From: damarkley@gmail.com   
      
   On Monday, August 10, 2020 at 1:39:51 AM UTC-4, JF Mezei wrote:   
   > There has been discussion on how methane could be produced on Mars.    
   > Carbon from air, H2 from water.    
   >    
   > However, when SpaceX fills its methane tanks in Boca Chica, where does    
   > that methane come from?    
   >    
   > Is "natural gas" functionally equivalent to methane when it comes to    
   > high perforance rocket engines?    
   >    
   > Do components in natural gas other than methane contribute positively to    
   > power ? dead weight? or contribute negativel to combustion, soot etc ?    
   >    
   > It is possible to "filter" natural gas to get pure methane, or does    
   > SpaceX have to make pure methane from Carbon and H2 that it buys from    
   > the local hardware store?    
   >    
   >    
   > If/when it switches its launch of the Raptor-based rockets to KSC, does    
   > this require a lot of new infrastriucture to get methane there? Is this    
   > something shipped in ships? I assume there are no "methane" only pipelines?    
   > Or would SpaceX make methane at KSC ?   
      
   Natural gas is essentially methane.  Depending on where it comes from it can   
   have varying proportions of other gases such as ethane, propane, butane,   
   hydrogen, carbon monoxide, carbon dioxide, nitrogen, helium and argon.    
   Anything can be purified of    
   course.  I do not know what SpaceX is using but I would bet it's rather pure   
   methane.   The other gases I listed would all decrease the energy content of   
   the fuel since methane has the highest ratio of hydrogens. Those other gases   
   (the hydrocarbons at    
   least) would all contribute to deposits which decrease nozzle efficiency.    
   Liquified methane, while still cryogenic is a lot easier to handle than   
   liquified hydrogen.  Now having written all that, I just found an excellent   
   explanation on Reddit:  "Why    
   use methane and not hydrogen?   
      
   Using methane+LOX (methalox) as propellants provides a lot of benefits over   
   traditional hydrogen+LOX (hydralox) launch systems. Hydrogen gives a higher   
   specific impulse (>400s), but all of the modifications required to deal with   
   hydrogen negate that gain.   
    Because methane requires much smaller tanks than hydrogen does, it makes the   
   overall design much lighter. Methane is more stable in space over long periods   
   of time vs hydrolox, and doesn't need such highly insulated cryogenic tanks   
   like hydrolox. It    
   also has a boiling point much closer to that of oxygen, allowing a simple   
   bulkhead design. Having a density closer to that of oxygen allows for a   
   simpler turbopump (hydrolox is very hard on a turbopump, see Space Shuttle   
   main engines). Higher thrust    
   level helps first-stage get off the ground easier. In addition, liquid   
   hydrogen causes hydrogen embrittlement, where hydrogen atoms alloy themselves   
   into their metal containers, and so weaken the structure. At high pressures,   
   this can be catastrophic.    
   Liquid hydrogen causes so many problems; Elon once eloquently said that   
   methane "doesn't have the pain-in-the-ass factor that hydrogen has."   
      
   Why use methane and not RP-1?   
      
   Methane also has benefits over SpaceX's current fuel, RP-1. It can be   
   manufactured on Mars by the Sabatier process. Methane also helps their   
   reusability aims, as RP-1 creates a lot of carbon when it burns, coking up   
   engines and slowing their reuse,    
   whereas methane has no such problem. Methane burns hotter and is lighter than   
   kerosene, so it has a slightly higher specific impulse than kerosene; an   
   engine with the same combustion pressure and efficiency will have a 10 second   
   higher specific impulse    
   when using methane instead of kerosene. However, methane is a lot less dense   
   than kerosene, which requires heavier tanks, which mostly offsets this   
   increase in performance. Still, methane is close to being an ideal "best of   
   both worlds fuel", and ticks a    
   lot of boxes for SpaceX."   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)