Question about gas giant piracy, escape windows, and playable intercept rules

[SolarGlitch]

I am unsatisfied with how this seems to work RAW, but doing difficult homebrew or orbital/vector calculations for every simple ship-combat scene also feels insane.

Traveller often teases pirates lurking near gas giants as a classic danger of wilderness refuelling. But how is this supposed to work at the table?

Example: a Jupiter-sized gas giant has a diameter of about 143,000 km, so the 100D jump limit is about 14.3 million km. A 2G ship starting near the gas giant and accelerating outward from rest needs roughly 10 hours, or about 100 six-minute combat rounds, to reach jump distance. That is not a playable pirate encounter.

But maybe that model is wrong. A ship probably does not “land” on the gas giant and slowly climb out. It is more likely doing a fast skimming pass, entering with velocity, dipping into the atmosphere to refuel, and leaving on an outbound trajectory.

So the real question seems to be:

Can the pirates obtain an intercept during the vulnerable skimming/outbound window?

Is there any official or common simple abstraction for this? Something like: “pirates have X combat rounds before the target is too far/fast to catch”?

This matters because of symmetry. Gas giant piracy should be dangerous if the PCs are refuelling, but also plausible if the PCs are the pirates. I want a tense, finite engagement window, not either instant escape or a 100-round combat.

 

[Terry Mixon]

I am unsatisfied with how this seems to work RAW, but doing difficult homebrew or orbital/vector calculations for every simple ship-combat scene also feels insane.

Traveller often teases pirates lurking near gas giants as a classic danger of wilderness refuelling. But how is this supposed to work at the table?

Example: a Jupiter-sized gas giant has a diameter of about 143,000 km, so the 100D jump limit is about 14.3 million km. A 2G ship starting near the gas giant and accelerating outward from rest needs roughly 10 hours, or about 100 six-minute combat rounds, to reach jump distance. That is not a playable pirate encounter.

But maybe that model is wrong. A ship probably does not “land” on the gas giant and slowly climb out. It is more likely doing a fast skimming pass, entering with velocity, dipping into the atmosphere to refuel, and leaving on an outbound trajectory.

So the real question seems to be:

Can the pirates obtain an intercept during the vulnerable skimming/outbound window?

Is there any official or common simple abstraction for this? Something like: “pirates have X combat rounds before the target is too far/fast to catch”?

This matters because of symmetry. Gas giant piracy should be dangerous if the PCs are refuelling, but also plausible if the PCs are the pirates. I want a tense, finite engagement window, not either instant escape or a 100-round combat.

Honestly, the inbound leg seems more likely than while refueling. The target ship has no jump fuel and is screwed no matter what they do. Let them come part way in, race out to catch them (with them running toward the 100D limit, if the pirate is lucky), then the pirate is off to the 100D limit to jump out once the deed is done. Never let them get close enough to refuel.

 

[Condottiere]

Actual wilderness, meaning that the starship can't access a secured source of fuel, usually would be undertaken with the knowledge that they could be ambushed by hostile, currently undetected, forces.

Since I suck at mathematics, I'd say that the captain or assigned navigation officer, will chart out a course that would minimize that vulnerability, ensuring that there is a possibility of escape if intercepted.

That could include trying to throw off pursuit by trying to hide in the gas giant's atmosphere.

 

[Quint]

Welcome to one of the great Eternal Debates of Traveller aka "how the heck can piracy actually work given the game engine/rules?1?"

No answer thusfar has been provided that everyone can agree upon (and a straight RAW of pretty much every edition is that it can't). Personally, IMTU piracy is less "pirates on the Spanish Main" and more "Viking coastal raiding" (also in tune with Space Vikings by H. Beam Piper) because the RAW supports that - Gas giant piracy I'd rules is about bracketing the merchant with mines and threatening to blow them to bits if they don't hand over the goods. In any case it requires getting quite inventive with the RAW to find something that works with a flavor that you like...'

D.

 

[hopsnbaer]

I think piracy works fine if you attack the prey on the inbound leg when they are out of fuel. The outbound leg might be rough getting beyond 10 diameters so you can try jumping with a penalty might be rough, but is completely doable, otherwise you need to aimed shot the J-Drive, or fuel tank before they can jump.

