DDE Posted January 29, 2019 Share Posted January 29, 2019 1 hour ago, tater said: Can you just imagine a civvie seeing such a vessel without knowing its purpose? Quote Link to comment Share on other sites More sharing options...
sh1pman Posted January 29, 2019 Share Posted January 29, 2019 18 minutes ago, DDE said: Can you just imagine a civvie seeing such a vessel without knowing its purpose? My first thought would be “Oh, that must be the infamous Space Trampoline!” Quote Link to comment Share on other sites More sharing options...
StrandedonEarth Posted January 29, 2019 Share Posted January 29, 2019 40 minutes ago, DDE said: Can you just imagine a civvie seeing such a vessel without knowing its purpose? “Someone must be planning one heck of a Human Cannonball shot...!” Quote Link to comment Share on other sites More sharing options...
Rakaydos Posted January 29, 2019 Share Posted January 29, 2019 Concerning methane disassociation products, someone on NSF explained that it wasn't a problem in hypersonic flight due to "frozen reactions". If I understood it right, a mach 5+ airstream is an effective "someone else's problem" field- by the time the methane can actually disassociate, it's been blown far from the spaceship. They also mentioned that this is a major problem with Scramjet design, as your jet fuel leaves the engine before it can burn. Quote Link to comment Share on other sites More sharing options...
sevenperforce Posted January 29, 2019 Share Posted January 29, 2019 14 minutes ago, Rakaydos said: Concerning methane disassociation products, someone on NSF explained that it wasn't a problem in hypersonic flight due to "frozen reactions". If I understood it right, a mach 5+ airstream is an effective "someone else's problem" field- by the time the methane can actually disassociate, it's been blown far from the spaceship. They also mentioned that this is a major problem with Scramjet design, as your jet fuel leaves the engine before it can burn. "Someone on NSF" is correct. Quote Link to comment Share on other sites More sharing options...
Mad Rocket Scientist Posted January 29, 2019 Share Posted January 29, 2019 13 hours ago, Cunjo Carl said: So, I'm still on the new heat shield. It's interesting from a technical perspective, so I can't really help it! The more I look at methane, the more it looks... completely intractable as a reentry coolant. The problem isn't just that it decomposes when hot, but what it pyrolyzes into at 1000C: Carbon Black (or a relative like graphite or carbon nanotubes). [...] I thought gasses at reentry temps would just disassociate into plasma, does anyone know the boundary between those processes? Or if I'm just completely misunderstanding? Quote Link to comment Share on other sites More sharing options...
Rakaydos Posted January 29, 2019 Share Posted January 29, 2019 1 hour ago, sevenperforce said: "Someone on NSF" is correct. User Semmel. Didn't want to take credit, was too lazy to double-check. Quote Link to comment Share on other sites More sharing options...
sevenperforce Posted January 29, 2019 Share Posted January 29, 2019 9 minutes ago, Mad Rocket Scientist said: I thought gasses at reentry temps would just disassociate into plasma, does anyone know the boundary between those processes? Or if I'm just completely misunderstanding? The atmosphere will be shock-compressed and undergo compressive heating. Some of that heat will mercifully be used up disassociating the nitrogen and oxygen into plasma. The rest will radiate away as a blackbody from the plasma in every direction. Radiation incident on the vehicle will heat the vehicle. Forcing methane through tiny pores accomplishes two things: first, it provides a heat sink to carry thermal energy away from the vehicle; second, it blocks some portion of the radiation incident on the vehicle. If the methane does pyrolize appreciably, that's a good thing, because carbon black is almost completely opaque to radiation and will pick up more than methane gas alone. 11 minutes ago, Rakaydos said: User Semmel. Didn't want to take credit, was too lazy to double-check. Heh! Wasn't trying to snark; I was just saying that whoever that guy is, he's right. Quote Link to comment Share on other sites More sharing options...
CatastrophicFailure Posted January 29, 2019 Share Posted January 29, 2019 In other news, due to the delay with Demo-1, SpaceIL’s lunar lander is now next on the manifest, and according to SFN, now has a date & time of 0158 GMT on Feb 19th (8:58 p.m. EST on 18th). Crap. That’s right in the middle of my work day. Quote Link to comment Share on other sites More sharing options...
