Can ships change their real space vector in Jump?

[Enoki]

Honestly Jump significantly alters the real space velocity/vector just by its nature. In that the point that you jump from has a different vector than the point that you arrive at.

This is why I have the ship arrive with a random vector, almost always NOT pointing at the gravity well (eg., planet) if the ship jumps with a vector. Thus, if the players want to be most efficient with a jump, they bring the ship to a halt then jump. That way when they arrive, the ship is stationary at that end too.

 

[mike wightman]

Only one edition, MegaTraveller, has a lanthanum grid so you must mean that edition.

 

[mike wightman]

This is why I have the ship arrive with a random vector, almost always NOT pointing at the gravity well (eg., planet) if the ship jumps with a vector. Thus, if the players want to be most efficient with a jump, they bring the ship to a halt then jump. That way when they arrive, the ship is stationary at that end too.

If you are stationary with respect to your departure world then you will not be stationary in your system of arrival.

 

[Condottiere]

So canon says.

Also, why we're supposed to transit at a dead stop.

 

[atpollard]

Well, if it's a rocket, the rocket exhaust is in the jump bubble with you, so there's conservation of mass.

But when you exit jump space, it's a really impressive jump flash as the ionized vapor expands

Actually, I was thinking of something different. The assumption always WAS that a ship entered Jumpspace and nothing effected it after that (other than a catastrophic failure that destroyed it). Once you open Pandora's Box that what happens INSIDE a jump bubble can change things OUTSIDE the jump in Normal Space ... one begins to question what other variables can be changed?

Say I build a ship with a 1000 dTon J1 drive and a 100 dTon Hull with a 900 dTon external "fuel bladder" inflated with air (hydrogen gas). Once inside Jumpspace, I deflate the bladder and exit Jumpspace as a 100 dTon craft. What happens if we change VOLUME while in Jumpspace? If VOLUME functions as MASS and ENERGY functions as TIME in Jumpspace, then changing the mass while conserving Kinetic Energy would increase VELOCITY ... would that be JUMP DISTANCE or VELOCITY upon transition back to Normal Space?

• I don't quite follow the Jump Space-normal space equivalence, but this is a question that somebody should ask if Jumpspace and Real space can interact the way T5 says they can.

 

[atpollard]

Only one edition, MegaTraveller, has a lanthanum grid so you must mean that edition.

JTAS 24 "Jumpspace" by Marc says there is a Lanthium Coil in the Jump Drive and there is a Grid in the hull that maintains the Jump Fields in Jumpspace. There may or may not be Lanthuim in the hull grid (JTAS 24 was silent on that) but there IS a grid of "cables". [like a Faraday cage?]

Personally, I suspect Iron cables to explain the Steel hulls and Crystaliron for advanced tech.

 

[RogerD]

Isn't jumping towards a planet and accelerating to near C while in Jumpspace an indefensible doomsday weapon ... a 100 dTon planet-killer super-torpedo with 1 week to impact and zero chance to intercept?

You don’t need to have the acceleration happen in jump space, right? Just accelerate to speed on the correct vector, then jump. The difficulty of the astrogation to actually hit should be pretty high though. Probably easier to launch your Size-7 KK missile from in-system to take out jump variability.

 

[kilemall]

Easiest rule change is jump leaves you zero inertia at the end of the trip. Still leaves an optimal zone to drop in ahead of the planet that virtually all incoming share assuming you aren’t doing the 100D propagation thing. Then you’re just doing jump shadow,

 

[Enoki]

If you are stationary with respect to your departure world then you will not be stationary in your system of arrival.

I take this as the ship lacks inertia. That is, it is truly stationary, not stationary relative to some external object. It shouldn't be difficult at all to achieve this.

 

[RogerD]

I take this as the ship lacks inertia. That is, it is truly stationary, not stationary relative to some external object. It shouldn't be difficult at all to achieve this.

I don’t understand this idea. Any concept of stationary must be with respect to some frame of reference. You can pick a convenient one to make the math easier so you don’t have to worry about the fact that we are spinning around on the surface of a planet that whirls around the sun which orbits the galaxy and so on.

