XPost: rec.arts.sf.written, sci.space.policy, rec.arts.sf.movies   
   From: jfindley@cinci.nospam.rr.com   
      
   In article ,   
   fjmccall@gmail.com says...   
   >   
   > Jeff Findley  wrote on Mon, 28 May 2018   
   > 10:40:55 -0400:   
   >   
   > >In article ,   
   > >fjmccall@gmail.com says...   
   > >>   
   > >> "Robert Clark"  wrote on Mon, 28 May   
   > >> 2018 07:19:51 -0400:   
   > >>   
   > >> >   
   > >> >With altitude compensation, allowing even first stage nozzles to achieve   
   the   
   > >> >highest possible vacuum Isp, and using also  lightweight structures such   
   as   
   > >> >composite tanks, the payload fraction of an SSTO can be comparable to   
   that   
   > >> >of multi-stage rockets, ca. 3%.   
   > >> >   
   > >>   
   > >> Sorry, but that doesn't follow.  If you can build an SSTO using those   
   > >> technologies you can also build a multi-stage rocket using the same   
   > >> technologies.  The multi-stage rocket will always do better.   
   > >   
   > >True for lightweight structures.  Not so true for altitude compensating   
   > >engines (e.g. aerospike engines and the like).   
   > >   
   >   
   > Why not?  What prevents the use of such engines on a multi-stage   
   > vehicle once you have them for an SSTO?   
      
   Other than complexity and cost, nothing.  For a reusable first stage it   
   might make sense to incorporate altitude compensating engines, if   
   development were already paid for, but the devil is still in the   
   details.  The higher cost and complexity of the engines would need to be   
   paid for, somehow.   
      
   For a reusable SSTO it's arguably an enabling technology due to the   
   razor thin mass margins.  A reusable SSTO seems to be the "performance   
   uber alles" solution in the reusable space.  Eventually they may prove   
   to be useful, but as you say any innovation which applies to them could   
   be applied to a TSTO, but only if it makes economic sense.   
      
   An example of something which makes no sense on a TSTO would be SABRE   
   engines.  Compared to a liquid fueled rocket engine, they're going to be   
   hideously complex, hideously expensive, and a lot heavier.  It just   
   doesn't make sense to take the cheaper reusable rocket engines off a   
   first stage and replace them all with SABRE engines.  Your vehicle will   
   need LOX in the 2nd stage anyway, so you're just eliminating the need   
   for it on part of the vehicle.  And LOX is so cheap and readily   
   available, it's just daft.   
      
   > >   
   > >For a two stage  to orbit vehicle, it's just a lot easier to optimize   
   > >the first stage engines for operation in the atmosphere and optimize the   
   > >upper stage engines for operation in vacuum.  This is exactly what   
   > >SpaceX has done with Falcon 9's Merlin engines.  The vacuum version of   
   > >Merlin has a different (longer) nozzle design.   
   > >   
   >   
   > 'Easier'.  In other words, not a thing in the world prevents using   
   > altitude compensating nozzles on the first stage other than 'harder'   
   > and 'more expensive'.  And you'd get a similar sort of advantage doing   
   > that as you would get on an SSTO, just to a slightly smaller degree   
   > because the first stage doesn't go quite as high.   
      
   Agreed.  Where, depending on the details, "easier" is laymen speak for   
   cheaper, less complex, and etc.  The devil is in the details.  Not all   
   enabling technologies for reusable SSTO make economic sense to   
   incorporate into the first stage of a reusable TSTO.   
      
   Jeff   
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