From: nuny@bid.nes   
      
   On Saturday, August 20, 2016 at 3:30:17 AM UTC-7, MrAnderson wrote:   
   > All right, next part, i finally managed to find some time.    
   > Hmm, what's the exact speed of sound in interstellar medium? I know it varies   
   > probably, but are there some numbers for that?   
      
     Various sources (including NASA and ESA) say from 100 to 400 km/sec.   
      
     Incidentally, 400 km/sec is also about the speed of the Solar wind near the   
   plane of the planets' orbits, so once the ship accelerates to about twice that   
   on its way out of the system (probably in the first few hours of main engine   
   operation) there *   
   will* be a shock wave. That might make an interesting story vignette- bridge   
   officers worrying a little about when they go transonic because of the kind of   
   buffeting ordinary aircraft experience when they go above Mach one; usually   
   the shock doesn't form    
   perfectly symmetrically because aircraft have wings and stuff that form their   
   own shocks which interfere with the main one from the nose. That might not be   
   an issue since the dust shield is more or less perfectly circular, or it might   
   because it isn't *   
   perfectly* circular. Jet pilots describe it as kind of like hitting and   
   penetrating a wall- it tends to jerk the aircraft around and that would be   
   Very Bad for a starship, but it is much much larger than a fighter so it   
   should be survivable, but still "   
   exciting".   
      
   > I still prefer the idea of one engine and turning around in the middle of   
   > trip, it's saving on mass a lot, so that's worth fighting.   
      
     Yes. Even if other alternatives are more workable they carry a huge extra   
   fuel penalty to lug them around.   
      
   > With those turbulences, I think we don't have enough data to say it will ne   
   > a problem or not.   
      
     I think it will. Also remember, the dust shield is there for a reason and   
   during turnaround it can't do its job.   
      
   > Theoretically you can deal with it by some magnetic / particle shields,   
   > create something like "bubble" with almost no interstellar medium.    
      
     Yeah. Thing is there's so much variation. Take the example of the Solar wind   
   vs. the interstellar medium- that produces a huge shock wave (the heliopause)   
   far from the Oort cloud, way beyond the magnetic field the Sun or the rest of   
   the bodies in the    
   system produces. That boundary flaps around a lot (slowly) because the density   
   and hence the speed of sound in the ISM varies along the System's orbital path   
   around the Galaxy. We think one of the Voyager spacecraft has finally reached   
   it, but it's not    
   moving fast enough to notice it except with instruments.   
      
     A magnetic field strong enough at any reasonable distance from the ship will   
   also flap around during turnover, but much faster because the ship is moving   
   so fast, creating back-reaction heating in the coils making the field.   
      
     Also, such fields act like an enormous sail or brake depending on the   
   direction and strength of the local magnetic field at any given point in the   
   ship's trajectory. Such a brake is mentioned on the Atomic Rockets page as a   
   possibly viable option to    
   turning around- you wouldn't even have to use the engines to brake. However,   
   if a magnetic bubble "brake" is only turned on during the turnover maneuver I   
   don't think it would be a deal-breaker, especially since you're wanting to   
   decelerate by then    
   anyway. If it screws up your trajectory that's another matter, but you still   
   have light years to correct that.   
      
     Oh. We've been assuming the ship fires main engines while still in the   
   origin system, haven't we? There's gonna be another couple of "exciting"   
   moments when the ship crosses out of their origin star's heliopause, and when   
   it crosses into the target    
   system's star's heliopause.   
      
   > "Another thing; the fountain-style radiator *won't work* when the ship is   
   > flying backward to decelerate because the molten metal coolant isn't going to   
   > "fall back" to the dust shield- it'll just spray off into space and   
   > eventually run out. If the ship decelerates with the dust shield facing   
   > forward the spray will just go a little farther before "falling back"   
   > (actually the ship runs into it)."   
   >    
   > Hmm,  but when ship is decelerating backward, it sprays coolant, and ship   
   > produces artificial gravity by deceleration process, so it will fall.   
      
     When flying frontward, yes, but flying backward the spray is being sprayed   
   where the ship was, not where it's going to be. It has no way to catch up.   
   Say, pure molten iron and magnets might work- the magnets will have no effect   
   on the metal until it    
   cools below its Curie point.   
      
   > This ship is like antimatter torchship, it needs high accel to get to 0.7 c   
   >or so and brake then.   
      
     Yes.   
      
   > Also, how effective is this type of radiator? There are plenty of radiator   
   > designs, we don't have to stick with this one, we can even combine two   
   > systems.    
      
     There are only three ways to shed heat; conduction, convection, and   
   radiation. The first two aren't usable because they need an external medium to   
   carry the heat away and the ISM just isn't dense enough- it might as well be   
   hard vacuum. All other    
   proposed radiators are just that- they shine in the infrared (and maybe   
   visible) with the rest of the Universe acting as a heat sink. The farther the   
   spray travels, the more time it has to shine before cooling down enough to be   
   able to accept heat from    
   the engines again. Solid radiator designs are huge, relatively fragile things   
   that work the same way, except the heat-carrying fluid is confined in tubes   
   and whatnot, and that gives it less time to shine before being reused. Also,   
   the confining tubes and    
   support structure are connected to the ship and will conduct heat back to the   
   ship at the attachment points. The spray method doesn't do that.   
      
     It does have the disadvantage of potential contamination though; the molten   
   metal droplets can catch dust, and the collection channels in the dust shield   
   will add more, requiring the filtering system I mentioned a few posts ago. A   
   confined system won't    
   need that.   
      
     However, if you have to run the engines hotter than normal for a while you   
   just run the coolant pumps faster so the coolant goes farther before falling   
   back, giving it more time to cool off. You can't do that with a confined   
   system either.   
      
   > "It's starting to look to me like it would be better to have two drive   
   > systems, one at each end of the ship, or better yet to have some way to   
   > rotate the engines so they faced into the direction of flight for   
   > deceleration."   
   >    
      
   [continued in next message]   
      
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