Spaceception Posted January 29, 2019 Share Posted January 29, 2019 2 hours ago, Rakaydos said: Concerning methane disassociation products, someone on NSF explained that it wasn't a problem in hypersonic flight due to "frozen reactions". If I understood it right, a mach 5+ airstream is an effective "someone else's problem" field- by the time the methane can actually disassociate, it's been blown far from the spaceship. They also mentioned that this is a major problem with Scramjet design, as your jet fuel leaves the engine before it can burn. That's cool, by the time it can form those products, it's not an issue anymore. What about on the return trip, will you need to clean it off, or will it do the same thing? Quote Link to comment Share on other sites More sharing options...
sh1pman Posted January 29, 2019 Share Posted January 29, 2019 1 hour ago, Mad Rocket Scientist said: I thought gasses at reentry temps would just disassociate into plasma, does anyone know the boundary between those processes? Or if I'm just completely misunderstanding? Pyrolysis will come first. Plasma is a gas that contains electrons, ions and neutral particles. Methane can’t really exist in a stable plasma form; hot free electrons will destroy all molecules and molecular ions, and the only things left will be carbon black and hydrogen plasma. I don’t know the temperatures required to vaporize and ionize carbon (probably several 1000s K), but pyrolysis happens at 1500K or so. Quote Link to comment Share on other sites More sharing options...
sevenperforce Posted January 29, 2019 Share Posted January 29, 2019 Of note -- methane pyrolysis takes place through a chain, as fusions yield successively ethylene, acetylene, and then finally carbon-carbon. While this takes place quickly, it is enough of a delay that total pyrolysis will happen well out away from the vehicle surface. 55 minutes ago, Spaceception said: What about on the return trip, will you need to clean it off, or will it do the same thing? We are talking about the return trip, right? Quote Link to comment Share on other sites More sharing options...
Cunjo Carl Posted January 29, 2019 Share Posted January 29, 2019 2 hours ago, Rakaydos said: Concerning methane disassociation products, someone on NSF explained that it wasn't a problem in hypersonic flight due to "frozen reactions". If I understood it right, a mach 5+ airstream is an effective "someone else's problem" field- by the time the methane can actually disassociate, it's been blown far from the spaceship. They also mentioned that this is a major problem with Scramjet design, as your jet fuel leaves the engine before it can burn. I couldn't find the article you're talking about, if it's handy could you provide the link? It sure sounds nice, but to be honest I'm not quite seeing how this would work. It sounds like the idea is that the air flow is so high, that actual time it takes for the methane to dissociate will be longer than its residence time near the craft.This makes a lot of sense for a combustion reaction which requires multiple collisions of large-ish molecules (kerosene), each of which involves a multi-step reaction with lots of reaction intermediates. Totally makes sense! That can take time. With methane though, it's a surface catalyzed decomposition we're interested in. The methane can happily stick to the surface covalently through the process. No collisions in the gas phase are strictly required, besides the methane initially hitting the surface. Certainly, hydrocarbons can pyrolyse in the gas phase (especially the big ones), but they'll happily do it attached to a surface as well. I'm sure there's a paper on it..... Here (see discussion) . It's actually pretty perfect, 1000C methane for 10min on Iron Oxide! (I realize stainless will have a lot of chrome-oxide on the surface as well, but still) The 'amorphous carbon' they talk about is also called carbon black, but for thermal purposes, the carbon nanotubes they grow are pretty equivalent, if not a bit worse. Any ways, the point is that chemistry can happen without gas collisions, and the methane is being injected directly into the boundary layer near the metal surfaces. (as a note, chemically, a medium speed reaction at 1000C will typically be a very fast one at 1200C) The relative amounts are also worrisome. For an engine, you want near-complete combustion, but in this case we'd be worried if even 0.001% of our methane did this. (Assuming maybe 10tons of methane used across a 750m2 surface, given 12/16 weight loss during pyrolysis, a final solid density of ~2, and ~50nm of film being enough to effect optical properties.) Finally, I'd heard the metal surface was set to be ~1200C, so the methane within the pores is bound to have portions being plenty hot enough for all this to happen as well. I'm not even sure how we'd avoid clogging the pores, to be honest! It's kinda too bad to be honest. Otherwise dissociation reactions like this are almost always endothermic (heat absorbing) and can really improve a gasses heat capacity! I'd still be happy to be wrong, I'd like to give your article a read! Quote Link to comment Share on other sites More sharing options...
Xd the great Posted January 29, 2019 Share Posted January 29, 2019 (edited) Well, MOAR flaming and reentry flame effects are always welcome. What is the temperature required to burn carbon soot? Will we see a burning starship due to these carbon soot? Edited January 29, 2019 by Xd the great Quote Link to comment Share on other sites More sharing options...
