Jumps, Time to Orbit, to 10D and 100D

[TheDS]

The view that only the entry and exit points matter is the one I have understood for all my Traveller time, as well as anyone I have ever met who discussed the physics, including those who wrote about it. However, in a discussion elsewhere on this board, the case is made that this does not hold true for the rest of the universe.

Thrash found out that there were two views, and pegged MWM on whether jumpspace has 1-to-1 relationship with n-space and MWM said it does. (Rather, he IMPLIED it does, if we're going to make such distinctions.) And that is why jump-masking came to be the official truth in GURPS Traveller. This led to a rather large discussion between the denizens of this board, one side taking things literally and one side taking them metaphorically, and neither side really willing to give up.

The fact of the matter is, GT's representation (clarification?) is now the "official" representation. Jumps are straight lines. Intervening objects are disasters. New military tactics are possible to defeat enemy invasion fleets, with nary a shot needing to be fired. Piracy gains a whole new life too. Ship designs REQUIRE 2 jump minimums to avoid some of the new tactics, but not all can be avoided so simply. The face of the universe has become unrecognizable due to this one little change.

But at least now there's more details to consider!

The Earth travels at about 30 km/s around the sun. Mercury, which is at about 1/3 the distance, is only about 50% faster, at about 45 km/s. A world whipping around a M8 star in its hab zone will probably be considerably faster, but I haven't calculated that out.

30 km/s is about 1 hour of thrust at 1 Gee. This means that if you are jumping between two worlds that are like Earth, but going in opposite directions, then you need to apply 2 G-hours of thrust to make up the difference. This is probably prefered for the outbound jump. There has never been any mention that your velocity affects jump, though I would believe near-c velocities matter if some one said so.

You want to have 0 velocity relative to the world you are coming to, or perhaps some velocity toward it. In either case, I think you want to aim for the tail end of it's path, where it "just was", so you don't bump into it.

Worlds really close to their star, which whip around their world in like 2 weeks, well, they're deep within the jump shadow anyway, so you're going to want something of a high velocity when you come out. You're going to need to know where the world is, so you can intercept it when you want to. You'll want your vector aimed so that you come in from behind it, so that it's not going to plow into you if you lose power suddenly.

Finally, any world is going to have a traffic net. Simply stompiong on the gas and saying you'll be there in an hour is going to be quite silly on a heavily trafficked world. Traffic control will ban you if you come in at the wrong time. Outgoing control tells you before you leave what the proper approach vector for your destination is. When you get there, they expect you to be on the right vector, though the vagarities of jump time are going to mess this up some.

Still, it is best if you assume jump USUALLY lasts 150 hours (or whatever it was), and that only non-optimal jumps will take more or less time. Getting there too soon CAN be a disaster, if the world isn't where it needs to be yet. It is also best to assume that jump lands you right where you wanted MOST of the time, unless you did something less than optimal.

As the vector difference between most worlds outside their parent-star's jump shadow is easy to make up with about 2-4 G-hours of thrust, I wouldn't worry too much about it, and say that the ship accelerates all the way out and all the way back in, minus a fudge factor on the inbound leg to cover inaccuracies and traffic control.

 

[The Oz]

Originally posted by TheDS:
As the vector difference between most worlds outside their parent-star's jump shadow is easy to make up with about 2-4 G-hours of thrust, I wouldn't worry too much about it, and say that the ship accelerates all the way out and all the way back in, minus a fudge factor on the inbound leg to cover inaccuracies and traffic control.

That's pretty much how I used to do things, in the days before jump shadowing even existed. You used the time before jump to match velocities (more or less) with the target star system and planet, and aimed your jump point to get to to a miniumum-time course for your destination.

Most of the time we never really bothered with it; jump was routine and so was the normal-space travel. It's only when every hour counts, or when that pesky pirate is chasing you that details like this matter, but then they're fun! At least for the ref, the PCs often had a different view.

 

[kaladorn]

How fast is a star this far out on the spiral arm actually moving wrt the core? Probably stupidly fast. That 32 hour window on jump emergence, if you want to count that, is probably a real deal breaker.

