Jumping away from gas giants

[MonkeyX]

Do ships still need to be 100 x diameter away from a gas giant to be able to jump or are there special rules for them?

[Galadrion]

Why would there be special rules for them? Nope, just like other planets, stars, and pretty much everything else, a ship needs to get outside the 100-diameter limit in order to be able to jump without complications.

[Reynard]

Same goes for stars.

[MonkeyX]

The reason I asked is that gas giant seem to be vast but have different gravity than planets would their size it at least that’s the conclusion I came to looking over the tables.

[Ursus Maior]

The 100D-rule is pretty sketchy at best as it treats every body of mass the same, careless about density. This makes jumping away from a star with the mass of 1 sol (e. g. our sun) much more difficult than jumping away from a black hole or a neutron star of the same mass, since their greater density will dramatically reduce their diameter. The guys over at https://www.freelancetraveller.com/ did a good job on jump-masking and all the shenanigans that follow from it.

My personal take is not to give it to much thought and play the rule as is written. This can make for extremely odd shadows in systems and thus long transit times to worlds, depending on their orbital distance from other bodies and how these bodies shadow the jump path in realspace. Good for some adventures, but if I don't want to work with it or my players don't actively seek out these orbital mechanics, I don't include them in a session. There is just no need to delay an arrival for 3 days because of orbital mechanics, if you just want to "get there".

[AnotherDilbert]

The 100 D limit is not the whole truth, it's just a simplified rule of thumb:

Entering jump is possible anywhere, but the perturbing effects of gravity make it impractical to begin a jump within a gravity field of more than certain specific limits based on size, density, and distance. The general rule of thumb is a distance of at least 100 diameters out from a world or star (including a safety margin), and ships generally move away from worlds and stars before beginning a jump.

Basically, jumping from 100 D works, jumping within 100 D might work, or might not...

[Reynard]

According to the Jump Drive section in the Traveller Companion:
"Jumping within 100 diameters of an object imposes
DM-4 on Engineer (j-drive) checks, and jumping within
10 diameters imposes DM-8. It is still possible to make
such a jump and survive… possible, but not likely."

Plus
"It is extremely dangerous to jump from a Lagrange
point; an additional DM-2 is applied to Engineer
checks on top of the normal DM for jumping within
a gravity well."

So yeah, it's possible but not very bright.

[Condottiere]

1. I wouldn't try transitioning within the atmosphere, but with Scotty at the engines, ten orbits is doable.

2. A good astrogator is worth his weight in gold, though to be fair, millenia of astrography would pretty much nail every gravity well in Chartered Space.

3. There is some form of critical mass, otherwise every atom in space would be a speed bump.

4. Speaking of which, black holes are exceptional.

[MonkeyX]

The -4 DM for jumping within 100 diameter still really sets the bar at rolling an 8. That’s standard difficulty. Is this something g that may actually be happening more in universe than previously realised?

[Old School]

The -4 DM for jumping within 100 diameter still really sets the bar at rolling an 8. That’s standard difficulty. Is this something g that may actually be happening more in universe than previously realised?

Probably, but still only in emergencies. The risks are too high. At easy difficulty and with extra time, success is automatic, as it should be. Of course, with some of the ship options from the great rift set and a good astrogator, you can stack on a whole bunch of positive DMs.

As for the original question, in my games we consider the density of the object, but without hard and fast rules. So for a gas giant we used 50 or 60 diameters last time it came up.

[Condottiere]

Gravity times actual diameter, times a hundred.

[paltrysum]

Same goes for stars.

Yep. Which can be problematic. Take a look at the red giant in Acrid/Trojan Reach and tell me where to place the mainworld. If you place it in the habitable zone, it's still weeks away by manoeuvre drive. You can place Acrid in the outer system, but that makes its corrosive atmosphere difficult to explain. But hey, that's half the fun, right? Finding a reason that such things exist.

[NOLATrav]

Gravity times actual diameter, times a hundred.

Yet in MgT 2e we don’t even have Small and Large GGs. I do appreciate the new skimming rules in the Companion however.

