Tuesday, April 23, 2013

Traveller: Jumping Out

Last week I covered jumping into a system. Amazingly, this week I cover jumping out. Unexpected, I know.

Before a ship can jump out of the system, it must move to outside of 100 diameters of a gravity well.  This is far easier to say than to do, because not only must the GM take into account the diameter of the world the PCs are leaving, but also the diameter of whatever it's orbiting -- if it's a moon of a gas giant, or close to a stellar body, it's possible to be outside 100 planetary diameters but not 100 stellar or gas giant diameters.

Annoyingly, Traveller (which is usually full of tables and formulae) has nothing in the rules to handle this. Fortunately, I have a workaround.

1. Look up the system on on the Interactive Atlas of the Imperium. 
In this example, we are using the Wonstar system.

2. Find the spectral class of the star. 
In this case, Wonstar is a type M0 V, but is a binary system with a type M1 D (dwarf star).

3. Determine the closest safe orbit using this table. 
Table, and in fact most of the information in this post, is taken from this article. The number in front of the / lists the first orbit outside of the 10-diameter limit (for dangerous jumps); the number after lists the first safe orbit outside the 100-diameter limit.

Dangerous/Safe Orbit Table
Spect   Luminosity 
Class    Ia        Ib        II        III       IV        V        VI 
B0       6/10      5/9       5/9       4/8       4/8       3/7      --- 
B5       7/10      6/9       5/8       3/7       2/6       2/6      --- 
A0       8/11      6/10      5/8       2/7       2/6       1/6      --- 
A5       8/11      6/10      4/8       2/6       1/5       0/4      --- 
F0       8/12      7/10      4/8       2/6       1/5       0/4      --- 
F5       8/12      7/10      5/8       2/6       1/5       0/4      0/4 
G0       9/12      7/11      5/9       2/7       1/5       0/3      0/3 
G5      10/13      8/11      6/9       3/7       1/5       0/3      0/2 
K0      10/13      9/12      6/10      4/8       1/6       0/3      0/1 
K5      11/14      9/13      8/11      6/10      ---       0/2      0/1 
M0      11/15     11/14      9/12      7/10      ---       0/2      0/1 
M5      12/16     12/15     10/14      9/12      ---       0/1      0/0 
M9      13/19     12/16     11/14      9/13      ---       0/0      0/0

Dwarf Stars
DB       0/0
DA       0/0
DF       0/0
DG       0/0
DK       0/0
DM       0/0

Wonstar Prime is an M0 V, which means that an unsafe jump may be performed anywhere in the system, but in order to reach the safe 100d limit, a ship needs to be in at least orbit 2. Since Wonstar Main is in orbit 3, this is not an issue. Wonstar Second is a dwarf (0/0) and therefore irrelevant.

4. Determine planetary diameter and safe jump distance. 
This is easy. The Interactive Atlas tells us that Wonstar Main is a small planet with a diameter of 6,400 km. Multiplying that by 100 give us -- surprise -- 640,000 km.  If it were a moon of a gas giant, then things get a bit vague and tricky, as these calculations are assumed to go from the beginning of the planetary atmosphere and moon orbits are significantly higher than that. Since I don't have a good solution, all I can do is say that small gas giants are approximately 400,000 km in diameter and large ones are 900,000 km. The ship won't need to go the full distance to reach 100 diameters, but how much less is up to the GM. Consult the section labeled "Surface-Habitable Moons" in this article and fake it.

5. Determine transit time. 
If you're a math nerd, go ahead and figure out how long it takes your ship to reach 640,000 km assuming a constant acceleration of whatever your ship's thrust rating is.  If you're like me, consult the table on p.145 of Mongoose Traveller and pick the closest value -- in this case, it takes 106 minutes for a 4-G ship to reach 400,000 km. So let's spitball and say it takes 120 minutes, or 2 hours, to reach a jump point. If your players complain, make THEM do the math.

6. Don't forget random encounters!
Pirates, collisions, aliens, etc.

7. Plot course; divert power; check to see if jump was successful or a misjump occurred.
Two protons expelled at each coupling site creates the mode of force, the embryo becomes a fish that we don't enter until a plate, we're here to experience evolve the little toe, atrophy, don't ask me how I'll be dead in a thousand light years, thank you, thank you. Genesis turns to its source, reduction occurs stepwise though the essence is all one. End of line. FTL system check, diagnostic functions within parameters repeats the harlequin the agony exquisite, the colors run the path of ashes, neuronal network run fifty-two percent of heat exchanger cross-collateralized with hyper-dimensional matrix, upper senses, repair ordered relay to zero zero zero zero. Counting down. All functions nominal. All functions optimal. Counting down. The center holds. The falcon hears the falconer. Infrastructure, check. Wetware, check. Everyone hang on to the life bar, please. Seized by God, they cry for succor in the dark of the light. Mists of dreams dribble on the nascent echo and love no more. Jump.


  1. And all these I've been going by whether or not it's interesting or dramatic to have them delayed. Otherwise it usually goes something like, "So after a bit you clear the gravity well and are ready to make the jump.. ". 😉

    Now, I do find it interesting that you could use this information to make a hazardous jump (such as one from a large asteroid body in a field of similar objects) seem more sound and logical then simply what would work for dramatic effect.

    Have you or your players encountered this is play?

  2. I dunno, this seems to be some sort of hybrid between Traveller and something else...

    I need to buy you a Geff. He did all sorts of wondrous maths in his head. We calculated how deep the well was at 100 earth diameters from earth and that was the new g threshold for jumping. 100d became a spacer "rule of thumb" not an actual scientific measure. Of course if you don't have a player who does integral calculus in his head...

  3. Wow. The text stream in step 7 was just....epic. Just reading it, I had a moment of mindshift. Be careful, kids, looking directly at the human mind processing data can result in blindness, elevated seratonin, erectile dysfunction, and/or hairy palms. Consult a psychologist before beginning/ending treatment. Side effects may vary. Your mother was wrong, you're *not* special, you're just another meatsack with delusions of grandeur.

  4. Encountered what, a hazardous jump? Not yet.

    I agree that most of the time it won't be necessary. However, it is inevitable that players will need to take off in a hurry (perhaps trying to outrun pursuing authorities) and it may be necessary to know how long it takes to get to a jump point.

  5. hybrid between Traveller and something else

    I see what you did there.;:)

  6. Wish I could take credit for that. It's from the rebooted Battlestar Galactica.


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