100 diameters

[EDG]

OK, to clarify - in the Traveller corebook:

The UWP digit tells you the diameter of the world in thousands of miles.

But the table next to that lists the diameter in kilometres.

So a size 8 world has a diameter of 8,000 miles, which is shown in the table as a diameter of 12,800 km.

 

[Xoph]

Okay I will have to pretend that miles is the translation from the old Vilani measurement system that the Solomani never completly eradicated. It has the added bonus that when ever a player uses an imperial measurement I can have NPCs say things like "Sorry I don't speak Viani" or something. I will definatly have my Viani nobles use measurements like Stones, Yards, and fortnights.

 

[EDG]

and fortnights.

as if weeks and days are metric...

 

[GamerDude]

1 Mile = 1.60934 km

1 Kilometer = 0.62137

If you need a quickie calculator got to http://www.onlineconversion.com/ it has all kinds of converters.

 

[Klaus Kipling]

The whole concept of days and weeks seems a bit redundant in a spacefaring society. You could (and we do) have a standard day but it's only relevant on one planet. So ship time will never (unless you're playing on the Rim) correspond with the cycle at your destination.

Interestingly, a million seconds comes out at 11.574 of our Earth days.

A megasecond is a pretty good substitute for a fortnight...

 

[Hemdian]

We had this debate (what and why 100D) on the old Traveller Mailing List back in 1995. The first post I ever made online was an analysis of the 100D rule where I equated the chance of a mismatch to the gravity exerted by a typical Earth-like world at 100Ds ... therefter called the minimum safe distance. I then recalculated using a range of densities. What I found was the MSD for Earth's sun was almost to the orbit of Jupiter! In the debate that followed the problem was discovered to be that gravity decreases with the square of the distance. However 'tidal force' (which is related to gravity) decreases with the cube of the distance. Recalculating again gave the Sun's MSD to be just passed the orbit of Mercury.

Anyway, the article covering both the simple gravity model and the tidal force model can be found here. It uses the MegaTraveller rules and hasn't been 'Mongoosed' yet (waiting to see the final 'Fair Use' policy) but you should still be able to follow it ("Routine" = 7+ on 2d6). Also the math gets a little hairy in parts and I'm not completely happy with the tidal force calculations (they are messy compared with those of the gravity model but they work).

So if you want something that sounds a little more plausable that just distance, should give more believable figures for neutron stars, and tells you what happens if someone jumps from a starport without even taking off ...

Enjoy.

 

[EDG]

If I can do some physical/mathematical nitpicking...

Anyway, the article covering both the simple gravity model

I'm clear on why you divided G by c in that simple gravity model to get your "picoStress" unit. Dimensionally speaking, it's nothing to do with stress - you're dividing m/s2 by m/s, which gives you a unit measured in "per second" (1/s). I'm also not sure what the logic is of going from G at 100D for Vland being about 2.94e-5 g's to using measurements of 'picostress' where 1 pS being about 3.06e-5 g's.

(more properly, physical stress is defined as force per unit area).

Also the math gets a little hairy in parts and I'm not completely happy with the tidal force calculations (they are messy compared with those of the gravity model but they work).

I've never been convinced that what you're using really is "tidal" there. If you differentiate something that is inversely proportional to the square of distance then you'll end up with something inversely proportional to the cube of the distance, but is that really "tidal"?

Or put another way: m/s is velocity. Differentiate that and you get acceleration (m/s2), which is rate of change of velocity (and gravity is an acceleration). Differentiate that again and you get (m/s3), which is rate of change of acceleration - so you're effectively measuring the gradient of the gravitational field. Measure that at a given point in space and you're saying that "the gravitational field is changing at this point at the rate of X m/s2".

I guess it's related to tides in the sense that something in a steep gravitational gradient is going to suffer from extreme tides, but I don't think it's accurate to actually call it "tidal force". The closest one gets to a "tidal force" is the physical stress caused by different gravitational acceleration across a single body - but you need more than one point in space to determine that.

 

[Hemdian]

If I can do some physical/mathematical nitpicking...

Be my guest.

I'm clear on why you divided G by c in that simple gravity model to get your "picoStress" unit. Dimensionally speaking, it's nothing to do with stress - you're dividing m/s2 by m/s, which gives you a unit measured in "per second" (1/s). I'm also not sure what the logic is of going from G at 100D for Vland being about 2.94e-5 g's to using measurements of 'picostress' where 1 pS being about 3.06e-5 g's.

(more properly, physical stress is defined as force per unit area).

