From: johnhare@tampabay.rr.com   
      
   "Gordon D. Pusch"  wrote in message   
   news:gik73tcg1w.fsf@pusch.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.   
   >   
   It is somewhat irritating that you snipped a single sentence from   
   the paragraph to make it seem that my position is far from where   
   it really is. I pasted the paragraph in below to show the difference.   
      
   >>>>>>>There are three performance curves on the graph. The low one is   
   what many people shoot for. Airbreathing is so desirable that   
   substantial performance penalties are going to be overlooked in   
   order to incorporate them. The middle curve is where performances   
   just match. The most difficult one is where not including airbreathers   
   must be justified in terms of simplicity or cost. I am interested in the   
    middle curve.<<<<<<<<<   
      
   This is my position. I am looking for useful answers to what airbreathers   
   have to achieve to match rockets. The many people that write papers   
   and advocate airbreathing at any cost should be keelhauled.   
      
   > 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 !!!   
   >   
   You should have noted that my post suggested that this would be the required   
   performance to match rockets, not that the 43-75 was feasable in the   
   forseeable planning horizon.   
      
   > 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.   
   >   
   Are you responding to my post or someone elses? I am comparing   
   performance usefullness of a subsonic airbreathing system. Supersonic   
   requires fairly heavy (by comparison) intakes.   
      
   > 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.   
   >   
   This is definately in response to someone else.   
      
   > 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...   
   >   
   My orriginal question stands unaddressed, What performance is required   
   of an air breathing engine in order to match an all rocket LV performance.   
   I questioned whether the numbers I have derived for a requirement are   
   accurate enough for reasonable decisions to be made.   
   You should note that I also said that T/W of 25 was in reach maybe.   
      
   While you are rolling on the floor laughing, care to make a small wager   
   on the capabilities of a concept demonstrater? Since you are so certain,   
   you should be willing to offer really good odds.   
      
      
   >   
   > -- Gordon D. Pusch   
   >   
   > perl -e '$_ = "gdpusch\@NO.xnet.SPAM.com\n"; s/NO\.//; s/SPAM\.//; print;'   
   >   
      
   --- SoupGate-Win32 v1.05   
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