 

[phavoc]

That's a general question about the efficacy of flying high and fast though a very thin atmosphere while skimming. Is that sufficient enough to gather the necessary hydrogen? Or must a ship slow down to just a few hundred kph before it opens up its scoops to collect from hydrogen-dense areas? The hydrogen must be collected, cooled and then shunted to your tanks. Bipping along at 1,000 kph and opening your scoops should blow out your internal piping due to the massive pressure increase. This isn't starship hull, it's piping and its subject to the vagaries of pressure like anything else.

So if you force a ship to limit its speed then it can be intercepted during are after refueling. The OG High Guard took its name from covering for ships doing the refuelling because they were more vulnerable... but is that due to just speed? Is there some sort of interference with sensors due to the makeup of gas giant atmosphere travelling over a hull charged (or charging) with electrical energy that disrupts a ships sensors? Multiple potential issues that could be put together to make the situation delicate for the refueler, thus vulnerable to the proverbial pirate pouncer who was lurking in the atmosphere under cloud cover for it's next victim.

 

[MonsterX]

I'm not sure what the difficulty is supposed to be here. Pirates can be hiding in the gas giant clouds; they'll have a small craft or sensor buoy to spot incoming prey; they just pull up alongside their prey as it is flying through the atmosphere and say "your cargo or your life". Or they are landed on a gas giant moon. Etc. The target ship decelerates to be able to enter the atmosphere - sure, it will still have velocity, but it doesn't want to burn to a crisp, these ships don't have heat shields (unless they do, but mostly they don't), and needs to spend enough time to force enough hydrogen in to fill the fuel tanks. Velocity also goes down from aerobraking, whether intentionally or not. Since it will be fighting gravity on the way out, acceleration is slow making it easier to intercept. GG have big deep gravity wells.

Of course, if the prey has a more powerful M drive than the pirate, or is able to detect the pirate before the pirate can detect it, it is hard for the pirate to intercept, but this all depends on the situation. A clever pirate will have this all gamed out, but so will the trader captain. If you detect the pirate early, the prey can just accelerate instead of decelerate, and stay at a distance from the gas giant, forcing the pirate to either give up or, if their M drive is much more powerful, they can try a lengthy stern chase, which might give the Space Patrol (if there is one) time to show up. In this case the trader doesn't get to refuel and has to find another way to do it.

You can play it out on a battlemap with vectors and gravity effects if you want to check its plausibility, but the short answer is "yes it is". These battlemaps are a bit hard to use since the gas giant is giant (its in the name) and there are probably moons and maybe some rings all at distances where they might plausibly affect the combat (i.e. there are places where combatants can hide - either on the objects or behind them), but you can also abstract the battle terrain and tactical impacts with sensor rolls, pilot rolls, astrogation rolls, ship tactics rolls, just ask the players what want to do tactically, and telling them if it is an option - that is much harder to visualize so I will usually make a virtual battlemap if there is a fight in a GG system because it is tactically so interesting. There is a lot of scrolling in and out to see things though - ships might be moving anywhere from 100s of hexes per round to 1 per round depending on their tactical decisions, and so scaling it is clunky.

It requires commitment, so how I handle it depends on whether the group seems to be Jonesing for a nitty gritty tactical fight which might last two sessions, or whether they are looking forward to something else, and this is just an obstacle on the way to the real fun.

 

[swordtart]

The game system abstracts a lot out, but on a thrust to midpoint turn and thrust to brake cycle, timing the intercept is going to be challenging.

At the mid-point of a 1g burn for 2 hours you are going so fast that a few minutes out either way and you won't be in sensor range. In a purely mathematical model where acceleration is perfectly constant you need to solve a few simultaneous equations, but if that thrust varies by even a fraction then your calculation becomes impossible.