To start with "stress" in this context is not conventional stress (force per unit area) but a measure of impediment between space and jumpspace. Instead of "space stress" I could have called it "jump impedence". And as the speed of light cannot be broken in normal space yet we're talking about FTL travel I decided to bring it into the equation. I thought it highly unlikely that MSD would be exactly 100D for Earth or Vland ... so when I saw that G/c and Vland's 100D = 0.96 (x10^-12) I chose to set my magical constant where G/c = 1 (x10^-12). Working back that sets the threshold gravity at 3.06 x 10^-5 ... neither Earth's 100D nor Vland's 100D.

I've never been convinced that what you're using really is "tidal" there. If you differentiate something that is inversely proportional to the square of distance then you'll end up with something inversely proportional to the cube of the distance, but is that really "tidal"?

I don't know if its really tidal but switching from the direct effect of gravity to changes in gravity as an explanation of "space stress" (aka "jump impedence") was sufficient for our needs.



At the end of the day its all made up BS. The goal was (a) to have a more gradiated misjump risk, (b) do away with the "exactly 100D" definition for MSD, (c) explain why LaGrange points don't allow ships to jump from nearer, and (d) wrap the whole thing up in some techno-babble gobbledygook.

How do you handle it IYTU?

 

[ninthcouncil]

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.

It all opens a can of worms, however, to try and apply that realistically. The stars are not fixed in relation to each other, so we'd have to figure in the relative velocities of the origin and destination systems (which, of course, we don't know) for any use to be made of the residual velocity of the spacecraft after exiting jump. It's really not worth the bother, along with a lot of other things which would be required to make Traveller space physically "realistic". And remember, this is a universe where interstellar space is 2-dimensional... :lol: If we can accept that, I think a bit of convenient non-conservation of energy is pretty easy.

Much as I love Traveller, sometimes there is a discontinuity between the essentially fantastic elements and accompanying attempts at "realism". The pettifoggery concering potential energy conservation during teleportation is a good example.

 

[EDG]

At the end of the day its all made up BS. The goal was (a) to have a more gradiated misjump risk, (b) do away with the "exactly 100D" definition for MSD, (c) explain why LaGrange points don't allow ships to jump from nearer, and (d) wrap the whole thing up in some techno-babble gobbledygook.

How do you handle it IYTU?

Yeah, at the end of the day it's all entirely arbitrary anyway. Personally in my SF games (I wouldn't call it "IMTU" since the only thing they have in common with Traveller are heavily tweaked worldbuilding and TNE shipbuilding rules) I set the limit where the local gravity is 0.01 m/s2. That puts the "minimum safe distance" for Sol at around 0.77 AU, and for Earth it's around 200,000 km from the centre of the planet. More to the point it means that the MSD is over 1 AU for a neutron star and within the surface of a red giant.

 

[Hemdian]

More to the point it means that the MSD is over 1 AU for a neutron star and within the surface of a red giant.

Inside a red giant ... wasn't that done in "A Mote In God's Eye"? (Its been many years since I read it.)

 

[EDG]

More to the point it means that the MSD is over 1 AU for a neutron star and within the surface of a red giant.

Inside a red giant ... wasn't that done in "A Mote In God's Eye"? (Its been many years since I read it.)

Possibly, it's been ages since I read it too . Though I thought the FTL drive in that involved wormholes that disappeared and reappeared depending on the gravitational stresses in the system?

Either way, it just means that you can jump out from any point beyond the surface of the red giant. For Antares that'd be darn useful since you don't have to go all the way out to 1000 AU (you'd probably still want to get to at least 10-20 AU from the star's surface though or you'll be toasted somewhat).

 

[Rikki Tikki Traveller]

FTL Travel involved Transline which was measured based on lines of constant Luminosity (supposedly). They only connected specific points in space, like a wormhole.

When the protostar ignites in "The Gripping Hand" it changes the locations of the Translines all over the region, including moving that Transline spot from inside the star to outside the star, opening up the system to Moties. :shock:

It was a fiction to limit the number of entry points within a system and provide an interesting reason for why the Moties were limited to their home system.

(just reread these books last year... )

 

[GamerDude]

More to the point it means that the MSD is over 1 AU for a neutron star and within the surface of a red giant.

Inside a red giant ... wasn't that done in "A Mote In God's Eye"? (Its been many years since I read it.)

Sort of..

The "wormhole" exit was very close to a sun.

That is until in the "Gripping Hand", a new start close to the Motes was born, then the entry point moved significantly.

And that is a very rough & short description of what happened in the book.