From: snidely.too@gmail.com   
      
   Just this Sunday, Jeff Findley puzzled about:   
   > In article , jfmezei.spamnot@vaxination.ca   
   > says...   
   >>   
   >> I ask this conceptually, not whether it makes sense or not for SpaceX.   
   >>   
   >> How difficult would it be to convert Falcon9 from kerosene/Merlin to   
   >> Methane/Raptor engines?   
   >   
   > Difficult.  Changing the fuel changes pretty much everything.   
   >   
   >> is this a question of rebuilding first stage from scratch and only keep   
   >> the grid fins, landing gear and software?   
   >   
   > Rebuild from scratch.  The lower density of liquid methane (430 kg/cubic   
   > meters) compared to kerosene (775-840 kg/cubic meters) would mean you'd   
   > need to increase the diameter since Falcon is already pushing the   
   > fineness ratio close to the limits.  So, you wouldn't be able to reuse   
   > any of the tooling and couldn't transport the stages with semi-trucks.   
      
   My back of envelope calculations are   
      
   -- you need 3.5 Raptors to match the thrust of 9 Merlins (depending on   
   which source you use for Raptor and Merlin thrust; WP for me)   
   -- my rough sketch suggests you can fit 5 Raptors in the skirt of an   
   F9, one of them in the center   
   -- for a density 438.9 g/l (which may be the super-chilled density at   
   -173 C), the existing RP-1 tankage could supply 5 Raptors for 91   
   seconds or 4 Raptors for 114 seconds.   
   -- F9 first stage time is typically 162 seconds.   
      
   I've made no corrections for differing oxygen consumption rates.  I   
   don't think we'd have to worry about tank walls, as the F9 LOX tank is   
   already the same material (an Al alloy), AIUI, and probably can skip   
   adding insulation to the tank given the just-in-time tanking policy.   
      
      
   >> Or just change engine mounts, and use the LOX tank design onto the   
   >> methane fuel tank ?   
   >   
   > See above.   
   >   
   >> Since the Raptor engines are already designed/tested, they already have   
   >> the tooling to make engines, and the fuselage for Falcon9, the software,   
   >> grid fins, landing gear, and interafce to stage2, just curious whether   
   >> it would represent an order of magnitute similar to building new rocket   
   >> or whether it would be relatively minor % of developping from scratch.   
   >   
   > Pretty much redesign from scratch.   
   >   
   >> And more generically from a market point of view: Will starship   
   >> obliterate the demand for smaller launchers such as Falcon9, or will   
   >> customers still want "their own" smaller launcher to get to their   
   >> desired obrit on their own timing without requiriong cooprdination with   
   >> many other customers who also want same orbit and are ready with their   
   >> payloads at same time?   
   >   
   > Time will tell.   
      
   The expectation is that certain payloads will keep F9 around for a   
   while; the two Dragon variants are among those payloads, and   
   speculation is that some NROL payloads will also mandate F9 for a   
   while.  This is a frequent talking point on NSF livestreams, but a 6   
   hour tanking test allows for a lot of talking points.   
      
   The other issue is that F9 has a well-understood deployment design,   
   while Starship satellite deployment  isn't really known outside of   
   SpaceX, although the "alligator" renders are one possibility.  We will   
   know more when the first Starship Starlink launch occurs.   
      
   /dps   
      
   --   
   You could try being nicer and politer   
   > instead, and see how that works out.   
        -- Katy Jennison   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)