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Old 04-07-2014, 01:45 PM   #21
Johnny1A.2
 
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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Originally Posted by malloyd View Post
Actually spaceships on a freight run 4 years long probably should meet the second of those, since there's no good reason to have any crew at all.

And the freight *better* be high value for its weight. Even at perfect efficiency, getting your ship up to 300 mps and back down uses energy thtn has never been cheaper than on the order of $1000 per pound. So if you can't charge $1000 per pound for it, your freight run is a money loser even if the other operating costs are zero and cargo (and ships!) are free at the point of origin.

It's next to impossible to make interstellar trade make sense in a realistic setting, since any civilization that can actually build a starship is going to be able to make anything at home more cheaply than they could ship it back. Even if they have to assemble it atom by atom. And you can send the atomic scale blueprint without a starship.
That's a bit of a non sequiter, because the ability to build fast STL starships doesn't automatically imply the ability to assemble anything you want atom by atom. Just because you have the energy doesn't mean you have the means to apply it to whatever you want. It's a bit like saying that the ability to build an Apollo moonship implies the ability to synthesize a steak from scratch.

It's true that shipping bulk ore or the like doesn't make sense over interstellar distances, but that doesn't mean that nothing could possibly be worth shipping.
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Old 04-07-2014, 02:14 PM   #22
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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Originally Posted by Johnny1A.2 View Post
That's a bit of a non sequiter, because the ability to build fast STL starships doesn't automatically imply the ability to assemble anything you want atom by atom. Just because you have the energy doesn't mean you have the means to apply it to whatever you want. It's a bit like saying that the ability to build an Apollo moonship implies the ability to synthesize a steak from scratch.
We may have had the ability to assemble things atom by atom before we launched Apollo. We certainly have it now well before we could launch a starship. Yes it would cost a completely insane amount to attempt to assemble a steak an atom at a time with the probe of an atomic force microscope, but it would cost an equally insane amount to ship it from Alpha Centauri.
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Old 04-07-2014, 02:26 PM   #23
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

Trade is going to be in luxury consumer goods and possibly hard to find metals/minerals. One thing I'm thinking might be shipped is food stuff's.
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Old 04-07-2014, 02:55 PM   #24
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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Trade is going to be in luxury consumer goods and possibly hard to find metals/minerals. One thing I'm thinking might be shipped is food stuff's.
There are essentially no minerals that are hard enough to find that a several hundred AU journey is a sensible option, and very very few consumer goods where trading blueprints for local manufacture doesn't make more sense. Foodstuff over a 4 year journey will usually do poorly, unless it's something that normally gets aged, and even then only the stuff that's $5,000/bottle is worth sending. The only materials I can really see being worth transporting are genetic materials.
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Old 04-07-2014, 02:55 PM   #25
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

There are a few things to consider. If you are looking at achieving 300 mps, you are likely using constant acceleration, and consequently a brachistochrone transfer. I havent do e the math, but you 4 yr figure may be inaccurate. Orbital transfers are never straight lines. Ill work this up when i get home for you.

The other thing is that jumps out is that for two planets to be 400 au apart, one needs to be in the inner oort cloud, assuming a system similar to our own. I personally dont thknk this is impossible, but it might be a little unusual. Also, orbits out there tend to be kind of eccentric, so the separation will vary considerable. Some launch windows may be cheaper than others. I can calculate the frequency of any particular launch window if you like.
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Old 04-07-2014, 02:56 PM   #26
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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Trade is going to be in luxury consumer goods and possibly hard to find metals/minerals. One thing I'm thinking might be shipped is food stuff's.
As others have said, what would someone buy for thousands of dollars a pound that they couldn't get locally for much cheaper?
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Old 04-07-2014, 02:57 PM   #27
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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Originally Posted by Humabout View Post
There are a few things to consider. If you are looking at achieving 300 mps, you are likely using constant acceleration, and consequently a brachistochrone transfer. I havent do e the math, but you 4 yr figure may be inaccurate. Orbital transfers are never straight lines. Ill work this up when i get home for you.
With 300 mps delta-V, straight line is usually close enough. You only get in trouble when your delta-V is fairly close to the orbital velocity of the object.
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The other thing is that jumps out is that for two planets to be 400 au apart, one needs to be in the inner oort cloud, assuming a system similar to our own.
Pretty sure it's a multi-star system.
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Old 04-07-2014, 03:59 PM   #28
Vaevictis Asmadi
 
