Jump astrogation question

[Spartan159]

If a jump can be plotted such that a ship arrives at an "ideal" arrival point to get to the target planet in the shortest time, could a jump arrival point be predicted given an estimate of when the target left the prior system? Astrogation in reverse as it were.

 

[boomslang]

If a jump can be plotted such that a ship arrives at an "ideal" arrival point to get to the target planet in the shortest time, could a jump arrival point be predicted given an estimate of when the target left the prior system? Astrogation in reverse as it were.

In theory, yes; in practice, probably no.

A competent astrogator could look at the relative position and velocity of the origin world to the destination world, and assuming a full-thrust outbound run with no midpoint turnover from the origin world to the Jump point, and likewise a full-retrothrust run inbound to the destination world, for a given starship's M-drive rating, the geometry should be pretty straightforward to reverse-engineer and solve for time and place.

But there are two problems.

First, you have to deal with the whole "variable Jump duration" angle. Planets can move a great distance, especially with respect to each other, in a >32-hour window of uncertainty.

(For this reason, I have always contended that the duration of a Jump must be predictable from the Jump calculations before the Jump begins, or else the margin of error is ponderously large otherwise and a starship can arrive quite far away from its intended destination or perhaps nearly on top of it; canon, of course, ignores this and lets Jump duration be a surprise only to be revealed once in Jumpspace, and perhaps then only with a few moments' notice once precipitation out of Jumpspace becomes immanent.)

So, if the position of the destination world can be reliably anticipated when calculating a Jump, it should be reverse-engineer-able from the arriving starship's position and vector (taking into account not only the two worlds' relative motion, but even things such any relative differences in planetary diameters and therefore specific differences in travel time out from one and in toward the other. But if it cannot be pre-calculated, then no, you probably cannot reliably predict the exit time, place, and vector within a limit-of-military-sensor-range envelope. (Terra in its orbit, for example, shifts its position relative to Sol by ~30km/s.)

This is the easy part, as I see it. The hard part comes next.

Second, given that messages from one system to another cannot travel faster than the speed of Jump, how is a camping fleet of would-be ambushers going to know where to expect the arriving ship to be arriving from?

If you can make a good guess (an Xboat or a Subbie or something like that on a scheduled run), you can -- within the constraints of the first issue above -- lay your ambush (or, more charitably in the case of an Xboat Tender, set up for a quick rendezvous and capture/refueling) fairly reliably.

On the other hand, this might work only so long as an enemy remains unaware of your hunting tactics. Once alerted to your ploy, it is a trivial matter to move to less than "best speed" courses, and start Jumping in all over the place with whatever vector might be needed in the new geometry.

tl;dr: It depends on how fussy and in what way you are about variable Jump durations, and, that constraint still permitting, how benign/hostile you want the "welcome mat" practice to be in the long run.

 

[Spartan159]

Hmm. Never really considered the effect planet velocity would have before. In this particular case I figured the bad guys had a heads up in the form of a scout jumping from where the characters are with news of when they are predicted to lift off. The system the characters is in has a naval base but the next system has no bases, a class B starport and is in the jump shadow of the primary star.

The goal of this mental exercise would be what range would be the encounter be at. Would the jump flash give the pirates enough warning to "surprise" the valiant crew, or would it be a mutual surprise given the variable time factor? This is pretty much a test combat, I'll be happy with a pirates driven off. Good guys have a 600 dTon Protected Merchant, with an escort 400 dTon Fiery Class. I rolled for jump time and the Escort comes out only 3 combat turns later. I was thinking the Pirates have a 800 dTon Merc Cruiser, a 440 dTon Nishemani corsair and 2 Scouts. I have not decided on their armaments yet.

 

[aramis]

Note that, in TNE, an astrogation crit can put you on a coast to orbit jump-exit.

This requires knowing the time of jump and vectors...

Which leads me to think the last few seconds before transition to jump, some fine tuning may be doable by sensing conditions in jumpspace.

