I'm not saying such a theory couldn't be formulated, but metastability is a very uncommon feature, so it's a long shot (as in, from here to Andromeda Galaxy ). What we can say for sure is that we have not observed metastability directly, and that the 70s theory had pretty much crashed and burned, not only because of that, but because its other, less earth-shattering predictions had also failed to come true. In particular, if it was true, then the team in question would have had very clear evidence of metallic hydrogen formation, even at much lower pressures.

20 minutes ago, Master39 said:

And you expect Devs that didn't know about SRB gimbaling to know that.

KSP1 devs aren't the ones making KSP2. KSP2 ones aren't quite as clueless about rocketry as KSP1 team was at the start.  I know because I was there, you know how Isp causes thrust of a rocket engine to increase with altitude? Well, you have me to thank for that. Before I pointed out how it should actually work, they used to decrease fuel flow, with thrust remaining the same. Of course, there's not a single dev from that time remaining on KSP1 team, either. 

The problems with engines that you mentioned are getting fixed. New models are have correct nozzles, turbopump exhaust pipes, and sometimes even proper gimbaling hardware. The old ones were always ugly placeholders.

25 minutes ago, Master39 said:

Basically some very obvious gameplay expansions when you want to have a game with something more to do on its celestial bodies than planting a flag and taking a screenshot of Jeb next to it.

We don't know that yet, but I suspect that they will not completely abstract colonies, and ISRU will be better done in KSP1. In fact, with how they approached the engines (discredited theory notwithstanding), I'm fully expecting them to put real challenges and real solutions to them, as far as colony making goes. A fully fleshed out colonization system has a chance of being very educational, and there's no reason for them not to take that chance. Gameplay actually goes quite well with realism, there are already several more or less realistic games about settling Mars.

Even KSP1's commnet isn't so bad from the educational standpoint. It's not all that good as a gameplay feature, but it's not completely trivial, and it's based on the way real antennas work.

Quote

And not even that hardcore at that, the game engine can't have underground structures and likely a colony will work exactly the same with the same modules and building either on Mun or Eve and let's not even start talking about what would it mean if they ever try to add a realistic life support that kills Kerbals.

Of course it can have underground structures. It cannot have underground terrain features. That is, caves and such. Nothing is stopping you from making a small bunker and giving it a huge IVA underneath. In fact, nothing is stopping you from clipping building through terrain, it works just fine in KSP1. You can also clip parts, like drills, under the ground. Underground colonies are very much possible even in KSP1, though obviously, they'd be visually unimpressive (and, as a flipside, light on system resources).

We don't know if there same modules will be used on the Mun and Eve, but for just about every planet other than Eve (and for every planet we've seen colonized so far), you could get away with using similar modules. There's a risk of that, but quite frankly, it'd be a missed opportunity. Given that there's so much water around, they should require Eve colony parts to use underwater colony parts.  TBH, with all the attention Venus had recently been getting, this is not even all that far-fetched.

[kerbiloid]

Spoiler

What if the KSP planets are made of superdense metastable metallic hydrogen, and the green popeye freeloaders just mine it from beneath...

 

[Guest]

13 minutes ago, Dragon01 said:

The problems with engines that you mentioned are getting fixed. New models are have correct nozzles, turbopump exhaust pipes, and sometimes even proper gimbaling hardware. The old ones were always ugly placeholders.

They're being fixed now, and most of them still aren't, 8 years after release.

That means that for most of KSP life and even now you need to verify everything you may learn about engine technology with a more reliable source. 

And the nozzle size was only the easier example, the behaviour of the engines is profoundly altered for obvious gameplay reasons (restartability and throttle capacity).

 

19 minutes ago, Dragon01 said:

We don't know that yet, but I suspect that they will not completely abstract colonies, and ISRU will be better done in KSP1.

I don't expect them to be completely abstract colonies, but they already said that they don't want to build "space sim city" and that you'll be able of abandon colonies to do other things without finding them dead when you go back.

I expect them to be more on the management game side than on the realism one and even then I expect that to be more a "minigame" than the real focus.

 

28 minutes ago, Dragon01 said:

Of course it can have underground structures. It cannot have underground terrain features. That is, caves and such. Nothing is stopping you from making a small bunker and giving it a huge IVA underneath. In fact, nothing is stopping you from clipping building through terrain, it works just fine in KSP1. You can also clip parts, like drills, under the ground. Underground colonies are very much possible even in KSP1, though obviously, they'd be visually unimpressive (and, as a flipside, light on system resources).

