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Old 12-12-2018, 03:35 PM   #1
AlexanderHowl
 
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Default FTL Assumptions [Space]

One of the most important decisions that a GM can make concerning interstellar settings are the assumptions surrounding FTL travel. If the assumptions are too generous, a GM could end up with PCs launching relativistic attacks from across the galaxy through hyperspace. If the assumption are too harsh though, interstellar travel can take so long that the PCs will die of old age (and the players of boredom) before they reach their destination.

I have decided to share my assumptions about TL10 FTL travel. First, I assume that objects have a hyperspace shadow equal to the eighth root of the mass in Earths × 0.1 AU (creating a hyperspace shadow roughly equal to 0.1 AU for the Earth and 0.5 AU for the Sun). Second, I assume that hyperspace jumps only preserve sufficient velocity to maintain the orbit of where they are traveling to, with excess kinetic energy being transformed into electromagnetic energy at the point of origin (insufficient kinetic energy causes the hyperspace jump to settle in the appropriate orbit instead of the destination orbit). Third, FTL velocity is equal to square root of drives × 100c and FTL communications do not exist.

The three assumptions create a workable framework for potential interstellar conflicts. The first prevents sneak attacks on planets, the second prevents relativistic attacks on planets, and the third prevents rapid deployment of military resources. If the maximum effective FTL speed is 200c, then it takes about a week for military spacecraft to go between systems and around two weeks for civilian spacecraft to go between systems.

So, what do you think? Do you use similar assumptions in your settings with FTL? If not, what assumptions do you use for FTL in your setting?
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Old 12-12-2018, 04:58 PM   #2
naloth
 
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Default Re: FTL Assumptions [Space]

Over many years, many things.

Our Trek campaign basically had warp shadows. You couldn't use warp drive until you were sufficiently away from a gravity well.

Our Star Wars campaign was basically slow FTP, where things go progressively slower as you got closer to things.

In a campaign that loosely resembled Uplift, we used jump drives that relied on jump points that trailed solar bodies.

In our 40k game, FTP travel was unpredictable and dangerous. It was more like sailing a giant ocean with a small raft and TL2 navigation. Quite a bit like how Dr Who travels around in the Tartis, actually.

For Sundog and Star Frontiers game, travel just kind of took place in the background. There might have been a formula or a table, but really it was plot/GM driven.

Of all the ones, jump drives probably worked best. Space combat is hard to envision where warp is possible and I tend to end up reducing fights to manageable speeds one way or another.
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Old 12-12-2018, 05:35 PM   #3
David Johnston2
 
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Default Re: FTL Assumptions [Space]

In Sun Divers, the STL propulsion system gathers "dark matter" and shoots it out through propulsion engines known as "dark jets". This gives the ships a maximum speed past which the drag on the dark matter scoops balances the jet acceleration. FTL ships must then dive for the local sun at high speed and draw upon the sun's energy to enable a jump drive that causes them to be transported in a "bubble" that dissolves at the destination having lost most of it's momentum in transit...for some inexplicable reason.

Also the emergence isn't quiet since the bubble's energy scatters in a burst of energy that would probably destroy the ship except that's it's all outward. It's possible to mistake the detonation of a nuke for an emergence.
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Old 12-12-2018, 06:02 PM   #4
Michael Thayne
 
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Default Re: FTL Assumptions [Space]

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Originally Posted by AlexanderHowl View Post
Second, I assume that hyperspace jumps only preserve sufficient velocity to maintain the orbit of where they are traveling to, with excess kinetic energy being transformed into electromagnetic energy at the point of origin (insufficient kinetic energy causes the hyperspace jump to settle in the appropriate orbit instead of the destination orbit).
I'm not sure I understand this. Kinetic energy can only be calculated relative to a frame of reference. And the relative velocity of any two stars can easily be 100 mps or more. Do ships regularly spend lots of delta-V to get the right pre-jump trajectory?
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Old 12-12-2018, 07:05 PM   #5
AlexanderHowl
 
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Default Re: FTL Assumptions [Space]

Yes, they would have to. Otherwise, they would 'gain' relative velocity to their destination, which could have some very dangerous effects from a military point of view. While it is to prevent PCs from doing relativistic attacks, it has consequences on military actions.

For example, without that assumption, military forces would always accelerate to 0.1c before jumping into hyperspace. When they arrived at the destination, they would release wings of AKVs going 0.1c before jumping away from the conflict. The AKVs could then use 10cm missiles to deal over 3d×93,000 damage to any target. Since every military would use the tactic, interstellar conflicts would result in everyone dying.

With the assumption though, jumping at 0.1c would be sheer insanity, as a 10,000 metric ton spacecraft would release as much energy as 2.5 teraton nuclear weapon (the fireball would destroy any spacecraft within 10,000 miles). Military doctrine would be to match relative velocities with the destination before a jump and then to accelerate to attack velocity. That would allow defenders to actual prepare a defense, which results in better stories that a 0.1c AKV splat.
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Old 12-12-2018, 10:21 PM   #6
RyanW
 
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Default Re: FTL Assumptions [Space]

The option I'm leaning to for my setting is gate network. Jumping independently of a gate is possible, but it is inefficient and requires a highly specialized ship.
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Old 12-12-2018, 10:36 PM   #7
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Default Re: FTL Assumptions [Space]

Quote:
Originally Posted by AlexanderHowl View Post
For example, without that assumption, military forces would always accelerate to 0.1c before jumping into hyperspace.
That requires other assumptions, too. 0.1c ain't easy. Without magic (or some very extreme staging), it requires TL 11 technology and years of acceleration.
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Old 12-13-2018, 03:41 AM   #8
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Default Re: FTL Assumptions [Space]

I presume that the use of 8th root of mass for the shadow radius was simply because it gave a pleasing range of distances?
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Old 12-13-2018, 05:34 AM   #9
AlexanderHowl
 
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Default Re: FTL Assumptions [Space]

Yes, and it was the only way to get close to one of the numbers from GURPS concerning a hyperspace shadow. It also means that any substantial mass creates a hyperspace shadow. As for 0.1c not being easy, FTL is superscience and most settings with FTL also have reactionless engines. Military spacecraft with TL10 reactionless drives could have 2g continous acceleration, allowing them to reach 0.1c in less than three weeks.
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Old 12-13-2018, 07:12 AM   #10
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Default Re: FTL Assumptions [Space]

Is there a minimum size to generate a warp shadow? I'll point out that a 150 human is 2.5E-23 times the size of the earth, but generate a 1/7,000 AU warp shadow. Which is about 13,000 miles.



Additionally, AU are massive. The moon is only about 1/400th of an AU away, and our fastest flyby of the moon from earth (ever) took 8 hours. Apollo took 3 days.
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