 

[Xerxeskingofking]

Hmm. Never really considered the effect planet velocity would have before. In this particular case I figured the bad guys had a heads up in the form of a scout jumping from where the characters are with news of when they are predicted to lift off. The system the characters is in has a naval base but the next system has no bases, a class B starport and is in the jump shadow of the primary star.

The goal of this mental exercise would be what range would be the encounter be at. Would the jump flash give the pirates enough warning to "surprise" the valiant crew, or would it be a mutual surprise given the variable time factor? This is pretty much a test combat, I'll be happy with a pirates driven off. Good guys have a 600 dTon Protected Merchant, with an escort 400 dTon Fiery Class. I rolled for jump time and the Escort comes out only 3 combat turns later. I was thinking the Pirates have a 800 dTon Merc Cruiser, a 440 dTon Nishemani corsair and 2 Scouts. I have not decided on their armaments yet.

few thoughts:


while jump is semi random, the rough exit time can be predicted too within, say, 48 hours. it might be possible for the pirate ships to be kept at a higher than normal readiness for that length of time, which means that while they cant predict the actual jump tome, they can be quicker off the mark when it happens (crews already at or near action stations, in unseal vac suits, etc)


second, assuming the arrivals come out at zero or near zero velocity relative to the target star, they might be able to "lie in wait" near the 100D line and rely on being faster than their prey to build a overhaul vector.

third, they could spread their ships out in a search "chain" which covers a wide area. the players may only be facing one or two ships at the start.

fourth, repairs are expensive, and slow. if you need your players to move off world again in a normal time frame, don't get them too banged up, or they will be stuck for weeks while the holes in the hull are patched up. or, if you need them stuck in system, slip in a "lucky hit" that puts their jump drive on the fritz for 2 weeks until they can fix it.

fifth: Pirates are in it for the money. They don't like to attack defended targets because repairs are just as costly for them. unless the players have something that is literally worth hundreds of megacreds, they are not worth losing a ship over, and the pirates will break off if they start taking serious damage.

 

[Spartan159]

few thoughts:


while jump is semi random, the rough exit time can be predicted too within, say, 48 hours. it might be possible for the pirate ships to be kept at a higher than normal readiness for that length of time, which means that while they cant predict the actual jump tome, they can be quicker off the mark when it happens (crews already at or near action stations, in unseal vac suits, etc)

I'll roll how long they've been on station. Fatigue might be a factor as well.

second, assuming the arrivals come out at zero or near zero velocity relative to the target star, they might be able to "lie in wait" near the 100D line and rely on being faster than their prey to build a overhaul vector.

This is what I had been thinking, one problem for them is the fact that the big ships have the same acceleration as the characters at 3G, the scouts are only 2G and will get left behind

third, they could spread their ships out in a search "chain" which covers a wide area. the players may only be facing one or two ships at the start.

This is likely the case, maybe a triangle with the cruiser as the center point

fourth, repairs are expensive, and slow. if you need your players to move off world again in a normal time frame, don't get them too banged up, or they will be stuck for weeks while the holes in the hull are patched up. or, if you need them stuck in system, slip in a "lucky hit" that puts their jump drive on the fritz for 2 weeks until they can fix it.

No real need to hurry them along although it would be easier if they don't get banged up too much.

fifth: Pirates are in it for the money. They don't like to attack defended targets because repairs are just as costly for them. unless the players have something that is literally worth hundreds of megacreds, they are not worth losing a ship over, and the pirates will break off if they start taking serious damage.

This is a real question, just what in heck are they carrying? This started out as a plot device to move the characters along in a specific direction, give them reason to be stuck on Aramis for a couple of weeks in lieu of an overhaul, and reason to leave for Natoko. The characters were at Celepina. The Premise was the Navy wanted 4x25 dTon modules shipped to Nutema, L'ouel d'Dieu, Aramis and Natoko along with technicians. The Navy chartered the ship but allowed them to use the remaining cargo for speculation along the way. Each stop would require 2 weeks, give or take, for the technicians to set up? train? something? with the modules, leaving one at each stop. Random rolls escalated the situation into this exercise in gunnery. Another is what will make the pirates break off? Do they want to capture the cargo or see it destroyed?