The most convenient frame of reference given the game physics as described is probably to pick the target or source planet and work out the velocity vector between them and treat that as something to be compensated for before, during or after jump. Whether that is the starting world or the ending world just affects the sign of the vector components so it doesn’t really matter much.

I think for normal merchant operations I would usually match vectors before jump so that I’m less likely to hit stuff when I exit jump. I can see where I’m going, basically. During jump would work too, but who carries that kind of drive?

 

[Enoki]

I don’t understand this idea. Any concept of stationary must be with respect to some frame of reference. You can pick a convenient one to make the math easier so you don’t have to worry about the fact that we are spinning around on the surface of a planet that whirls around the sun which orbits the galaxy and so on.

The most convenient frame of reference given the game physics as described is probably to pick the target or source planet and work out the velocity vector between them and treat that as something to be compensated for before, during or after jump. Whether that is the starting world or the ending world just affects the sign of the vector components so it doesn’t really matter much.

I think for normal merchant operations I would usually match vectors before jump so that I’m less likely to hit stuff when I exit jump. I can see where I’m going, basically. During jump would work too, but who carries that kind of drive?

This is not true. Inertial navigation systems that already exist can determine if the vehicle they are used on is moving without outside reference. The outside reference only comes into play when you want to know where you are relative to other "stuff." That is it can determine the velocity and vector of the vehicle it is on without outside reference.
Basically, a combination of gyroscopes and accelerometers can determine if the ship is moving. No need to compare that to anything else.

So, the ship comes to a stop. Motion ceases. It then jumps.

 

[mike wightman]

I take this as the ship lacks inertia. That is, it is truly stationary, not stationary relative to some external object. It shouldn't be difficult at all to achieve this.

Stationary relative to what?
What is the reference point that is not moving while every galaxy cluster, galaxy, star, planet and moon are in motion?

 

[mike wightman]

Easiest rule change is jump leaves you zero inertia at the end of the trip. Still leaves an optimal zone to drop in ahead of the planet that virtually all incoming share assuming you aren’t doing the 100D propagation thing. Then you’re just doing jump shadow,

Zero inertia relative to what? From what you have written it appears to mean zero inertia relative to the planet you are heading for.

 

[mike wightman]

This is not true. Inertial navigation systems that already exist can determine if the vehicle they are used on is moving without outside reference. The outside reference only comes into play when you want to know where you are relative to other "stuff." That is it can determine the velocity and vector of the vehicle it is on without outside reference.
Basically, a combination of gyroscopes and accelerometers can determine if the ship is moving. No need to compare that to anything else.

So, the ship comes to a stop. Motion ceases. It then jumps.

An inertial nav system uses its internal sensors to track movement - movement in the outside world. The computer then uses this tracking data to determine position in the real world.

From wiki:

Gyroscopes measure the angular velocity of the sensor frame with respect to the inertial reference frame. By using the original orientation of the system in the inertial reference frame as the initial condition and integrating the angular velocity, the system's current orientation is known at all times. This can be thought of as the ability of a blindfolded passenger in a car to feel the car turn left and right or tilt up and down as the car ascends or descends hills. Based on this information alone, the passenger knows what direction the car is facing, but not how fast or slow it is moving, or whether it is sliding sideways.

Accelerometers measure the linear acceleration of the moving vehicle in the sensor or body frame, but in directions that can only be measured relative to the moving system (since the accelerometers are fixed to the system and rotate with the system, but are not aware of their own orientation). This can be thought of as the ability of a blindfolded passenger in a car to feel themself pressed back into their seat as the vehicle accelerates forward or pulled forward as it slows down; and feel themself pressed down into their seat as the vehicle accelerates up a hill or rise up out of their seat as the car passes over the crest of a hill and begins to descend. Based on this information alone, they know how the vehicle is accelerating relative to itself; that is, whether it is accelerating forward, backward, left, right, up (toward the car's ceiling), or down (toward the car's floor), measured relative to the car, but not the direction relative to the Earth, since they did not know what direction the car was facing relative to the Earth when they felt the accelerations.

