# jumping times

#### katta

##### Mongoose
Hello all! I checked the jumping times in the core rulebook and I still have a few points that, for me, are ambiguous. I understand that all this part is just a gross approximation of celestial mechanics (I am an aerospace engineer IRL) and that is totally fine. However, there are a few key corner cases that can be treated in various ways and I would like to have your opinion on the matter.

Questions:
- does the jump drive preserve the speed that you had before the jump? Or maybe you can jump only if you are "still" in space? Still compared to what? The star? The planet?
- If the latter is true, after jump are you "still" compared to the new planet? Or the new Star? Or the old ones?

I am thinking about the case where a spaceship is chased and needs to reach jump distance and jump as fast as possible. In that case the formula (for m drive) in the book (t = 2* sqrt(d/a) ) would not apply because that is correct only with a flight profile where you start from 0 speed, accelerate (+a) for half of the trip and decelerate (-a) for the remaining half reaching, again, 0 speed. Decelerating while chased is not the best strategy. What would be the m drive and j drive time calculations in that case where the characters use a sensible strategy (of not slowing down)? Or maybe it is NECESSARY to have 0 speed for jumping...

Most versions of Traveller allow you to preserve your current velocity relative to the planet/station/star you are moving relative to. Note that at the velocities attainable by ships the star is the most useful reference point.
They then totally ignore the issue of the relative movement of the system you are jumping from to the system you are jumping to since their relative velocites are likely to be small on the scheme of things.

There's no accounting for velocities (I might as well use the right word, and its all relative - but that's another thing) other than those of the ship itself. No vectors for planets or stars. Or ships in combat for that matter, but at least some of that made it into the High Guard update. The travel times on page 163 assume turn-around, which makes sense planet to planet (again ignoring relative velocity between planets and the delta-V need to escape and establish orbits).

Personal opinion, but not written anywhere, is to treat emergence velocity as zero relative to the 100D bubble you are jumping into (so if jumping into a world's 100D bubble, you are effectively starting at zero relative velocity to that planet). That way you can't do the surprise relativistic attack trick of building up velocity in another system and jumping in at a good fraction of lightspeed, giving the enemy minimal time to react as you drop onto the world like a dinosaur-killer (not recommended for crewed ships). Though this also provides incentive for pirates to loiter near likely 100D emergence points to bounce on ships appearing at zero velocity, rather than zooming on past the pirates. I think using the zero velocity assumption makes the game more playable in the way it's intended - though referees can use whatever assumptions they want, as long as they're consistent and understood by both referee and players.

That doesn't speak to outbound velocity and transit time, though. Especially if the above is house-ruled in, it becomes most effective to leave using d = 0.5aT^2, reaching maximum velocity as you hit the big red jump button.

Traveller canon for the Third Imperium setting does allow for emerging from jump space with a high velocity and conducting a drive by....

Traveller canon for the Third Imperium setting does allow for emerging from jump space with a high velocity and conducting a drive by....
Yeah, it does, but again, my opinion only, it makes it less playable and worsens the condition of ignoring relative vectors between planets and stars - for stars, it's normally in the dozens of kilomteres per second, but it can be as high as 300kps, and Earth in its orbit is more than 30kps, so are you going coming out in the same general vector or in an opposite one? It's easier and avoids the dinosaur killer scenario - which is mentioned, I think explicitly in the Third Imperium book as a justification for Short Bow - but going off possible faulty memory here - so I agree, it's not exactly canon to assume zero velocity emergence, but it handwaves off all the possible velocity permutations and explains why there isn't a specific prohibition against an attack that makes nuclear weapons look like toys.

- does the jump drive preserve the speed that you had before the jump? Or maybe you can jump only if you are "still" in space? Still compared to what? The star? The planet?
- If the latter is true, after jump are you "still" compared to the new planet? Or the new Star? Or the old ones?
Momentum is conserved.

Hence velocity relative the centre of the universe (or something like that) is maintained.

You should consider the relative velocity of the star systems, but in practice no-one ever does.

You can plot a direct course from the origin planet to the target panel, but standard practice is to jump "at rest", i.e. presumably low velocity relative both star systems.

It's likely that most systems have departure and arrival spacelanes, based on precalculated time schedule, and it's easier to figure out the exit point at slow speed.

Premature precipitation can happen to the best of us, though you could base it on the angle of collision, when it either deflects further, or you have bounce back.

I believe the Starship Operator’s Manual says that momentum is preserved. You come out of jump at the same velocity you were traveling prior to jump. In all honesty, I prefer Geir’s idea: jump strips the object of its velocity for the simple fact that it would eliminate a large number of deep impact attacks.

Hello all! I checked the jumping times in the core rulebook and I still have a few points that, for me, are ambiguous. I understand that all this part is just a gross approximation of celestial mechanics (I am an aerospace engineer IRL) and that is totally fine. However, there are a few key corner cases that can be treated in various ways and I would like to have your opinion on the matter.