 

[boomslang]

few thoughts:
second, assuming the arrivals come out at zero or near zero velocity relative to the target star, they might be able to "lie in wait" near the 100D line and rely on being faster than their prey to build a overhaul vector.

This is an excellent Special Case. When a world in question is within the Jump Shadow of its primary -- be that a star or a GG -- it does significantly constrain the default Jump entry/exit points for a "best speed" Jump course to and from that world.

But we should bear in mind that stars too have relative motion vectors with respect to each other, and it might take some M-drive use to establish a proper zero-relative motion exit vector: stationary with respect to the destination star might mean Jumping with a long realspace vector relative to the star in the origin system, which might or might not be an easily-reversed calculation for an observer.

In the scenario as described by the OP, there is probably sufficient knowledge available for this to be fairly reliably worked out -- assuming a competent astrogator/navigator is on hand.

 

[boomslang]

The goal of this mental exercise would be what range would be the encounter be at.

This points out one of the key factors in a blockade; the blockade force knows where the incoming vessels will be headed: namely, the blockaded world.

And furthermore, the incoming vessel will be decelerating to a final vector relative to that world of zero.

So the challenge for the blockade is to intercept the incoming vessel while it is still far enough away from the planet that the blockade force can destroy or capture it without having to get too close to the world's defenses.

Since there is a Class B starport at the mainworld in question, we can assume there should be at least an ersatz planetary navy (some sort of Coast Guard, perhaps) and/or some planetary defensive batteries (fairly common) that make it necessary for the pirates to press their attack before the battle gets too close to the mainworld.

And given that the mainworld is within the Jump Shadow of its primary, if the pirates lose, they will have to outrun the planetary navy to make it back to Jump altitude and escape.

Why do the pirates need to stop the vessel on the way in? Why can they not simply let the target land, refuel, and then depart, quietly lying in wait to ambush it as it climbs out of the primary's Jump Shadow? The planetary defenses will be too far away to help, and unless the planetary navy is mobilizing in response to previous corsair activity, the target is unlikely to have an SDB or Fighter escort while departing (and those types of forces would themselves have already been stationed out near the limit of the Jump Shadow as a picket for incoming ships).

 

[AnotherDilbert]

You can always jump, if you have the fuel. Jumping from within the 100d shadow just increases the risk of misjump a little. You do not want to do this every time, but if someone is shooting at you it can be worth it.

So, it difficult to intercept a ship when it's leaving the starport with full tanks.

 

[boomslang]

Jumping from within the 100d shadow just increases the risk of misjump a little.

I suppose that depends on one's definition of "a little" as it were...

 

[mike wightman]

LBB2 - 13+ for a misjump, DM +5 if within 100D

 

[AnotherDilbert]

Oops, that is a little worse than I remembered. Last time I checked it was Mongoose, it's a bit more benign there.

Ok, a commercial ship misjumps on 8+ and is destroyed on 11+. You have a 58% chance not to misjump. Not something you will try if you expect to survive a pirate encounter, but if the local pirates are reputed to space their victims it might be worth it...

 

[Xerxeskingofking]

I'll roll how long they've been on station. Fatigue might be a factor as well.

Or just decide that they are not. by managing shifts right, it would not be unreasonable for a ship to keep a "heightened" alert state for a day or two without serious loss of performance

No real need to hurry them along although it would be easier if they don't get banged up too much.

Well, your the DM. with total control over your own dice rolls. Just fudge the pirates dice rolls to ensure they don't get too banged up.

