From: zenadsl6186@zen.co.uk   
      
   Jordin Kare  wrote   
      
   > Greg  wrote:   
      
   >> I seems like a bad idea to me IMO. It is just a version of the   
   >> piston-less pump really. He has just replaced the valves with a   
   >> rotating slide thing..... ok so i'm not good at explaining. IMO slide   
   >> seals are not easier than valves.. well valves have sliding seals too.   
   >> I see no advantage in the design over piston-less pumps.   
      
   Nor me. Neither do I see anything patentable, but I won't argue that.   
      
   One thing that might be done would be to introduce a very light piston, to   
   thermally seperate the drive gas from the pumped fuel/propellant. A disk of   
   foamed plastic, or the high-tech equivalent.   
      
   >> While on the subject. Positive displacement pumps don't have the NPSH   
   >> problems that dynamic pumps have. Pistons have advantages over   
   >> piston-less. But what about other designs. Like a screw pump with a   
   >> screw expander? That way you get the gas to do some work while   
   >> expanding, rather than "workless" free expansion...   
   >   
   > Not sure what you mean by workless free expansion, but...   
      
   The gas is stored at 6,000 psi and gets expanded to 600 psi before feeding   
   the cylinder - the expansion from 6,000 psi to 600 psi does no work.   
      
   At least that's what I'd guess he means. There are other possibilities, for   
   example the later expansion from 600 psi to atmospheric does no work either   
   .. Energetically this is not a very efficient pump.   
      
   > We did look a bit at rotary positive displacement pumps (and rotary   
   > positive-displacement motors) for Mockingbird.  They tended to be   
   > heavier than piston pumps, and there's less integration of the drive and   
   > pump sections, but they weren't out of the question; we just didn't have   
   > the time and resources to look at them in detail.  One problem with   
   > rotary expanders is that they tend to be hard to seal; they need tight   
   > tolerances which are difficult to maintain over a wide temperature   
   > range.   
      
   Quimby expanders are good for tolerance requirements, as the pressure   
   difference is split between several seals.   
      
   If the shafts aren't parallel then more work can be extracted as well.   
   Another way is to vary the depth, groove/land ratio, or the pitch.*   
      
      
   --   
   Peter Fairbrother   
      
      
   * Reminds me a bit of triple-expansion steam engines - as a little kid I   
   once sat on the end of the pistonshaft/driveshaft coupling thingy and   
   travelled in 24-feet circles. Be totally illegal nowadays, from safety pov,   
   but it was great fun.   
      
   (On the "Waverly", if anyone wants to know. She was (still is) a sea-going   
   paddle steamer with magnificent triple expansion engines, she used to ply   
   the Scottish island routes, but her engine room is covered in plastic   
   windows now, the new engineer isn't as amenable to a friendly dram as the   
   old Chief was, and her Captain isn't a friend of my Grandpa anymore)   
      
   pps anyone know what happened to the erstwhile engineers's daughter? She was   
   a cutie. Ahh, memories.   
      
      
   *all independant variables, even with non-parallel shafts.   :)   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)