Sorry for the confusion..
Yes but don't all the small hydrocarbons (e.g. methane, ethane, propane, butane) have fairly high vapor pressures, i.e. aren't they gasses under standard conditions? Bigger ones are liquids....
so I would think that tanks with the necessary strength eliminate the problem by allowing pressurization.
Getting LOX cold enough on Mars is what I see as the biggest hurdle. (I am assuming methane as the fuel, not LH2.) As it is compressed, a lot of heat will have to be dumped and you have to get it to -183C (and the methane -162C or so) at STP to liquify and even colder to densify. You can do that, but I just wonder about the mass requirement for such hardware on Mars. When you look at down to 20K for LH2, how much more mass is required for the necessary hardware?
In short, there is no way a hydrolox rocket massing 1.6t dry will boost the same payload as a methalox rocket massing 4t dry. It simply cannot hold enough propellant. (Upper stages are different since they do not have to lift their own mass and thus can have tiny engines and tiny thrust structures. They also don't have to store LH2 for more than a few hours).
Quote from: envy887 on 01/10/2017 09:59 pmIn short, there is no way a hydrolox rocket massing 1.6t dry will boost the same payload as a methalox rocket massing 4t dry. It simply cannot hold enough propellant. (Upper stages are different since they do not have to lift their own mass and thus can have tiny engines and tiny thrust structures. They also don't have to store LH2 for more than a few hours).I don't want to derail this thread any, but has anyone ever worked up what the Space Shuttle would have looked like as methalox instead of LH2? That would be instructive.
Keep in mind all tanks on Earth are "pressurized" to 1 atmosphere absolute. The fact that we breath that pressure normally means we forget about it.
OTOH the atmosphere is 1/160 that of Earth. Not a vacuum but I suspect actual radiation would be a bigger part of how a radiator would dump heat on Mars.
I fear chilling and storing prop on Mars may be more complicated than many people realize due to these issues.
propane melts at 144K and boils at 309K
SpaceX plan to produce Methane on Mars but to do so, they need to harvest water and break it.Step one - produce hydrolox 2H2O -> 2H2 + O2Step two - produce methalox 2H2 + O2 + CO2 -> CH4 + 2O2 My back of the envelope (attached) shows that using the HydroLox from stage one gives almost as much dV as the methalox of both steps, which makes me wonder, does it worth the extra effort of producing methane?
The Sabatier reaction is 4H2 + CO2 -> CH4 + 2H2O. The water goes back into step one. The entire process does not produce sufficient LOX for methalox propellant so additional LOX is obtained by electrolysing CO2 directly. People tend to concentrate on the fuel component, but it's the LOX that dominates the mass of the propellant. Plus Oxygen has other uses!
Quote from: dror on 01/10/2017 07:22 pmSpaceX plan to produce Methane on Mars but to do so, they need to harvest water and break it.Step one - produce hydrolox 2H2O -> 2H2 + O2Step two - produce methalox 2H2 + O2 + CO2 -> CH4 + 2O2 My back of the envelope (attached) shows that using the HydroLox from stage one gives almost as much dV as the methalox of both steps, which makes me wonder, does it worth the extra effort of producing methane? Step two is not how it is usually proposed to produce methalox on Mars. It's generally not a good idea to conflate steps as this can lead you astray.The Sabatier reaction is 4H2 + CO2 -> CH4 + 2H2O. The water goes back into step one. The entire process does not produce sufficient LOX for methalox propellant so additional LOX is obtained by electrolysing CO2 directly. People tend to concentrate on the fuel component, but it's the LOX that dominates the mass of the propellant. Plus Oxygen has other uses!
Quote from: john smith 19 on 01/17/2017 07:23 amOTOH the atmosphere is 1/160 that of Earth. Not a vacuum but I suspect actual radiation would be a bigger part of how a radiator would dump heat on Mars.That is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into space. If you could afford the mass, you could drill vertical shafts into the ground and make an earth-coupled (Mars-coupled??) heat-pump with tubes full of ethylene or propylene glycol, ammonia, or some class of freon and dump the heat into the ground. If you had a trencher or backhoe you could lay out tubes in a back and forth pattern a couple of meters down. My WAG is that something like the ammonia radiators on ISS would be required.
I personally think the ideal mission for the first Red Dragon (other than landing) would be a basic experimental exercise in making and storing LCH4 and LOX. That is the most important thing that has to happen to get back off The Red Planet.
Quote from: TomH on 01/18/2017 02:20 amThat is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into spaceI'd be careful here. While roughly 640Pa is nothing by human standards that's nowhere near hard vacuum. Anyone who's tried to draught proof a house will know that even small draughts can carry a lot of heat.
That is exactly my point. There is almost no atmosphere to use as a heat sink nor to convect heat off of radiator fins. The thermal energy must be radiated into space
Yep, heat transfer coefficient isn't linear to pressure. Martian atmosphere carries heat surprisingly well.https://www.researchgate.net/profile/Alvaro_Soria-Salinas/publication/299600459_Convective_Heat_Transfer_Measurements_at_the_Martian_Surface/links/570361ee08aedbac126f4bc4/Convective-Heat-Transfer-Measurements-at-the-Martian-Surface.pdfEstimates h of almost 10 W/m2K for 4m/s wind. Forced convection would increase that a lot.
The first step beyond methane would likely be ethylene. It is the base of many chemical products. There is also extensive research going on to transfer the production of ethylene from methane from lab status to practical application. This is driven by the fact that many remote oil production facilities still burn methane byproduct because of transport problems. If methane can be processed on site to ethylene this waste would go away. Incidentally it would be extremely useful for Mars.
Yes. When someone said it'd take about 50 000 tonnes of propellant to propel ITS back to Earth I wondered if people realize just how big a system they are talking about.
Quote from: john smith 19 on 02/13/2017 07:40 amYes. When someone said it'd take about 50 000 tonnes of propellant to propel ITS back to Earth I wondered if people realize just how big a system they are talking about.Do you have a link? Back of the envelope suggests that for a minimum energy unmanned return ~1500 tonnes should be enough. Which is still a substantial amount to make on Mars.