Also, there is no suggestion in current rules that you spend time putting in the 'acceleration compensation' before jump. So, my conclusion is that your vector is preserved, but preserved does not mean wrt core or even the primary in most cases, but wrt the destination/source. So, if you arrive close to your destination point, whenever you actually come out there.

That's poorly explained.... but maybe you can see what I mean.

 

[TheDS]

If you're thinking what I think you're thinking, then I think you're thinking wrong. And right.

Vector is preserved when you jump. That's relative to the universe, the core, the world you're at, the world you're going to.... everything. if my vector is coreward at 300 kps in relation to the world I'm leaving, when I arrive, it will still be 300 kps coreward in relation to the world I left. The world I'm now AT is not (likely to be) traveling at the same exact vector as the world I left, so I may seem like I'm traveling only 200 kps in relation to this world. In relation to the core, I may be standing still in both cases!

Even though I used "coreward" as a direction, and that's in relation to the core, and I've messed up my example a little by mixing my relativity,

I think you get the point.

 

[kaladorn]

Hmmm.....

Noodle time.

I J-6 it. I'm jumping outward from core. My velocity is 2000 kph, in the system I'm in, wrt the world I just left. But that world was rotating through its system at 75,000 kph. And the system was rotating around the galactic core at <insert ludicrious speed>.

I jump....

I spend a week out of sight.

I come out, doing whatever speed I was doing, but druing that week, the planet I'm shooting at is rotating at the aforesaid 75K kph. *And so is the entire system its in, rotating at the rotation-around-galactic-core speed*.

So, the 32 hour window of uncertainty translates to
- a big distance wrt the location of a planet in the system
- a bigger distance wrt the location of the system as it rotates around the galactic core

So, if either of these factors is actually considered, and the vector preserved is wrt to core, then that 32 hour window is a huge problem.

Ergo, they must not be considered.

Am I mistaken? If so, why? If not, why not? Your answer should be 500 words or less and use monosyllables....

 

[The Oz]

Stars close to each other (within Jump-6, in TRAVELLER terms) tend to have small relative motions wrt each other. They are all orbiting the galactic core at the same distance (compared to their distance from the core) and unless one of the stars is not from the local stellar group, they all formed from a gas cloud that gave them roughly the same initial velocity. Looking around on the web I find maximum values for relative stellar velocities of about 30-100 kps (kilometers per second), and often less, when stars are part of the same stellar group. The highest is about 500 kps.

I also get a speed of roughly 30 kps for Earth's orbit around the Sun. Sounds impressive; it's over 100,000 kph (kilometers per hour).

A TRAVELLER starship accelerating at 1G can match that speed in .85 of one hour, or about 51 minutes. Suddenly the relative velocities of the planets doesn't look so bad. Since it takes a 1G ship over 6 hours to get to the 100D limit (assuming a turnover halfway) I always just assumed that part of the time spent in accelerating to the limit was also spent in matching the velocity of the target planet. Even if you allow your ships (as I did) to accelerate continuously to the 100D limit, there's still time to match velocities before you jump.

I told my PCs that normally they could assume that routine jumps would be set up to match the velocity of the target, without any need to roll for it or anything. It was only when special circumstances were in place that we bothered with more detail about this.

 

[kaladorn]

1. I don't think the relative star velocity is the important thing.

2. I don't think the velocity of the planet is the important thing.

I think the *location* is the important thing in both of these cases.

The relative velocity of stars says nothing wrt the coordinates of the stellar system wrt the only static reference point (the galactic core, assuming this isn't undergoing some shift...). So, the fact they only have a limited relative velocity isn't so relevant, so to speak. It is the question "Given 32 hours of time, how far in space has your system travelled, and the planet within it that you are going to?"

That's the real issue. If the system has covered 57.6 Million km in that time (500 kps) and the planet has covered 3.2Million km wrt the system primary, that's gonna make your exit point a darn far away spot.....

 

[The Oz]

Do the math:

In your example, 57.6 million kilometers works out to less than two days travel time at 1G, assuming no initial vector and a turnover halfway.

3.2 million kilometers is much less, of course, only 10 hours at 1G.