Questions:
- does the jump drive preserve the speed that you had before the jump? Or maybe you can jump only if you are "still" in space? Still compared to what? The star? The planet?
- If the latter is true, after jump are you "still" compared to the new planet? Or the new Star? Or the old ones?

I am thinking about the case where a spaceship is chased and needs to reach jump distance and jump as fast as possible. In that case the formula (for m drive) in the book (t = 2* sqrt(d/a) ) would not apply because that is correct only with a flight profile where you start from 0 speed, accelerate (+a) for half of the trip and decelerate (-a) for the remaining half reaching, again, 0 speed. Decelerating while chased is not the best strategy. What would be the m drive and j drive time calculations in that case where the characters use a sensible strategy (of not slowing down)? Or maybe it is NECESSARY to have 0 speed for jumping...
Per CT, a ships relative velocity and heading are the same emerging from jump space as they were when entering. The IRL problem here is that systems have different velocities and are travelling in different directions, so unless you line everything up correctly properly prior to entering jump, you will be going the wrong direction when you emerge. Plus planetary and star locations may prohibit direct-line travel. Miller published this explanation in JTAS, issue 24.

MGT rules, at least, also state that your relatively velocity and heading are reset to zero in the event of a ship encounter. Which, of course, directly contradicts the heading/velocity rule. It makes sense for gaming purposes, but then, if you are going to do that, why bother with trying to explain how it works - and then immediately toss the explanation out the window? It's a sad tradition in Traveller!

It would make far more sense to have ships come to rest in the departure system and then emerge with zero velocity. The faster you are going during the jump the worse your accuracy is upon emergence. Which would mean most ships would slow to a halt, then jump. That actually gives you more opportunities for encounters with pirates. Otherwise no ship would EVER be intercepted since they would be travelling too fast and could just blow past any pirate - who would not be able to match velocities.

HG80
Although jumps are usually made at low velocities, the speed and direction which a ship held prior to jump is retained when it returns to normal space...
Suppose, for instance, that a fleet were to jump into a system with its black globes on and its velocity set upon a predetermined course. It could drift unseen past any defending fleet and drop its screens at a preplanned moment, to bombard a planet or to engage enemy fleets by surprise. Further tactical possibilities are left to the imaginations of the referee and players.

Black globes are rare, and expensive (also in HG 80, they were artifacts. And yeah, how do you equip a fleet with rate artifacts?) A fleet would be close to whatever its job is to protect because there is no real purpose in fighting away from your post (if orbital bombardment is your plan you can do it at a distance and use math and guidance systems to strike from 10 AU.

Also, since we are referring to MGT rules, 2nd edition posts a grav detection system tamhat can detect any ship, even light-years away. Ships cannot jump while a globe is active (I think it's a question for the rules lawyer if they could activate while in jump), and upon emergence their course and speed would be picked up, thus rendering a globe useless for stealth. Math would predict their course and a defending fleet could destroy them one at a time and none of the defenders would even know till their globe failed.

And then there were the various question about just how you could do a fleet jump when the mode itself had a random variable for actual travel time.

It all sounds cool and interesting if taken separately, but when taken as a whole it's clear that the rules were not written, or corrected, to be non-conflicting. Which one normally expects by the 2nd or 3rd edition.

1. Friction - jump bubble.

2. Black globe - in theory, you could transfer any energy/heat accumulated during transition to any leftover hydrogen and vent it as gas. Though how much energy a black globe accumulates through transition of an unknown dimension and with the jump bubble, who knows?

Ignore the shiny thing for a moment (black globe) and instead concentrate on what the rules as written say.
Velocity is retained through jump. Civilian ships appear to routinely jump with a zero velocity (relative to what is up to the referee).
Note the black globe rule indicates the fleet jumps together as a unit.

I'm a bit of a completist, so I've dug further into this issue and here's what I've found thus far. Parsing the CT80 sentence - "Although jumps are usually made at low velocities...", begs the question - "why?"

In the JTAS #24 article, Miller wrote "Commercial ships, for safety reasons, generally reduce their velocity to zero before jumping. Such a procedure eliminates some of the danger of a high velocity collision immediately after leaving jump. Military ships and high speed couriers often enter jump at their highest possible speed and they aim for an end jump point which directs their vector toward their destination in the new system. Such a maneuver allows constant acceleration in the originating system, followed by constant deceleration in the destination system."

If we look at the 1981 CT rulebook, under Transit times to the 100D from a size 8 world - it states 5hrs. The same book has the ubiqutious travel time table which lists approximately the same 5hrs for a 1G to reach a million Km. Miller writes further in his jump space article, that ships make the journey from starport to 100D so that are at zero velocity when they reach the 100D limit.

If you do the rough math, it should take the roughly 5hrs to travel at 1G to reach the 100D and come to rest at zero velocity relative to your current system. Not slowing down means you'd make the 100D limit in maybe 2.5hrs, give or take. This brings up a potentially new contradiction - the travel time table is just for commercial vessels that follow all the safety rules (and surely would not be applicable to PC's and their typical min-max playing style).

