Planet density and diameters

[dccarles]

I was hit square between the eyes by an attack of the blindingly obvious last night. Without going into any mucky equations, the average density of a gas giant or star is about 1 ton/cubic meter. The average density of a rocky planet (like Earth) is about 5 tons/cubic meter[1]. Yet, while Traveller clearly assumes that planetary gravity (not volume) inhibits the safe use of jump drive, it doesn't account at all for planetary density.

If you're running a space combat scenario where one side is badly outmatched, this can have great consequences. Multiplying (or dividing) the distance to safe jump by 5 more than doubles (or halves) the time required to reach it.

(I came up with this because I was designing a research ship for Traveller 5, designed to observe a neutron star as closely as possible. In High Guard it would have an armour factor of about 50.)

--Devin

[1] If you have better stats, please share.

 

[Chaos]

You´re right, IMHO.

IMTU I do not use a fixed 100-diameter limit; rather, the limit is the distance at which the gravity exerted by the planet drops below 0.0001g - which for Earth is 100 diameters, for Jupiter (off the top of my hat) would be about 700 diameters, for Mars would be about 60 diameters.

 

[Peter Schutze]

I calculated something like this out for 2300AD where it is a specifically gravity determined "FTL shelf"

From memory:
the sun's gravity shadow covers out past the orbit of venus. trying the formula on a few other stars seems to bear out that the habitable zone starts just within the outer edge of a star's gravity zone
venus and earth are both about the same = 100 diam
saturn is less dense than jupiter
the mercury, our moon or mars was only about 30 diam (forgot which)

simplest approximation was to multiply 100 by either density or surface gravity can't remember which but it came to within a few percent of being right

what it proved to me was that you need to be really unlucky to get interference with jumping straight to your target planet (if its not a gas giant moon) as there is so much "gravity free" space in a solar system that "masking etc" is hugely unlikely ..... contrary to one of the rulebooks.

for example you can aim between earth and the moon with a fair amount of space that isn't ever going to be in either gravity well. it then becomes a matter of timing to avoid the few minutes that the moon's tiny area of effect crosses your path.

 

[dean]

Or you just aim due north of the ecliptic plane and come out of jump at 100D's "above" the planet. Since the vast majority of planets we've observe occur with orbits oriented on one plane, you'd pretty much never encounter the problem of encountering a close planet or moon unless it was unusually close in orbital distance, or massively huge, or both.

 

[Chaos]

I calculated something like this out for 2300AD where it is a specifically gravity determined "FTL shelf"

From memory:
the sun's gravity shadow covers out past the orbit of venus. trying the formula on a few other stars seems to bear out that the habitable zone starts just within the outer edge of a star's gravity zone
venus and earth are both about the same = 100 diam
saturn is less dense than jupiter
the mercury, our moon or mars was only about 30 diam (forgot which)

simplest approximation was to multiply 100 by either density or surface gravity can't remember which but it came to within a few percent of being right

what it proved to me was that you need to be really unlucky to get interference with jumping straight to your target planet (if its not a gas giant moon) as there is so much "gravity free" space in a solar system that "masking etc" is hugely unlikely ..... contrary to one of the rulebooks.

for example you can aim between earth and the moon with a fair amount of space that isn't ever going to be in either gravity well. it then becomes a matter of timing to avoid the few minutes that the moon's tiny area of effect crosses your path.

Remember, gravity diminishes with the square of distance, so you cannot simply multiply by surface gravity - instead, you should multiply by square root of surface gravity.

 

[Icosahedron]

for example you can aim between earth and the moon with a fair amount of space that isn't ever going to be in either gravity well. it then becomes a matter of timing to avoid the few minutes that the moon's tiny area of effect crosses your path.

Are you sure? I thought the Moon was well within 100D of Earth - about 30D I think.

I could be wrong, I haven't got the figures to hand.

 

[aramis]

Are you sure? I thought the Moon was well within 100D of Earth - about 30D I think.

I could be wrong, I haven't got the figures to hand.

The Moon is the only natural satellite of Earth:

orbit: 384,400 km from Earth
diameter: 3476 km
mass: 7.35e22 kg​

Earth is the third planet from the Sun and the fifth largest:

orbit: 149,600,000 km (1.00 AU) from Sun
diameter: 12,756.3 km
mass: 5.972e24 kg​

Terra is approximately 110.6 lunar diameters from luna, and luna is 30.1 terran diameters from terra.

Heck, the far side of luna's 100 lunar diameter blocking zone is only 57.4 terran diameters from earth... it's not even something one has to account for when jumping in!

 

[Theophilus]

Would the suns diameter/zone being past venus mean a ship jumping in reverts outside venus's orbit?

How does this effect travel time to earth?

Under CT/MT a ship accelerated half way, then decelerates half way, under TNE a ship has limited acceleration, compute travel times for TNE.

 

[Chaos]

Would the suns diameter/zone being past venus mean a ship jumping in reverts outside venus's orbit?

How does this effect travel time to earth?

That means you can only jump to Earth if it´s on the same side of the Sun as you´re coming from. If you´re coming from the opposite side, you´ll drop out of jumpspace on the far side of the sun... up to 285 million km from Earth.