Since you're more likely to be only 16 hours off your intended jump exit time, you'll only have half these distances.

So yes, if you have a bad jump and come out way early or late from your intended time, your destination will have moved and you'll have to play catchup, but it'll only be a day or so. And that's for the worst case, where the star has a very high proper speed. Most stars aren't moving anywhere near 500kps so the travel time from a bad jump will be even less.

It does make for interesting interstellar racing, though, because you'd rather come out of jump early than late. Early gives you more time to accelerate and make up for any distance issues. Coming out late costs you double: the extra time in jump plus the time you have to spend catching up with your destination.

Hmmm. From all I've ever read, time in jump is treated as 168 hours plus or minus 16 hours, and Navigator skill didn't affect this. What if it did? What if you set things up so time in jump was 152 hours plus a maximum of 32 hours, and the Navigator's skill could get you closer to that minimum time? That would make Navigation skill highly desireable.

 

[Richard Saunders]

Hello.
Just some idle thoughts.
Maybe jump space has very little to do with real space, when Solomani left earth and jumped to Centari matbe it was a 1J jump, They where flighing an experimental craft so they realy didnt want to try it deep in the grav well so they flew it out past neptune or pluto to try it.
when they came out of jump they where a long way from Centari so they asummed that you needed to be a long way from a grav well for it to work, Yes they would have realised the error after they meet the Vilani but it still mean they got out of Sol system with j1 (not the hand wave we discovered j2 first).

The time difference for each jump could be from Navigation but it also could be from the engineer or both (all engineers tinker) sometimes you get it right sometimes not, head wind or tail wind.

Sol system is currently diving back towards the plane of the galaxy (several million years before we get there) we are travelling faster than the plane in real speed but slower latteraly.

The difference in speed between systems within 18ly is going to be Bugger all, Transit speeds would be 1.5 times the speed required to catch a planet if you come out behind it (16h late) if you come out early you decelerate and wait for the planet to catch you (16h minus whatever you want to cutof).
Bye.

 

[TheDS]

I just thought of another problem, which MIGHT be what kaladorn is trying to tell us:

Let's say I'm orbiting the Sun, just ahead of Earth. That puts my motion with respect to the sun at 30 kps. Let's say I jump, and I want my location to be exactly where I am now, with respect to the sun and the Earth. We'll say I'm 100 diameters ahead of it, just for gits and shiggles.

So, what point in space do I plot as my destination?

First, we need to know where the earth will be in 168 hours, with respect to the sun. Second, we need to know the sun's orbit around the galaxy, and calculate where it will be in 168 hours. Third, we need to know where the galaxy will be in respect to the universe... but I think we will simplify and say all of our calculations can be made with respect to the galaxy.

Ok, so we've got two numbers. We jump. Assuming we arrive at the exact time we calculated, and our jump was exactly where we intended, we are now in the same position we were, with respect to sun and Earth. However, our vector is 1 week old, and the Earth is not going in the same direction it was before; it's course has changed by about 7 degrees. (360 degrees in a circle, 365 days in a year, so 7 days is about 7 degrees.)

That's not the problem. We can make up for that easily. The problem is, what if the jump takes 16 hours too much or too little time? In either case, the sun and earth will not be where we expect them. The error depends on how fast the sun orbits the galaxy. (My rough calculation puts this at 100 kps.) This gives us about a 6 million km error. Not that huge, half a day or so at 1G.

This means that any relative motion between stars could make a big difference; you saw the difference made with NO relative motion.

I like the idea The Oz puts forth, about jumps taking a certain minimum, and Navigation skill getting you closer to this. I use something almost exactly like that in my own game.

For those interested, the drive I use requires you to be at a certain distance from a solar pole. There is an optimum distance to be, depending on the star's mass, and being there, with everything good to go, allows you to travel in the minimum time (just under 800,000 seconds, if you want to know). You can be further from this ideal point by up to a factor of four; this is necessary for some of the larger Giant stars, if you don't want to arrive within the photosphere, that is. Some stars are simply too large even for this, and so there is no way to get to them.

