From: johnhare@tampabay.rr.com   
      
   "Kelly St"  wrote in message   
   news:20040202233304.22792.00001284@mb-m18.aol.com...   
   >   
   >   
   > Sorry for the delay in responce.   
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   >   
   > Ok, now we get down to some numbers.   
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   > ;)   
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   > For a LOx/Kero VTVL SSTO using off the shelf engines you get a 14 ration   
   >   
   > between in orbit weight and GLOW.  I.E. if you want 50 tons of loaded ship   
   >   
   > in orbit, your glow is about 700 tons.  Using off the shelf (with water   
   and   
   >   
   > LOx injection ) turbo-jets up to Mach 6 drop that to a 8 to 1 ratio, and   
   >   
   > your 50 ton in orbit in orbit craft has a glow of about 425 tons.   
   >   
   I have been using 16 ratio to LEO with 9,000 m/s required. From Mach 6   
   there is about 6,000 m/s remaining to orbit plus fairly minor gravity and   
   drag   
   losses. At 3,500 m/s exhaust velocity of a vacuum optimised engine, you are   
   looking at a rocket ratio of <6. I assume you are figuring the remaining   
   ratio   
   is feeding the jets?   
   >   
   >   
   > Now with a 10-1 t/w ration jet engine set, you add 10 tons of jet engine   
   >   
   > but lose about 7 tons of rocket and tank dry weight.  (If you assume 15-1   
   >   
   > t/w engines their no weight gain.)  As a nit the water and Lox injection   
   >   
   > also increases thrust in the same engine, so effective T/W improves, but I   
   >   
   > can't find out by how much.  So I assume T/W stays at about 10.   
   >   
   High Mach jets are a bit more complex. The 10/1 jet also has an intake   
   system of similar mass. The variable ramps, boundary layer bleed,   
   subsonic diffuser, and various Mach scheduled actuators are just   
   as heavy as your base engine according to my references. The afterburner   
   with variable throat   and nozzle geometries that vary with Mach,   
   dynamic pressure, and temperature also masses about half as much as   
   the dry turbojet.   
      
   The result tends toward a 1,000 lb engine producing 10,000 lbs of thrust   
   dry. Add 1,000 lbs for the intake system, and 500 lbs for the afterburner/   
   nozzle system, and you end up with a (generic) 2,500 lb system producing   
   15,000 lbs of thrust with afterburner. This would net a T/W ratio of 6.   
   Mass injection increases T/W at the expense of Isp. It quickly becomes   
   a trade study beyond the scope of usenet engineering.   
   >   
   >   
   > Now the weight per say isn't really interesting. The important thing is   
   >   
   > that you wind up with a smaller and more resilient craft.  Its easier to a   
   >   
   > working structure for it and likely you get more reliability.   
   >   
   We probably disagree here.   
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   >   
   > The reason I pushed the point to a HTHL SSTO (aside from just seeing if it   
   >   
   > would work) is that it added operational flexibility, effectively lowered   
   >   
   > the necessary delta-v, lowered thrust needs on take off and low altitudes   
   >   
   > over a VTVL, and (since the LOx was added in flight) lightened the landing   
   >   
   > gear weight.  The wings do double duty as fuel tanks and extra reentry   
   >   
   > surface to allow a cooler surface temp on reentry. Note the wing in my   
   >   
   > configuration are NOT scaled for take off with the full mass of 425 tons   
   >   
   > needed for a boost to orbit. Since it gets LOx in mid air (from mid air   
   >   
   > "refueling" or LOx mining from the air), its take off weight came to about   
   >   
   > 170 tons.   
   >   
   Your article was not online when you posted the link last time. Just going   
   from   
   your posts. It would seem to me that you need at least 10% of take off mass   
   for   
   airframe and landing gear. That would be 17 tons of airframe plus 10 tons of   
   jet engines. ( I think jet mass is light) 3 tons of rocket engines and 7.5   
   tons of   
   various tankage for another 10.5 tons. I see a minimum 75% of your mass in   
   orbit being airframe, tanks, and engine systems. Not including a possible   
   ACES   
   or LOX refueling mass penalty. By the time you add TPS, cargo handling,   
   and control systems, you could reach negative payload.   
   >   
   >   
   >   
   >   
   > The fact that the cost of the jets would buy a lot of LOx is REALLY   
   >   
   > irrelevant.  Fuel is currently a virtually negligible fraction of launch   
   >   
   > costs.   
      
   Full agreement.   
      
    Servicing the craft (or building in the case of expendables)   
   >   
   > currently dominates launch costs, and a high dry mass fraction of   
   structure   
   >   
   > and hull rather then bulky tanks is likely to dramatically lower servicing   
   >   
   > costs.   
      
   Agreement would be conditional on actually achieving the goals you   
   suggest.   
      
   Also of course the jet engines you add - replace nearly half of the   
   >   
   > Rocket engines you'd need for the far heavier (700 tons versus 425 tons)   
   >   
   > all rocket VTVL SSTO.  Rocket engines are about as costly as, and far less   
   >   
   > durable then, jet engines.  So you save money on the total engine system   
   >   
   > purchase and servicing costs.   
   >   
   Currently your arguement seems to make sense. If anyone produces a   
   reasonable   
   RLV rocket engine in some quantity, then no. Rockets are a relatively   
   immature   
   propulsion compared to jets. I can buy a well used 5k jet engine for ~$40k.   
   If I were to place a quantity order (100 units?) with XCOR, I could  beat   
   that   
   price for new 5k rockets. The guts of a pump fed rocket total an easier   
   problem than a high Mach turbojet, if a mature RLV unit is produced. This   
   includes servicing access. The durability an cost problems can be made   
    to go away.   
   >   
   >   
   >   
   >   
   > As to your statement that   
   >   
   >  =>=="I would like to find the point where the operational flexibilities   
   >   
   > possible   
   >   
   >  => with an airbreathing engine do not cost performance in real terms."==   
   >   
   >   
   >   
   > Depends on what performance your concerned with, and how it works in your   
   >   
   > design.  A 15-1 T/W ration jet engine in my configuration will allow you   
   to   
   >   
   > boost to orbit with no added dry mass since the added weight of the jet   
   >   
   > will be offset by the lower weight of tanks and rocket engines needed.   
   But   
   >   
   > is that the critical performance number?  Is operational flexibility,   
   >   
   > operational costs, development cost, etc more critical?   
   >   
   Your last sentence describes the critical areas. Our disagreement is   
   on whether *current* jet engines are capable of doing what you suggest.   
   >   
   >   
   > Also how the craft ascends effects your numbers.  My HTVL configuration   
   >   
   > needs a lot less take off thrust, hence engines, then a VTVL   
   configuration.   
   >   
   > So the trade off in weight and engines would be very different for VTVL   
   >   
   > jet/rocket SSTO.  Even a HTVL that does a fast acceleration up into vacuum   
   >   
   > like Rascal would need a lot more jet engine thrust (hence weight), vrs a   
   >   
   > HTHL that flies a more moderate ascent to max airbreathing altitude and   
   >   
   > speed.  So at what point a given engine is acceptable, depends on the   
   >   
   > details of the craft that needs it, and what its design priorities are?   
   >   
   Details are the thing.:-)   
      
      
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