Ethylene may be great once you get it inside the combustion chamber but will it behave properly when driven thru turbopumps and coolant channels? Thermal stability, tendency to polymerize and all that?
What's the route from basic Martian resources to synthesize ethylene?
3. Relative ease of ISRU methane production on Mars compared to heavier hydrocarbons (though this argument does not make make much sense to me); and
That the synthesis of methane is easier than that of heavier hydrocarbons makes perfect sense. What I doubt is that plans for Martian ISRU in the future much influenced SpaceX's choice of propellants for the near term (Raptor).
Try subcooled propylene as a propellant. Even better than ethylene.
Liquefied natural gas enhances affordability and reusabilityLiquefied natural gas is commercially available, affordable, and highly efficient for spaceflight. Unlike other rocket fuels, such as kerosene, liquefied natural gas can be used to pressurize a rocket’s propellant tanks. This is called autogenous pressurization and eliminates the need for costly and complex pressurization systems, like helium. Liquefied natural gas also leaves no soot byproducts as kerosene does, simplifying engine reuse.
You need a catalyst for ethylene to polymerise, so the liquid may still be stable under heat and pressure. According tohttp://www.chemguide.co.uk/mechanisms/freerad/polym.htmlyou need 200 C and 2000 atm of pressure. Typical staged combustion engines are only 200 atm, so this is probably well below what could cause a problem.
Robotbeat, Niloff: what I'm thinking is that lots of people are keen on lox-methane these days, like BO, ULA and Firefly (last I heard Firefly had switched to lox-RP-1, but methane was originally baselined), and they're not all focused on Mars. On the BE-4 info sheet, BO says this about its choice of LNG:Quote from: Blue OriginLiquefied natural gas enhances affordability and reusabilityLiquefied natural gas is commercially available, affordable, and highly efficient for spaceflight. Unlike other rocket fuels, such as kerosene, liquefied natural gas can be used to pressurize a rocket’s propellant tanks. This is called autogenous pressurization and eliminates the need for costly and complex pressurization systems, like helium. Liquefied natural gas also leaves no soot byproducts as kerosene does, simplifying engine reuse..Another factor is that even if SpaceX succeeds wildly, it's going to launch one heckuva lot of mass from the the surface of the Earth before it produces its first liter of ISRU methane. Since there's a lot of hardware to be developed on the way to that first liter of ISRU methane anyway, I would think it would make sense to optimize for Earth launch for the time being and then tweak propulsion systems as needed later for ISRU methane.So, I could believe that if there were several propellant combinations that were approximately equally attractive, SpaceX would probably choose the one that's best for Mars. But if there were other fuels nearly as attractive as methane, why are none of the less-Mars-obsessed players pursuing them?
Very generally on the top of specific impulse and impulse density, I was thinking about optimal mixture ratios. If oxidizer and fuel have different densities, then impulse density will peak at a mixture ratio corresponding to a higher propellant bulk density than where the specific impulse peaks. The larger the difference in the densities of oxidizer and fuel, the larger will tend to be the difference in the mixture ratios of the two peaks. Consider lox-hydrogen. The attached figure shows specific impulse1 and impulse density as a function of both bulk density (lower horizontal axis) mixture ratio (O/F: upper horizontal axis)2.Obviously specific impulse peaks at a density of about 317 kg/m3 (O/F=4., while impulse density peaks at 633 kg/m3 (O/F=17.. Lox-hydrogen stages usually operate at mixture ratios of about 5 or 6. You can imagine situations where you'd want to go significantly higher than that. But you'd never want to go to a bulk density lower than that of maximum specific impulse or higher than that of maximum impulse density. Anyway, there's nothing very profound about this, but I thought I'd mention it, because it hadn't occurred to me before.1. Specifically, these values are 95% of the ideal vacuum values calculated with RPA Lite; the densities of both propellants correspond to those at their respective normal boiling points.2. Note, though, that the mixture-ratio axis is non-linear, because bulk density is not a linear function of mixture ratio (though specific volume, the reciprocal of density, is).
Good point. For perfect burning, mixture ratio should be 16.
It would seem that RS-68 uses too small mixture ratio;