XPost: rec.arts.sf.written, rec.arts.sf.science, sci.space.policy   
   From: jfindley@cinci.nospam.rr.com   
      
   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.   
      
   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".   
      
   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.   
      
   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.   
      
   Jeff   
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