Xd the great Posted February 11, 2019 Share Posted February 11, 2019 How on earth are they going to prevent an explosion when the methane mixes with air halfway through reentry? Quote Link to comment Share on other sites More sharing options...
sh1pman Posted February 11, 2019 Share Posted February 11, 2019 2 minutes ago, Xd the great said: How on earth are they going to prevent an explosion when the methane mixes with air halfway through reentry? By flying very fast Quote Link to comment Share on other sites More sharing options...
tater Posted February 12, 2019 Share Posted February 12, 2019 6 minutes ago, Xd the great said: How on earth are they going to prevent an explosion when the methane mixes with air halfway through reentry? One, oxygen density will be quite low that high. Two, compared tot he kinetic energy of the spacecraft, I think the chemical energy of the small amount of methane leaking out the pores is trivial. Quote Link to comment Share on other sites More sharing options...
Xd the great Posted February 12, 2019 Share Posted February 12, 2019 8 minutes ago, sh1pman said: By flying very fast I am talking about the break even point, where they stop supplying methane but some is still left in the sandwitch part. The methane and oxygen may mix, and I am not sure if a piece of stainless steel can withstand that much force. Quote Link to comment Share on other sites More sharing options...
RedKraken Posted February 12, 2019 Share Posted February 12, 2019 BFR REentry math : by sebaska https://www.reddit.com/r/spacex/comments/ain1kk/elon_musk_why_im_building_the_starship_out_of/eepp18r/ "The bow shock pressure is not immense. The compression ratio is immense (hundreds to thousands times) but if you are compressing from 0.0001 bar ambient, the end result is small. Edit: Actually you can easily estimate pressure: Loaded Starship has about 200t mass. It had about 500m² surface. As hypersonic lift is predominantly caused by bottom/windward surface (contrary to subsonic, transonic and low-mid supersonic, where upper wing surface provides 80+% of the lift), so 500m² surface supports 200t mass at re-entry g-load. With 5g it means 20kN/m² i.e. 20kPa, i.e. ~0.2 sea level pressure." Quote Link to comment Share on other sites More sharing options...
StrandedonEarth Posted February 12, 2019 Share Posted February 12, 2019 14 minutes ago, Xd the great said: I am talking about the break even point, where they stop supplying methane but some is still left in the sandwitch part. The methane and oxygen may mix, and I am not sure if a piece of stainless steel can withstand that much force. I'm thinking it would burn on contact, not mixing enough to explode. Quote Link to comment Share on other sites More sharing options...
tater Posted February 12, 2019 Share Posted February 12, 2019 (regarding the NASA call for crew lander designs from the private sector). Also, regarding the AF contract where SpaceX got nothing (and there are questions about why): Quote Link to comment Share on other sites More sharing options...
Cunjo Carl Posted February 12, 2019 Share Posted February 12, 2019 I found a fun anecdotal talk from Mueller about the development of the Merlin engine, which I thought I'd share. Part 1 Part 2 He talks briefly about the pintle injector he used for Merlin, so I did some research to find they're singularly great for throttlability, low-cost, and combustion stability. Apparently Sea Dragon was going to use pintle injectors, and there's (unsubstantiated) speculation that Raptor may use them as well. The engine's rapid tuneability kinda suggests it is, and the fact raptor is gas-gas suggests it isn't. It's apparently still a mystery! Here's a nice easy-read review paper on pintle injectors, and how they do what they do so well. http://www.rocket-propulsion.info/resources/articles/TRW_PINTLE_ENGINE.pdf The Evelyn Woods is, the fuel is sprayed radially from a single spray nozzle at the center (like the cone setting on a garden hose), and the oxidizer is sprayed straight through a cylindrical nozzle around that. The two flows meet at 90degrees to each other, so they hit hard and mix mid-flight into a cone shaped flow. A little bit of the radial fuel spray will sneak through, and cause 'film cooling' along the walls of the combustion chamber, helping prevent the engine-rich exhaust we saw in last week's Raptor test. Probably the biggest bonus for SpaceX, the combustion can be quickly throttled right at the injector head with a single hydraulic actuator for both fuel and oxidizer. Anyways, I hope someone finds it an interesting watch/tidbit like I did. Cheers! Quote Link to comment Share on other sites More sharing options...