On the one hand, trying to get the many disparate statements in the many books correct seems to be a herculean task. And versioning hell makes it even worse. From a gaming perspective it's preferred to have quick and easy explanations that are universally applicable - jump drives work this THIS and travel works like THAT. On the other hand, as you increase the complexity of any system, it gets harder and harder to make it actually work - especially when you continue to bolt on things without having a distinct framework already in place.

The explanation used by Miller (i.e. safety) seems reasonable on the surface, but like many Traveller issues if you pick at it then things start to unravel somewhat. If civilian ships/crews run with this safety margin, then how or why would military ships do this in non-wartime regular operations? And if it's the norm, why don't we see this as reasonable tactic for merchants looking to shave half a day off the clock, or that are jumping into systems with pirate activity and are looking to zip past any potential encounter?

That's why I like the non-canon answer of zero velocity emergence. Removes all the 'what ifs'. And if you ask how this is possible, then, well, Traveller represents the universe in a 2D format with quantized stellar distances so: mumble, mumble, holographic universe... anti-de Sitter space, quantization of jump spaces... mumble, mumble, handwave, shrug).

Actually it makes them worse.

What do you have zero velocity relative to? Say you emerge at zero velocity relative to the planet - the star is moving, the galaxy is moving, the galaxy cluster is moving.

It's actually much simpler to state that the ship jumps with a velocity that is retained and then you make course corrections with your M-drive upon arrival.

That said I could see a case for deliberately aiming for inside the 100D limit so you precipitate out at the 100D limit with your velocity at zero relative to the object you 100Ded.You could even call it a Ded jump plot.
Ded as in dead stop, Ded as in (100)Ded

One difference between Mongoose and the previous editions is that the jump drive is turned off and we coast through jumpspace.

In theory, we could keep the manoeuvre drive on, but how that effects effective velocity, either during transition or exit, who knows?

That's why I like the non-canon answer of zero velocity emergence. Removes all the 'what ifs'. And if you ask how this is possible, then, well, Traveller represents the universe in a 2D format with quantized stellar distances so: mumble, mumble, holographic universe... anti-de Sitter space, quantization of jump spaces... mumble, mumble, handwave, shrug).
I think this works much better, too. As I prefer it, a ship may enter jump space while underway, but the faster it is travelleling, the more inaccurate its entry in the other system is going to be. So ships wishing to make ACCURATE jumps would want to come to a relative halt and then jump. Calculations can still be made while in-transit, and then your emergence point would be more or less accurate. I came up with an idea for each system to set aside areas at the 100D limit for ships to use to target their re-entry points. Since you can't have traffic control for something you can't know about, ships departing the other system use an incremental system that assigns the next block to the next ship departing from System A to System B. Easily works for any system with a simple navigation satellite. Systems without that, or that have very little traffic have their own, larger, block to accommodate those few ships that are travelling. Its essentially space traffic control and you can further minimize risks With a set of clever players they might be able to predict where a ship they want to attack/raid based on predictable traffic patterns.

If I'm playing space opera-like systems, then sure, I'd be ok with more mumble-mumble/handwave, but regardless of system, I prefer to keep it to a minimum and keep the underlying structure firm so that every game isn't loosey-goosey with the rules. There will ALWAYS be some that has to be done, so why waste it on things you don't need to?

Actually it makes them worse.

What do you have zero velocity relative to? Say you emerge at zero velocity relative to the planet - the star is moving, the galaxy is moving, the galaxy cluster is moving.

It's actually much simpler to state that the ship jumps with a velocity that is retained and then you make course corrections with your M-drive upon arrival.

That said I could see a case for deliberately aiming for inside the 100D limit so you precipitate out at the 100D limit with your velocity at zero relative to the object you 100Ded.You could even call it a Ded jump plot.
Ded as in dead stop, Ded as in (100)Ded
If you go by MGT rules, any ship encounter resets relatively velocities of both ships to zero to play out the encounter. Otherwise ships able to retain their velocity upon jumping wouldn't be able to be intercepted in their arrival system since they'd have too much of a head start for a ship lying doggo waiting for prey to emerge.

Zero velocity is, generally speaking, for the system you are IN. But you could change that to being relative to the galaxy you are in (based upon the core, and of course the system you are in is going to be different), or you could make relative to the system you are going TO. Which is why for most it's zero velocity relative to the system you are in - and that is typically defined by the local star. In space everything is always in motion, and as Miller correctly stated in his JTAS article, it's all relative to something somewhere else.

One difference between Mongoose and the previous editions is that the jump drive is turned off and we coast through jumpspace.

In theory, we could keep the manoeuvre drive on, but how that effects effective velocity, either during transition or exit, who knows?
How so? CT had the drive turned off - it only activated for the jump, and powered the jump grid in the hull to precipitate entry into jump space. MGT has the jump drive off as well - but it creates a bubble of exotic particles around the ship, and the bubble slowly degrades, and when it fully degrades you pop back into normal space.

I believe in all examples your M-drive will not affect movement in jump space. And in the newer version they've made it so that a M-drive won't even work when it's past a system heliosphere.

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