We don't know if there same modules will be used on the Mun and Eve, but for just about every planet other than Eve (and for every planet we've seen colonized so far), you could get away with using similar modules. There's a risk of that, but quite frankly, it'd be a missed opportunity. Given that there's so much water around, they should require Eve colony parts to use underwater colony parts.  TBH, with all the attention Venus had recently been getting, this is not even all that far-fetched.

Of course all of those things could be there, as could all the technologies you listed as metallic hydrogen alternatives and I'd want them (as I said I'd prefer having more choice with metallic hydrogen and one alternative that differ in crafting and resources requirements than having only one option for realism reasons), but I have no reason to give them for granted since we have no indication of any of that being in the game except for that time Nate Simpson smirked while saying "no comment" on the question about water parts by Shadowzone (and that it could just mean "we have a ballast part and a propeller you can strap to the rover cockpit and pretend it's a boat/submarine").

 

 

[Guest]

2 hours ago, Master39 said:

And the nozzle size was only the easier example, the behaviour of the engines is profoundly altered for obvious gameplay reasons (restartability and throttle capacity).

This is actually one of the lesser transgressions. Deep throttling (though not that deep) is possible and so is basically infinite restartability. Merlins do both, it's not quite infinite restarts, but they can take a lot. We just don't usually build engines that way, and we certainly didn't in Apollo era, but here, it's a quantitative thing, and not nearly as bad as promoting a discredited theory.

KSP1 did dropped the ball on engines, especially in the visual department. However, this is already acknowledged as an issue. There is no reason to take KSP1's flaws as justification for KSP2 not to fix them. In fact, KSP1 is trying to fix them, and many people complained about the old placeholders. 

2 hours ago, Master39 said:

I expect them to be more on the management game side than on the realism one and even then I expect that to be more a "minigame" than the real focus.

I don't think it'll be a "minigame", we're talking about a core feature of the game. They might be more management game side once set up, but remember that the most challenging part of setting up a colony IRL is getting one going in first place. That's where I'd expect challenges to crop up, it definitely shouldn't be trivial to create a colony. What the devs said could very well mean that building things will require you to have a small, but self-sufficient settlement already in place, built the old-fashioned way, by landing and connecting modules.

Regardless of how deep the system is, I think it's too core. Face it, orbital mechanics are so KSP1. KSP2 can't be "same, but more, with a few more systems tacked on", because you can have all that with mods. It needs to go well beyond that.

Edited October 7, 2020 by Guest

[OHara]

We acknowledged up-thread that imagining usefully-metastable metallic hydrogen
bothers some players, but not others.

I can understand some reasons why some players might be bothered a lot.   Other metastable materials, like hydrazine and hydrogen peroxide, make fine monopropellants.   But the energy per molecule released from H→H₂ is ten times larger, near the top of the scale for energies in chemical bonds.  Chemical bonds struggled to hold together the diamond anvils that (probably) formed monatomic metallic H, and the energy released overcomes chemical bonds trying keep solid the material in the nozzles.  It would be very surprising if the metastable state could be usefully confined by normal materials.  In 2010 two academics speculated at a conference (which is fine) that it might in fact be practical.

The recent speculation, within the last decade, of something almost too good to be true, with no supporting experimental evidence, makes metallic hydrogen rockets feel just a bit like pseudo-science.  My reaction, as a solid-state-physics PhD, to the metallic hydrogen engine as in-game lore was to cringe -- but I can get over it if it fits well in KSP2.

Obvious silliness like "mystery goo" doesn't make me cringe, nor do game-play compromises like restartable engines and a dense home planet.   I think the reason is that they are obviously fantasy.  Metallic hydrogen rockets are in the uncomfortable middle between realistic and fantasy.

KSP2 will probably lose only very few sales to the people too embarrassed to be associated with a metallic hydrogen engine.  But with this thread now in the KSP2 section, it might be useful to suggest an alternative engine for this niche: a ~200-MJ/kg propellant with exhaust temperature requiring magnetic confinement.

My only idea is to imagine some not-yet known isotope (thus obviously fantasy) of hydrogen that decays to ¹H in some controllable way, maybe under irradiation, because nuclear decay is where you usually find that high energy density.   Probably there is a reasonable alternative at the Atomic Rockets page but it is large and I am unfamiliar.

[Edit: looking at Atomic Rockets I see that the more advanced, more speculative, nuclear engines with hydrogen propellant easily have the energy density in their fuel+propellant, and I see a liquid-core nuclear engine was suggested up-thread.   In some cases Atomic Rockets reports graphite nozzles were suggested for the very hot exhaust -- I suppose the nozzle has a short lifetime.