zolotiyeruki Posted February 12, 2019 Share Posted February 12, 2019 5 hours ago, sh1pman said: But he said “build”, any sane person would parse that as “the cost of making the entire rocket from ground up, incl. engines”. I don’t know how 38 Raptors + SS hull can be cheaper than 10 M1D + aluminum hull, but it seems that’s what he meant. Ah, for some reason I was thinking the tanks on F9 were CFC. Probably got that mixed up because BFR was originally going to use 9m carbon fiber tanks and F9 used COPVs for some stuff. Quote Link to comment Share on other sites More sharing options...
StrandedonEarth Posted February 12, 2019 Share Posted February 12, 2019 3 minutes ago, zolotiyeruki said: Ah, for some reason I was thinking the tanks on F9 were CFC. Probably got that mixed up because BFR was originally going to use 9m carbon fiber tanks and F9 used COPVs for some stuff. Well, F9B5 did start using a CFC interstage Quote Link to comment Share on other sites More sharing options...
tater Posted February 12, 2019 Share Posted February 12, 2019 Quote Link to comment Share on other sites More sharing options...
Nightside Posted February 12, 2019 Share Posted February 12, 2019 9 minutes ago, tater said: Now they just have to get back to Florida one more time. Do we know anything about the survival chances of the center core? Quote Link to comment Share on other sites More sharing options...
tater Posted February 12, 2019 Share Posted February 12, 2019 3 minutes ago, Nightside said: Do we know anything about the survival chances of the center core? Moar TEA-TEB! Quote Link to comment Share on other sites More sharing options...
Xd the great Posted February 13, 2019 Share Posted February 13, 2019 If the launch price of the starship is 1 million, it will be the cheapest rocket ever. Even cheaper than cubesat launchers. Biggest rocket launch smallest sats. Nice. Quote Link to comment Share on other sites More sharing options...
Cunjo Carl Posted February 13, 2019 Share Posted February 13, 2019 (edited) Oh, I figured out why they aren't worried about their 1200C steel surfaces sooting up into a black mess with carbon from the Methane! First, I found a paper confirming that sooting occurs on metal oxide surfaces even with 0 reaction time, so the winds from reentry really aren't going to make the problem completely go away. Above about 800C, we'd slowly get soot even from methane. Grown in this way, the soot would be harder and more chemically resilient than the stainless steel, so cleaning it would be a bear. But then I had an epiphany (well, perhaps it was more of a 'no duh' moment :D). Above 1000C even stainless steel will 'carburize'- it will actually soak up Carbon into the metal! This is great because above 1000C it will allow the surface to stay nice and shiny. However, it's also bad because if done wrong it can change the structure's properties during reentry, which doesn't sound like the greatest even in the context of SpaceX's joy for cowboy engineering. Done correctly, carburization hardens steel surfaces by increasing Carbon content, permanently improving both scratch and corrosion resistance. However, too much carburization causes brittleness and enormous internal stresses. An additional concern, once the heat shield is carburized, keeping it between 500-1000C for longer than about 10 minutes will cause the Chrome to precipitate out (as Chrome Carbide) and result in all sorts of nastiness. Heating it back above 1000C will reset the timer though, so in the context of reentry to sea level this isn't so difficult, but it's still an added requirement. Methane is great at carburizing steel, and can work in the 1s of minutes timescale at 1200C, causing problems for repeated use. Fortunately, the carburization reaction can also be reversed. H20 and CO2 are excellent at decarburizing steel even in moderate concentrations, effectively removing the excess carbon from the steel in the form of CO. Now, if only we had some way to make H2O and CO2 midflight (nudge nudge, wink wink @cubinator ) . So if the correct levels of CH4, CO2, and H20 are maintained, the carburization (carbon content) in the steel will also be maintained, and the shiny hot steel will remain intact for many uses. As a result, the steel heat shield would stay at a carbon equilibrium and would be effectively be catalytic for: CH4 + CO2 -> 2CO + 2H2 and CH4 + H2O -> CO + 3H2 This would be a nice side bonus because these reactions should be endothermic (absorbing heat), and are also fantastic for increasing the specific heat of the resulting gas! Generally for holding heat, the more moles of gas you can split your molecules into the better they become at soaking up heat. The above reactions both double the moles of gas from their reactants, so they're great from a thermal standpoint. The Oxygen from the reentry wind will also be decarburizing, but I'm guessing it's to a lesser extent in context, and the SpaceX engineers would need to account for carburization inside the pores and heat shield as well. This is a much nicer looking picture now, but it brings up a whole new interesting set of questions. How will they manage carburization/decarburization during reentry? Or will the just accept the heavy surface carburization for what it is and maintain/swap heat shields accordingly? This sounds like the sort of nuts-and-bolts problem SpaceX excels at solving, and I'm interested to see what they come up with. Images care of JFP Technical and Phase-Trans respectively. Images show carburization (left) and decarburization (right) in the surface of stainless steel. SpaceX will probably want to be somewhere in between! Edited February 13, 2019 by Cunjo Carl Quote Link to comment Share on other sites More sharing options...