 

[CosmicGamer]

That means you can only jump to Earth if it´s on the same side of the Sun as you´re coming from. If you´re coming from the opposite side, you´ll drop out of jumpspace on the far side of the sun... up to 285 million km from Earth.

Even on a flat sheet of paper you don't need to drop out on the far side. (see image)

Luckily the universe is not flat, so I would guess the odds that the location you are traveling to is directly opposite one of the suns (for multi star systems) it orbits is pretty low.

For purposes of discussion, the ship is departing from the blue planet which is just outside the suns 100d range and going to the green planet that is just inside the 100d range of it's sun.

Personally, I don't want to have to compute (not that I could) the location and orbits of everything in the subsector(s) being used and then update it constantly as time moves forward in a campaign.

 

[Fersboo]

Personally, I don't want to have to compute (not that I could) the location and orbits of everything in the subsector(s) being used and then update it constantly as time moves forward in a campaign.

Isn't that the job of the humungous CPU?

 

[CosmicGamer]

Isn't that the job of the humungous CPU?

Size doesn't matter....

if you don't have a program to do it.

for clarity: I'm referring to the fact that I (player or GM) do not have the ability to make the calculations to know exactly where everything in a system currently is so that this information can be used within the game. Specifically, how would you know if the world you are traveling to is on the near side or far side of the sun in relation to your current location? How about two months (game time) from now when your travels have you going between these locations again?

 

[Fersboo]

Size doesn't matter....

if you don't have a program to do it.

for clarity: I'm referring to the fact that I (player or GM) do not have the ability to make the calculations to know exactly where everything in a system currently is so that this information can be used within the game. Specifically, how would you know if the world you are traveling to is on the near side or far side of the sun in relation to your current location? How about two months (game time) from now when your travels have you going between these locations again?

I understand. I was being facetious.

 

[Chaos]

All right, my mistake, I wasn´t thinking of jumping to either side of the star´s 100 diameter limit.

And I wouldn´t actually track everything... simply make a random roll every time you jump to a system. Say on a roll of 7+ you have a free line of jump to the target, on a roll of 6 or less you don´t; for each point below 7 you closest possible jump approach is 50 star diameters further away than the optimum.
On the departue point, you can acquire up-to-date system information on the target, meaning you can make that roll before you jump, and alter your jump destination accordingly - i.e. you can jump to one side of the star, reducing the distance from the exit point to the target to 20 star diameters per point below 7.

How does this sound for a quick-and-dirty solution?

 

[CosmicGamer]

...Say on a roll of 7+ you have a free line of jump to the target, on a roll of 6 or less you don´t; for each point below 7 you closest possible jump approach is 50 star diameters further away than the optimum.
On the departue point, you can acquire up-to-date system information on the target, meaning you can make that roll before you jump, and alter your jump destination accordingly - i.e. you can jump to one side of the star, reducing the distance from the exit point to the target to 20 star diameters per point below 7.

How does this sound for a quick-and-dirty solution?

This could very well work for you. In case there are science 'geeks' that are knowledgeable, I would leave things vague.

"The jump calculation will put your ship such and such distance away from your destination."

Whether this is because of the location of the sun in the system you are departing from or going to, a gas giant, another planet, or other possibilities does not have to be detailed.

 

[Icosahedron]

I designed an Orrery spreadsheet to do those calculations once, but I couldn't be bothered using it. I just make a vague random roll.

 

[aramis]

That means you can only jump to Earth if it´s on the same side of the Sun as you´re coming from. If you´re coming from the opposite side, you´ll drop out of jumpspace on the far side of the sun... up to 285 million km from Earth.

Not at all.

It depends on which level of masking you use:
if you use terminal masking only, then you could jump from venus to earth, but you'd take the ≤100 diameter penalty anywhere past 10 venus diameters. Note that terminal masking is canonical; it's the minimum.

If you use full-route masking (which I think is stupid, BTW), it means jumping to earth while the sun is in the way is two jumps: one up, and one back down.

When jumping in from outsystem with full-route, you simply aim high or low to avoid said star, as diagrammed by CosmicGamer.

 

[Peter Schutze]

>That means you can only jump to Earth if it´s on the same side of the Sun as you´re coming from.

actually what it means is that jumping close to venus or mercury .... up to the inner edge of the habitable zone in any system is problematical because of the STAR more than the PLANET 100D/ .0001g etc limit

>Personally, I don't want to have to compute (not that I could) the location and orbits of everything in the subsector

been done .... not a recommendation one way or the other .... AstroSynthesis program tracks everything in each system and can do m-drive time calculations for you. If you want a traveller 2D universe, just remember that all z positions are zero. personally for traveller I'd just create the important systems for the local details info and not worry about trying to get the above system stuff to work at all.

 

[JAFARR]

For sake of playability - just use 100 diameters and forget the rest. Remember that was the beauty of CT, just enough reality to be playable. Isn't this whole discussion about why you have the navigator make his navigation roll in the first place, to see if he(she) did his job right including obtaining all the pertinent information?

 

[shadowdragon]

chaos- my calculations bring me to 99 * radius for g to equal .0001, or 50ish diameters. what formula are you using or are you using 100 radii rather than diameters? or am i doing something wrong here?