Changing vector is also very difficult, so once you have chosen your route you are pretty much stuck with it after just a few minutes thrust. Anyone who has played asteroids with the ship thrust locked on will recognise how within seconds you are zipping around out of control. It is not like driving a car where you velocity handily drops to zero when you step off the gas. Dogfighting is something that happens with aircraft, not spacecraft. You will be given the opportunity to heave to and be boarded (or just dump some cargo). If you refuse you will be destroyed from as afar away as they can manage and the pirates will sift through the wreckage. If you are really lucky they may disable you before they destroy you. If you fired on them they may well space you as an example to others. If you fail to escape, they may let you go so you can tell the tale of Gentleman Jack the pirate who lets you go with your life if you do not resist. The outcome of a fight with pirates might be pre-determined in the starport bar by reputation rather than in space combat.

When actually skimming your speed is not going to be that high, you are going to have to stay within escape velocity of the gas giant so you can orbit within atmosphere or you will be wasting fuel turning round each time. Matching lower velocity is easier and you are probably not going to leave sensor range (though your signature may be attenuated by the atmosphere).

Pirates hanging out at the gas giant is a thing because it is at least a probably destination rather than trying to locate a ship is in the infinite vastness of inner space. Even the 100D limit is usually a vast area where thousands of ships can be out of sensor contact with one another.

Whilst interception is like finding a needle in a haystack, at least a pirate hanging out at the gas giant knows where the haystack is.

 

[Condottiere]

It's about the only time you'd want a high burn thruster.

And, ironically, it's probably cheaper to just those the cargo overboard.

This doesn't lighten the load, unless it's attached externally.

 

[SolarGlitch]

I agree that I would also like to avoid complicated orbital/vector math at the table.

Catching the target on the inbound leg makes sense to me. But I am still not sure how this should be resolved in a simple, fair way.

If the pirates are just sitting near the gas giant and detecting ships at long range, that feels too easy. I had imagined that they would need to hide: maybe in the gas giant atmosphere, behind a moon, among ring debris, etc. But if they are hiding deep in the gas giant environment, then presumably their own sensors are limited too. By the time they detect the target, the target may already be close, moving quickly, or committed to a trajectory that is hard to intercept.

I also tend to assume that fuel scoops are purpose-built systems, not just “opening a cargo hatch into the atmosphere.” So while skimming should absolutely be dangerous and constrained, I am not sure I would assume that the ship must slow down to almost aircraft speeds just to collect fuel.

What I am really missing is not a full physics model, but a simple table procedure. Something like:

- roll to see whether the pirates are well positioned
- roll to see whether the target detects the ambush early
- determine whether the encounter happens inbound, during skimming, or outbound
- determine roughly how many combat rounds the pirates have before the target can break away

With some nice modifiers depending on how the PCs set them selves up.
That would make gas giant piracy feel dangerous and playable without forcing the referee to invent the intercept geometry every time. My concern is less about justification in fiction and more: How do I resolve it in a way that feels fair and not arbitrary?

 

[MonsterX]

I agree that I would also like to avoid complicated orbital/vector math at the table.

- roll to see whether the pirates are well positioned
- roll to see whether the target detects the ambush early
- determine whether the encounter happens inbound, during skimming, or outbound
- determine roughly how many combat rounds the pirates have before the target can break away

With some nice modifiers depending on how the PCs set them selves up.
That would make gas giant piracy feel dangerous and playable without forcing the referee to invent the intercept geometry every time. My concern is less about justification in fiction and more: How do I resolve it in a way that feels fair and not arbitrary?

You don't need the math to do the vectors; you can get it close enough using the method in Mayday - 1G is one 1250 hex per turn, per turn in a given direction. So your 1G ship moves 1 hex in Direction 1 on turn 1, 2 hexes on turn 2, 3 on turn three (so on turn three it has moved a total of 6 hexes since the combat beginning. Onto turn four, it has the option of moving only 2 hexes in Direction 1, or 3 hexes (coasting) or 4 hexes. Or it could add a 1 hex burn in any other chosen direction. There are a lot of tactical options this gives which are not immediately obvious unless you've played out a few of these scenarios. It also closes out some tactical options you might think you have, if you haven't played with vector movement before.