But your suggestion is a good metric should we start detailing gas giants.

[NOLATrav]

Same goes for stars.

Yep. Which can be problematic. Take a look at the red giant in Acrid/Trojan Reach and tell me where to place the mainworld. If you place it in the habitable zone, it's still weeks away by manoeuvre drive. You can place Acrid in the outer system, but that makes its corrosive atmosphere difficult to explain. But hey, that's half the fun, right? Finding a reason that such things exist.

We touched on Heya earlier, I believe it exhibits the same problem - it orbits a K6 III orange Giant. IMTU I put it in the “habitable” zone of the Giant, which gave it an orbital period of just over 20 years (using GURPS First In rules several years ago). So this low tech agri world with moderate population and law level has a summer that is FIVE YEARS long... imagine all the fests and parties...

[Geir]

Gravity times actual diameter, times a hundred.

Yet in MgT 2e we don’t even have Small and Large GGs. I do appreciate the new skimming rules in the Companion however.

But your suggestion is a good metric should we start detailing gas giants.

I’m overthinking it, since it’s a game construct, but *if* the 100D value is based on the force of gravity at that point in space, then it would be the square root of gravity times diameter times a hundred.

Let’s take the example of Earth and Jupiter:
The formula for force is GM/(r)^2 with G being the gravitational constant, but honestly using real numbers like I did is overthinking it. In any case, for Earth it comes out to F=2.455 X 10^-4. For Jupiter it comes out to 6.48 X 10^-4 or 2.63 times more force, which also happens to be its gravity. But since force is proportional to r^2 (or distance) then if you multiple the distance (r) value by the square root of 2.63 you get the same force value as Earth at 100D. So it’s 162.17D for Jupiter.
That’s if everything is governed strictly by force, which in the old “rubber table” visualization would be how far down the table is warped. If instead jump space safety is governed by the slope of the surface being at or below a certain value, then I’d need calculous and I haven’t done that since the 80s, so I’ll leave it to someone else to figure out.

Or: It’s a game. The rules say 100D, so use 100D.

If you apply the gravity rule to stars, then... Don't. The gravity at the surface of the sun is 28G and the square root of 28... well, it would put the jump shadow out around Jupiter's orbit.
So 100D. Maybe make an adjustment the other way for big stars. After all a Type III giant can be just a bit more than Sol mass, so a good cheat is to base the star 100D jump shadow instead on ~1AU * star mass.
(had to edit my own post three times to correct typos... haven't even been drinking...maybe that's the problem)

[Ursus Maior]

Same goes for stars.

Yep. Which can be problematic. Take a look at the red giant in Acrid/Trojan Reach and tell me where to place the mainworld. If you place it in the habitable zone, it's still weeks away by manoeuvre drive. You can place Acrid in the outer system, but that makes its corrosive atmosphere difficult to explain. But hey, that's half the fun, right? Finding a reason that such things exist.

Good hint, thanks for that one. My Travellers will soon go to Acrid as part of our PoD-campaign. So, wow, a jump shadow of over 273 AU is insane. That's 5.57 times the distance to Pluto.

That is going to need some work in preparation today.

[Sigtrygg]

Gravity times actual diameter, times a hundred.

That would make the safe jump distances from gas giants and stars even greater than 100D...

(gas giant diameter/their gravity) may give better results...

[Sigtrygg]

Yet in MgT 2e we don’t even have Small and Large GGs. I do appreciate the new skimming rules in the Companion however.

But your suggestion is a good metric should we start detailing gas giants.

You want to increase the safe jump distance from gas giants?

Condottiere's suggestion would almost triple the safe jump distance of Jupiter for example.

[Sigtrygg]

Instead of trying to explain it due to 'the force of gravity' us the tidal force instead, here's a link but I am sure there was a more in depth essay on this topic..
https://vectormovement.com/2010/05/

[Condottiere]

I was looking for an easy way out.

So what creates gravity, iron?

We take the diameter of the solid core in the centre of a gas giant, times it by the surface gravity, and multiply that by a hundred.