Hydrogen propellant doesn't ionize very easily so doesn't fit well with magnetic nozzles anyway.  Also, maybe a rare off-Kerbin resource is part of the in-game niche to be filled, so maybe that resource could be some other imaginary isotope that produces hot ions. ]

Edited October 7, 2020 by OHara

[DStaal]

30 minutes ago, OHara said:

My only idea is to imagine some not-yet known isotope (thus obviously fantasy) of hydrogen that decays to ¹H in some controllable way, maybe under irradiation, because nuclear decay is where you usually find that high energy density.   Probably there is a reasonable alternative at the Atomic Rockets page but it is large and I am unfamiliar.

It's a bit hard to guess what exact specs they are thinking of for a MMH engine - and they appear to have two modes of it, from the images - but yeah, generally the estimate would be similar to a large swath of the liquid/gas core nuclear engines.  Which design would be best as a replacement largely depends on what your goals are - if you want size, TWR, or ISP as your primary concern you'd pick slightly different designs.  You may want something else as a landing/takeoff engine as well.

[mcwaffles2003]

14 minutes ago, DStaal said:

It's a bit hard to guess what exact specs they are thinking of for a MMH engine - and they appear to have two modes of it, from the images - but yeah, generally the estimate would be similar to a large swath of the liquid/gas core nuclear engines.  Which design would be best as a replacement largely depends on what your goals are - if you want size, TWR, or ISP as your primary concern you'd pick slightly different designs.  You may want something else as a landing/takeoff engine as well.

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Chemical--Metastable--Metallic_Hydrogen

Quote

Injecting enough water propellant to bring the temperature down to 3,500 to 3,800 K will lower the I_sp to 460 to 540 seconds. Doing the same with liquid hydrogen will lower the I_sp to 1,030 to 1,120 seconds.

 

[Guest]

59 minutes ago, OHara said:

I can understand some reasons why some players might be bothered a lot.   Other metastable materials, like hydrazine and hydrogen peroxide, make fine monopropellants.   But the energy per molecule released from H→H₂ is ten times larger, near the top of the scale for energies in chemical bonds.  Chemical bonds struggled to hold together the diamond anvils that (probably) formed monatomic metallic H, and the energy released overcomes chemical bonds trying keep solid the material in the nozzles.  It would be very surprising if the metastable state could be usefully confined by normal materials.  In 2010 two academics speculated at a conference (which is fine) that it might in fact be practical.

Do note, the results disproving the 1970s metastable metallic hydrogen theory are from 2019 (links to study in sig). This means any speculation dating before that had been superseded. Before that date, we had no good evidence (though there were clues) that this theory was wrong. Its predictions have now been tested and found incorrect. AFAIK, it's the only theory that ever predicted metastable metallic hydrogen. Other models don't predict it.

Also, hydrazine and HTP are not metastable. They decompose chemically into different substances, which is a different thing. Metastability in this context refers to a phase change, where energy is stored in phase shift of a single substance. So far, the only substance exhibiting notable metastable states is water, I believe solid HF and ammonia should have such states, too (in water, hydrogen bonds are the cause), but I don't know if they do. As mentioned, a theory predicting this in hydrogen has been disproven.

The replacement engines had been suggested. Vapor core NTR has a broadly similar design, and a liquid core NTR is an almost exact match for performance (and hence my personal favorite for the niche). Advanced nuclear engines are the way to go here.

[OHara]

30 minutes ago, DStaal said:

It's a bit hard to guess what exact specs [the KSP2 developers] are thinking of for a MMH engine

Yes, it is difficult.  Looking back, the strongest source I see for linking that engine to some magnetic nozzle is an informal dinner conversation about the resources they were planning to have in the game, over one year ago.

I would guess the game-idea is to have off-Kerbin resources that enable interstellar engines, but the developers may very well have substituted something else for metallic hydrogen long ago, and we probably wouldn't know yet.

 

10 minutes ago, Dragon01 said:

Metastability in this context refers to a phase change, where energy is stored in phase shift of a single substance. So far, the only substance exhibiting notable metastable states is water,

Okay, I suppose you're considering this context to be physical metastability.   So the the dry-ice form of CO₂  would be analogous.  But usually the physical phase change from solid to gas requires energy input.

But metallic hydrogen, as probably found at the core of Jupiter, certainly has different chemical bonds than the low-pressure form of H₂.  Those H-H bonds are happy to form, and in forming the release that huge amount of energy.  That's why I was thinking it more analogous to chemical monopropellant like 2H₂O₂→2H₂O + O₂ 

[Superfluous J]

1 hour ago, OHara said:

the developers may very well have substituted something else for metallic hydrogen long ago, and we probably wouldn't know yet.

"Advanced Liquid Fuel" or something similarly vague is my hope.