XPost: rec.arts.sf.written, sci.space.policy, rec.arts.sf.movies   
   From: fjmccall@gmail.com   
      
   Jeff Findley  wrote on Tue, 29 May 2018   
   06:19:11 -0400:   
      
   >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.   
   >   
      
   The fact that you NEED it for an SSTO doesn't mean it doesn't make   
   sense on a TSTO once you have it.   
      
   >   
   >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.   
   >   
      
   Note that all your arguments would also seem to apply to an SSTO.  The   
   engines are just as complex and heavy there as they are on a TSTO. And   
   your SSTO is still going to have to carry LOX, since SABRE changes to   
   a pure rocket at around Mach 5.5 at 28 km of altitude.  First stage   
   separation for Falcon 9 is around Mach 10 at 80 km of altitude.  What   
   SABRE brings to the table is a preposterously high ISP below that 28   
   km Mach 5.5 point.  That's an advantage whether you're doing a SSTO or   
   a TSTO with a fly-back booster.  Carrying less LOX gives you a lighter   
   stage, because while LOX is cheap it isn't massless.  So once SABRE   
   exists and is reliable, I don't see any reason why someone wouldn't   
   use it on the first stage of a TSTO launcher.   
      
   >> >   
   >> >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.   
   >   
      
   Perhaps not, but once they're developed I'm having a lot of difficulty   
   finding many that don't.   
      
      
   --   
   "Millions for defense, but not one cent for tribute."   
                              -- Charles Pinckney   
      
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