From: nuny@bid.nes   
      
   On Thursday, September 29, 2016 at 3:20:46 AM UTC-7, elie....@gmail.com wrote:   
   > > > 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.   
      
     Remember you're constantly accelerating to get maximum bang on impact and to   
   minimize chance of interception. That means you're going to build velocity   
   rather more quickly than today's burn-a-little-then-coast-a-lot space travel.   
   You're going to need a    
   lot of lateral deltavee to do even the most minor course corrections the   
   closer you get to a target, especially for precision targeting like   
   bunker-busting.   
      
     Your assumption that the spaceframe only needs to handle lengthwise thrust   
   needs re-examining. Below you mention a point that makes it worse, turnaround   
   before impact. I think turnaround is a very bad idea, and I'll go into why   
   below.   
      
   > > 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.   
      
   (Include correction of "expandable" to "expendable"- I wondered about that too)   
      
     Simple is good but that's too simplistic to work. Just because a subsystem   
   has to run at a specific *temperature* during cruise doesn't mean it's   
   emitting more *heat* than another system that can run at a lower temperature,   
   or that either one will    
   remain constant throughout the mission profile. Consider a small transmitter   
   pointing aft (to let the launchers know where it is, I assume there's no stray   
   signal going forward to give it away to defenders) that has to run at 300 K   
   but only emits 1 watt    
   of waste heat, and a nuke sleeping at 100 K but emitting 10,000 W. It's   
   possible to equalize heat flow rates by carefully sizing the plumbing actually   
   in contact with the heat sources *for constant heat flow rate* but when the   
   rates change...   
      
     See, before impact the radio can be shut down and allowed to freeze solid   
   but the nuke has to come up to ~300 K so its explosives and timers will work.   
   That's going to change the total heat throughput, changing the available   
   thrust when the coolant is    
   chucked out of the exhaust. You can mitigate that somewhat with a variable   
   nozzle, which adds complexity and a potential breakdown point.    
      
     (Remember, the ship as a system has only one heat sink, the exhaust nozzle.)   
      
     That's only two minor but essential subsystems. If you have cryosleeping   
   crew, they'll constitute a possibly convenient auxiliary heat sink while   
   waking up, but their life support will definitely be an inconvenient heat   
   source. The assault shuttles or    
   whatever to get them off the ship before it hits its target will need to warm   
   up too.   
      
     All of those take different amounts of time as well, screwing up the   
   available thrust numbers even more.   
      
     Internal heat budgeting is a lot more complex than just overall solar   
   collector/hull coolant-input-to-exhaust-output budgeting. That latter isn't   
   constant either- the closer to Sol you get the more heat you collect. Unless   
   your collector area is    
   variable too...   
      
     Seems to me parallel, switchable *mutually insulated* cooling subsystems are   
   unavoidable.   
      
   > Plumbing-wise, it seems simpler than having the heat-sink itself be at   
   > multiple temperatures.   
      
     Not heat sinks so much as heat reservoirs, each at different temperatures   
   and different heat capacities that change as the mission progresses.   
      
     (Which reminds me- when its course is no longer sufficiently sun-bound for   
   the collector to work, the only heat input is to the skin, dropping available   
   thrust unless internally stored heat can make up the difference. That's a very   
   good reason to have    
   internal heat reservoirs.)   
      
     It's been said that things should be as simple as possible, but no simpler.   
   That's still true here but this is an engineering example of a situation where   
   things must be as complex as necessary. Same thing, different emphasis.   
      
   > > 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.   
      
     Exactly...   
      
   > 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 molecular hydrogen is the least-dense known solid-phase element,   
   meaning it's the least-effective for shrapnel. If these are mass produced   
   one-size-fits-all missiles all with the same amount of hydrogen loaded before   
   launch, unavoidably the    
   farther they travel the less hydrogen will be aboard at impact.   
      
     The whole idea of shrapnel is pretty much inapplicable for things near a   
   target but not directly impacted anyway. I'll go into that below.   
      
   > > >...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.    
   >    
      
   [continued in next message]   
      
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