From: mikkelhaaheim@gmail.com   
      
   Le jeudi 29 septembre 2016 12:20:46 UTC+2, elie....@gmail.com a écrit :   
      
      
   >    
   > With expandable coolant like here, (as described above), I would use the   
   opposite design: use the expandable coolant to first cool the coldest parts,   
   then the warmer ones in order. This way, it is always colder than what it is   
   supposed to cool down,    
   until the solar-thermal drive part where it is heated up to possibly thousands   
   of K.   
      
   First, I think you mean "expEndable".   
   Second, what you are describing is essentially the multi-stage heat sink being   
   discussed.   
   Third, you have to be somewhat careful with your use of multi-stage heat   
   sinks. On the one hand, as mentioned, there are some things that have very   
   narrow ranges of temperature tolerance. If the initial coolant temp is too   
   cold, they freeze, and don't    
   work. If the initial coolant temperature is too hot, they overheat, and don't   
   work. Sometimes the difference is on the order of tens of degrees, or even   
   less. Conversely, some things require extremely cold coolant, which they   
   quickly heat to very high    
   temperatures. What this means is that you can not just sequence the things   
   that need to get cooled... you also need to run some of the coolant flows in   
   parallel.   
      
      
   > > How much of that impact mass will be from unused hydrogen fuel?   
   >    
   > Ideally none or a small margin: hydrogen that was not used is wasted. Either   
   the craft itself could have been smaller, or it could have been replaced with   
   a deadlier payload.   
      
   OTOH, the H2 makes a very nice explosive. Remaining H2 would allow for a   
   mission reserve in case of fluid loss, and reacts vigourously with a large   
   variety of oxidisers. This actually adds to the argument for having a closed   
   H2 loop.   
      
   > But if you mass-produce those or are not sure of the target as you build   
   them, hydrogen will carry kinetic energy as well. If it is solid, it may even   
   work as decent shrapnel, otherwise it is limited to full-tank-sized shrapnel   
   fragment.   
      
   Solid H2 would act very poorly as shrapnel. The low density would give it   
   about as much shrapnel potential as a very soft snowball... in hell,   
   considering its extremely low melting point.   
      
   >    
   > > >...being a long thin cone it can be built as an armour-piercing   
   projectile,    
   >    
   > > Which requires a pointy, armored tip, the opposite of the solar   
   concentrator.    
   >    
      
      
   > I wouldn't hit one farm or atmosphere generator - I would hit all of them.   
   Similarly to an ICBM attack, launch a multitude of those and put MIRVs on   
   them, optionally with a few nukes if you need the extra punch. With that, you   
   can blanket several areas    
   with the fists of God. This should be enough to kill most of everyone and   
   wreck any concerted terraforming effort below the 'robust planet-wide   
   self-sustaining ecosystem' or 'breathable atmosphere' phases.   
      
   I would point out that if a multitude of projectiles are required to do the   
   damage, it no longer qualifies as a WMD (nor a "world killer". That said, I   
   agree that a KKV can be used as a WMD if it is detonated before impact, and   
   therefor spreads the    
   impact over a large area. I am less confident that such attacks will hit   
   everything, unless ou launch multiple salvos several hours apart. The high   
   relative velocity pretty much means that you can only hit one side of a planet   
   or moon, which means you    
   will need to time at least one other volley to impact when the other side   
   rotates into position. Even then, if we are talking about a large moon or   
   planet, especially with assets spread throughout orbit, it will be very   
   difficult for even hundreds of    
   such weapons to hit all the targets.   
      
   >    
   > > > The point is precisely that you can't intercept it, as you don't see it    
   > > > coming.    
   >    
   > > That's still arguable. It won't take long for someone to figure out how to   
   see continuous clouds of cold hydrogen "exhaust" that weren't there a day ago.    
   >    
   > But how visible is this exhaust?   
   > If ultra-low flow exhaust at a few hundreds K is too visible, we can   
   increase flow to decrease exhaust temperature. It will decrease autonomy and   
   dV, so it exhaust temperature should be kept as high as possible.   
   > As the Universe is full of hydrogen, I would expect it to be very difficult   
   to detect.   
      
   I tend to agree on this point. At the low mass rate on the order of tens or   
   even a few hundred g/s, you will have an extremely low density of articles   
   that will be virtually impossible to detect unless you are drifting through   
   them.   
      
      
   >    
   > > Consider how the "Greek camp" and "Trojan camp" of Trojan asteroids are    
   > actually closer to Earth, (in terms of travel time, physical distance is    
   > about the same), than they are to Jupiter even though they are out in    
   > Jupiter's orbit.    
   >    
   > Indeed, and this was something I had never realized until Zane Markowsky's   
   Chidren of a Dead Earth(*) pointed it out: to one part of the Belt, the inner   
   planets are closer than other parts, travel-wise.   
   > He describes how coherent polities form based on asteroid families (that are   
   relatively close to each-other), instead of a 'Belt culture'.   
   > Orbital mechanics are a funny thing.   
   >    
      
   It is not quite that simple. First, when you are refering to a "culture", you   
   are refering to various collections of characteristic that make some groups   
   more alike than others. Even if you have clan structure government, you still   
   have a belt culture,    
   because the different clans will be more alike to each other (will have more   
   in common with one another) than they are to those remaining on the Earth, the   
   moon, Mars, etc. However, you could be correct that they might not be   
   organised into a unified    
   government. Second, the central governent of Mexico might be closer to Texas   
   than the central government of the US. In the 19th century, you probably had   
   mexican settlements close to border texan settlements than other texan   
   settlements. However, you had    
   a chain of supply (etc) to the various texan settlements that was stronger   
   than what the mexican government could muster. Yes, Mexico had a temporary   
   advantage with the Alamo. But the US held the territory, and retook the Alamo,   
   in a very short time.   
      
   [continued in next message]   
      
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