100 diameters

[IanBruntlett]

Hi,

I've just finished reading the spacecraft operations chapter of the core rules.

I've stumbled across a phrase that I think I've seen before but can't remember where and I can't find it in the index.

"A ship can only safely Jump when it is more than one hundred diameters distant from any object".

Could you 1) tell me how far is a diameter? 2) tell me where it is discussed in the core rules?

TIA


Ian

 

[klingsor]

You can find the diameter of a planet in the size table in the world creation bit of the core book.

 

[Rikki Tikki Traveller]

For Earth/Terra, the 100D limit is about 1.2 Million Kilometers. It would take about 6 hours at 1g to get there.

Why it is a SIZE based limit rather than a mass based limit has never been fully explained. It appears that Jump Drives are volume dependent not mass dependent like conventional rockets.

 

[far-trader]

Why it is a SIZE based limit rather than a mass based limit has never been fully explained.

Best and most likely explanation is it's a simplification. The size of worlds was already there and an excuse was needed to actually require some space travel to allow a chance for encounters with pirates

Otherwise you have everybody just jumping from port to port and the pirates all die of loneliness

 

[EDG]

It's still fiddly to determine for worlds (you still have to find the size, and then multiply by 200 to get 100D because the size is in radii. And then convert to km if you're not American).

For stars (which are much more obstructive when it comes to 100D limits, since their 100Ds could be beyond the orbit of the habitable planet) it's not remotely obvious because you first have to convert the sizes (only available in CT book 6 so far) from solar radii to AU. A red dwarf's 100D limit would be around 0.3 AU (outside its habitable zone), Sol's 100D limit is at 0.93 AU, but the supergiant Antares's 100D limit is at 1000AU!

Tying it to size rather than gravity makes for some daft results too. A neutron star is only about 10 miles in diameter, so it's 100D limit would be at 1000 miles, despite it having several times the mass of Sol (whose 100D limit is around 0.93 AU). And Antares' 100D limit is really far out despite the fact that its density is actually really low (if you think about it, you have a 15 solar masses worth of star spread out over a volume of nearly 525 cubic AU!). So as a "simplification" it becomes nonsense at extreme values.

 

[Xoph]

If you are just going to jump and aren't going to fly your ship you might as well use the stargate world.

 

[far-trader]

It's still fiddly to determine for worlds (you still have to find the size, and then...

Ah but I was spoilt rotten early on in my Travelling. I got The Traveller Book back in '82 and it had all that nicely tabularized with travel times for all the G ratings to 100d for all the planet sizes and large and small gas giants. It even has travel time to orbit. Before that I think we just ignored it:

PCs "We leave the planet."

Ref "No encounters on the way out."

PCs "Then we jump at 100d."

Ref "OK."

 

[klingsor]

The, lets be charitable and say confusion, over mass and volume is an old issue in Traveller with starships being the best/worst example. Really given constant thrust acceleration would increase as a ships mass goes down whether from fuel use or empty holds. Unfortunately the maths gets interesting, not really difficult but more work than anyone wants - though playing closer to reality might be fun with ships dumping cargo or fuel (if they can afford it, sometimes you just need to jump) to increase their acceleration. This rather invites the question of whether a ship maintains momentum during jump travel, will it emerge with the same vector and velocity as it had when it entered jump, and would that be absolute or relative? It probably tells you somewhere in one of the books.

I think there used to be a rule that you could risk a jump between 10 and 100 diameters but it increased the chance of a misjump.

 

[EDG]

Really given constant thrust acceleration would increase as a ships mass goes down whether from fuel use or empty holds. Unfortunately the maths gets interesting, not really difficult but more work than anyone wants - though playing closer to reality might be fun with ships dumping cargo or fuel (if they can afford it, sometimes you just need to jump) to increase their acceleration.

I think it would make more sense to link acceleration to mass rather than volume... but doing it totally accurately would be a pain. Maybe the current volume-based calculations assume that each dt is a certain mass or something?

This rather invites the question of whether a ship maintains momentum during jump travel, will it emerge with the same vector and velocity as it had when it entered jump, and would that be absolute or relative? It probably tells you somewhere in one of the books.

I'm pretty sure the canonical answer is "yes, the same vector is preserved". Relative to what, though, is somewhat fuzzy - but the idea is that if you jump with a velocity of 100 km/h, you'll still have that velocity when you come out the other end.

I think there used to be a rule that you could risk a jump between 10 and 100 diameters but it increased the chance of a misjump.

I'm pretty certain that there was - is that not in the current Traveller rules anymore?

 

[RandyT0001]

It's still fiddly to determine for worlds (you still have to find the size, and then multiply by 200 to get 100D because the size is in radii. And then convert to km if you're not American).

The size of the world listed in the UWP is in 1000's of miles in diameter, not radius. (p.170)

 

[far-trader]

I think it would make more sense to link acceleration to mass rather than volume... but doing it totally accurately would be a pain. Maybe the current volume-based calculations assume that each dt is a certain mass or something?

Agreed, and yes there is a relationship of dton to mton in several bits of canon. But it's inconsistent and vague, so no real help. It's really just a simplification.

I'm pretty sure the canonical answer is "yes, the same vector is preserved". Relative to what, though, is somewhat fuzzy - but the idea is that if you jump with a velocity of 100 km/h, you'll still have that velocity when you come out the other end.