However, by tracking both the current angular velocity of the system and the current linear acceleration of the system measured relative to the moving system, it is possible to determine the linear acceleration of the system in the inertial reference frame. Performing integration on the inertial accelerations (using the original velocity as the initial conditions) using the correct kinematic equations yields the inertial velocities of the system and integration again (using the original position as the initial condition) yields the inertial position. In our example, if the blindfolded passenger knew how the car was pointed and what its velocity was before they were blindfolded, and if they are able to keep track of both how the car has turned and how it has accelerated and decelerated since, then they can accurately know the current orientation, position, and velocity of the car at any time.

 

[Baroun Tardis]

Actually, I was thinking of something different. The assumption always WAS that a ship entered Jumpspace and nothing effected it after that (other than a catastrophic failure that destroyed it). Once you open Pandora's Box that what happens INSIDE a jump bubble can change things OUTSIDE the jump in Normal Space ... one begins to question what other variables can be changed?

Say I build a ship with a 1000 dTon J1 drive and a 100 dTon Hull with a 900 dTon external "fuel bladder" inflated with air (hydrogen gas). Once inside Jumpspace, I deflate the bladder and exit Jumpspace as a 100 dTon craft. What happens if we change VOLUME while in Jumpspace? If VOLUME functions as MASS and ENERGY functions as TIME in Jumpspace, then changing the mass while conserving Kinetic Energy would increase VELOCITY ... would that be JUMP DISTANCE or VELOCITY upon transition back to Normal Space?

• I don't quite follow the Jump Space-normal space equivalence, but this is a question that somebody should ask if Jumpspace and Real space can interact the way T5 says they can.

OK, I see where you're trying to go here.. and I'm not sure this path takes you here.
There's an old CT TNS story about a ship with battle damage losing a chunk of hull while in jump, and suffering a misjump that lasts 40-odd years objective time, so there's a bit of canon in your favor there. Might want to research that for more details.

T5 canon though....
Once you pinch off the jump bubble, that bubble stays there for 168 +/- 10% hours. There's a knack mentioned in the book that some astrogators pick up that makes if always -10%, but you're still looking at fixed time. The fixed time situation that I think most applies here though is using gravity well interference to force a Short Hop. Hop drives can make a sub-10 parsec jump if the astrogator forces the jump line to cross the 100D limit of something large. You can also do the same trick with J-drives on ships without jump governors.
What's creepy to me there is the jump duration is .... 168 +/- 10% hours.
Yeah, ponder that...
You're hanging out at my home in Carmel, and set up a jump for Pretora, 3 parsecs away, but you either accidentally or deliberately set your course to go through Monte Ray, the companion star in the system.... so you're gonna precipitate 6300 AU or so from your jump entry, rather than the 3 parsecs that you just burnt fuel for.
If you're "moving" through jump space, then jump duration should be pretty darn close to zero before you hit that wall, right?
......except.... the jump duration is .... 168 +/- 10% hours.
And that right there is some creepy canon.

T5 also has a very clear line: T5.10B2p112: "A course can not be changed once begun." Later that same page, it talks about the above method for getting around not having a jump governor by plotting courses that can not be completed.... and then explicitly says they have the whole duration.

There's also stuff in T5 about jump bubble diameter calculation. I'm gonna focus on coil drives rather than grid drives, but I suspect they'd work out the same. The bubble forms at a set diameter based on the tonnage of the ship entering jump:
D= (Tons * 13.5 )^1/3 *20 (in meters) For example, a functioning jump drive in a 100 ton ship creates a jump bubble with a diameter (=100*13.5 ^1/3 * 20 = 11.05 *20 =) 220 meters. For example, a functioning jump drive in a 2400 ton ship creates a jump bubble with a diameter (=2400 * 13.5 ^1/3 *20 = 32.3 *20 =) 640 meters.

Your 1000 ton ship is going to make a 460 meter diameter jump bubble on entry... and keep it.

What happens if there's other stuff in the field?
Difficulties. Occasionally, a Jump Bubble will enclose nearby debris. If the delicate balance of total ship tonnage is disrupted, the ship may experience a misjump. The Jump Bubble affects each continuous object within it separately. A jump drive operates on the ship tonnage contained within its jump field. The ship with its Jump Drive almost always operates properly; other objects within the bubble suffer Jump Mishaps.

