From: henry@spsystems.net   
      
   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.)   
      
   >Maybe this is a dump question too but I have always wondered that: How   
   >much powerful Saturn 5 would have been LH2/LOX in the first stage?   
      
   Much depends on what assumptions you make, but it could easily have ended   
   up less capable, rather than more capable.  LH2 is overrated:  it adds a   
   lot of extra problems, and they increase a stage's dry mass to the point   
   that overall stage performance usually is no better than competitive with   
   LOX/kerosene, except that the stage gross mass is lower (which can matter   
   for upper stages but is almost irrelevant to first stages).   
   --   
   "Think outside the box -- the box isn't our friend."    |   Henry Spencer   
                                   -- George Herbert       | henry@spsystems.net   
      
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