07112022, 07:52 PM  #11  
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Re: [Spaceships] Getting A Few Thousand Extra MPS
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07112022, 08:49 PM  #12  
Join Date: Jun 2013

Re: [Spaceships] Getting A Few Thousand Extra MPS
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Of course, given Spaceships treats fuel tanks as having zero mass aside from their contents, you could fairly readily build a vessel that doesn't have multiple stages, by applying the rocket equation. With one fuel tank of hydrogen giving you a deltav of 60 mps (TL 10 fusion rocket), you need the equivalent of 60 fuel tanks to be able to get up to 1800 mps, activate your ramscoop, then later slow down by 1800 mps. Given you need to scale your drive and ramscoop to the smaller vessel to get 3600 mps, however, you're looking at an over200year journey. A better option is to not use all of the hydrogen the ramscoop grabs immediately, but rather use it to refill your fuel tanks. That only calls for 1800 mps of deltav to start with, and you can do that with the equivalent of 30 fuel tanks. That calls for around an 80% mass fraction being reaction mass (16 fuel tanks). That means you could scale your ramscoop and drive to the full size of the vessel, as you have the room. So, at 0.005G, you spend the first ~1.9 years accelerating to reach 1800 mps, then activate your ramscoop. You travel for a time with the ramscoop on and using all the hydrogen it provides to accelerate further. Eventually, you'll need to start slowing down, which if the ramscoop has to be in the front of your ship (which I think is the case) will be... interesting. Basically, you'll turn the drive off and slow yourself down with the ramscoop, storing the hydrogen until your fuel tanks are full. You then flip around and use your drive to decelerate further. Once your fuel tanks are empty again, you flip back around and use the ramscoop to decelerate, and basically repeat this until you hit the 1800 mps limit for the ramscoop, timing things so this happens when you have full tanks. You then flip around one more time for a final deceleration burn. I think the way the deceleration phase would work, at least for a basic approximation, is that you build your reserves back up in the same amount of time it took to initially deplete them; assuming kinetic energy stays constant, increasing 5x in mass should correspond to x(0.2)^(0.5) to velocity, or roughly x0.45. So, 1.9 years to get up to 1800 mps, then an asyetundetermined amount of time accelerating using the ramscoop+drive, then turn off the drive but leave the ramscoop on for 1.9 years for x0.45 to velocity, then flip, then 1.9 years for 1800 mps to velocity, then flip, then 1.9 years for x0.45 to velocity, and so forth. Another complication to the above is that, while we're running on just the ramscoop, the vessel is actually 1/5th its starting mass, and thus should accelerate more rapidly, to the tune of roughly 2.2x (square root of 5, so kinetic energy increases at the same rate as before). Technically, this means we should probably be achieving our initial deltav of 1800 mps in less than the above 1.9 years (and similarly refilling our tanks more quickly as well), but for simplicity, we should probably just stick with 1.9 years. I think I'm going to call it a night rather than get into any calculations; I'll pick this up tomorrow unless someone beats me to it (or lets me know I'm completely on the wrong track).
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07112022, 09:52 PM  #13  
Join Date: Aug 2007

Re: [Spaceships] Getting A Few Thousand Extra MPS
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I don't think anyone here has been using any SF writer as a science advisor.
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07112022, 10:48 PM  #14  
Join Date: Oct 2019

Re: [Spaceships] Getting A Few Thousand Extra MPS
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07122022, 03:05 AM  #15  
Join Date: Mar 2013

Re: [Spaceships] Getting A Few Thousand Extra MPS
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07122022, 03:46 AM  #16 
Night Watchman
Join Date: Oct 2010
Location: Cambridge, UK

Re: [Spaceships] Getting A Few Thousand Extra MPS
Keeping it aimed at a ship more than a few lightseconds away is impractical. The Sun's magnetic field fluctuates, and will cause the beam to move around.
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07122022, 04:28 AM  #17  
Join Date: Mar 2013

Re: [Spaceships] Getting A Few Thousand Extra MPS
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And what's the accel rate? 2G like for laser rockets? Or limited to the much lower rate for MagSails? 

