Belts and safe jumping

[rancke]

I'm not interpreting - that's the mistake others have made for years. They interpret and then other people use their interpretations rather than the rules as written.

If you read Marc's article there is nothing to interpret - it clearly states the crew in the game universe know the jump duration.

But it also states that jump duration is established when the jump is initiated. You are interpreting that to mean that it is eshtablished before jump is initiated, far enough in advance to aim for where the destination world will be the moment the ship arrives. As Orr pointed out, that is very far from being a given. So, yes, you are interpreting, if not downright misreading.


Hans

 

[BytePro]

Most ships have sensors that should be more than adequate to avoid the 'oops, we jumped within 100D of a missed rock.' - even in a fairly unrealistic density asteroid belt (more like a recent debris field how Hollywood depicts it, of course).

The most reasonable explanation for this not working would be if two rocks were in the process of a high-speed collision, and one was obscured from view during a scan (which would have to be a very odd set of circumstances involving either a really quick scan, and/or a ship with no relative velocity in just the right direction).

Leaving the plane of the belt, wouldn't always be the fastest bet - as one could already be on an inner or outer edge on the plane, for example.

If the ship is not in a hurry, and has functioning sensors - I see no reason it is even an issue. If one wants it to be, then a skill check is all that is needed. (Or you can make your players play a level of Asteroids to succeed! :devil Exact time and distance is really just Referee fiat unless you want to involve a computer...

 

[aramis]

Is there anything canonical regarding safe jump distances for asteroid/planetoid belts? Or anything resembling a consensus on the topic if there's no canon?

Real world asteroid belt data (which may not be representative) put average separations on the order of several light seconds; average size of item puts 100 diameters well under 100,000km (1/3LS) across. At worst, a 1/27 chance of hitting a dropout zone... unless using full-jump gravity interference (masking), in which case, it's more like a 1/3 chance.

 

[nDervish]

Read Marc Millers's Jump space article - not other peoples' interpretations of it.

Link?

In the article it states that the crew know the jump duration and man their bridge stations at the appointed time.

The sentence I've seen quoted repeatedly from the article states that the duration of the jump is fixed at the time the jump is initiated or, as you yourself put it, "You know your time in jump from the moment you jump." (emphasis mine) This does not imply that you know the jump duration prior to the moment you jump. (In the real world, when you make a skill roll for your character, you know whether you succeeded or failed from the moment you roll the dice. But do you know whether you succeeded or failed before you roll them? I don't.)

This has no bearing on the crew being able to man their stations at the appropriate time, as they definitely do know when they will precipitate after the jump has been initiated.

No, there's no adjustment of course going on. The jump program takes into account all the orbital positions at your destination. It's why you need that 3dTon computer next to the bridge - to hold all the orbital information necessary to make those calculations.

Please note that my speculation about course corrections in jumpspace was predicated on two "if"s, the first of which was "If you don't know the jump duration at the time the jump is plotted". If you don't know when you will precipitate, then all the computing power and astrographic data in the universe aren't going to tell you where the planet will be at that time.

By using masking and shadows in his game, he left himself the possibility as a GM to keep the players from routinely jumping to and from wherever they chose.

I actually don't use masking/shadows IMTU, I just mentioned them in the OP for the sake of completeness.

 

[aramis]

Marc's jump space article in ... ?

And how does that interact with misjumps, or is that also in the article?

JTAS. Issue 24.

 

[Carlobrand]

JTAS. Issue 24.

Ooh, I have that one! Thanks!

 

[Fritz_Brown]

Please note that my speculation about course corrections in jumpspace was predicated on two "if"s, the first of which was "If you don't know the jump duration at the time the jump is plotted". If you don't know when you will precipitate, then all the computing power and astrographic data in the universe aren't going to tell you where the planet will be at that time.

The positional calculations are based on the exact time of 168 hours. An iterative process then tweaks that time. Perturbations unknown to the astrogation program then cause a further slight variation. It's part of why jumping with a computer is better than just using an astrogator - it can do the iterative work much more easily (besides the data it can include).

That's just my take - not canonical. It's what makes sense to me.

 

[Carlobrand]

Okay, so - according to Marc's article - jump duration is variable but not random (within the game universe, and assuming it's not a misjump) since, he says, a computer can compute a 1-parsec jump solution that is accurate to within 3000 klicks, which isn't possible unless the computer knows both the precise details of the target system and the time of exit to within a couple of minutes or so.