That said, you don't NEED to get involved in all that. You don't need to know the math to do it, but if you don't have a sense for how the movement workers, it is hard to position your ships like you want. And it can get fiddly too.
The good thing is once they've done it, they start to get a feel for how space ship battles work.
____________________________
Soooo, most of the time I would just abstract it, unless your group is really in to space tactics. If the pirates have time and a free hand (i.e. no Space Patrol nearby) where the encounter happens is up to the pirates, but you can place the moons and rings (just draw the image on paper, to get an idea, put the moon in random places, and draw the players ship coming in). Pirate might be waiting under the clouds on a big moon, or in a depression of an asteroid sized moon, or powered down except passive sensors among the rocks of a ring.

Ask the players if they are using active sensors I'd give multiple sensor checks going in, but the first ones far out have lots of minuses - the farther out they do it the more options to disengage they will have. If it is active, the pirates know they are coming for sure, but the players get a easier time to detect them. Maybe keep it in the dice "tower", but it is nice if they get to roll the dice since the outcomes are important. For the pirates set up , I'd use a ship's tactics roll - use the effect as a sensor DM on the players' ship. You can also give the PCs a tactics roll: if it is high enough, then clue them in as to where the pirate might be hiding - reduce the number of places mentioned as the effect gets higher. Or tell them any other subterfuge the pirate might use. (optionally, both captains can make a Leadership roll to impose comms discipline; a -6 effect or worse means someone used comms and the gig is up, they are detected).

When the pirates engage depends on when the detection happens. If the players see them and start thrusting away instead of slowing down (which they can do - this means thrusting sideway, so they keep their incoming vector, but add one to the side - once they are past the GG they step on the gas. ) The pirate has to come out and chase them down or give up. Opposed astrogation checks: the relative effect modifies how far away the pirate starts the stern chase. If it happens early, it will start at distance or very long range, but reduce it by range bands per failed check.

If the players are coming in on the opposite side of the gas giant from the pirates, then the encounter happens when they are leaving. Their possibilities to get away are less, since they have a long boost out of the gravity well.

 

[hopsnbaer]

That's the point of the encounter roll or just make it arbitrary if the story needs a pickup. No issue with moving the story along.

If tehre is an encounter the pirates are positioned well enough to reach the PC ship. Are they using a lure ship?

 

[swordtart]

Opposed sensor checks should tell you who spots who. You may wish to give the pirates a slight bonus as they are actively looking for a target. Pirates might well also have better sensors and at least rudimentary stealth if they are relying on ambush. Finally they might well deploy virtually undetectable drones in the upper atmosphere to relay sensor information.

With respect to atmospheric interference the pirates also have an opportunity to calibrate since they presumably have been in system for a while. If the players choose to do a deep scan of the gas giant as a precaution then they might need to spend a few hours checking. A pirate encounter should be avoidable but not casually.

The Companion provides more rules for operations in gas giants including the effects on sensors, and difficulty to actually conduct operations.
High Guard provides lots of rule on sensor operations (p74 onwards). If you are risking having PC jumped by a pirate that can destroy their ship and everyone on it, you need to be scrupulously fair and give them every realistic chance. Sensor cat and mouse is not a simple process and is a mini game in itself.

However fast and dirty...
The pirate makes a sensor check when the target jumps in. If it detects the ship, it will be able to tell that the ship is jumping into the 100D of the Gas Giant and can presume that is therefore the destination and can be presumed to begin monitoring. If the target ship is approaching from in-system under M-Drive then initial detection is more complex (HG p76).

If the pirate detects the inbound ship they should be able to choose a sensor penalty for checks into and out of the atmosphere which applies to both ships going forward (this represents diving into the atmosphere to hide). You could simply roll 1D and use that as the modifier.

The pirate may deploy probe drones if it has them and may use other sensor improvement measures to stack the odds in its favour. Any of this will make the encounter much more deadly and you should refer to the relevant sections of HG.

