A Question about Jump Drives

[Winterwolf]

I have been reading a lot of Sci Fi recently, and I have a question when a ship jumps into jumpspace does it retain the speed it was moving at or does it lose all speed when it exits jumpspace.
I mean if a ship is travelling at 3G's running from someone. When it hit the jump limit and jumps a week later it drops out of jump at it destination is it still travelling at 3G's

 

[HG_B]

I have been reading a lot of Sci Fi recently, and I have a question when a ship jumps into jumpspace does it retain the speed it was moving at or does it lose all speed when it exits jumpspace.
I mean if a ship is travelling at 3G's running from someone. When it hit the jump limit and jumps a week later it drops out of jump at it destination is it still travelling at 3G's

That depends on the version of Traveller.

 

[robject]

That depends on the version of Traveller.

What he said.

That said, the usual answer is that exit velocity is equal to entering velocity, with respect to the target solar system, but the vector is randomized.

 

[rancke]

That said, the usual answer is that exit velocity is equal to entering velocity, with respect to the target solar system, but the vector is randomized.

I thought the usual answer was that the ship reduces its vector relative to the destination world to zero before jump. (Well... IIRC the text says that the ship reduces its vector to zero before jumping. I added the 'relative to the destination world' because reducing the vector relative to the departure world doesn't make anywhere near as much sense).

I don't have a reference, sorry.


Hans

 

[HG_B]

I thought the usual answer was that the ship reduces its vector relative to the destination world to zero before jump.

Hans

That is a procedure rather than what happens physical law wise.

 

[jawillroy]

IMTU, I assume that a ship comes out of jump moving at the same length and direction vector as it was traveling when it went into jumpspace - and that part of the navigator's job is to calculate what the ship's vector will be relative to the destination world.

Aaaaand since that's all dependent on the relative velocities of the two star systems in question, not to mention the destination world itself, I just handwave that all away and say you end up where you want to end up (unless there's a good reason otherwise, e.g., the GM need the PCs to be hurtling towards the outer system at a hellacious velocity in order to aid the Plot.)

 

[Vargas]

I thought velocity was retained through Jump as per Marc Miller's Jumpspace article in JTAS.

 

[Andrew Boulton]

I'm pretty sure all versions agree that velocity is conserved.

 

[HG_B]

I'm pretty sure all versions agree that velocity is conserved.

MGT rules don't take a position on this subject.

 

[Andrew Boulton]

MGT rules don't take a position on this subject.

I sit corrected.

 

[whartung]

I've always assumed like the others. The vector is maintained, velocity and direction.

The issue is how useful that vector will be in the target question.

As a "generic" simple rule, the "maintain velocity, random direction" is a nice hand wavy game mechanic, that's playable and useful.

From the hard science guys, there's a question of relative vector relationships.

Specifically, a ship with a velocity V, really has a velocity relative, most likely, the local star or other large gravity well. When a ship is landed on a planet, it has a velocity of 0, but that's relative to the planet. The planet has a velocity of PV relative to the star, and the star has a velocity of SV relative to it's galaxy, and on and on.

I don't even know if there's some universal constant reference for velocity. I also really have no idea how different the velocities of stars are, relative to the galactic core, but more importantly for this discussion, relative to each other.

From a mechanics point of view, we don't really consider this problem at all (for good reason). But if the vector is truly maintained, then it actually becomes an issue. System A may be moving dramatically "faster" compared to System B. The concern is whether that velocity becomes an issue in terms of matching vectors for the ship once it arrives.

In practice, it likely doesn't matter. I did a little simulation, and when you have a constant thrust 2G drive, especially with unlimited acceleration, i.e. thruster plates, then the planetary velocities are ALMOST zero in the big picture. By the time a ship gets from Earth to Jupiter, Jupiter has literally "hardly moved".

But you can that if you have a velocity of "0" in System A, and you arrive in System B, that "0" may now be 100,000km/hr or whatever because of the relative differences between the two systems. And, especially for fuel limited games (i.e. TNE), that could be an interesting consideration since they'll have to compensate for that velocity difference either by accelerating in the source system or in the destination system. The work is the same, just a matter of where it's done.

Some quick google fu brings up http://www.enchantedlearning.com/subjects/astronomy/planets/earth/Speeds.shtml

Our system is moving at 155 miles/sec vs the Galactic core, and the Galaxy is moving 185 miles/sec. Not insane velocities, but in TNE from "0" to 155 miles/sec is 14 G Turns of acceleration. So, there is room "adventure" here in fuel starved environments.

I imagine the relative velocities of systems is pretty minor. Just like the relative velocities of our planets are pretty minor, and the stars pretty much all go in a similar direction around the galaxy.

Still, fun thought experiment.

 

[HG_B]

System A may be moving dramatically "faster" compared to System B.

No, not for stars within 27 light years of each other.

 

[RainOfSteel]

I would not personally worry about the relative movement of solar systems. Your ship's software handles this when generating jump coordinates.

What's really interesting is that every solar system's ecliptic is on its own angle. The Sol system ecliptic is at a considerable degree of tilt off of the Milky Way galaxy's ecliptic.

While this doesn't matter for jumps between systems, it would matter for matching orbits with any planet once you jumped in-system. I personally imagine that most ships would go for an insertion into the orbit of the main world; they just wind up with and oddly angled orbit around the planet. For docking with local highports or outports, you would need to a) maneuver into the plane the ecliptic or b) swing your achieved planetary orbit around to match the planet's ecliptic. Or at least I imagine high and outports would be in the local planet's ecliptic.

If you have gravity drives, you can descend to the surface however slowly you like to handle the conservation of angular momentum, bleeding it off a bit at a time; so I think your starting planetary orbit angle would not so important. If you're using non-gravitic thrusters, I'm thinking the conservation of angular momentum will require that you enter the atmosphere from what resembles a latitudinal orbit.

This is hand-waved away (in more than just Traveller) with the generally assumed, but incorrect, idea that all solar systems have the same ecliptic plane which is itself the same as the Milky Way galaxy's ecliptic plane.

However, for the purposes of a role-playing game, introducing this level of realism doesn't add a lot of fun more most, would add needless complexity for everyone else, and you can bet trying to use it would launch arguments about how solar systems work from at least some gamers.

 

[HG_B]

If you have gravity drives, you can descend to the surface however slowly you like to handle the conservation of angular momentum, bleeding it off a bit at a time; so I think your starting planetary orbit angle would not so important.

Simply adjust on your trip to the 100D limit from the originating system.

 

[Icosahedron]

As Whartung and RoS suggest, conserving a vector (which seems to be preferred in the OTU) can lead to problems.

Personally, I work it so that the velocity is relative to the local gravitational field (which is usually dominated by the nearest gravitating body). This makes scientific sense in relativistic terms.

I also don't conserve vectors, having all exits at zero velocity relative to the local field. This not only reduces the navigational headaches of drections and ecliptics, but also removes (or greatly reduces) the problem of near-c rocks.