XPost: rec.arts.sf.written, sci.space.policy, sci.physics   
   From: jfindley@cinci.nospam.rr.com   
      
   In article ,   
   tkoenig@netcologne.de says...   
   >   
   > Jeff Findley  schrieb:   
   >   
   > > We've had this debate a long, long time ago in the sci.space newsgroups.   
   > > The fact is that there are many stages which are theoretically capable   
   > > of performing SSTO with minimal payload.  No one has ever done it   
   > > because the cost to get that payload into orbit is typically higher than   
   > > it would be if you just used a smaller TSTO (or higher number of   
   > > stages).  In other words, it just makes no sense to fly a disposable   
   > > SSTO.   
   >   
   > There are also other considerations to take into account.   
   >   
   > One of them is mechanical. A first stage flying with a much smaller   
   > payload than the current second stage + actual payload would   
   > have much lower mass towards the end of the burn, resulting in   
   > higher acceleration and higher mechanical stress.  Buckling could   
   > be something that needed to to be guarded against, as could be   
   > outright failure due to compression stress.   
      
   That's a solved problem.   
      
   > Possible solutions if it turns out to be a problem:   
   >   
   > Turn down the engine thrust (How far is that possible? Can the   
   > engines even do that? What will it do to the specific impulse?)   
   > or switch off individual engines (what happens if you swith off   
   > one half a second later than the others?).   
      
   If you have multiple engines, yes you shut a few down to keep the Gs   
   under control.  It's quite easy to shut down a liquid fueled rocket   
   engine in a controlled manner.  This isn't an issue.   
      
   If you still have too high of a thrust near the end of the burn, you   
   need an engine that you can throttle.  Yes you can lose some performance   
   by doing this, but you can design the engine to minimize this (e.g. the   
   SSME had a crazy high chamber pressure to begin with, so when it was   
   throttled, you didn't run into huge problems like flow separation and   
   the like).   
      
   Note that Falcon 9, the first stage that sparked this conversation, has   
   both options available to it.  Merlin can throttle (it does so on every   
   flight by throttling down a bit at max-Q).  And, the Falcon 9 first   
   stage has 9 engines total.  You can easily shut them down in pairs to   
   keep the thrust "balanced".  And since it has one engine in the center,   
   it can run on as few as one engine.   
      
   For a Falcon 9 based SSTO, you might not even need all 9 of those   
   engines at all.  An "optimized" Falcon 9 based SSTO might be able to   
   delete a pair or two of engines to save mass and lengthen the burn time   
   to make orbit.   
      
   > Strenghten the strucutre. This would add mass, and it would not   
   > be the same first stage anymore.   
      
   It's already overbuilt for the SSTO role since it won't be carrying the   
   mass of an upper stage or as massive of a payload.  So "strengthening   
   the stage" beyond that really isn't necessary.   
      
   Jeff   
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