This is a real question, just what in heck are they carrying? This started out as a plot device to move the characters along in a specific direction, give them reason to be stuck on Aramis for a couple of weeks in lieu of an overhaul, and reason to leave for Natoko. The characters were at Celepina. The Premise was the Navy wanted 4x25 dTon modules shipped to Nutema, L'ouel d'Dieu, Aramis and Natoko along with technicians. The Navy chartered the ship but allowed them to use the remaining cargo for speculation along the way. Each stop would require 2 weeks, give or take, for the technicians to set up? train? something? with the modules, leaving one at each stop. Random rolls escalated the situation into this exercise in gunnery. Another is what will make the pirates break off? Do they want to capture the cargo or see it destroyed?

30 second plot hooks:

a) Thiers nothing important in the containers, and the priates were just trolling for work when the players exit jump. their were just unlucky.

b) Thiers nothing important in the containers, but the priates believe thiers something worth billions of credits on the players ship, and are willing to take serious damage to take it.

c) Thiers nothing important in the containers, but the navy has fed the pirates false intel to draw them out. the navy thinks it has a leak or mole in one of the stops, and is using the players as bait to work out which base the mole is in.

 

[Spartan159]

Or just decide that they are not. by managing shifts right, it would not be unreasonable for a ship to keep a "heightened" alert state for a day or two without serious loss of performance




Well, your the DM. with total control over your own dice rolls. Just fudge the pirates dice rolls to ensure they don't get too banged up.





30 second plot hooks:

a) Thiers nothing important in the containers, and the priates were just trolling for work when the players exit jump. their were just unlucky.

b) Thiers nothing important in the containers, but the priates believe thiers something worth billions of credits on the players ship, and are willing to take serious damage to take it.

c) Thiers nothing important in the containers, but the navy has fed the pirates false intel to draw them out. the navy thinks it has a leak or mole in one of the stops, and is using the players as bait to work out which base the mole is in.

It did not even occur to me that the cargo might be relatively without value. Option c looks good to me, the pirates just happen to be a bit stronger than Naval Intel thought. The mole could even turn into an enemy if he escapes. The characters of course won't know anything about that right off.

 

[Xerxeskingofking]

It did not even occur to me that the cargo might be relatively without value. Option c looks good to me, the pirates just happen to be a bit stronger than Naval Intel thought. The mole could even turn into an enemy if he escapes. The characters of course won't know anything about that right off.

after the battle, have a Nav Int operative contact the players and explain the true situation (this is the same group from the AHL thread, that has a ex IN cap as its leader, isn't it?), then enlist their help in outing the mole. no one is going to raise any eyebrows at the victims of a pirate attack poking around and asking questions, or a former IN captain demanding answers form the local IN branch, so the players could act as a "overt" cover for NavInt to do some "covert" digging, or he may need a "deniable" asset to enact some "viliglante" justice on the mole.

Also, their is no need to explain the pirate attack during or immidately after the attack. it can wait as a back burner or ready to go sideplot that can be brought out if they kill off the main plot and their still time for a bit more fun on a planet. Maybe 2months down the line the Nav Int guy shows up with a man in cuffs and says "this is the man who told those pirates who attacked you over Natoko what you were doing. I need a private place to....question him. you don't have a spare cargo hold I can use?"


my line of throught on the cargo was that it was low priority enough that the IN was willing to subcontract the players to transport it around rather than task a navy ship to take it, so it cant have been that valuable.

 

[whartung]

(For this reason, I have always contended that the duration of a Jump must be predictable from the Jump calculations before the Jump begins, or else the margin of error is ponderously large otherwise and a starship can arrive quite far away from its intended destination or perhaps nearly on top of it; canon, of course, ignores this and lets Jump duration be a surprise only to be revealed once in Jumpspace, and perhaps then only with a few moments' notice once precipitation out of Jumpspace becomes immanent.)