07122022, 07:12 AM  #18  
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Join Date: Oct 2010
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Re: [Spaceships] Getting A Few Thousand Extra MPS
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07122022, 08:06 AM  #19 
Join Date: Jun 2013

Re: [Spaceships] Getting A Few Thousand Extra MPS
So... one issue with my above approach is that I'm not certain how reducing and increasing the mass of the ship (by expelling and gathering up hydrogen) should influence acceleration and velocity, respectively. If we're looking at kinetic energy (1/2*m*v^2), my previous suggestions of x2.2 to acceleration and x0.45 to velocity work. If we're looking at momentum (m*v), however, x5 and x0.2 are more appropriate. But both of those are supposed to be conserved. I'll do the calculations for each.
Accelerating the whole way, while ignoring relativity, would actually put us a bit south of 3c when we reach our destination. With that in mind, and the fact that we can slow down pretty readily with the ramscoop, I think it may be safe to assume we can get up to 10% lightspeed, which I think is somewhere around the fastest you can go without being likely to destroy yourself (I think it's also slow enough that relativity can largely be ignored, simplifying things for us). With x1 to acceleration, once we're at 1% light, it would take 9x as long to reach 10% light, or 17.1 years  total 19 years (traveling 0.95 light years in the process). At x2.2 acceleration, it would only take 7.8 years, for a total of 9.7 years (traveling ~0.45 light years). At x5 acceleration, only 3.4 years, for a total of 5.3 years (traveling ~0.2 light years). We then turn off the ramscoop and coast at 10% light for some time. Eventually, we need to start slowing down. The first slowdown cycle is simply opening the ramscoop and gathering up hydrogen, taking 1.9 years to refill our holds. Starting with the x0.45 option, that drops us to 4.5% light (traveling 0.138 light years), and then we flip around (we'll treat this as taking no time) and spend another 1.9 years accelerating in the opposite direction, dropping us to 3.5% light (traveling 0.076 light years). We flip, and hit the brakes (activate ramscoop) again, dropping to 1.575% light over the next 1.9 years (traveling 0.05 light years), then flip and accelerate to drop to 0.575% light over the next 1.9 years (traveling 0.02 light years). That would drop us below the minimum for the ramscoop, so what we'd actually do is a shorter burn, then refill the ramscoop such that we hit 1% light just as we fill up (or maybe just dump the hydrogen without accelerating it); we'll assume that takes another 1.9 years (and we travel 0.02 light years in this time). A final 1.9 years to drop all our velocity (traveling 0.005 light years in the process), and we're there. That's a total deceleration time of 9.5 years (and distance of 0.29 light years, we'll round up to 0.3). Going with the x0.2 option, our first braking maneuver would drop us to 2% light over 1.9 years (traveling 0.114 light years in the process), and then a flip and burn would put us at 1% light over 1.9 years (traveling 0.03 light years). Lets just assume we can alternate dumping hydrogen and gathering it up again to slow down to 1% light over the course of 1.9 years (still 0.03 light years travel), then a final 1.9 years (0.005 light years distance) to drop all our velocity. That's a total deceleration time of 5.7 years (and distance of 0.15 light years). So, with x2.2/x0.45, we take 9.7 years to accelerate, and 9.5 years to decelerate. We cover 0.75 light years during this time, and coast at 10% light for the remaining 3.25 light years of distance  which takes another 32.5 years, for a total of 51.7 years. With x5/x0.2, we instead take only 5.3 years to accelerate, and 5.7 years to decelerate. We cover 0.35 light years during this time, and coast at 10% light for the remaining 3.65 light years  which takes another 36.5 years, for a total of 47.5 years. Thus, with this scheme, we can probably travel 4 light years in around 50 years. Note above that simply using the ramscoop is actually decelerating us faster than our drive is able to accelerate us. That's the drag that makes it so ramscoops don't really give infinite deltav, so realistically it should take longer than the ~50 years calculated above (and we probably never reach 0.1c).
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07122022, 09:07 AM  #20 
Join Date: Nov 2015

Re: [Spaceships] Getting A Few Thousand Extra MPS
I don't understand what the problems would be (according to the rules in Spaceships) with building say an SM+10 fusion drive ship, grabbing an ice rock 10x the size of the ship and heading off with about 4k mps to spare.
Acceleration at the start of the journey might be quite bad (1/11 of what the engines usually provide) but it would improve over time. If a rock/asteroid is too unstable and possibly not pure enough, you can still build a "ship" made of 20 fuel tanks for about 600M and push that. 
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