Accuracy of Jump

[Drifter]

Is there canon, or widely held opinion, about how accurate a Jump exit can be plotted?

For sake of argument lets say we are dealing with a world deep inside the 100D limit of its primary. Jump masking and other items seem to imply that a ship makes a straight line Jump from system to system. So a ship coming in from one system would have to cross to the other side of the 100D sphere to Jump to a system on the 'other side'.

For example, a ship coming from Dojodo to Mora would have to cross the 100D sphere (if Mora were inside such a Jump mask) to get to the Jump area for Jokotre.

Jump is a straight line - once in jumpspace you are committed to one direction only; forward.

OR

Can a Jump be plotted from any point on the 100D limit sphere? So you just aim the ship for the closest part of that limit and Jump from there. And when you arrive you can have already have plotted your exit at the closest point the your destination?

That is, your actual Jump can have curves and turns and isn't exactly a straight line, just mostly.

For either I assume piloting or navigation skills determine exactly how accurate your jump is. I don't see ships jumping out one on top of the other, or all heading to the one exacty coordinate in space. But if its a totally straight line then you wil see a pretty close cluster of breakouts and exits.

[Malenfant]

Given that it's impossible to be temporally accurate (IIRC the jump time is 168 hours +/- 1d6 hours?) I'd imagine that spatial accuracy isn't too reliable either.

[Hans Rancke-Madsen]

Originally Posted by Malenfant

Given that it's impossible to be temporally accurate (IIRC the jump time is 168 hours +/- 1d6 hours?) I'd imagine that spatial accuracy isn't too reliable either.

Marc Miller's jumpspace article specifies a spatial accuracy (I can't remember how small, but it was something like hundreds or thousands of miles per parsec -- very accurate). What varies with time must be the position relative to the world you're aiming for.


Hans

[Mark Laiho]

Hi,
Just some ideans.

You don't change your actual momentum from the begining of a jump to the exit of the jump. However the system you jumped to has its own vector usually different from the one you came from. So rathing than figuring that all out they assign you a random vector when you come out of jump as well.

So you might do an accurate jump but unless you planned for the difference in vectors between systems you could end up going in the wrong direction.

Having said that you should be able to build up the proper vector then jump so you come ouf of jump heading in the right vector. This doesn't mean you will land in the right spot though.

I assume the 100Diameter limit drops you out of jump space early if you plotted jump point is suppose to be further along....

One of the reasons for plotting a jump point carefully is to avoid anything dropping you out of jumpspace early i think.

The random time value might be to take into account the sight difference in lengths of the jumps. It might also be a variable property of jump space.

[Anaraxes]

Originally Posted by Malenfant

Given that it's impossible to be temporally accurate (IIRC the jump time is 168 hours +/- 1d6 hours?) I'd imagine that spatial accuracy isn't too reliable either.

That's only true if you imagine that you jump to a fixed point in an absolute coordinate scale, so that being late means your planet has moved on.

But we're dealing with timey-wimey jumpy-wumpy FTL. Jumps could just as easily be targeted to a specific point on the 100D limit of some body, the only interesting features in jumpspace, and exactly spatially precise, yet variable in time. You know where you're going, but you don't know how long it takes to get there.

Perhaps you _must_ appear somewhere on that 100D sphere, but the actual point on that sphere is completely random.

We might wave our hands in a vaguely Heisenbergian direction, and declare that there are errors in both time and space, the product of which can't be driven below some fixed value. And that you can trade them off. The customary and popular tradeoff is the time variance given in the rules, which still leaves the spatial variance to be defined. But perhaps you can tighten that up at the cost of even more variable time of transit -- or conversely, make the time specific at the cost of spatial accuracy.

There are lots of possibilities. What would be fun for your game?

[Malenfant]

Originally Posted by Anaraxes

That's only true if you imagine that you jump to a fixed point in an absolute coordinate scale, so that being late means your planet has moved on.