Once the ship either reaches the range the pirate wants to attack or the target spots the pirate, the encounter can begin proper. You can then just run it as a normal ship encounter/combat per the CRB. If you are not using the gas giant operation rules in the Companion then you could just apply the sensor penalty you determined earlier as a penalty to any attack rolls. The ship in atmosphere can reduce these by 1 per turn if it wishes by emerging from the depths.

Assuming the target spots the pirate at the normal sensor ranges (<300,000 km) then it will already be on its final approach to the gas giant and significantly slowed so you can hand wave off the complex physics of ships trying to intercept at a gazillion km per hour.

The story of the encounter will be more important than the physics of it. Be clear in your mind why the pirates are attacking and how committed they are (and how effective you can afford them to be for campaign purposes). Overconfidence on the pirates part can allow a fair fight, but no sensible pirate is going to allow a fair fight if they can possibly avoid it.

 

[Sigtrygg]

Are there any rules in MgT for planet templates and gravity zones like in LBB:2?

It strikes me that if you are operating around a gas giant then you should make a gas giant template with gravity zones around it...

If you make a "hex" 1250km then use the table in LBB:2

large gas giant R 57 hexes, then it has gravity zones extending a very long way...

 

[SolarGlitch]

Thank you for you answers.

@MonsterX thank you for pointing me in the direction of Mayday hex rules, I have to check it out asap.

@swordtart I was not aware of the additional section in the Companion, this helped a lot. Especially the statement:

Fuel skimming takes a number of ‘passes’, during which the ship opens its scoops and forces gas into them by moving fast through the atmosphere [...] with a pass typically requiring 2D minutes

now this changes a lot for me! For simplicity lets assume a 7 min pass in 1000 km depth in my previous Jupyter example. Then this give roughly a speed of about 57 km/s. This changes a lot. For a pirate sitting on the gas giant it would take about an hour to accelerate to this speed.
Assuming a somewhat static pirate, a simple calculation give me, that my ship would only spend 30 minutes within the "very long" range band of the pirate. Excellent! This is very playable! Depends of course on the depth, the 2Dmin role, the pirate trying to match speed, gas giant size, etc. but it feels much more reasonable!

 

[Terry Mixon]

Thank you for you answers.

@MonsterX thank you for pointing me in the direction of Mayday hex rules, I have to check it out asap.

@swordtart I was not aware of the additional section in the Companion, this helped a lot. Especially the statement:



now this changes a lot for me! For simplicity lets assume a 7 min pass in 1000 km depth in my previous Jupyter example. Then this give roughly a speed of about 57 km/s. This changes a lot. For a pirate sitting on the gas giant it would take about an hour to accelerate to this speed.
Assuming a somewhat static pirate, a simple calculation give me, that my ship would only spend 30 minutes within the "very long" range band of the pirate. Excellent! This is very playable! Depends of course on the depth, the 2Dmin role, the pirate trying to match speed, gas giant size, etc. but it feels much more reasonable!

Mongoose really needs a master index of all the products, @MongooseMatt. It’s almost impossible to know what is out there or find it when needed. It would be simple to add new products, and a pain to add old ones, but it only gets harder the longer you wait.

 

[Anstett]

From a thread on this previously:

Okay, Geir’s-Total-Unofficial-But-Workable-and-Consistent-with the-Core-Book rules for gas giant refueling:

Refueling at a Gas Giant: Difficult (10+) Pilot check (Special duration, DEX).

Duration: 2 hours for each 10% of total hull volume refueled. Each positive point of Effect reduces time by 5%. For example, to refill 20 tons on a 100 dton ship, or 40 tons on a 200 ton ship, requires 4 hours, with each positive Effect decreasing time by 12 minutes.

This task can be performed carefully, resulting in an Average (8+) check requiring twice the indicated time, or rushed, resulting in a Very Difficult (12+) check requiring half the indicated time.

If the task is successful, the ship emerges from the gas giant and continues on its way.

If the task is failed, the operation is aborted one hour into the operation, with no fuel collected. The pilot must make a Difficult (10+) Pilot check (1 hour, DEX) with a DM equal to the negative Effect of the initial failed check to avoid damage and to allow a second refueling attempt, if desired. Positive Effect on this check will reduce the time interval by 6 minutes to a minimum time of 30 minutes.

