XPost: rec.arts.sf.written, rec.arts.sf.science, sci.space.policy   
   From: fjmccall@gmail.com   
      
   Jeff Findley  wrote on Wed, 30 May 2018   
   06:30:51 -0400:   
      
   >In article <9tlsgdhofb6sah360nbr4jpigmeahl9vjg@4ax.com>,   
   >fjmccall@gmail.com says...   
   >> 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.   
   >>   
   >   
   >You're falling into the "performance uber alles" trap.  That's great,   
   >for missiles and other military systems where size is a constraint.  But   
   >for reusable launch systems, which really aren't constrained much by   
   >size, it doesn't make any sense.   
   >   
      
   Not really.  If it's there for the taking, why not take it?   
      
   >   
   >Above Mach 5.5 it's going to be operating as a rocket engine anyway, but   
   >a very inefficient one at that due to the much higher mass of both the   
   >engine and the more aerodynamically complex vehicle.  I'd argue that you   
   >might as well drop that stage at Mach 5.5 and not even bother with   
   >"rocket mode".   
   >   
      
   So your argument seems to be that SABRE is an even worse idea for an   
   SSTO than it is for a TSTO?   
      
   >   
   >Ignoring that, that wonderful ISP of SABRE comes at the cost of inlet   
   >drag and all of the machinery needed to handle that 80% nitrogen being   
   >sucked into the engine along with the 20% O2.  Plus it is itself a very   
   >expensive and very heavy engine.  Your stage becomes much more complex   
   >because the propulsion/aerodynamics guys are going to want to   
   >"optimize" the heck out of the design.  Suddenly the thing sprouts wings   
   >and landing gear and becomes even more complex as someone has the bright   
   >idea of making the stage HTHL.  Ultimately, your SABRE powered lower   
   >stage ends up being far more expensive than a bigger, but simpler and   
   >less expensive, LOX/kerosene liquid fueled rocket engine powered stage   
   >that operates in VTVL mode.   
   >   
      
   Note that all that is true regardless of where you use it.  In fact,   
   for an SSTO the 'poor rocket engine' is being used for an even greater   
   part of the time, so your position is that SABRE makes less sense for   
   an SSTO than it does for a TSTO booster.   
      
   >   
   >I just don't buy it.  SABRE would be much more suited to hypersonic   
   >cruise (i.e. a weapons system) than for acceleration to orbital   
   >velocity.   
   >   
      
   If you never need the 'rocket' part, why would you bother with SABRE   
   at all?   
      
      
   --   
   "The reasonable man adapts himself to the world; the unreasonable   
    man persists in trying to adapt the world to himself. Therefore,   
    all progress depends on the unreasonable man."   
                                         --George Bernard Shaw   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)