It's not "ponderously large". Can it add several hours to the trip? Yes. But it just adds hours, not days or weeks. Even for ships with 1G drives. Even for ships with 1G drives and limited reaction mass. The duration of the Jump is learned when the ship enters jump, not before. All of this has been canon forever. T5 clarifies jump scatter for coordinated fleets, allowing them to arrive relatively close to each other (within a couple of hours as I recall).

Earth, at 30km/s takes 12+ hours to traverse one edge of its 100D shadow to another. If you "aim" your jump to hit Earth directly, assuming no drift, 12 hours of the 32 hour potential window will have you precipitate (safely) out of Jump at the 100D marker (bearing will vary, but you'll be 100D from the planet). With Jump scatter you can now arrive at most 10 hours ahead of the planet, or, at most, 10 hours behind it. So, with rough math, you will arrive, at worst, 183D from the planet. Again, hardly "ponderous", and nothing even a meek 1G drive can't handle.

Second, given that messages from one system to another cannot travel faster than the speed of Jump, how is a camping fleet of would-be ambushers going to know where to expect the arriving ship to be arriving from?

If it's simple trade ambush, you park chunks of your fleet drifting around the 100D limit, and simply take target of opportunity. From a single point in space, a single fleet can probably control 10+% volume of space surrounding a planet. Depending on traffic volume, this can have a considerable affect. And you only need a couple of ships to be effective against small traders. 8 destroyers can interdict 30%-40% of space I think. And once the word gets out, ships will stop coming, save for blockade runners.

They don't have to hit every ship. Just some. And note, they won't necessarily control 30-40% of the TRAFFIC, just the operational volume. Lots of traffic will mean more will get through, can only shoot at so many ships at once.

 

[boomslang]

So, with rough math, you will arrive, at worst, 183D from the planet. Again, hardly "ponderous", and nothing even a meek 1G drive can't handle.

Regardless of drive and planet sizes, imprecision still adds hours of in-system travel; something like 4 hours in your example. And that is assuming no additional margin of error added by the Astrogator not getting it exactly right.

Pirates can cover a lot of distance and get a lot of shooting in given an extra 4 hours, and this needs to be factored in to the scenario.

If it's simple trade ambush, you park chunks of your fleet drifting around the 100D limit, and simply take target of opportunity. From a single point in space, a single fleet can probably control 10+% volume of space surrounding a planet.

It is worth mentioning at this point that for a typical Size 7 planet outside its primary's Jump Shadow, ambushers will need to spread their fleet over slightly more that 6 trillion-with-a-"t" square kilometers to cover the entire 100D envelope (the equivalent of a square 2.5 million km on a side), and that will require a relatively large amount of materiel, even if they choose to leave large gaps.

Or at least it will call for numerous fast, heavy-hitting ships. It will take a pokey 1G Trader about 6 hours to make planetfall (assuming the above typical Size-7 planet and a Jump exit somewhere reasonably in the neighborhood of the 100D limit); at 6Gs, ambushers might be able to intercept it in a couple of hours and still have time for a couple more hours of combat before getting too close to the mainworld's defenses. Fleet Tactics would seem to be the relevant skill in this situation, I venture.

Note also that the prey, if attacked on the first, accelerating leg of the inbound trip, may have significant momentum toward the general vicinity of the mainworld (presumably on a parabolic or hyperbolic orbital course to avoid the risk of smacking directly into the planet by default), and, either on its own initiative or upon prompting from planetary defense control, might exercise the option of continuing to accelerate toward the mainworld, eventually slinging around behind the planet in a "missed approach" course, now happily protected by the planet's defensive weaponry and any small craft sent up to rendezvous and escort it around the other side of the planet. This would further reduce the window in which the ambushers could press the attack while remaining out of range of mainworld defenses, and again, Fleet Tactics skill looks like it would be relevant.

 

[aramis]

It's not "ponderously large". Can it add several hours to the trip? Yes. But it just adds hours, not days or weeks. Even for ships with 1G drives. Even for ships with 1G drives and limited reaction mass. The duration of the Jump is learned when the ship enters jump, not before. All of this has been canon forever.