If this task is failed, the ship suffers hull damage equal to 1D times the Effect of the failure, times the tonnage of the ship divided by 100. For example, if a pilot of a free trader (200 tons) fails this second check with Effect -2, then the ship takes 2D x 2 points of hull damage, a range of 4 – 24 Hull points. As a free trader has 80 Hull points, critical hits occur when damage exceeds 8, 16, and 24 Hull points – the referee can roll a Severity 1 critical for each 10% interval or may choose to roll once at a higher Severity (for example a single Severity 2 if 16 Hull points of damage occur). This may result in more Hull damage, more criticals and other bad things.

After a failed refueling, the ship remains at the layer of the refueling attempt. By default, a refueling takes place in the Deep (4) layer. Careful refueling occurs at the Shallow (3) layer. A rushed refueling also takes place at the Deep Layer, but is performed at higher velocity, resulting in more fuel gathered, but at greater risk.

Descriptions of gas giant Layers are as in the Traveller Companion (p.163-164), but layer penalties are not applied to the refueling task, only to other Piloting tasks conducted at that layer. For non-refueling operations or the aftermath of a failed refueling, task DMs as indicated in the Companion (for instance DM-1 in the Shallow and DM-2 in the Deep), but only one (Usually, Average (8+)) Pilot task check at the deepest layer (or per day of gas giant operations at that layer) is required. Failures result in the damage indicated above, based on the Effect of the failure, not the Layer. This damage result balances piloting skill and size of the craft, causing less percentage damage to an almost successful check but more absolute damage to larger vessels.

 

[Condottiere]

 

[phavoc]

Thank you for you answers.

@MonsterX thank you for pointing me in the direction of Mayday hex rules, I have to check it out asap.

@swordtart I was not aware of the additional section in the Companion, this helped a lot. Especially the statement:



now this changes a lot for me! For simplicity lets assume a 7 min pass in 1000 km depth in my previous Jupyter example. Then this give roughly a speed of about 57 km/s. This changes a lot. For a pirate sitting on the gas giant it would take about an hour to accelerate to this speed.
Assuming a somewhat static pirate, a simple calculation give me, that my ship would only spend 30 minutes within the "very long" range band of the pirate. Excellent! This is very playable! Depends of course on the depth, the 2Dmin role, the pirate trying to match speed, gas giant size, etc. but it feels much more reasonable!

There's a challenge here in travelling at this speed. 57km/s is 205,200 km/hr (or 127,500 MPH). That's Mach 167. So while you can do this in a vacuum with no issue, you certainly cannot do that in any kind of atmosphere. In any sort of atmosphere it would be VERY thin, and that kind of goes against the idea of "hiding" in the gaseous clouds as a ship attempts to refuel by hitting hydrogen-rich pockets of gas.

Basically you'd be so far above any sort of cloud layer that it would be impossible for any ship to hide within a gas giants clouds. So they might as well be in space.

 

[swordtart]

Thank you for you answers.

@MonsterX thank you for pointing me in the direction of Mayday hex rules, I have to check it out asap.

@swordtart I was not aware of the additional section in the Companion, this helped a lot. Especially the statement:



now this changes a lot for me! For simplicity lets assume a 7 min pass in 1000 km depth in my previous Jupyter example. Then this give roughly a speed of about 57 km/s. This changes a lot. For a pirate sitting on the gas giant it would take about an hour to accelerate to this speed.
Assuming a somewhat static pirate, a simple calculation give me, that my ship would only spend 30 minutes within the "very long" range band of the pirate. Excellent! This is very playable! Depends of course on the depth, the 2Dmin role, the pirate trying to match speed, gas giant size, etc. but it feels much more reasonable!

Why does a 7 minute pass equate to 57km/s?

It could as easily be 7 minutes travelling at 100 km/h. The 2D minutes per pass is the same at every depth, you just collect more dense material. For example in the extreme shallows you need to make 10 passes to get the same amount of fuel as conducting operations in the deep.

MGT2 rarely gives actual speeds as it generally treats ship movement as relative thrust.