Correct again.

I think there used to be a rule that you could risk a jump between 10 and 100 diameters but it increased the chance of a misjump.

Yep, as EDG notes you could but with about a 50/50 chance of misjump. You could even jump within 10d if you felt suicidal (the chance of misjump was all but certain and you'd likely be destroyed in the process).

 

[Xoph]

The size of the world listed in the UWP is in 1000's of miles in diameter, not radius. (p.170)

I'm sure you mean Kilometers right?

 

[far-trader]

The size of the world listed in the UWP is in 1000's of miles in diameter...

I'm sure you mean Kilometers right?

No unfortunately

One of my pet peeves with CT and all it's metrification, and they go and make planetary size code rolls in miles :roll:

Unless you mean Mongoose "fixed" that? But they couldn't very well. It'd change everything.

 

[AKAramis]

Really given constant thrust acceleration would increase as a ships mass goes down whether from fuel use or empty holds. Unfortunately the maths gets interesting, not really difficult but more work than anyone wants - though playing closer to reality might be fun with ships dumping cargo or fuel (if they can afford it, sometimes you just need to jump) to increase their acceleration.

I think it would make more sense to link acceleration to mass rather than volume... but doing it totally accurately would be a pain. Maybe the current volume-based calculations assume that each dt is a certain mass or something?

10Tons Metric per Td (Brilliant Lances Technical Architecture)

This rather invites the question of whether a ship maintains momentum during jump travel, will it emerge with the same vector and velocity as it had when it entered jump, and would that be absolute or relative? It probably tells you somewhere in one of the books.

I'm pretty sure the canonical answer is "yes, the same vector is preserved". Relative to what, though, is somewhat fuzzy - but the idea is that if you jump with a velocity of 100 km/h, you'll still have that velocity when you come out the other end.

Again, TNE era clarification; this time, in the core book.

I think there used to be a rule that you could risk a jump between 10 and 100 diameters but it increased the chance of a misjump.

I'm pretty certain that there was - is that not in the current Traveller rules anymore?

It's a DM-8 on a 2d6 roll for 8+ (DM+Effect Mod of a prior task in the chain); failure results in an inaccurate jump dumping one in the outsystem.

It discusses misjumps, but doesn't tell you when they occur...

 

[daryen]

The size of the world listed in the UWP is in 1000's of miles in diameter, not radius. (p.170)

I'm sure you mean Kilometers right?

No, miles.

He is right. The UWP size number is in 1000s of miles in diameter.

As for the 100D limit, Traveller is completely schitzo on it. They specifically state that an objects mass in realspace somehow interacts with jumpspace, but then the limits are based on volume.

Personally, I have always like to think that jumpspace is inherently irrational. Realspace mass has no interaction with jumpspace. However, volume does. So, the 100D limit is fully in effect. The 100D limit of a (relatively) low mass red giant sun is absurd, but the 100D limit of a neutron star is dinky. (Better have that will filed somewhere if you try a 100D jump to a neutron star, though.)

Yes, this doesn't "make sense." But then, this is jumpspace we are talking about. It isn't supposed to make sense. It uses completely different physics and rules and physical laws. No one knows why it interacts with realspace volume (but not mass), but it does. (Hey, jumpspace is "magic" and a volume interaction makes as much sense as "mana" or other magic systems. Go for it.)

BTW, I do wish that realspace maneuver drives were at least remotely related to mass. That is supposed to at least pay lip service to to real physics, so that would be helpful. It doesn't need to be as complicated as GT:Starships or FF&S or anything, but at least giving mass a cursory level of inclusion would be helpful.

 

[Xoph]

I'll have to check this in my book tonight, but using miles, what ever a mile is, for this one thing while using metric for everything else(including the calender) is insane. I'm considering it a typo.

 

[far-trader]

I'll have to check this in my book tonight, but using miles, what ever a mile is, for this one thing while using metric for everything else(including the calender) is insane. I'm considering it a typo.

Heh, I wish

I did contemplate chucking the CT rule and calling it radii in kilometers, ending up with a close average of the same median sizes at the bottom and middle but more extreme at the top.

Where the ATU Size Code is thousands of KM in radius and CT Size Codes are approximate and rounded:

ATU Size Code vs CT Size Code

0 - 0
1 - 1
2 - 2
3 - 4
4 - 5
5 - 6
6 - 7
7 - 9
8 - A
9 - B
A - C

 

[Xoph]

What real difference would it make if you just used the numbers as written and called it metric. Sure, the planets would be a bit smaller and such, but I don't think anyone would ever notice. Besides it has to be a typo of some sort, I can't think of any system that has used more then on system of measurment in this way.

 

[far-trader]

What real difference would it make if you just used the numbers as written and called it metric.

You mean just say it's kilometers in diameter instead of miles in diameter?

The problem is you end up with all your worlds smaller than Earth. That's why I figured when I thought about it that it would have to be radius. The bonus being that you get more variety of worlds since you gain bigger ones with higher gravity.

No, I don't recall any mixed unit systems either. Well, not counting many real world examples

If it's a typo it's a danged persistent one (going on 32 years now)

 

[Xoph]

okay little planets would be silly. I guess the radius solution will have to do, but it makes everything more complicated. This is definatly one of those things Mongoose should have fixed. :x