That does raise the question to me of what happens if there's a 100 ton scout nuzzled up to your ship when it jumps: it's going into jump space too, but gets an automatic Mishap roll. Looking at the tables, it implies damage to the jump drive, which kinda reminds me of Jump Inducers. Anway, back to topic.

A misjump due to a drive failure takes 7+Flux days with exit location as planned... so, in theory, you could have a two day jump rather than 7. There's a mishap roll for damage to the J-drive. (T5.10B2p115)

What I think is happening is that the time to go from jump entry point to jump exit point is zero.... but it takes time for the jump bubble to decay and return to N-space. In the case of occlusion jumps, you zero-time to the occlusion point and then get stuck and wait around for jump bubble decay ,which would explain why you're still looking at normal jump time even if it's near 0% of the planned/powered jump distance. That also explains the misjump issue above and reaching your destination in 2 days.

I suppose you could rule that the system you're proposing ("Balloon Jump"?) is a deliberate attempt to force a misjump due to drive mismatch, and take it directly to the misjump table with a note that it's 7 days minus ABS(Flux) and then a damage roll to the jump drive. At high TL, you can even go with Ultimate drives with very high QRebS and hope they make the rolls. Or you could put in several (possibly cheap) J-drive modules, and just switch to a backup as they fail out. Or both.

 

[RogerD]

Basically, a combination of gyroscopes and accelerometers can determine if the ship is moving.

The gyroscopes will detect rotation relative to where the gyroscopes spun up. I suppose that rotation can be zeroed out prior to jump and it would remain that way after jump. I’m unclear on what effect gravity has on a gyroscope.

An accelerometer measures if the ship is accelerating or not. It can be integrated over time to get velocity. However, it will not pick up on acceleration due to gravity. If you set that integrated velocity to zero before jump, it is relative to some arbitrary velocity like the velocity you had when you turned it on.

At TL 25 there is an inertialess maneuver drive, which I think is meant to be like the drive in the Lensman series. It has the effect of setting the ship’s mass to zero which means it can instantaneously accelerate to the speed of light (faster in the Lensman books) and colliding with something would just instantly stop it. I could imagine using something similar in a Traveller universe for jump drives - you assume the inertia of the destination gravity field’s source on exiting jump.

 

[Grav_Moped]

a deliberate attempt to force a misjump due to drive mismatch, and take it directly to the misjump table with a note that it's 7 days minus ABS(Flux) and then a damage roll to the jump drive.

I like this idea... a bit impractical, but interesting.

 

[Grav_Moped]

I’m unclear on what effect gravity has on a gyroscope.

Maintain constant altitude over a gravity source and you can use its deflection from the direction of local gravity to determine "horizontal" (actually circumferential) displacement. Don't need to hold constant altitude if you know altitude of origin and present position.

 

[Jame]

Yes, and that's what I was trying to say. Thanks for the clarification!

Easy to measure and keep track of, in-universe (and essential!) Not tracked in the game system, and doing so gets complicated if you're trying to do it right.

Stars' relative motion is easy, planetary relative motion requires calculating orbits and keeping track of where each planet is in its orbit... but you might want to do that anyhow as part of keeping track of stellar jump occlusion.

And if it's tracked in Travellermap (which would probably be the only place it could be), it only works where its procedurally-generated star system matches the one from canon (wiki or direct source material).

If I ever have to make a rule for that, I'd say IMTU that astrogation would normally take stellar motion into account.

What happens if it isn't taken into account? Well, that could be a misjump!

 

[Enoki]

Now, one thing I do differently than canon is you can jump to nothing. That is, you can make the end of a jump into open space if you want. What you cannot do is jump close to a gravity well at either end. My version of jump has it that the ship is essentially bending time-space using gravity and it is pulled to the far end of the well it creates where it emerges a week (7 days) give-or-take later. The time in jump is fixed, the distance varies depending on the ship's ability to create the well. The stronger the field, the deeper the well. But there are limits on the depth you can create and that is why J6 is all that's allowed.
Thus, you don't need a planet or something at the far end of a jump. In fact, you are limited to keeping far enough from one during the entire jump to only have microgravity--call it background gravity if you will--during the entire jump. Get too close to a gravity source, and you risk a misjump due to either the well forming wrong, bending, or the like.