From: johnhare@tampabay.rr.com   
      
   Until recently I have considered air breathing engines on VTVL to   
   not be worth consideration. Now I believe they *might* be worth   
   consideration if they can reach certain performance goals. The   
   performance goals I have in mind are,   
      
   Dry mass/payload ratio to remain similar if, and only if, some   
   substantial performance or operational gain is made.   
   or   
   Cost of construction, testing, and operations drop a considerable   
   amount for similar mission capability.   
      
   I would like to quantify the performance requirement for   
   the curve in which the addition of a subsonic air breathing   
   engine reaches break even vs an all rocket VTVL.   
      
   There are 3 places I am aware of to get the mass for the air breathing   
   engine  I am interested in. Replacement of some rocket engine mass   
   from launch for a minute or so. Minor fuel savings during launch.   
   Elimination of some landing fuel required by all rocket landing.   
   For figuring purposes, a 160,000 lb GLOW vehicle with 10,000   
   lb dry mass including payload and landing fuel. This is a dense fuel   
   SSTO as I am not sold on hydrogen for various reasons.   
      
   If terminal velocity is 100 m/s and 3 gee deceleration is used with no   
   margin, then a 30k rocket engine will burn 500 lbs of fuel in 5 seconds   
   before   
   landing. To match this a 15k air breathing engine will have a 20 second   
   burn using 150 to 300 lbs of fuel at Isp=2,000 and Isp=1,000 respectively.   
   Those two points on the curve suggest that the engines could mass between   
   350 and 200 lbs to match the pure rocket performance. This requires a T/W   
   of 43 and 75 for the 2 cases.   
      
   Replacement of some rocket engine mass on launch can only be a contribution   
   to the available air breathing engine mass. If the orriginal rocket is 200k   
   at   
   sea level, then the thrust replaced can only be about half of the 15k as the   
   air breathing engine loses thrust during the climb and craps out entirely at   
   30,000 to 50,000 feet. The gravity losses after that cut the gains some.   
   Say 7.5k of rocket thrust traded at T/W 125 for a 60 lb savings applied to   
   the air breathing engine.   
      
   Fuel savings on launch are argueable. Say 60 seconds at 10k average thrust.   
   This is 300 to 600 lbs of fuel vs 2,000 lbs for the rocket. This minor   
   savings   
   on fuel is during the low slow part of the trajectory. The best I can guess   
   is   
   it translates to 100-150 lbs in orbit.   
      
   This suggests that there would be 550 lbs available to the 2,000 second   
   air breather and   360 lbs available for the 1,000 second air breather.   
   T/W ratios required would be 28 and 42 respectively for these systems   
   to match pure rocket performance on a VTVL.   
      
   Leaving aside whether it is worthwhile to use air breathing at all, are   
   these numbers accurate enough to start an honest decision curve?   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)