When you jump out, it's best to have no motion relative to the star, otherwise you increase your jump time. When you arrive, your motion is equal to that of the star; however, now you're falling toward the star, so it's a good idea to get moving. (In each calculation I've done, the entry-point is well beyond the 1G threshold.) The difference in relative motions is made up with the stars.

A jump ALWAYS takes the minimum time, subjectively. That means that the objective time might be 8 million seconds, but to the crew, it was 800,000. This allows the ship to always use the same amount of fuel, and encourages ships to try and make the optimal jump, so they don't lose time unnecessarily. (Either way, they have to reset their clocks each jump.)

Jumps of up to 4 ly access the first jump level, Jumps up to 8 ly access the second level, and so on, up to 16 ly (j4). You cannot reach ALL stars within J4 if you have a J4 drive; the stars are connected with "links", giving the name "jump-link drive" to the system. A star that is "in the way" will cut a link between two stars. There are no links between stars further than 16 ly. This GREATLY reduces the number of worlds that border each other, and it also reduces the higher JL drives' utility. If you've got other stars in the way, you can't make full use of your drive.

"In the way" is defined as being within about 60 degrees of the intended path and closer. I have a template I use to tell me exactly what stars can have links, since putting it into words doesn't work well. Basically, the links serve to give a topography to space, allowing choke points to be formed, and giving worlds a chance to be an inportant crossroads.

My star maps have a low density, about 1 in 5 or 6, so that the map isn't too cluttered, and obvious, defensible borders can form. Higher jump-link drives allow new paths across previously uncrossable borders. There's a few more details, but I think I've gone over the important parts.

 

[TheEngineer]

Hi Jumpers,

as a believer and practicer of the "my jump may not be straight" religion I would cope with the time uncertainty problem in some ways:

Apart from plotting a jump route, there it is likely to hit the 100D sphere of a destination world even if the jump take 16 h more or less, the navigator could be able to control and to modify the jump route during the jump. He is of course not able to control the TIME spend in jump, but he is still able to make mircomodifations of the route.
So he basically expects the best exit time, meaning something around 152 h.
If he recognizes, that the jump time seems to be longer (perhaps he could also recognize special jump grid load patterns), he can perform minor corrections of jump vector in order to compensate movement of destination environment during the time lag.
Its like driving a car with a fixed amount of fuel and you can just stop and leave the car then the tank is empty. If you pass the destination then you would have to drive around the blocks for a little while.

Well, its weird theory time, so I will also provide my thoughts regrading the interesting effect that an interupted jump takes as much time as a regular one.
I would suggest, that its the jump transition process, which takes the time. So its something around 78 h for in-transition and 78 h for out-transition. In between jump space is something like "superconductive" regarding time (but still time consuming!).
So, if you hit an intervening object you still have to account in/out transition time but since the in-between jump time is neglegtible the jump takes as long as a jump takes.
This theory does not negate anything of the first one. Its just bad luck for the navigator, who may
perform worthless jump vector corrections here....

Hmm, jumpspace is funny.
This theories are accepted by a vast majority of Traveller players.
At least by those I have talked to lately (two of two)

But here is another question related to the effects of jump masking:
As in one post mentioned a large amount of destination mainworlds are likely to be affected by jump masking caused by the primary star. O.K.
Has anybody an idea how many of these affected are "habitable" worlds, so that they are supposed to be somewhere in the habitable zone of the star ?
And if a mainworld is habitable and is placed in a jump masked orbit, wouldn't it be reasonable to provide trade outposts and bases outside of the masked area in order to serve interplanetary trade, so that normal trade and travel procedures can proceed as the usually do (CT/MT/TNE...) ?


Regards,

Mert
Glisten Jumpspace Institute

 

[robject]

Ye cats, I'm getting confused. I thought the jump exit vector was random with respect to the destination system... when did this change OR was I wrong all along?

 

[TheDS]

You might be getting your terms mixed up a little there. Not surprising, since they are so similar.

Jump shadow is the thing that an object projects into jumpspace with its diameter. In jumpspace, an object looks 100 times bigger than it does in our space. A planet can be permanently inside a star's jump shadow. A moon can be permanently inside a planet's jump shapdow. I often refer to the 100 diameter distance as the "jump barrier", and it's basically the same thing as the jump shadow.

