Question on the 100D jump limit

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Old School
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Question on the 100D jump limit

Postby Old School » Mon Jan 14, 2019 9:37 pm

So a 100 diameter boundary around a planet isn't a fixed point, but rather a very large globe surrounding the planet. How do you treat this in your games?

I believe past materials have referenced that it is frowned upon to come out of jump space with any velocity, so a ship is expected to slow to a stop before jumping. A stop a relative to what? Relative to the planet they are jumping to would make sense, although the game mechanics appear to simplify this by having the ship stop relative to the planet it departed from before jumping. I would think we could just say that the ship adjusts its speed and heading to match that of the destination planet before jumping, and approximate (hand wave) this by having it stop relative to the planet it left.

More importantly, where is the ship relative to the destination when it arrives? Thinking in terms of a two dimensional map, the jump limit is a big circle around a planet. Do ships arrive and depart from anywhere outside of this ring? I think planetary authorities in high traffic systems would prefer to have more control than that, but you can't have one designated jump point for departures an arrivals , (i.e. a point 102 diameters from the planet directly downstream on the planet's orbital path), because some traffic would be masked by the planet itself and therefore couldn't jump from or two that point.

Again, thinking in two dimensions because that's the traveller map that we've accepted, I would see a planet having 4 "jump points" that incoming and outgoing traffic is expected to use. Perhaps slightly separated points for arrivals and departures, but close enough that for SDBs and customs vessels to cover both at the same time. This makes traffic control, customs, pirate patrol, etc. all much easier than having ships take any route they desire to or from the 100d limit. The farther you are from one of these points when you arrive, the more suspicious you are.

How do you folks handle this in your games, or has it come up? Thanks.
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Re: Question on the 100D jump limit

Postby MonkeyX » Mon Jan 14, 2019 9:49 pm

My assumption is the astrogation check is the means of deciding where you exit jumpspace. The idea that the planet itself would pull the ship out of jump early is a good one but without having every planet mapped I’ve always averaged travel time to any location on the surface from the exit point. The idea that the authorities may put out jump locations to arrive and depart is a great idea to keep some sort of structure to the whole thing.
The old TSR Buck Rogers game had a map of the solar system that tracked orbits so that travel from one planet to another could sometimes be longer or shorter. You could do this in Traveller too but it would need it done for each and every system.
Old School
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Re: Question on the 100D jump limit

Postby Old School » Mon Jan 14, 2019 9:51 pm

and as you are thinking up your brilliant answers, I see that some of you have discussed this before. . .until the thread went wildly off course, as they tend to do.

viewtopic.php?f=89&t=58152
Old School
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Re: Question on the 100D jump limit

Postby Old School » Mon Jan 14, 2019 9:58 pm

MonkeyX wrote:
Mon Jan 14, 2019 9:49 pm
My assumption is the astrogation check is the means of deciding where you exit jumpspace. The idea that the planet itself would pull the ship out of jump early is a good one but without having every planet mapped I’ve always averaged travel time to any location on the surface from the exit point.
Well sure, a good astrogation check means you came out more or less exactly where you planned to. But here's what came up in my game: You expect a ship to be arriving from another system one parsec away. If you want to wait on that ship, perhaps to attack it, do you know approximately where it will arrive at? Let's say it arrives in the expected time window, 102 diameters from the planet. In two dimensional terms, 102d is a very large circle. Do you know where on that circle they will be arriving? Because barring an accepted "jump point", they could be millions of km away from you, even if both of you are 102d from the planet.
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Re: Question on the 100D jump limit

Postby MonkeyX » Mon Jan 14, 2019 10:14 pm

I think the answer is no, you can’t preempt where a ship is going to jump in system. The one instance this came up in my game so far was to ambush an Aslan trade convoy. The reason I allowed the players to ambush them was because the information they gathered told them the astrogation route they would use barring a normal variation in distance. Without that info I’d guess the only way would be to sensor sweep for the ships transponder code and jump bubble as it entered the system then rush towards it in the hopes you can intercept it before it gets within any safety the planet may offer.
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Re: Question on the 100D jump limit

Postby phavoc » Tue Jan 15, 2019 12:23 am

I have come up with a house rule that every planet within jump range (previously just J-6) had a set of pre-determined sections of space allocated to each planet. So if you had three planets with starports in System A, each would be assigned it's own orbital area outside the 100D for arrivals. Each arrival section was 50k section, and ships were to arrive at zero relative velocity for safety reasons. That way there was organization for the arrival planet and to organize traffic patterns as best as they could be. For smaller planets/systems, they would get a larger section, but not divided up.