Elthy Posted February 13, 2019 Share Posted February 13, 2019 Wow, this sounds like something the SpaceX reddit would love. Very good find! Quote Link to comment Share on other sites More sharing options...
DDE Posted February 13, 2019 Share Posted February 13, 2019 1 hour ago, Elthy said: Wow, this sounds like something the SpaceX reddit would love. Very good find! What doesn’t kill Starship makes it tougher. Quote Link to comment Share on other sites More sharing options...
tater Posted February 13, 2019 Share Posted February 13, 2019 Quote Link to comment Share on other sites More sharing options...
sh1pman Posted February 13, 2019 Share Posted February 13, 2019 Something's happening there, or about to happen Quote Link to comment Share on other sites More sharing options...
tater Posted February 13, 2019 Share Posted February 13, 2019 Probably a visitor. Quote Link to comment Share on other sites More sharing options...
tater Posted February 13, 2019 Share Posted February 13, 2019 https://hackaday.com/2019/02/13/the-impossible-tech-behind-spacexs-new-engine/ Quote Link to comment Share on other sites More sharing options...
sh1pman Posted February 13, 2019 Share Posted February 13, 2019 13 minutes ago, tater said: https://hackaday.com/2019/02/13/the-impossible-tech-behind-spacexs-new-engine/ Still don’t quite understand why regular staged combustion closed cycle scheme (like in RD-180) is inherently less efficient than full flow staged combustion. Good article, though. Great for showing to people who don’t know a thing about rocket engines. Quote Link to comment Share on other sites More sharing options...
magnemoe Posted February 13, 2019 Share Posted February 13, 2019 Just now, sh1pman said: Still don’t quite understand why regular staged combustion closed cycle scheme (like in RD-180) is inherently less efficient than full flow staged combustion. Good article, though. Great for showing to people who don’t know a thing about rocket engines. Problem is that the turbo pump don't burn as efficient as the rocket as it would melt the turbine blades. So they have to burn fuel rich and dump un-burned fuel who acted as coolant. In jet engines nitrogen act as coolant the same way. They also pump air inside the turbine blades to cool them. Quote Link to comment Share on other sites More sharing options...
sh1pman Posted February 13, 2019 Share Posted February 13, 2019 2 minutes ago, magnemoe said: Problem is that the turbo pump don't burn as efficient as the rocket as it would melt the turbine blades. So they have to burn fuel rich and dump un-burned fuel who acted as coolant. No that’s open cycle. I’m talking about closed cycle, with preburner gas being sent to combustion chamber. Quote Link to comment Share on other sites More sharing options...
sevenperforce Posted February 13, 2019 Share Posted February 13, 2019 17 minutes ago, sh1pman said: Still don’t quite understand why regular staged combustion closed cycle scheme (like in RD-180) is inherently less efficient than full flow staged combustion. Good article, though. Great for showing to people who don’t know a thing about rocket engines. The article wasn't entirely correct; from a fuel consumption standpoint, an ORSC or FRSC engine is just as efficient. However, the preburner must be oversized in order to run both oxygen and fuel turbopumps, and using the same driveshaft runs into seal issues. You also have mixture problems, and you can't run chamber pressure nearly as high as a FFSC because you're running split. Quote Link to comment Share on other sites More sharing options...
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