(As example, Alpha Centauri's moving toward Sol at about 20 kps. A couple minutes off has the ship emerging about 3600 klicks [edit: oops, 2400 - sorry] from its intended exit point within the system. Assuming jump masking - which not everyone uses - one would also need to know the positions and orbital characteristics of the significant bodies in both the origin and target systems in order to chart a course that avoids inadvertently precipitating out at some unanticipated and inconvenient point between here and there.)

Now, if a computer can generate a solution to that degree of accuracy given a set of starting data, then it can also generate a solution for some specific future time at some specific starting point in space - thereby explaining the CT Book-2 comment, "...starports have single-use flight plans (in self-erasing cassettes) available for all worlds within jump range for Cr10,000 per jump number;" you just have to be certain to hit your window, or you've just wasted Cr10,000. It also explains the ability of Navy fleets to jump and arrive at the destination at more or less the same time. Further, if a computer can generate a solution that is accurate to within a couple of minutes, and it can do so for some chosen future time and point in space, then it can also inform the crew of the anticipated jump duration before jump.

 

[Orr]

Okay, so - according to Marc's article - jump duration is variable but not random (within the game universe, and assuming it's not a misjump) since, he says, a computer can compute a 1-parsec jump solution that is accurate to within 3000 klicks, which isn't possible unless the computer knows both the precise details of the target system and the time of exit to within a couple of minutes or so.

No, you're looking at it backwards.

When you jump from Sol to Alpha Centauri, you don't plot your exit point to be a certain distance from Alpha Centauri. Instead you plot your exit point to be a certain distance from Sol which Alpha Centauri just so happens to be passing.

It's like the initial ranging round fired by an artillery piece.

 

[Fritz_Brown]

SNIPPETY

I would agree with you, for the most part. I would simply say that practically, the duration isn't known until right at jump is because the computer method continues iterating until the very last second, possibly causing some perturbation. This isn't true with the pre-planned flight plans or with the hand-calculated ones. (And, makes them very slightly more dangerous, IMTU.) (Not arguing - just giving it some ... nuance. )

When you jump from Sol to Alpha Centauri, you don't plot your exit point to be a certain distance from Alpha Centauri. Instead you plot your exit point to be a certain distance from Sol which Alpha Centauri just so happens to be passing.

It's all relative, man.

 

[nDervish]

Okay, so - according to Marc's article - jump duration is variable but not random (within the game universe, and assuming it's not a misjump) since, he says, a computer can compute a 1-parsec jump solution that is accurate to within 3000 klicks, which isn't possible unless the computer knows both the precise details of the target system and the time of exit to within a couple of minutes or so.

That can still be interpreted to go either way, depending on the frame of reference in which that 3000km accuracy is measured.

If you're measuring in the destination frame of reference, then, yes, the jump duration is clearly known prior to jumping, since you know how much to 'lead' the target location by.

If you're measuring in the origin frame of reference, then it doesn't imply anything one way or the other.

 

[mike wightman]

Ok, one other piece of evidence.

In High Guard the tactical use of the black globe is mentioned - jumping into a system with the globe already on (implying no jump flash for an arriving ship) and attacking a target.
This is an impossibility if the jump duration isn't know to the crew. Note that this section also implies fleets jump together.

 

[Whipsnade]

Ok, one other piece of evidence.

Go ahead...

In High Guard the tactical use of the black globe is mentioned - jumping into a system with the globe already on (implying no jump flash for an arriving ship) and attacking a target. This is an impossibility if the jump duration isn't know to the crew.

This time you failed to remember that Traveller starship weapons have ranges measured in light-seconds and that jump time variation is distributed on a bell curve. Given those "facts", the Project Blackheart cruisers can plan on jumping to a point somewhere "ahead" of the planet Zho warships are orbiting and catching them somewhat unaware.

Note that this section also implies fleets jump together.

MT in the person of DGP cleared that one up long ago, although not as well as they should have. A squadron jump mechanic was introduced which squeezed jump time variation down to hours rather than tens of hours if the vessels in question shared jump plotting data. Where DGP failed, as usual, was in the details.

They narrowed the entire jump exit window for all the vessels in a squadron rather than narrowing the "scatter" of jump exit times between vessels. In doing so, DGP opened up a huge can of worms because all sorts of nasty tactics like aiming near-c rocks through jump space now became easier.

If DGP had kept the original jump time variation equation for the squadron as a whole and then narrowed the exit time variation between the vessels comprising the squadron, fleets could jump together and predicting jump duration would remain the same.

