From: henry@spsystems.net   
      
   In article ,   
   johnhare  wrote:   
   >> ...the oxygen content of air is *four orders of magnitude*   
   >> less, per unit volume, than that of LOX.  So you inevitably need big heavy   
   >> machinery to handle air.   
   >   
   >What is the maximum possible T/W you see from a turbine based   
   >air breathing engine. How inevitable is the question. Do you see   
   >a fundamental T/W limit at 100, 40, 15, or some other number?   
      
   I'm not a turbine-engine guy, so it's a little hard for me to call.  My   
   understanding is that the fighter-engine guys are now in the 10-11 range,   
   and it's taken them thirty years to get there from the 7-8 range.  The air   
   temperature at the turbine inlet is now well above the melting point of   
   the turbine blades (!).  (The blades are single crystals of very stubborn   
   alloys, with cooling vents blowing [relatively] cool air out onto their   
   surfaces to keep the hot stuff at a distance.)  That technology isn't too   
   far from its limits.  15, maybe?   
      
   Radical design changes might perhaps take it farther.  But that's harder   
   to predict.  I'd be surprised to see 25.  (I do get surprised sometimes.)   
      
   Systems which don't use turbomachinery can do better on mass, but they   
   have a hard time doing as well on air handling, and they generally don't   
   work at low speeds.  (Mind you, the turbomachinery tends not to work very   
   well beyond about Mach 3.)   
      
   Hybrid systems, rocket/airbreather combinations, can do still better.   
   The question there is whether there's enough Isp gain to be worth it.   
      
   >At what T/W do air breathing engines become performance   
   >competative with the lower stage rocket thrust they replace?   
   >Competative does not necessarily mean desirable in this case,   
   >just not a penalty.   
      
   Given the other constraints they impose -- for example, they tend to need   
   reasonably clean airflow, which is not easy to come by on the surface of a   
   lower stage -- I think I'd call for at least 40, and that's not going to   
   be easy, especially as speed builds up.  (Good LOX/kerosene rocket engines   
   with sea-level nozzles are up around 125.)   
      
   >During a previous discussion I accepted that 120/M seemed to   
   >be a reasonable break even for an air breather that supplies all   
   >the acceleration  from the ground. I suggested a few weeks ago   
   >that for a VTVL SSTO, 28 to 43 might be a reasonable requirement   
   >for units designed for the landing mass only, not operating supersonic at   
   >all during launch phase. Would you agree with these requirements   
   >for break even performance?   
      
   I wouldn't strongly *disagree*, but that reflects limited feel for the   
   problem rather than deep conviction that those are good numbers. :-)   
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