Jump masking is the effect of trying to travel from one side of a shadow to the other side. Say if Earth and Mars were on opposite sides of the sun, but still visible to each other (like say a 165 degree seperation) and I wanted to jump rom one to the other. The sun itself isn't directly between the two, but the sun's shadow IS directly between them. Mars is masked as seen from Earth. When Earth and Mars are close to each other, on the same side of the sun, the sun's shadow is not in the way, and is not masked.

In the first case, to plot our jump to Mars, we need to basically go half way around the sun, or we could jump to a point half way around the sun and travel the rest of the way "on foot". Either way, we can't jump straight to it, because of that big ball of plasma in the middle. Note also that if we're wanting to jump, we have to make sure we're still about an AU away from the sun. It would be faster to just swing around inside the orbit of Mercury.

For Main-sequence stars that are cooler than our sun (about G5 or less), their hab-zone is permanently within their shadow, so a ship always comes out at 100 diameters from the star. For really big stars, this is a major problem, and exists whether you believe in masking or not. Masking plays a part only when you are jumping to a destination on the other side of a large object. For planets, well, that's no big deal since you're leaving it and can go in any direction you want (giving your pursuers an idea of where you're going). But stars are really big, and require a couple extra days (or more) to go around them if your destination happens to be blocked. The odds of it being blocked even a little are about 1 in 3 or 4 (sorry, I forgot), and of having to go all the way around are considerably smaller. Forces you to have detailed astrographic maps, and to be able to plot out where you're going well in advance to avoid shadows if you can't spare the time to go around them.

So, does this help answer your question?

 

[TheEngineer]

Ye cats, I'm getting confused. I thought the jump exit vector was random with respect to the destination system... when did this change OR was I wrong all along?

We finally managed to get another one confused. As TheDS stated, I am already confused too regarding masking and shadowing but perhaps you know what I mean

Dealing with the vectors most rulesets say, that the speed and direction of a vessel just before the beginning of the jumps is kept. So after exit from jumpspace you have got the same speed and direction relativ to hmmm... the universe.
That represents a very classical view of the universe but thats the text.

But as we know nearly nothing about the orientation and rotation etc. of the destination system the vector is indeed a kind of random with respect to this system.

Regards,

Mert

 

[Richard Saunders]

Originally posted by TheEngineer:
</font><blockquote>quote:</font><hr />
Ye cats, I'm getting confused. I thought the jump exit vector was random with respect to the destination system... when did this change OR was I wrong all along?

We finally managed to get another one confused. As TheDS stated, I am already confused too regarding masking and shadowing but perhaps you know what I mean

Dealing with the vectors most rulesets say, that the speed and direction of a vessel just before the beginning of the jumps is kept. So after exit from jumpspace you have got the same speed and direction relativ to hmmm... the universe.
That represents a very classical view of the universe but thats the text.

But as we know nearly nothing about the orientation and rotation etc. of the destination system the vector is indeed a kind of random with respect to this system.

Regards,

Mert </font>[/QUOTE]Hello.
Your vector would not be random, it may just not be towards the planet you want to go to.
If you jump from earth and its in the bit of its orbit where its headed away from the galactic core (at this point think of a flat screen just like your monitor, the top is toward the galactic core the right is spinward, now sun is in the middle of the screen, the earth is moving counterclockwise around it, it's now at 9oclock, (you jump) the destination system is the same setup but the planet is at 3oclock, your ship is moving south the planet is moving north (sensably you would plot to come out sunward of the planet but it may be jump masked).

Jump masking is the 100d distance around every material object in the universe.
Jump shadow is what you cant see from where you are (hold your hand up in front of you the bit of wall you cant see is in shadow, you cant go there but the rest of the wall is not shadowed).

I dont think there would be a system in the imperium or anywhere else for that matter that would not have the major bodies stellar mapped and orbits inserted into a database for jump calculations.
Even if you have to bye the updated jump route data for every system you go to (dont they sell jump route data crystals at starports).
This explanes why you need a bigger computer for a bigger jump number.
Hope this helps.
Bye.