As LKW normally points out, they never did quit their day jobs...

 

[Carlobrand]

No, you're looking at it backwards.

When you jump from Sol to Alpha Centauri, you don't plot your exit point to be a certain distance from Alpha Centauri. Instead you plot your exit point to be a certain distance from Sol which Alpha Centauri just so happens to be passing.

It's like the initial ranging round fired by an artillery piece.

The problem with that interpretation is that it renders the "accurate to within 3000 kilometers" statement meaningless for all practical purposes. A plus-or-minus 10% variation in a 168-hour jump under those terms has the potential to result in a variation in excess of 1.2 million kilometers from the intended exit point with respect to the target planet, possibly double that for a Jump-2. I don't recall anything in canon suggesting that your jump exit could be as much as a million or two kilometers off. If some salesman told me my jump drive was accurate to within 3000 klicks and I instead found myself a million klicks from my intended exit, I'd be hiring an attorney - whether true or not, it's misleading as heck.

I would agree with you, for the most part. I would simply say that practically, the duration isn't known until right at jump is because the computer method continues iterating until the very last second, possibly causing some perturbation. This isn't true with the pre-planned flight plans or with the hand-calculated ones. (And, makes them very slightly more dangerous, IMTU.) (Not arguing - just giving it some ... nuance. )

Why wouldn't it be true of the pre-planned flight plans? And, while that's fine for your TU, the canon TU doesn't make the flight plans any more or less risky than your computer-generated ones. Ergo, the canon TU presents the capacity to generate a flight plan of equal effectiveness well before the jump is attempted, and if it is possible to know the duration at the instant of jump, then it is also possible to know the duration of jump well beforehand from those pre-planned fight plans.

Again, the duration must be known at the time the plan is developed or the "3000 kilometer" bit is an utterly pointless statement. Who would care how accurate you are with respect to the place you left a week ago when the only thing you care about at that point is where you end up when you arrive?

I'm not seeing a good rationale for why a computer can do something seconds before jump but is incapable of doing the same thing an hour or a day before jump. Planets and stars don't move unpredictably on that scale, there's no data you have in the minute before activating the jump drive that you wouldn't have an hour or two earlier, and canon seems quite clear that the flight plan is being developed before jump, not in the instant of jump. Absent those single-use flight plans, there's a basis for argument, but since canon says they exist and Marc says you're accurate to within 3000 klicks, it becomes much more difficult to sustain the argument. I'm also not seeing why it's felt to be an important point - what's wrong with the crew knowing beforehand?

 

[aramis]

Ok, one other piece of evidence.

In High Guard the tactical use of the black globe is mentioned - jumping into a system with the globe already on (implying no jump flash for an arriving ship) and attacking a target.
This is an impossibility if the jump duration isn't know to the crew. Note that this section also implies fleets jump together.

Again, proves little - just that the same solution puts you at the same window in the same relative formation. Note that a fleet with 10% flicker could theoretically set a "peek time" to check their location, and a second "peek" to generate the coordinated motion.

But also note: A fleet under black globe can't accelerate, decelerate, nor react... so sending a fleet in under globe is a fool's errand.

 

[samuelvss]

I'm not seeing a good rationale for why a computer can do something seconds before jump but is incapable of doing the same thing an hour or a day before jump. Planets and stars don't move unpredictably on that scale, there's no data you have in the minute before activating the jump drive that you wouldn't have an hour or two earlier, and canon seems quite clear that the flight plan is being developed before jump, not in the instant of jump.

The logical answer is that the uncertainty is in the travel of the ship through jumpspace, or even the condition of jumspace, that cannot be known before jump.

I think about throwing a ball (I've got a lousy arm); I know where I want it to go, I interface my ballistics software with the hardware, and I chuck it. I am much better at knowing whether it's a good throw, right when it leaves my hand, than I am at either producing such a good throw, or predicting it.

 

[rancke]

The problem with that interpretation is that it renders the "accurate to within 3000 kilometers" statement meaningless for all practical purposes. A plus-or-minus 10% variation in a 168-hour jump under those terms has the potential to result in a variation in excess of 1.2 million kilometers from the intended exit point with respect to the target planet, possibly double that for a Jump-2. I don't recall anything in canon suggesting that your jump exit could be as much as a million or two kilometers off. If some salesman told me my jump drive was accurate to within 3000 klicks and I instead found myself a million klicks from my intended exit, I'd be hiring an attorney - whether true or not, it's misleading as heck.

