Traveller defines a radius equal to 100 diameters of an object for the minimum safe jump distance.
It normally mentions it being because of gravity.
Last night, with help from FuzzyGeff, I did the maths for various objects in our solar system.
It doesn't come out.
The 100D distance from Earth gives a pull of 0.000025g.
The 100D distance from the Sun gives a pull of 0.000700g. Almost 30 times stronger than the Earth distance, yet still safe?
If 0.000700g is the safe distance down the well, then 19D is the safe jump altitude for Earth.
If 0.000025g is the safe distance down the well, then 525D is the safe jump altitude for The Sun. That's 4.9 AU, y'all. That's getting near to Jupiter's orbit. Jupiter needs about a 160D distance.
I want more elegant double-talk for what's going on.
Update: Fixed an error on my spreadsheet!
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Does it call out jump as multidimensional travel? Perhaps gravity interacts differently with respect to a different dimension as part of the jump process.
ReplyDeleteJumpspace are different dimensions (there's different levels for different speeds).
DeleteClearly something different is going on than pure depth of the gravity well, but for years it's implied that gravity is the reason.
"Sometimes the magic works,and sometimes it doesn't." Chief Dan George.
ReplyDeleteWell, when I was doing Traveller, there was an understanding amongst us that jumping or travelling within maybe 1/2 the distance of the first orbital shell from a star was a no-go due to radiation.
ReplyDeleteBut I can see the point you made. 100x(Diameter) is kind of arbitrary. It works as a general rule, and isn't too klunky.
But, hmmmm, maybe some logarithmic scale relating to overall mass would work. That way a more massy object, like a stellar remnant or a heavy-element rich planet would have a deeper jump well than a less massy object, even though the less-massy object may be bigger than the denser object. Which would make an interesting adventure, scouting a previously unscouted system, only to be forced out of jump because something about the system is significantly off from what was previously observed (from another system, maybe even another sector.) But I am lazy, so I'll stick with the 100 diameter constraint and toss in the IMTU (in my Traveller universe) rule about not jumping close to a star due to radiation and other bs justifications :)
Some more thoughts to clog the double-talk filter in your J-Drive:
ReplyDeleteA few years ago I was playing around with this whole concept, because the inconsistency and arbitrariness of "100 diameters rule" kept nagging at me.
I went through a process similar to the one that you did, and ran some numbers on the assumption that the jump limit would be proportional to some threshold "g" (gravitational acceleration) limit. I played around with the limits at the "big and fluffy" end (like stars or gas giants) and at the "small and dense" end (from rocky planets to asteroids--though I never crunched numbers on "neutron stars"). All of this would have been a pain in "the old days"--I bought one of the original Traveller sets hot off the press in 1977--but with Excel and similar spreadsheets this becomes easy, once you get the equations right.
So I did the same thing you've done: On the assumption that "100 diameters, *somewhere*" would be the threshold "g" limit, one can take a "100 diameter rule" for Earth and see what that "g" limit implies for Sol (in Sol diameters), Jupiter (in Jovian diameters, etc.). Likewise, one can take a "100 diameter rule" for Sol and see what *that* "g" limit implies for Earth (in Earth diameters), etc.
However, I also considered that one could assume that it was NOT a "g" limit--that is, that more than [insert number] micro-G's starts to mess with the jump process. My alternate assumption was that one could say that it was a gravity *gradient* limit--that is, that more than [insert a different number] of *nano-G's per kilometer* is what starts to play hell with the jump.
I proceeded to run the numbers again, using gravity gradient as the limiting value. I looked at the gravity gradient value for Earth at 100 diameters, Sol at 100 diameters, Jupiter at 100 diameters, etc., just to play around with the numbers. The implications of "gravity limit" vs. "gravity gradient limit" make significant differences in application--that "4.9 AU" value for a gravity limit will be significantly different if you look at the implications of a gravity *gradient* limit.
As this type of analysis demonstrates, putting a more rational *pretext* for a "100 diameter rule" and choosing either to be driven by the "big and fluffy end" *or* by the "small and dense" end (*or* choosing some value between these extremes) can drive you into situations where either the star's jump limit dominates much of the star system *or* jumps from within the system are limited mostly by the planets because the star's jump limit is trivially small.
Having dumped all that chum in the water, I have to say that I have no idea what I've done with my original spreadsheets on this analysis. You'll have to work out the gravity gradient analysis details for yourself, then come to your own conclusions about which of these choices works best for your own Traveller universe.
And if you *really* want to drive some more subtle implications, you can crunch the numbers behind the idea that "big ships experience more gravity gradient than small ships, because they stretch out over more space". That gets you things like "Hey, my little Maltese Falcon [sic] smuggling ship can jump out of this gravity well long before that big Grand Inquisitor Star Cruiser passes its own jump threshold."
On that note, you can call me...
-- The Wandering Troublemaker