Cunjo Carl Posted January 30, 2019 Share Posted January 30, 2019 39 minutes ago, Xd the great said: Well, MOAR flaming and reentry flame effects are always welcome. What is the temperature required to burn carbon soot? Will we see a burning starship due to these carbon soot? You're right! I should be more Kerbal about this. Most organics will happily ignite somewhere between room temp and ~500C (that is to say, they'll become flammable but necessarily ignite without provocation 'autoignition') Carbon (like carbon black, carbon nanotubes, etc) tends to oxidize but not neccesarily completely burn in the 500-650C range, and will burn shortly above that if there's a good oxidizer present. I've been assuming the sides of the Starship will be mostly ensconced in methane during reentry, so the oxidation potential would be poor, but could definitely be wrong about that. Pyrolysis/Evaporation of carbons like these without an oxidizer happens above 3000C! It's quite special. I've heard people say that the energy of the methane burning will be so much less than the energy of reentry that it won't make a visible difference. It might be interesting to see a smoke trail though, if perhaps the conditions are right? ... That would look really cool, actually- like in the movies! Quote Link to comment Share on other sites More sharing options...
Spaceception Posted January 30, 2019 Share Posted January 30, 2019 1 hour ago, sevenperforce said: We are talking about the return trip, right? Yes. I'm assuming you will need to clean it, but I'm making sure. Quote Link to comment Share on other sites More sharing options...
Mad Rocket Scientist Posted January 30, 2019 Share Posted January 30, 2019 4 minutes ago, Cunjo Carl said: You're right! I should be more Kerbal about this. Most organics will happily ignite somewhere between room temp and ~500C (that is to say, they'll become flammable but necessarily ignite without provocation 'autoignition') Carbon (like carbon black, carbon nanotubes, etc) tends to oxidize but not neccesarily completely burn in the 500-650C range, and will burn shortly above that if there's a good oxidizer present. I've been assuming the sides of the Starship will be mostly ensconced in methane during reentry, so the oxidation potential would be poor, but could definitely be wrong about that. [...] Is this Kerbal enough though? There's plenty of oxidizer very nearby... Quote Link to comment Share on other sites More sharing options...
cubinator Posted January 30, 2019 Share Posted January 30, 2019 57 minutes ago, Mad Rocket Scientist said: Is this Kerbal enough though? There's plenty of oxidizer very nearby... Like I said before, if the methane and oxygen explodes in front of the rocket it will accelerate her towards retrograde, shortening reentry time. Delightfully counterintuitive! Quote Link to comment Share on other sites More sharing options...
Xd the great Posted January 30, 2019 Share Posted January 30, 2019 3 hours ago, Spaceception said: Yes. I'm assuming you will need to clean it, but I'm making sure. "Our starship has failed, because the cost of cleaning carbon soot are too high" Quote Link to comment Share on other sites More sharing options...
CatastrophicFailure Posted January 30, 2019 Share Posted January 30, 2019 poop. Quote Link to comment Share on other sites More sharing options...
Xd the great Posted January 30, 2019 Share Posted January 30, 2019 Its a new ship. What did you expect? Quote Link to comment Share on other sites More sharing options...
DDE Posted January 30, 2019 Share Posted January 30, 2019 7 hours ago, cubinator said: Like I said before, if the methane and oxygen explodes in front of the rocket it will accelerate her towards retrograde, shortening reentry time. Delightfully counterintuitive! Probably not. It could instead eliminate much of the air resistance. Quote Link to comment Share on other sites More sharing options...
Xd the great Posted January 30, 2019 Share Posted January 30, 2019 1 hour ago, DDE said: Probably not. It could instead eliminate much of the air resistance. How? By breaking the shck waves in front? Quote Link to comment Share on other sites More sharing options...
kerbiloid Posted January 30, 2019 Share Posted January 30, 2019 17 hours ago, DDE said: Can you just imagine a civvie seeing such a vessel without knowing its purpose? It's an ark. Spoiler It plays ark'anoid. Quote Link to comment Share on other sites More sharing options...
Xd the great Posted January 30, 2019 Share Posted January 30, 2019 17 hours ago, DDE said: Can you just imagine a civvie seeing such a vessel without knowing its purpose? Someone skydiving without a chute. Quote Link to comment Share on other sites More sharing options...
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