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Old 07-21-2011, 12:24 AM   #11
teviet
 
Join Date: May 2005
Default Re: [Spaceships] Voyage Time Calculation with Different Drives

Hohmann transfer orbits aren't so bad (the formulae are all on Wikipedia, no need to reproduce them here), but unfortunately you're not in such a nice, simple case here. In this case the fusion rocket has more than enough Δv for a Hohmann transfer orbit, but not so much that you can ignore the Sun's gravity. So you'd want a constrained minimum-time orbit, not a minimum-energy orbit, which would be an exercise in numerical integration rather than simple formulae.

If you want to make sails more appealing than rockets, may I suggest putting the jump point very close to the star? If the escape speed is more than about 2.5 times the typical Δv of your rockets, then a sail is the only efficient way to get there (it gets more efficient as you get closer to the star).

TeV

EDIT: To clarify, the orbital speed at a distance D (in AU) from a star of mass M (in Solar masses) is:

v_orbital = 18.5mps x square_root( M / D )

The escape speed is 1.41 times higher, so the required Δv is 0.41 times this number, or:

Δv = 7.67mps x square_root( M / D )

If your rocket's Δv is less than this, you can't approach or escape the jump point.

But if your jump point is fixed, rather than orbiting, then a sail is pretty much useless to approach it, unless it has an acceleration greater than the star's gravity at its location.

TeV

Last edited by teviet; 07-21-2011 at 01:10 AM. Reason: Added formula and clarification.
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Old 07-21-2011, 04:24 PM   #12
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Default Re: [Spaceships] Voyage Time Calculation with Different Drives

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Originally Posted by teviet View Post
[...] If you want to make sails more appealing than rockets, may I suggest putting the jump point very close to the star? [...]
That is actually a very, very interesting idea, thanks!
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Old 07-21-2011, 04:49 PM   #13
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Default Re: [Spaceships] Voyage Time Calculation with Different Drives

Unless you're dealing with extremely high performance drives that can basically treat the solar system as flat (acceleration > 0.001 Gs, delta-V > 100 km/sec) you're likely to wind up with some fairly messy orbital mechanics problems. For the high-thrust low-delta-V drive options, hohmann transfer orbits can be optimal, but they're also prone to being very slow.

As far as piloting skill, you're pretty much out of luck unless you give your human pilots precognition; a computer is simply better than a human at deep space navigation, and the problems are mostly solvable (for sails, since they're inconsistent, you'll need to solve in a statistical manner, but that won't make much difference). A computer won't necessarily think of the various multi-pass solutions for gravitational assist, but you're looking at much simpler problems.
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Old 07-21-2011, 05:27 PM   #14
teviet
 
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Default Re: [Spaceships] Voyage Time Calculation with Different Drives

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As far as piloting skill, you're pretty much out of luck unless you give your human pilots precognition; a computer is simply better than a human at deep space navigation, and the problems are mostly solvable (for sails, since they're inconsistent, you'll need to solve in a statistical manner, but that won't make much difference). A computer won't necessarily think of the various multi-pass solutions for gravitational assist, but you're looking at much simpler problems.
I suspect nondescript is thinking of a human intuitive ability to predict solar activity: exactly where and when the next flare or CME will be, and how strong, so that he can maneuver into an optimal position and reel in/out sail cable as necessary. Like space-weathersensing. It's mildly cinematic, but probably less so than having a human pilot on other sorts of spacecraft.

TeV
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Old 07-21-2011, 09:20 PM   #15
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Default Re: [Spaceships] Voyage Time Calculation with Different Drives

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Originally Posted by teviet View Post
I suspect nondescript is thinking of a human intuitive ability to predict solar activity: exactly where and when the next flare or CME will be, and how strong, so that he can maneuver into an optimal position and reel in/out sail cable as necessary. Like space-weathersensing. It's mildly cinematic, but probably less so than having a human pilot on other sorts of spacecraft.

TeV
He should also consider making the FTL travel the more skill-based part of the trip. There are plenty of fictional examples of Star Drives or Jump Gates that simply cannot be computer-controlled, relying on some mysterious human factor to activate or navigate.
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Old 07-22-2011, 04:36 AM   #16
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Default Re: [Spaceships] Voyage Time Calculation with Different Drives

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I suspect nondescript is thinking of a human intuitive ability to predict solar activity [...] It's mildly cinematic, but probably less so than having a human pilot on other sorts of spacecraft. [...]
Yes, exactly.
"Plotting the best course through the ever shifting solar winds and reacting to the unpredictable fluctuations on helm is more of an art than an exact science. Human intuition has a little edge over computers here."
That isn't very hard SF and is probably factually wrong, but it is still within my willing suspension of disbelieve for this setting.
Especially if it serves a meta goal.

Quote:
He should also consider making the FTL travel the more skill-based part of the trip. [...]
Of course. but I would really like to spread around the skill influence among the crew/party even more:
The FTL part will have two skill rolls (Navigation (Jumpspace) and Jumping [a psi skill, the jump drive is psi powered for this very reason]) which will actually govern how far the ship arrives from the destination planet, and two skill rolls for the STL part (Navigation (Space) and Spacer (for helm)/Piloting (Low-Performance Spacecraft)) which will vary real space travel time by +/- 5% or so depending on combined margin of success/failure.
Even with the small, cross trained crews/parties for PC-tramp freighters that should give three PCs influence over one of the cost variables (trip length) without being super soft space opera.
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