Adzling’s Guide to Chasing Starships

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Adzling’s Guide to Chasing Starships

Postby adzling » Mon Feb 08, 2021 10:00 pm

It can be very challenging to adjudicate a starship chase across multiple systems due to the large number of variables and the rules being spread out across 2 books (Core & Companion).
Collecting all these disparate rules is almost impossible to do in-game and is a core failure of many RPGs including Traveller.
IMHO Traveller would benefit greatly from a collection of “how-tos” for GMs so I built this tutorial on Starship Chases.
“How to Steal a Starship” will be my next guide. If you’ve got any ideas on other how tos please let me know!

1). Transit from Jump to Refueling Point:
This is affected by various factors:
The effect of the Astrogation check can alter where you arrive in relation to the gravity well you’re targeting (Traveller Companion p.142). While the default aim point is 105 Diameters there is a small variance depending upon your Astrogation check. A perfect check will drop you precisely at the 100D limit and a bad check can throw you significantly off course even result in a bad jump.
High-port, down-port and Gas giants all have very different times to reach from the standard 100 diameter entry point due to the varying size of their jump shadows and whether the ship needs to transit from orbit to the planet.

Due to their large mass (and correspondingly large jump shadows) it can take days to reach a Gas Giant from their 100D limit, while it’s typically only a few hours to reach a high-port orbiting a planet (planets are much smaller and hence have a much smaller jump shadow). You would typically add @24 more minutes to reach a down-port from orbit (core p.153).
Core p.153 also has some transit times for various distances, note these times assume full burn at the noted G until the halfway point then constant deceleration until arrival.

This can also be stated with the following formula:
((sqrt ((2 x planet diameter in meters x 50) / (G x 9.8 )))2)/60* = transit time to refueling point.
*If refueling at a down-port don’t forget to add the time from orbit to surface.

2). Refueling:
Houserule: my table ruled that it takes at least 30 minutes after landing/ docking/ arriving in orbit to begin refueling. This represents the time to make arrangements and prepare to refuel.
It takes 1d6 hours to fuel a ship with water, unrefined fuel or refined fuel (core p.147).

Refueling with ice would take a lot longer as you have to melt it first (no notation on how long it takes to melt or what you would use, our table decided it’s at least as long as refining fuel)

Refueling via skimming also takes 1d6 hours (core p.147) but requires that you refine the fuel or suffer -2dm to your Jump check. You can reduce this time as you would with any other skill check.

Unrefined fuel (water or fuel from a gas giant) takes hours or days to refine once it is loaded on the ship. This refining time is governed by the capacity of the craft’s Fuel Processors (example: a typical far trader can process 40/ tons of fuel per day).

3). Transit to jump point:
This can be done faster than arrival as you can accelerate all the way to the jump point without breaking (you cannot use the transit times in the core book for this as they assume constant deceleration from the halfway point).

This can be calculated using the following formula:
(sqrt ( (2 x Planetary Diameter in meters x 100) / (G x 9.8 )))/60* = time to jump point under constant acceleration.
*If refueling at a down-port don’t forget to add the time from surface to orbit.

4). Reducing Transit Times:
This is mostly a factor of your ship’s max G and the size of the gravity well/ 100D limit.

However you can significantly reduce transit times by overloading the M Drive (Core p.160) at the start of your transit. Because there is no friction to reduce speed in space, increasing your M drive output just one G can significantly reduce transit times (math tbd, stay tuned).

Alternately you can jump early, while within the gravity well’s 100D limit. This can get very risky however (DM-4 to J-drive check if within 100D limit, Core p.148 OR DM-8 if within 10D limit, Companion P.140).

Note: you can never jump INTO a gravity well on purpose, you will just precipitate out of jump at the edge of the 100D limit.

5). Jump Prep Times:
While plotting a jump course requires 1d6x10 minutes to complete the Astrogation check this is almost always irrelevant as they are typically performed during the transit out-system to the jump point (core p.148)

In order to initiate a Jump a J-drive check is required (core p.148). This takes 1d6x10 minutes to perform. You can reduce the time to perform this check to minutes or even seconds (Core p.60).

6). Jump Times:
Jumping takes 148+6d6 hours (core p.148)

This can be reduced by comparing the effect of the engineer’s J-drive check when he initiated jump, (Traveller Companion p.142).
A perfect jump will put the ship on target at exactly 160 hours.

Note 1: Velocity on entering and exiting Jump
Although it is not stated anywhere in MG2e that you exit jump at the same velocity you entered, this was the case in earlier Traveller versions.

Retaining velocity after exiting Jump is a huge lore/ setting problem as it would enable any terrorist to use a 100 ton scout ship to wipe out an entire planet with close to zero chance of being stopped (accelerate to relativistic speeds, jump, enter system at 100d limit and hit the planet with only a few minutes warning/ no time to react for any system defenses).

MG2e implies that you exit jump with zero velocity as the transit time tables on Core p.153 assume a starting point of 0 velocity.
So unless someone can point to something within MG2e rules that stipulates conservation of momentum on exiting Jump we are going to assume that all ships exit jump with zero velocity regardless of how fast they were going when they entered jump (the excess energy would just be used to inflate the jump bubble).