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

I expect realistic, reasonable interstellar trade between similar civilizations* to be information, live people, and very valuable biological materials. Biological substances can probably be cultivated in artificial environments cheaper than shipping. Even genetic data may be cheaper to ship as raw info rather than samples, if their biotech is high-tech enough to build living systems from genetic blueprints. If that isn't possible, cell lines, biological samples, and even (non-synthetic) drugs might be physically traded.

Bulk raw materials are very unlikely to be shipped, unless they are produced by some bizarre natural condition unique to one system, or by an organism that has never been successfully cultivated off of its homeworld (like Arrakeen sandworms). If the 6 star systems all formed together in the same open cluster from the same nebula, they should have similar enough composition that whatever you can mine in one system, you can mine just as easily in the neighboring systems too.

However, unreasonable trade might involve manufacturing secrets guarded by jealous corporations, allowing corp. A to manufacture/grow whatever and then charge out the nose to ship it. Not sure how they can prevent corp. B from re-engineering it on the other side, or finding cheaper alternatives, though. Even if it's a drug produced by an organism that can't be cultivated in artificial environments, expect people to search for a synthetic formula or alternative, or even brute-force a GMO one codon at a time to produce the stuff.


* If the civilizations are not similar, especially if they have separate origins, then there may well be things one can make that the other can't.



EDIT: Alright I thought up a few odd cases in which people might ship large quantities of stuff between the stars:

1. One star system has ruins of an alien civilization. Some artifacts can be repaired enough to make them useful, but nobody can recreate them.

2. One star system contains a bizarre, apparently unique source of exotic matter that can be mined.

3. People build a gigantic particle accelerator in space, or an exotic matter factory, or even discover an abandoned alien factory and manage to sort of get it going without understanding how it works. Essentially, a manufacturing 1 or more TLs higher than the rest of the civilization. It produces exotic matter, stable superheavy elements, or other weird things in significant quantities. It may be cheaper to ship this stuff rather than build a second hyper-expensive factory in another star system, or perhaps they are building a second factory but it will take a long time and isn't done yet.

4. Not relevant to your setting, but I could imagine a setting in which FTL travel is possible, but FTL communication isn't (or is very unsecure). In this case even data itself may be physically shipped between stars on ye olde data storage devices.
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Last edited by Vaevictis Asmadi; 04-07-2014 at 04:32 PM.
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Old 04-07-2014, 05:09 PM   #29
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

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With 300 mps delta-V, straight line is usually close enough. You only get in trouble when your delta-V is fairly close to the orbital velocity of the object.

Pretty sure it's a multi-star system.
Yes, I was using straight line math, and yes it is a multi-star system. That said after some comments about how far apart things were I started investigating if I could decrease the distance between them, the net result of that is the loss of a few unimportant Gas Giants and the addition of two extra habitable planets around the, so far, two mapped stars, bringing the total up to 4
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Old 04-07-2014, 05:10 PM   #30
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Default Re: [Space/Spaceships] Getting the velocity/Delta V I want with the tech I want

As promised, Numbers! For lack of better data, I used an initial orbit of 1 AU and a final orbit of 400 AU. If you give me more exact figures, I can give you better information back.

Assuming you achieve a top speed of 300 mps, as you stipulate, you will need about 600 mps worth of delta-V and be capable of about 0.0004 g acceleration. The trip will take you about 8 years - not 4.

Various launch windows will occur about once an earth year, assuming the primary is about one solar mass.
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