See post #11 of this thread for my suggestion for fixing that problem.

Again, the duration must be known at the time the plan is developed or the "3000 kilometer" bit is an utterly pointless statement. Who would care how accurate you are with respect to the place you left a week ago when the only thing you care about at that point is where you end up when you arrive?

Think of it as chrome. Useless, but interesting, and it's not really a problem (see the abovememntioned post #11).



Hans

 

[Carlobrand]

See post #11 of this thread for my suggestion for fixing that problem.



Think of it as chrome. Useless, but interesting, and it's not really a problem (see the abovememntioned post #11).



Hans

So Mr. Miller declares, "The exact time of emergence is usually predicted by the ship's computer and the bridge is well-manned for the event," ... but we've collectively decided that this information is unavailable until the instant one enters jump. And Mr. Miller declares, "Over a jump distance of one parsec, the arrival point of a ship can be predicted to within perhaps 3,000 kilometers," ... but we've collectively decided this is - "chrome"? - that you can indeed exit space much, much farther than 3000 kilometers from the anticipated exit point but it's okay because statistically you're most likely to precipitate out somewhere on the surface of the 100-diameter limit.

Shall I ask where in canon it actually says that the time in jump is unknown until you activate the drive, or that the exit point can vary by hundreds of thousands of kilometers, or have we collectively decided that democracy should prevail over canon? Are there particular adventures that might possibly suggest this view? Or perhaps some reference in Library Data or another supplement, a JTAS article, anything?

I will be the first to admit I have not memorized every word of CT canon, so it's entirely possible there are large bits I've forgotten or don't know about.

 

[rancke]

So Mr. Miller declares, "The exact time of emergence is usually predicted by the ship's computer and the bridge is well-manned for the event," ... but we've collectively decided that this information is unavailable until the instant one enters jump.

No, Marc Miller said that too in the same article. The duration is established at the moment of jump. Logically it cannot be known before it is established. Therefore the information is not available at the time jump calculations are made. Therefore it cannot be taken into consideration when performing jump calculations.

And Mr. Miller declares, "Over a jump distance of one parsec, the arrival point of a ship can be predicted to within perhaps 3,000 kilometers," ... but we've collectively decided this is - "chrome"? - that you can indeed exit space much, much farther than 3000 kilometers from the anticipated exit point but it's okay because statistically you're most likely to precipitate out somewhere on the surface of the 100-diameter limit.

No, we decide that of two possible interpretations, one of which contradicts a piece of canon and the other of which makes a piece of canon moot, but doesn't actually contradict anything, the second interpretation is the one to go with.

I do, anyway.

Shall I ask where in canon it actually says that the time in jump is unknown until you activate the drive, or that the exit point can vary by hundreds of thousands of kilometers, or have we collectively decided that democracy should prevail over canon? Are there particular adventures that might possibly suggest this view? Or perhaps some reference in Library Data or another supplement, a JTAS article, anything?

"The duration of a jump is fixed at the instant that jump begins, and depends on the specific jump space entered, the energy input into the system, and on other factors. In most cases, jump will last a week." [Jump Space by Marc W. Miller]​

EDIT: Almost forgot: We also avoid all the questions and problems associated with being able to calculate a jump duration and then deciding not to use it after all.


Hans

 

[aramis]

Carlo:

assuming a typical earthlike orbit, and earthlike star... and earthlike planet... ±16.5 hours is not too big a deal.

Earth orbits at 107,000km/h.
Earth, a size 8 world, has a 100 diameter exclusion of 2,560,000km
Earth crosses that exclusion zone in about 24 hours. target point ±12 hours.

If we assume the ±16.5 to be 4th std deviation, with even width std dev's, it's ±4 hours 7 minutes per std dev. so ±3 Std Dev would be the barrier. That's a rather nice 99% for earth. For a size 1 world, it's ±1.2 hours - rather likely a miss - but not a bad one.

See, it's about 3 G-hours to match course to the world, plus up to 3 hours to set neutral for the local system, plus up to a dozen G-hours to match up solar vectors (tho usually under 4)... you often won't want to hit the 100D limit... because it often means no time to enter orbit!

Edit: using 6d6 to generate gives a ±8 hours for ±2 StdDev (90%), ±13 hours for ±3rd StdDev (99%), and ±14 for 4th (99.9%)... with ±5 hours being the 1st SD (66%), and doesn't actually go to the full 16.5. 7d6 will, but has the issue of an odd centerpoint. Using 2d8+4d6 fits nicely....