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Old 06-09-2019, 06:04 AM   #1
dataweaver
 
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Default Semi-Realistic Interstellar Travel

I'm considering the possibility of a space setting featuring Interstellar travel, but without Stardrives: everything's STL. My first attempt involves something along the lines of the Einstein Class Exploration Ramship (Spaceships 5, p.11); but the write-up doesn't specify how fast the Einstein can go: it's got a fusion rocket with a ramscoop option, and it states that it needs to get up to 1% of lightspeed to start using the scoops, at which point it can accelerate up to “50% of lightspeed or more”. The rocket is capable of 0.005G acceleration.

So, how do I calculate the ship's top speed in interstellar space? And how does that top speed change if I replace the fusion drive with something that has better acceleration? I'm willing to upgrade to a Standard Reactionless Engine if I have to.
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Old 06-09-2019, 06:42 AM   #2
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Default Re: Semi-Realistic Interstellar Travel

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Originally Posted by dataweaver View Post
So, how do I calculate the ship's top speed in interstellar space? And how does that top speed change if I replace the fusion drive with something that has better acceleration? I'm willing to upgrade to a Standard Reactionless Engine if I have to.
It doesn't really have one other than the speed of light. Some kinds of ramscoop designs do have a limit caused by drag of the scoop field on the interstellar gas, but it varies a lot with the specifics of the design, and with the local density of the interstellar medium. For a reactionless thruster, even that goes away. It's also true that as your velocity increases your acceleration in your starting frame falls because of time dilation, but that's asymptotic, it never falls to zero.

There's also the fact that as you get faster, whatever particles you happen to collide with are hitting you faster, which eventually starts to be a radiation hazard, or even a source of serious heating. That's probably where the about 50% of lightspeed limit notionally comes from. If you aren't carrying anything that cares about radiation or being cooked you can go faster.

The other issue is that the speed of light is high, and 0.005 G is not. It takes about 100 years to get up to 50% of c with that acceleration. If you are going to a close by star, within a couple hundred light years, your maximum speed is how fast you can get up to before you are halfway there and need to start decelerating.

STL interstellar travel is essentially a plot device for one-way scene transitions anyway, not something you need a ship design for. If the PCs are conscious during the trip, they're isolated from everybody not on the trip, and die of old age before it ends. If they're in some sort of stasis, they arrive, but their destination has had centuries to change during their trip and therefore is effectively a new setting for them anyway.
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Old 06-09-2019, 06:43 AM   #3
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Default Re: Semi-Realistic Interstellar Travel

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Originally Posted by dataweaver View Post
I'm considering the possibility of a space setting featuring Interstellar travel, but without Stardrives: everything's STL. My first attempt involves something along the lines of the Einstein Class Exploration Ramship (Spaceships 5, p.11); but the write-up doesn't specify how fast the Einstein can go: it's got a fusion rocket with a ramscoop option, and it states that it needs to get up to 1% of lightspeed to start using the scoops, at which point it can accelerate up to “50% of lightspeed or more”. The rocket is capable of 0.005G acceleration.

So, how do I calculate the ship's top speed in interstellar space? And how does that top speed change if I replace the fusion drive with something that has better acceleration? I'm willing to upgrade to a Standard Reactionless Engine if I have to.
It doesn't sound as if the rocket's Acc should affect the top speed at all. What it does affect is how long it takes you to get to ram speed.

You can start the ram at 1% of c, which is 3,000,000 m/s. If you were accelerating at 1g, or 10 m/s^2, it would take you 300,000 s to get there. That's 83.3 hours, or about 3.5 days. If your acceleration is 1/200 g, then it takes 200 times as long, or 16,666.7 hours, which is about 695 days, or a bit under 2 years.

At that point, you go to ram, and you get whatever acceleration the ram provides. Is that specified?
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Old 06-09-2019, 07:22 AM   #4
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Default Re: Semi-Realistic Interstellar Travel

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Originally Posted by whswhs View Post
At that point, you go to ram, and you get whatever acceleration the ram provides. Is that specified?
The ramscoop simply provides fuel for a fusion drive by scooping up the interstellar medium as the ship flies through it. Thus the acceleration is that of whatever fusion drive you're using, presumably 0.005G from the fusion rocket in this case.

A bigger reason than acceleration to 'cheat' and use a super-science reactionless thruster is probably the huge fuel requirements of a fusion rocket - the sample vessel, at TL11, requires 45% of its mass to be fuel just to get up to ramscoop velocities. Fortunately the scoop will refill the tanks, and can be used to do a lot of the de-acceleration by braking, saving the fuel for the next voyage.
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Old 06-09-2019, 08:09 AM   #5
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Default Re: Semi-Realistic Interstellar Travel

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It doesn't sound as if the rocket's Acc should affect the top speed at all. What it does affect is how long it takes you to get to ram speed.
This is true of pretty much any semi-realistic space drive. If you don't have enough fuel to accelerate the entire trip, and for realistic drives you mostly do not, your acceleration has negligible effect on the thing you really care about - which is how long does the trip take.

The importance attached to it in SF novels and games is presumably because going really fast makes the math simpler - you get to ignore orbital mechanics, and the fact that if you want to make interplanetary trips something your characters do more than once in a lifetime, you *need* superhigh velocities you can reach in really short runs to get fast enough trips that makes sense.
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Old 06-09-2019, 09:07 AM   #6
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Default Re: Semi-Realistic Interstellar Travel

The design of the ships can be important even if the PC's aren't going interstellar. The existence of the old colony ship, or even its drive module, in a system can be important, either as a military objective, or a goal to find and restore the long lost Voyager.
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Old 06-09-2019, 09:11 AM   #7
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Default Re: Semi-Realistic Interstellar Travel

One issue with STL interstellar travel is that it assumes that space is empty of everything but gas and dust with a density between 10^-26 to 10^-16 grams per cubic centimeter. The problem will that assumption is that it is based on the hope that there are not large numbers of chuck of ice and rock out there (which are undetectable with modern astronomy). Of course, there also may be as much as 100,000 rogue terrestrial planets per star, which do tend to be difficult to detect in front of you with a gigantic ramscoop blocking your view.

Even if you do not run into anything that would end your trip prematurely, the acceleration of realistic fusion drives capable of a delta-v of 1% c is pretty low. At 0.01g (two engines), it takes 1 year of acceleration to reach 1% and then another 4 years to reach 5% c (that is pretty much the hard limit for realistic fusion, as it would take more energy to accelerate the capture interstellar medium to your velocity than you would get from the fusion due to neutrino losses and other inefficiencies above that velocity).

So, 5% c in five years is about as good as you can do for realistic interstellar travel, meaning that any STL interstellar spacecraft will be reserved for colonization purposes because it would just not be a worthwhile investment to trade with another location with minimum 200 year turnaround. Even at 2% interest, you could increase your savings by 50x by just putting it into a savings account during the same time period, so the return on any STL interstellar trading venture would have to be 100x the cost of the mission to make it worth the risk.
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Old 06-09-2019, 11:39 AM   #8
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Default Re: Semi-Realistic Interstellar Travel

You may want to consult GURPS Planet Krishna, which are based on L. Sprague de Camp's Viagens Interplanetarias series.

The novels take place in an STL interstellar nation, using torchships accelerating at 1g over interstellar distances. The social aspect of long journeys were minimized with the free government distribution of anti-agathic drugs. All the books were written before 1958, so the science is less rigorous than one would expect. But they're a fun read if you can locate them, and the GURPS supplement has all sorts of cool and useful stuff.
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