Not quite true.

It's certainly learned only after jump-commit, but, at least in TNE, between jump commit and jump entry, the jump can apparently be fine tuned a little bit - The navigator's roll can result in the difference between A jump with a few G-hours and a freefall-to-orbit course (on a crit). And the roll is made after commit and before entry, as it's explicit that once entered, NOTHING can change the course.

 

[whartung]

Regardless of drive and planet sizes, imprecision still adds hours of in-system travel; something like 4 hours in your example. And that is assuming no additional margin of error added by the Astrogator not getting it exactly right.

Pirates can cover a lot of distance and get a lot of shooting in given an extra 4 hours, and this needs to be factored in to the scenario.

That doesn't make the jump scatter any less a reality. What was posited was that the jump scatter was "wrong". It's not wrong, it's a reality that has to be dealt with, like a bad trade wind.

Your premise about windows of vulnerability perchance to piracy shows how terrestrial defenses are not an adequate solution to the problem, that it takes active patrols in order to thwart pirates lingering at the 100D limit.

Note also that the prey, if attacked on the first, accelerating leg of the inbound trip, may have significant momentum toward the general vicinity of the mainworld (presumably on a parabolic or hyperbolic orbital course to avoid the risk of smacking directly into the planet by default), and, either on its own initiative or upon prompting from planetary defense control, might exercise the option of continuing to accelerate toward the mainworld, eventually slinging around behind the planet in a "missed approach" course, now happily protected by the planet's defensive weaponry and any small craft sent up to rendezvous and escort it around the other side of the planet. This would further reduce the window in which the ambushers could press the attack while remaining out of range of mainworld defenses, and again, Fleet Tactics skill looks like it would be relevant.

There's no reason for any spectacular indirect approach. The ship has 100D to adjust it's course for a direct route. Obviously, for very small worlds, this can be a different issue. But for "average" worlds, it's not a problem at all. Pirates have the problem of having to make the ship STOP. For normal planets, there's no reason the approach vector of the incoming ship can not be set up specifically to be leveraged as "free" velocity on arrival. A ship can plot it's arrival so that it's always on the leading edge of the planets orbit, in order to get even more "free" closing velocity. At this point, it becomes very difficult for a pirate to force a ship to stop before they reach the terrestrial defenses.

They can certainly damage the ship, but they can't force the ship to stop. An arriving ship can plot to have an arrival vector that under full burn will guarantee that the ship will not stop until it's within range of the terrestrial defenses. The pirate can ask the ship to stop, the ship can put it's best efforts in to stopping, but in the end, when it does, it's within terrestrial range, thus potentially robbing the pirate of it's prize -- leaving only the role as a spoiler vs an opportunist. In the end making piracy unprofitable.

"Normal" systems don't have piracy problems, the frontier systems might, so standard procedure could be different for the particular destination. Yes, these techniques will prolong the flight, but the countermeasures are there to help ensure a safe flight by deterring the pirates in the first place, so likely well worth the the cost.

Obviously planets without terrestrial defense have different problems, but even then the system navy can provide corridors for entry to lower the volume they ostensibly have to protect.

Finally, due to the ease of damage, the threat of piracy is enough to keep traders away as well. Not wanting to risk MCr in damage for KCr in profit.

 

[boomslang]

There's no reason for any spectacular indirect approach.

Because once the pirates shoot out the M-drive, the planet's gravity will not pull the incoming target vessel moving toward it ever inwards to eventually crash on the surface?

How's that work, exactly?

(What I am saying is all planetary approaches need to be indirect and orbital -- rather than surface -- approaches for safety; in case of drive failure, the need to escape pursuers [sandcasting in particular requires not decelerating when being pursued], or whatever. Otherwise a ship on a direct-to-surface course that does not decelerate enough for some reason will crater itself unwelcomely for all concerned.)