From: g_d_pusch_remove_underscores@xnet.com   
      
   "johnhare"  writes:   
      
   > Airbreathing is so desirable that substantial performance penalties   
   > are going to be overlooked in order to incorporate them.   
      
   I very much question this claim --- especially for VTVL.   
      
   Air-breathing T/W ratios are so wimpy that they almost always force wings   
   and horizontal lift-off in the final analysis, since the engine cannot   
   lift the weight of the fully loaded vehicle. You can dream all you want   
   about air-breathing engines with a T/W ratio of "43 to 75," but I very much   
   doubt that you or _anyone_ will be shipping one any time soon !!!   
      
   Furthermore, you appear to have made the common false assumption that   
   air-breathing performance is independent of airspeed. In point of fact,   
   the effective I_sp of an air-breathing engine is roughly inversely   
   proportional to airspeed above roughly Mach 1, so that at Mach 6,   
   the effective I_sp of an air-breather is only a few times better   
   than a rocket burning the same fuel. And since you are also assuming   
   water _AND_ LOX injection, you must include these in your propellant input,   
   so that your effective I_sp is even further degraded. At this point,   
   your engine is starting to look more like a bad rocket than a   
   air-breather. And as Henry Spencer has pointed out many times   
   in this newsgroup, when a careful performance analysis is done,   
   one usually finds in the end that it is better to build a good rocket   
   that can double as a bad air-breather than an air-breather that can double   
   as a bad rocket.   
      
   Finally, since most of the propellant will still be consumed after   
   air-breathing has become useless, unless you go to two stages or otherwise   
   drop off your fancy air-breathing engines when you reach Mach 6 or so,   
   all that heavy turbomachinery becomes so much useless dead mass for most   
   of the trajectory to orbit.   
      
   In summary, I continue to remain unconvinced that air-breathing is even the   
   _least_ bit desirable for anything except possibly the first stage of TSTO.   
   Furthermore, the claim that air-breathers can achieve a T/W exceeding 40,   
   and  will be useful for VTVL makes me fall down and roll on the floor,   
   laughing my head off...   
      
      
   -- Gordon D. Pusch   
      
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