> > Bulk is a drawback, but not such a big one I suspect. By far the most   
   energy    
   > > received is from the Sun, and this is taken care of with the solar-thermal    
   > > engine. For the rest, you end up with a long, thin cone, but this craft    
   > > doesn't have to manoeuvre anyway.    
      
   > Why not? You cannot aim your launches from several AU away to guarantee a   
   hit without course corrections just from stray gusts of solar wind, never mind   
   magnetic and gravitational perturbations or enemy action.   
      
   I meant high-thrust manoeuvres, like a chemical or even nuclear rocket would   
   give. It already has manoeuvring capabilities akin to a ion drive. But the   
   minimal acceleration allows for the less thrust-optimised shape of a long,   
   thin cone.   
      
   > I mentioned earlier that there will be components you can't allow to get   
   very cold at any time or that have a significant required warmup time, so they   
   can be buried in the warmer parts of the heat sink.    
      
   > OTOH having different parts of it at different temps may also be   
   problematic, if it's even possible. A multi-stage heat sink arrangement could   
   mitigate that at the expense of more tankage and plumbing.   
      
   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.   
   Plumbing-wise, it seems simpler than having the heat-sink itself be at   
   multiple temperatures.   
      
   > 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.   
   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.   
      
   > >...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.    
      
   Indeed, the pointy tip is on the opposite side of the solar concentrator,   
   which is the base of the cone.   
   The cone itself is the shape of the solar concentrator shadow. At Earth   
   distance, that's about half a degree. It doesn't need to cover the complete   
   shadow, so it can be a truncated and/or wider cone (wider would allow to go   
   closer to the Sun). It can    
   even be a cylinder for ease of manufacture, accepting the losses as some   
   sunlight will hit the black part.   
   But whatever the shape, a pointy end will fit well at the end of the black   
   side.   
   This will require turning the craft at the last moment so the pointy end   
   impacts first, but a one-time rotating system shouldn't be too complicated,   
   and it's OK to take some sunlight on the side a few instants before impact.   
      
   > Why bother with a nuke? The point of KKVs is to use momentum, not explosives.   
      
   Depending on what you are trying to destroy, I agree. A nuke would only be for   
   the toughest targets, where shrapnel wouldn't be enough and a single KKV   
   impact wouldn't do enough damage. Then the KKV itself is to bypass armour, and   
   the nuke is to do the    
   real damage - KKVs are great at putting tremendous energy on a very small   
   surface (good to punch through things) and the mass of the spacecraft itself   
   is 'free', contrary to a specific nuke payload, but nukes will carry more   
   energy per kg than even    
   interplanetary KKVs.   
   I am sure there are others ways to effectively use a KKV against this type of   
   target, this was more of an example of solution - my point being that properly   
   configured, no passive defence can protect against a properly configured KKV.   
      
   > Why? You'd have to hit something critical, like a farm or "atmosphere   
   generator", and you've foregone terminal guidance. In Mars' atmosphere a dust   
   cloud raised by such an impact would be a nuisance only. We've seen natural   
   Mars-wide dust storms    
   lasting a couple of months (average, every three Earth years) come and go   
   already.   
      
   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.   
      
   > > 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.   
      
   > And since you've foregone maneuvering, course prediction for purposes of   
   interception will be trivial.   
      
   I am forgoing high-acceleration manoeuvring, but not continuous low burn -   
   this is precisely the point of this design: stealth dV from the constant   
   exhaust of expandable coolant. Thrust is incredibly low, but it is compensate   
   by being always-on - in fact,   
    it cannot be turned off, it will have to rotate if it doesn't want to change   
   trajectory.   
      
   For launch, I am in favour of a shrouded catapult described earlier (so others   
   can't see where you are aiming at or when you are firing), to go further and   
   hide the launch itself.   
   This further favours unmanned crafts, as those can take way higher   
   acceleration. This is assuming the craft can be designed to take short,   
   extreme acceleration with one-time measures (this wouldn't play nice with some   
   ways to cope with solid hydrogen,    
   for example). Not impossible, I suspect, but significantly more complex.   
      
   > > Even if other sides can build their own, this wouldn't work as a MAD   
   system,    
   > > as MAD requires second-strike capability.    
      
   > Only if you can aim it, or guarantee that the kill radius includes all of   
   your targets.   
      
      
   [continued in next message]   
      
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