From: smithm@SPAMBLOCKnetapps.com.au.retro.com   
      
   On 7 Dec 2004 16:53:19 -0800   
   "jmbrock@gmail.com"  wrote:   
      
   > Has anyone here heard of a mass driver?   
      
   Yes   
      
   > I believe it is basically a long, sloping track on which sits a   
   > platform, atop which is the payload (with booster) that you wish to   
   > launch. I also have heard that the platform would be accelerated down   
   > the track (electromagnetically?) and at a certain point, the payload's   
   > booster would fire, and the payload would be released, supposedly   
   > saving fuel costs and payload costs.   
      
   Correct.   
      
   > Anyone have any thoughts on this?   
      
   The main advantage is that the launch vehicle can be externally powered for   
   part of the launch. Because of this the main power source (and its fuel   
   supply) does not have to be accelerated during launch, thus requiring less   
   kinetic energy to reach orbital    
   altitude.   
      
   The disadvantages are many:   
      
   1. The launch vehicle has to go very fast close to the ground where the air is   
   thick. This causes drag and energy loss.   
      
   2. The mass driver has to be many 10s of kilometres long, representing a   
   substancial investment in infrastructure. Each part of the static mass driver   
   only does a small amount of work. So utilisation is low.   
      
   3. Nobody really knows how to accelerate large masses to km/s speeds over   
   large distances in an atmosphere. Rockets are better understood.   
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   Michael Smith   
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