From: daellis94@gmail.com   
      
   The above responses will probably give better insight into the mechanics   
   involved, but as far as I can tell, it's all about optimizing nozzle geometry   
   for various ambient pressures.     
      
   Of course, at its core, a nozzle is a device--or, I suppose more   
   appropriately, a structure--along a flow channel that converts the enthalpy of   
   a fluid (ie, the energy represented by the combination of temperature and   
   pressure) into kinetic energy in one    
   direction.  The shape of a rocket nozzle is optimized to try and get as much   
   of that energy out as possible.     
      
   So, ideally, you want a nozzle that can expel gas with a pressure of zero, at   
   least in a vacuum, but if the pressure outside the nozzle is greater, you're   
   going to be wasting a bit of energy in expanding the gas too much, since now   
   ambient pressure wants    
   to start gas flowing into the nozzle.     
      
   I'm aware this isn't a perfectly accurate description, but I think it is close   
   enough to the truth be a helpful way to visualize the situation.     
      
   (For those of you who know the fluid mechanics of this more clearly than I,   
   have mercy.  I'm only trying to help hahaha)    
      
   You've probably seen mention of rocket nozzles being over- or under-expanded   
   for certain conditions.  Traditional De Laval rocket nozzles tend to be   
   optimized more for low pressure conditions.     
      
   The aerospike design aims to be "more effective" simply by pursuing an   
   arrangement that allows for efficient optimization for all (or at least many)   
   pressures, and thus all altitudes.   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)