From: bondage@frontiernet.net   
      
   Henry Spencer wrote:   
   >   
   > In article ,   
   > Ilpo Lagerstedt  wrote:   
   > >Why not any first stage with LH2/LOX before Delta IV?   
   >   
   > Because it's a lot of trouble -- LH2 is a pain to work with -- and not   
   > particularly beneficial.  First stages benefit more from high thrust than   
   > from high specific impulse, and hydrogen engines generally have very high   
   > specific impulse but are relatively heavy for the thrust they provide.   
   >   
   > Just why McDonnell Douglas decided to use LOX/LH2 for Delta IV is not   
   > clear, although it may have been a question of US engine availability.   
   > (There has been essentially no LOX/kerosene engine development in the US   
   > for a long time, but significant recent work on LOX/LH2 engines not too   
   > far from what Delta IV needed.)   
      
      
      While we're at it, I note that the Delta IV exhaust was a pretty   
   easily seen orange-ish color, as opposed to the virtually invisible   
   (save for shock diamonds) exhaust of an SSME or RL-10 (as seen on   
   DC-X/A) at launch. Is this because Delta runs more hydrogen-rich, or is   
   it something else?   
      
      
      
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