 

[TheEngineer]

Hi,

of course Lionel is right.
The out vector is not random regrading the point that the players ARE able to exactly predict in which direction it will point in the destination system. (as L. stated by using large databases and simulation software, which is used to get exact positions and vectors of any known space objects, like a Mega-version of redshift or Starry Night Backyard..).

I just meant, that we (players,refs) have to hard information about the orientation of any system in the TU (except our solar system). So we could not make a exact specification about the vector orientation related to some system.

Regards,

Mert

 

[robject]

Isn't it likely, then, that our desired vector will always be significantly different than the vector we take to the 100D limit to jump? Thus an astrogator will not normally recommend a running jump at high speed, but rather would propose a small final vector.

In fact, more than half the time she may recommend such a small vector that it marks no significant difference from no vector at all.

In the case of an exciting chase scene, the captain will likely not follow the astrogator's recommendation.

Perhaps Astrogation is a specialized training program, like that of Dune's mentat, where the brain is trained to be an efficient parallel processor of astrometric data for jump calculations, along with that highly organized database mentioned above.

"But what about the map?"

"Relax. *I'm* the map."

 

[TheEngineer]

Hi,

if there is no nasty star nearby preventing you from heading into a special direction you can leave the departure world 100D limit in any direction you want.
So you leave one system in a vector that fits best to the destination system.
You might have to run very fast in order to compensate system velocity differences.
Or you might have to accelerate to the opposite direction to compensate negative differences.

I´m really thinking about writing some kind of jump calc sim software....with some graphics

Regards,

Mert

 

[Hal]

Originally posted by TheEngineer:
And if a mainworld is habitable and is placed in a jump masked orbit, wouldn't it be reasonable to provide trade outposts and bases outside of the masked area in order to serve interplanetary trade, so that normal trade and travel procedures can proceed as the usually do (CT/MT/TNE...) ?

Hi Mert,
I've got in mind that very idea. Buy a space station, transport it to a gas Giant, put it far enough away that it is in the leading trojan point to avoid the constant radiation barrage - and maintain storage facilities for fuel or maintain refinery facilities there as well. Use normal space tankers between the gas giant refinery and ferry your product back and forth. Either that or just have a small refinery at the space station and an onboard refinery in the tankers.
This way, thru traffic can stop at the outer base and then push on through. Minimal time required to reach the station, no travel time required to get out of a Gas Giant jump well, and best of all? A place where crews not actively engaged in the dangerous practice of fuel skimming can rest and sleep away from the Giant.

 

[themink]

All I can say is from MTU so read everything with a pinch of salt.

IMTU, when you jump, you have a grav-potential. You rip your hole in the universe and hurtle in a line (That is straight in Jump space and smoothly curved in Real Space). When that line intersects a Gravi-potential that is the same as the one you started with, you precipitate out of jump space.

The correlation between realspace and Jump space changes slowly over time.

Velocity isn't preserved (the math got too hard for regular jumping - it meant that I used to have to keep 3D vector graphs of all my stars)

The nett result of all the above is that:-

i) You can jump between planets fairly easily because you can "see" tham accurately enough to aim at where they are likely to be in a week (ish)

ii) When jumping between stars, aim at the star itself not one of the planets. It is a much bigger "target" (100D)

iii) A misjump divides into "hits something before getting to the target" which is likely to be either a local planet, a target planet or something heavy betwixt the two. Alternatively, your astrogator/pilot missed the target and you keep going until you hit the next thing with the right Gravi-potential - could be a very long way away.

iv) I also had that the bulk of fuel was used to tear the hole in space but 10% was slowly released whilst in jump space. The rate of fuel release effects your "effective" real space speed, so if you are releasing fuel as for a jump 3 but you miss your target then, after 7 days, when the ship is dragged fromward from your jump 3 distance to the actual egress point, the lack of fuel means that the hull is abraded.

Two nice things about the above: It gives an area to patrol (the 100D radius line) so it is much harder to smuggle/invade etc etc. The other nice thing is that, when you come out of misjump, you are likely to be throwing sparks, leaking air and generally messed up - otherwise people had threated to deliberately misjump.

All the above is just IMTU (when I use jump drives)