These arrival areas were on all charts and available at any starport, with updates being sent out on as needed basis. I have always thought that ships should just show up wherever they wanted was recipe for accidents. With the amount of ship traffic incoming to a planet (plus all the in-system traffic), it made no sense that some organization of this wasn't attempted. If the Imperium has standardized a bolt across 10,000 systems, they would be organized enough to set up a navigational process for systems as well.

Beyond that I would say arrival ships would, for safety reasons, arrive "behind" the planet just in case they suffered a drive loss and they could deorbit out of control (assumes a system without rescue capabilities of course). Since most traffic is going to be along the plane of the ecliptic you can assume that roughly 50% of all traffic will be here. But arrivals/departures should depend on where the systems and planets are aligned, so it's also not unreasonable to have ships departing above and below the ecliptic.
Old School
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Re: Question on the 100D jump limit

Postby Old School » Tue Jan 15, 2019 12:43 am

I think we are more or less on the same page, Phavoc. Some sort of organization is desirable. I don’t think there is a safety issue with ships jumping into the same general area. There is obviously some variation in where you come out of jumpspace, and spaceships, even the biggest ones, are incredibly small compared to the distances we are talking about. Coming out of jumpspace at speed, however, would greatly magnify whatever tiny risk there is, and also make traffic control, inspections, etc harder.
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Re: Question on the 100D jump limit

Postby ShawnDriscoll » Tue Jan 15, 2019 1:27 am

Old School wrote:
Mon Jan 14, 2019 9:37 pm
So a 100 diameter boundary around a planet isn't a fixed point, but rather a very large globe surrounding the planet. How do you treat this in your games?
As masked-off areas on a system map that ships cannot jump through. Travellers with high Astrogation skills avoid them.
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Re: Question on the 100D jump limit

Postby baithammer » Tue Jan 15, 2019 7:50 am

There is a section in the Travellers Companion about Jump drives and some extra rules, including a section on Jump Masking.

The only way to land close enough to another ship is Synchronised Jumps, which is in High Guard. ( On purpose that is.)
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Re: Question on the 100D jump limit

Postby Condottiere » Thu Jan 17, 2019 10:16 am

1. It's one of those circular discussions.

2. I'm pretty sure you can drift down the rabbit hole; accelerating while transitioning might be considered an unnecessary complication by the astrogator, and presumably doesn't affect the time spent in hyperspace, unless it throws off the astrogator's calculations, and might be regarded a traffic hazard at the off ramp.

3. Pretty sure that most star ports map out, possibly for years ahead, jump lanes for all possible approaches and times, and inform nearby systems of them, like a train schedule.

4. Masking by known interstellar objects/hazards doesn't require an astrogation check; maybe a memory one, if you don't check the astrogation charts, and likely the ship's computer would register as impossible course when entered.
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Re: Question on the 100D jump limit

Postby Linwood » Sat Jan 19, 2019 1:26 pm

That last point might be an adventure seed. “Wait a minute - I used a similar course back in the day when I last came through here and now the computer says it’s impossible! What changed?”

Could be the basis for a survey mission to the “new” planetary body (possibly a long-period comet?). Or an investigation - is there an error in the database or is something more sinister going on?
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Re: Question on the 100D jump limit

Postby NOLATrav » Sat Jan 19, 2019 3:19 pm

Regarding Old School’s original question, T5 posits that Jump is a straight line from departure point to destination point. So on a 2D map, you could draw a line from hex center to hex center to understand where the jump route between those two worlds are. That will also give you a point on the 100d circle.

IMTU I use the appropriate hex face on the subsector map. So an arc that is 1/6 of the 100d circle is where a ship will precipitate, with variance depending on the success, Effect, etc. This allows the authorities to have some idea of where incoming ships will arrive and also provides clues to where ships may be headed.

