From: johnhare@tampabay.rr.com   
      
   "Henry Spencer"  wrote in message   
   news:Hs65IH.4n3@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.)   
   >   
   I am actively working on surprising you. Not building a full up unit in   
   my garage. Just demonstration hardware of a different approach that   
   should reach that goal after real engineers do some analysis and a little   
   CFD work.   
      
   > 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.)   
   >   
   Mach 3 is past time to get out of the air anyway. IMO, that is where thermal   
   issues on the rest of the airframe begin to clearly out weigh any   
   theoretical   
   gain from an airbreather. Unless the airbreather can provide much more   
   acceleration than commonly assumed. Which implies a better T/W including   
   intakes than I believe likely in the near term.   
      
   > Hybrid systems, rocket/airbreather combinations, can do still better.   
   > The question there is whether there's enough Isp gain to be worth it.   
      
   That is the question. There is obviously a curve in there. Less clear is   
   exactly where the curve is.   
   >   
   > >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.)   
   >   
   Sounds like a reasonable requirement if the airbreather has other uses.   
   At one point I figured that an airbreather would have to exceed the   
   T/W of a rocket if only used in the launch phase.   
      
   > >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. :-)   
   > --   
   I have been sort of digging for a well researched paper that would give   
   a range of honest curves. It seems quite strange that the vast quantity   
   of material on the subject would not have a clear requirements breakdown   
   somewhere handy. A generic set of go-no go curves would be nice.   
   All I have been able to find so far is single project justifications,   
   usually   
   GLOW based.   
      
   > MOST launched 30 June; science observations running     |   Henry Spencer   
   > since Oct; first surprises seen; papers pending.        |   
   henry@spsystems.net   
   >   
      
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