Monday, June 02, 2008

Planetary Relative Speeds With Regard to Jump

I recall there was some good discussion on CoTI of this in a thread which I can't find at all.

Someone had pointed out that because
A) stars move in relation to each other and
B) planet certainly move as well,
therefore C), a ship with no vector relative to planet Y jumping into the vicinity of planet Z will arrive with a vector equal to the relative speed of those two planets in relation to one another.

On a day to day basis, this is not really something that PCs and Referees really need to be worrying their little heads about. But from time to time, it might be relevant. SO:


(Gentlemen, please check my numbers and assumptions)

Thought about this a bit, and did some googling. Near as I can tell, stars move anywhere from a few KPS to 500+kps; planets in our system move from a few kps to somewhere near 50.

Now, taking this to Book 2 combat speeds, where each turn is 1000 seconds and a vector produced over a turn's worth of 1G acceleration is 10,000kilometers long (a ship traveling on such a vector is then moving 10kps.)

The really fast stars seem to be in the very core of the galaxy; slow ones are in globular clusters. So on average, they ought to be between, on the low side. Right?

for system Y, (6D6)*10 speed in kps. (60-360kps.)
Roll D6; make Y's speed a negative number if 1-3
for system Z, (6D6)*10 speed in kps. (60-360kps.)
Roll D6; make Z's speed a negative number if 1-3
Add the speeds of systems Y and Z; the resulting number is the relative speed of the two systems. It will vary from This should be recorded, as it will not change over the course of the game.

For a given jump, do the same process with 1D6, and add that result to the stellar relative speed to take planetary movement into account.

So say that system Y has a speed of 300 (moving away from Z at 300kps)
and system Z has a speed of -250 (moving towards from Y at 250kps)
they'll have a relative speed of 50kps.
Figure planetary speeds for y and z at 30 and 50 kps; add that for a total of 130kps.

A free trader jumping from one planet to the other from a relative standstill will arrive moving at 130kps, or in book 2 game terms, will have a vector 130,000km per turn. It will take that free trader about 13 turns to decelerate and match speeds with the planet in order to achieve orbit.

On the other hand, the free trader might have decided instead to match speeds with her destination at launch, and spent 13 turns accelerating prior to jump. A scout could do this in half the time, either way.

This would have interesting tactical repercussions, if a fleet's navigator chose to come out of jump moving towards the target at half a light-second per turn. No?

It also means that ships coming in and out of system can be expected to be moving at absolutely hellacious speed about half the time.


Blogger Omer Golan said...

In a standard Traveller campaign, I'd keep star/planet motion strictly in the realm of Handwaving/Referee Fiat and assume the incoming ships enter a high orbit around the mainworld after a jump (and a few real-space manouvers). Otherwise things would becoming quite complicated (especially with the CT space-combat system involved) without adding enough to the fun to justify this.

The only type of a campaign where I'd deal with planetary movements is in a single-system low-tech interplanetary one, where the PCs end up travelling all (or most) of the time between the planets of the same star. In that case you should have a system to simulate the changing distances between planets because all (or most) travel would be STL.

9:24 AM  
Blogger Festeria said...

Oh, this isn't something to hash out every darn jump, so much as to explain some standard processes:

1) Commercial starships virtually always run for jump, because starports HATE it when freighters come screaming in at 450kps. Some would fine heavily for that. The cr10,000 jumpcharts would all be calculated based on a run to jump, and would dump a ship near the mainworld's 100d. Most PC-level captains wouldn't think twice about it: no ship without a dedicated navigator would mess around with that, anyhow.

With regard to the CT space combat system, I don't know that it confuses issues more than it clarifies.

It establishes that origin world vectors are retained when you come out of jump, and is controllable as long as it's calculated into the jump.

So if a PC says, "I want to jump to the Biknwadle system, and come out of jumpspace drifting towards the mainworld, running silent" I can tell him "yeah, you can do that" and give him a Nav roll to program it all in.

For MTU background, it helps me envision a lot of fleet tactical stuff: I can picture a fleet jumping in with the main body coming in far out and slow to as assemble, but have a few waves of ships coming in at ridiculously high velocities to run initial system scans, sow disorder and hit targets of opportunity. You could have a fast frigate come out of jump near 100D, moving past from the planet at half a light second per turn; it'd be presenting a target for one, two turns at the most; and it wouldn't be able to do much damage, but it could click off a few shots, scan the vicinity, and be completely out of weapons range before any of the natives built up any steam at all. Arrange to have a squadron coming in at very high vectors from a variety of directions, and you might have a beautiful hit-and-run scenario that even a 3-5000 ton, 1G ship could pull off.

11:53 AM  
Blogger Sam said...

Try this: Maneuver for two weeks at 1G before jumping into Terra right at the 100 diameter line.

By my calculations, you reach Terra in .105 turn, so if you were vectored to do a slingshot orbit, you would be in and out in a little over 210 seconds, or .210 of a turn.

That would give a 4G ship with 1 week of max acceleration under two minutes before it slingshotted back out, less than a quarter of a turn of exposure.



9:05 AM  
Blogger Festeria said...

Cool idea, Sam! I've thought that might be a useful thing to try, but it would have to be for a pretty specific situation.

With a vector that long, it seems pretty likely that any attempt to hit anything with a missile will fail. Laser fire's the best bet - and there had better be a lot of 'em, because that turn's all you get. Also, depending on the defender's state of readiness, there'll be return fire to contend with as well.

From there, it's all a question of what you're prepared to do next: decelerate and stick around the system to provide future harassment? Might be best to jump out if you've got the fuel to do it.

I could see a squadron of hit-and run cruisers scheduled to come in close to a homeworld and GG while the main fleet jumps in out in the system and gets organized: the defenders take some losses and contend with the high speed attack while the carriers come in and launch their squadrons.

9:34 AM  

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