Vector after Jump

[Trixbat]

If a ship goes into jump while traveling at a given velocity does it maintain that velocity?

For example, could a J-4, 4G warship accelerate continuously for 20 hours or so, to reach about 1% of light speed, use half its jump capacity to jump 2 to a target system, appear 300 diameters from the target world with known coordinates, take 10 minutes to target the world and launch a full missile salvo (or release a strike force of riders or fighters), then jump out again about 10 minutes later just before it reaches the 100 D limit, with its missiles then striking the world a few minutes afterward?

The manned variant for carriers is to microjump to a point a week's travel on the far side of the system to rendezvous with the survivors of its fighter or rider squadron).

(Obviously it would be theoretically possible to boost to higher speed, but I don't want to get into the issues of near-C rocks, targeting at relativistic velocities, etc. A velocity achievable by a day's travel at 4G seems perfectly normal, however - the question is whether you can do that.)

[Hans Rancke-Madsen]

Originally Posted by Trixbat

If a ship goes into jump while traveling at a given velocity does it maintain that velocity?

For example, could a J-4, 4G warship accelerate continuously for 20 hours or so, to reach about 1% of light speed, use half its jump capacity to jump 2 to a target system, appear 300 diameters from the target world with known coordinates, take 10 minutes to target the world and launch a full missile salvo (or release a strike force of riders or fighters), then jump out again about 10 minutes later just before it reaches the 100 D limit, with its missiles then striking the world a few minutes afterward?

The variation in time would mean that the ship could not be sure that it would be pointing right at the target world on arrival, nor what the distance to it would be.


Hans

[Apache]

Also, in MongTrav, slight misjumps are common.....you don't always appear at your destination where you think you are going to appear.

So while you can enter J-space with a high velocity.....you are going to exit J-space with that velocity......and you may have to change course really quick after you come out of J-space.

Which is why, IIRC, in almost all versions of Traveller travel time to the Jump point includes you braking at the halfway point and making the Jump with near-zero velocity......

[Trixbat]

Originally Posted by Hans Rancke-Madsen

The variation in time would mean that the ship could not be sure that it would be pointing right at the target world on arrival, nor what the distance to it would be.


Hans

Thanks, that makes sense - I'd hoped there was some reason for it. I was a bit worried as Marc Miller's "Jumpspace" article claimed warships sometimes didn't dump velocity. But given typical planetary velocities, the average deviation looks like it works out to around 50-100 diameters, which is convenient.

[Trixbat]

Originally Posted by Apache

Which is why, IIRC, in almost all versions of Traveller travel time to the Jump point includes you braking at the halfway point and making the Jump with near-zero velocity......

While the standard formula in, say, CT or Mongoose does include the half-way turnaround, I am not sure if GT, Mongoose Traveller or CT actually went out and explained that is what ships do when travelling to a jump point!

An interpretation of the CT travel time rules -- which I've seen used by at least one group I played with - was turnaround was calculated from the total distance - e.g., 800,000 miles to size 8 world jump point + 500,000 miles from Size 5 world jump point = 1,300,000 miles.

Ignoring such oddities as boosting to 1% of light, are there any Traveller rules anywhere besides Marc's JTAS article (which leaves open both options as viable) which actually says ships will accelerate to midpoint then decelerate to relative zero at the 100 d limit? I'm not saying it's not logical, but it would be nice if I could find whatever rule actually discusses it!

[Apache]

Originally Posted by Trixbat

While the standard formula in, say, CT or Mongoose does include the half-way turnaround, I am not sure if GT, Mongoose Traveller or CT actually went out and explained that is what ships do when travelling to a jump point!

An interpretation of the CT travel time rules -- which I've seen used by at least one group I played with - was turnaround was calculated from the total distance - e.g., 800,000 miles to size 8 world jump point + 500,000 miles from Size 5 world jump point = 1,300,000 miles.

Ignoring such oddities as boosting to 1% of light, are there any Traveller rules anywhere besides Marc's JTAS article (which leaves open both options as viable) which actually says ships will accelerate to midpoint then decelerate to relative zero at the 100 d limit? I'm not saying it's not logical, but it would be nice if I could find whatever rule actually discusses it!

Not AFAIK.

Every version of Traveller I've ever seen, the travel time formula to the jump point included turnaround/braking to set velocity to zero for the actual jump.

[Hans Rancke-Madsen]

Originally Posted by Trixbat

While the standard formula in, say, CT or Mongoose does include the half-way turnaround, I am not sure if GT, Mongoose Traveller or CT actually went out and explained that is what ships do when travelling to a jump point!

They all say that the ship accelerates for the first half of the journey and decelerates for the second half, implying (saying?) that it comes to rest with respect to the departure world. Which does not really make sense, since what the ship needs for maximum efficiency is to be at rest with respect to the arrival world. I've always assumed that this is what the ship does in "reality", and that the rule is a simplification for game purposes that is close enough and much simpler to calculate.


Hans

[Trixbat]

Originally Posted by Apache

Not AFAIK.

Every version of Traveller I've ever seen, the travel time formula to the jump point included turnaround/braking to set velocity to zero for the actual jump.

So far, I've checked GT and CT and neither seem to indicate this is the case.

Sure, both give turnaround/braking time travel formula, but these are general formula for space travel, not specific to jump point travel. I have yet to find a specific "travel time formula to the jump point." Neither specifically says you should or should not calculate distance from world to jump point.

(I believe you're right and the standard method of travel is to set velocity to zero - I just think it's weird that the rules don't make this obvious...)

Hmm, after some looking, I find GURPS Traveller does distinguish the standing jump and the running jump (the latter is what I discussed) - it says the latter is trickier to perform but actually doesn't say which version is the more common or provide useful examples of either being calculated. Probably because GT enshrines the evil of jump masking...


.

[Trixbat]

Originally Posted by Hans Rancke-Madsen

They all say that the ship accelerates for the first half of the journey and decelerates for the second half, implying (saying?) that it comes to rest with respect to the departure world. Which does not really make sense, since what the ship needs for maximum efficiency is to be at rest with respect to the arrival world. I've always assumed that this is what the ship does in "reality", and that the rule is a simplification for game purposes that is close enough and much simpler to calculate.

Hans

Hi Hans. Could you be kind enough to point out where they actually DO say this? In actual page references in any of the books? Because I can't find this, and I really need to find this to deal with some arguments regarding pirate intercept maneuvers. And I can't!

(If you dig really hard and do the math you can work it out from some buried examples, but I can't actually find any clear statements to this effect, and certainly nothing that actually say outright that you can't do the constant acceleration trick ... and two references that say the navy actually does it.)

I've only looked in CT, Mongoose Traveller, and GURPS Traveller for this which are the ones I play; I don't consider TNE canon for this question due to changes in their M-drive rules, and I don't have other editions handy.

[Hans Rancke-Madsen]

It IS only by implication (actually, I'm fairly sure the bit about standing and running jumps predate GT, but as I can't trace down the reference, I may be mistaken), but the implication is IMO strong enough to be irrefutable.

On p. 54 of TB and p. 92-93 of IE are tables listing typical travel times to safe jump distances around worlds of various standard diameters. If the typical way to travel to a safe jump distance wasn't to a standing jump, the typical travel times would not be based on the time to accelerate for half the distance and decelerating for half the distance.


Hans