Of course, you can fly to the wrong area and jump from there to throw the coppers of the scent...
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Re: Question on the 100D jump limit

Postby Condottiere » Sat Jan 19, 2019 3:33 pm

Gravitational wells might well curve that straight line.

I joked that hyperspace is duodimensional, and that jump bubbles are tridimensional intrusions.

There is some control over the exact length of a jump, otherwise microjumps wouldn't be an option.
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Re: Question on the 100D jump limit

Postby Reynard » Sat Jan 19, 2019 10:31 pm

You can go less than your rated Jump max including less than one parsec.

"A Jump Drive produces a Jump approximately equal to its Jump number in parsecs. A Jump Governor modifies the operation of a Jump Drive allowing any jump performance equal to or less than the drive’s rating, including micro jumps. It is theoretically possible to use a Jump Governor to emerge from jump at exactly the same location as jump entry occurred. Effectively, the ship spends a week in jumpspace but doesn't actually move."
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Re: Question on the 100D jump limit

Postby steve98052 » Fri Feb 15, 2019 5:36 pm

I would rule that there is a preferred entry point for jumps, and an expectation that ships arrive at a fairly low velocity relative to the destination world. The entry point would depend on the starting system, and be designed to optimize travel times to the extent that is reasonable, and that would vary with planetary motion in both systems. Some system pairs might have relaxed velocity rules, if thestars have high relative motion, but probably not many.

Other than that, I wouldn't bother getting into the details much.
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Re: Question on the 100D jump limit

Postby Old School » Fri Feb 15, 2019 5:49 pm

That’s basically where I’m at Steve, but I add the one wrinkle that becuase a preferred entry point might be masked for a particular departure system at any given time, there would be several arrival “gates”, i.e. designated areas that you are expected to use when jumping into a system. If you don’t appear out of jump within a reasonable distance of one of those points, you are considered suspect.
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Re: Question on the 100D jump limit

Postby steve98052 » Fri Mar 22, 2019 5:58 pm

That makes sense. Just knowing that there are guidelines can be a story seed: seeing something out of the ordinary is a sign of something up.
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Re: Question on the 100D jump limit

Postby paltrysum » Fri Mar 22, 2019 6:17 pm

Old School wrote:
Mon Jan 14, 2019 9:37 pm
I believe past materials have referenced that it is frowned upon to come out of jump space with any velocity, so a ship is expected to slow to a stop before jumping.
It would probably take me hours of digging through various books from previous incarnations of the game, but I seem to remember that this is not a requirement at all. It may be preferable to come out of jump without the ship moving, but it is not required. You can be hauling when you come out, but you just need to include that in your plan: You're going to have to decelerate if you expect to land, perform wilderness refuelling, or whatever it is you want to do in the next system.
My published Traveller adventures on DriveThruRPG:
https://www.drivethrurpg.com/browse.php ... %20Griffen
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Re: Question on the 100D jump limit

Postby Reynard » Fri Mar 22, 2019 7:14 pm

Just make sure you don't exit directly into known or unknown traffic at speeds that preclude any ability to avoid impact. This is especially true with the 100D around populated worlds. Remember that the jump exit is a window not an exact point and you are always exiting blind. Do you really want to be moving, let alone moving very fast, if you can never be sure exactly where your exit point is? It seems more than reasonable or at least courteous to come out at zero vector speed or at very low speed, recon location, make communication contact then proceed at reasonable speed. Telling traffic control you have a pirate phobia won't cut it.

I swear some gamers think they'll come out moving at a fraction of light speed then instantaneously react to a collision alert by veering safely away or screeching to a halt in seconds or maybe minutes.
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Re: Question on the 100D jump limit

Postby Sigtrygg » Fri Mar 22, 2019 7:50 pm

MWM definitive Jumpspace article JTAS 24:
The laws of conservation of mass and energy continue to operate on ships which have jumped; when a ship exits jump it retains the speed and direction that it had when it entered jump.
Commercial ships, for safety reasons, generally reduce their velocity to zero before jumping. Such a procedure eliminates some of the danger of a high velocity collision immediately after leaving jump. Military ships and high speed couriers often enter jump at their highest possible speed, and they aim for an endpoint which directs their vector toward their destination in the new system. Such a maneuver allows acceleration in the originating system, followed by constant deceleration in the destination system.

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