I was thinking more along the lines of "aim at a point on the 100D sphere, but you might pop out a few thousand km away from that point outside the sphere".

[malloyd]

Originally Posted by Anaraxes

That's only true if you imagine that you jump to a fixed point in an absolute coordinate scale, so that being late means your planet has moved on.

That's actually yet another pretty strong indication jumps can't be tied to real universe directions and hence that jump masking makes no sense at all. You could imagine jumping to something in a fast enough orbit that whether or not the 100D limit of primary is in the way depends on what the roll is for the time variance. Even at more typical orbital speeds for habitable planets, there's no overlap at all between the 100D spheres for smaller worlds with higher variance rolls - it takes Earth a little less than a day to move 100D.

[Anaraxes]

Originally Posted by malloyd

That's actually yet another pretty strong indication jumps can't be tied to real universe directions and hence that jump masking makes no sense at all.

Not to exhume this particular dead horse, but yes -- at least in realspace geometry. On the other hand, that might be exactly how it works in jumpspace coordinates. You might stop at the first sufficiently steep gradient you hit in jumpspace. But even if true, if the two spaces don't correspond that point still tells you nothing useful about whether any particular object is "in the way" in jumpspace. Just a handwave / GM fiat when you need that plot point.

[Hans Rancke-Madsen]

Originally Posted by malloyd

That's actually yet another pretty strong indication jumps can't be tied to real universe directions and hence that jump masking makes no sense at all. You could imagine jumping to something in a fast enough orbit that whether or not the 100D limit of primary is in the way depends on what the roll is for the time variance. Even at more typical orbital speeds for habitable planets, there's no overlap at all between the 100D spheres for smaller worlds with higher variance rolls - it takes Earth a little less than a day to move 100D.

23 hours? I thought it was less.

Anyway, just assume that the game rules for jump are simplified. If you want to jump to Earth, you aim for where it will be in 168 hours. If you arrive 17 hours ahead of shedule, you'll hit Earth's 100D limit and be precipitated out ahead of Earth. If you arrive 17 hours late, you'll hit the jump limit and be precipitated out at the limit. If you arrive at any time in between those two extremes, you'll be precipitated out somewhere along the jump limit. Just as the rules has it.

But what if you're aiming for a world that moves 100D in, say, 15 hours? That's where the simplification comes in. Since jump duration follows a bell curve, we can assume that you will usually arrive within the +/- 15 hour window and ignore the possibility of arriving more than 15 hours before or after.


Hans

[Drifter]

Well, I see that there is no widely held opinion :)

I despaired about the Marc Miller's article until I searched JTAS for it, and lo and behold!

Among other things it says you come out within 3000 km of your desired exit, for a jump 1, with other factors combining to possibly make that distance worse 'by a factor of 10'.

So it is a function of timey-wimey, jumpy-wumpy physics. To me that does not necessarily exlude jump masking, as seen in Far Trader (not sure if anywhere else). That does seem to imply that you don't absolutely have to end your jump at a 100D limit area. Although I've always taken it as a given you can jump to anywhere you want, such as deep space, just no inside a 100D of a substantial body.

If your destination is masked, you exit jump in deep space, 3000km give or take from the spot you've calculated as the closest you can get from a straight line from your departure point.

The whole point of this being - this adds a tremendous amount of time to a trader's time in a system. Days in some cases - a fully loaded freighter can take over 3 DAYS to reach a port at a G star system. AND anothere 3 days to get back to a jump point.

All this discussion of trade rules got me wondering just how fast freighters can be processed in a system. Its seems to me that just jumping into a star system, landing at the starport, doing tradey stuff, then launching and jumping again is actually rare. That is, the way I envisioned it in the Classic Traveller books. You spend nearly as much time in deep space as in jump space (depending on the stars you visit and the luck of the dice).

Astography is a very real consideration, deciding which ports to visit or avoid. Freighter liners and X boats would want to avoid ports where you need to spend days getting to maintinance and refueling facilities.