Note 2: How to determine where a ship is jumping to
It is possible to determine the destination of a ship you observe initiating a jump by making a Sensor and Astrogation check.
See PoD p.109.


I think that’s it, if you find any errors or have additions/ changes please let me know!
Last edited by adzling on Tue Feb 09, 2021 9:13 pm, edited 1 time in total.
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Re: Adzling’s Guide to Chasing Starships

Postby AnotherDilbert » Tue Feb 09, 2021 10:43 am

adzling wrote: 3). Transit to jump point:
This can be done faster than arrival as you can accelerate all the way to the jump point without breaking (you cannot use the transit times in the core book for this as they assume constant deceleration from the halfway point).
Yes, but note momentum is preserved in jump. You maintain the same velocity after jump as you had before jump. The relative motion of the star systems involved can be added or subtracted from that.

Due to jump masking you may not be able to plot a straight direct course from planet of origin to destination planet (with a jump in the middle). So, you may arrive with an inconvenient velocity vector pointing in the wrong direction, affecting Point 1 above.
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Re: Adzling’s Guide to Chasing Starships

Postby Condottiere » Tue Feb 09, 2021 12:03 pm

1. I'm going to assume you have to point the jump drive in the direction you want to transit, and that when the rabbit hole opens, it's also an indication of the direction.

2. The power measured of the jump flash should indicate how much power was used; divide that by the volume of the starship, and you can guess how far they had intended to jump.

3. Publicly available information would detail where refuelling stations and gas giants were positioned at the destination system.

4. The major variable, assuming pursuer starship performance matched that of the pursuee, would be time spent in transit.

5. You could viably transit after ten diameters, especially if you minimize any other penalties, or add bonuses.

6. One way to throw off pursuit would be an insystem jump, which would register the same amount of energy as a factor one.
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Re: Adzling’s Guide to Chasing Starships

Postby Old School » Tue Feb 09, 2021 12:34 pm

The companion also has more detailed rules for time in jump and also distance from destination when exiting jump. Have used them a couple of times and found they didn’t make a huge difference.

There are also some rules for detecting/guessing the destination of another ship when it jumps out. It’s mentioned in POD, but I don’t remember if they are pulled from somewhere else.

Ships carry their velocity into jumpspace with them. IMTU, ships are expected to arrive at rest relative to their destination world. Emerging from jumpspace moving at several hundred km/second is bad for several reasons. We abstract that by having ships be at rest relative to their departure world prior to jumping. In other words accelerate halfway to the jump point, then decelerate the second half. Ships in a hurry can skip this, but traffic control in civilized space frowns heavily upon commercial traffic emerging at speed. This has the effect of reducing the importance of M rsting on time to get to the jumppoint. 4G acceleration gets you there in half the time that 1G does.
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 3:51 pm

AnotherDilbert wrote:
Tue Feb 09, 2021 10:43 am
adzling wrote: 3). Transit to jump point:
This can be done faster than arrival as you can accelerate all the way to the jump point without breaking (you cannot use the transit times in the core book for this as they assume constant deceleration from the halfway point).
Yes, but note momentum is preserved in jump. You maintain the same velocity after jump as you had before jump. The relative motion of the star systems involved can be added or subtracted from that.

Due to jump masking you may not be able to plot a straight direct course from planet of origin to destination planet (with a jump in the middle). So, you may arrive with an inconvenient velocity vector pointing in the wrong direction, affecting Point 1 above.
Do you have a reference for speed being preserved on jump exit?
I think you're correct but the only reference i found was on the traveller wiki (https://wiki.travellerrpg.com/Jump_Drive) but i can find no reference in the mongoose 2e rules.
This does create a huge in-universe issue however; starships as planet killers.
If speed is preserved on jump exit and you can accelerate infinitely before entering jump then a starship becomes a highly effective, jump-capable, nigh-unstoppable planet killer.
It's almost unstoppable because of the huge speed you could build up and the short 100D distance it would appear near the planet target.

Thoughts?

Regarding your second point there is no rules or mention of this in mg2e anywhere i can find, do you have a citation?
My understanding is that jump shadows are only an issue if you fail your astrogation check/ attempt to jump to a moon or other planet in direct shadow from a nearby gravity well.

thanks!
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 3:56 pm

Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
1. I'm going to assume you have to point the jump drive in the direction you want to transit, and that when the rabbit hole opens, it's also an indication of the direction.
Although i think this makes sense, adds tactical options, and is how we will play at our table in traveller wiki it specifically notes that you can jump from anywhere, pointing anywhere.
Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
2. The power measured of the jump flash should indicate how much power was used; divide that by the volume of the starship, and you can guess how far they had intended to jump.
That is a very cool idea. There is a rule in PoD that allows you to predict the destination if you can sensor scan the target as it jumps. I'll add that reference in.
Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
3. Publicly available information would detail where refuelling stations and gas giants were positioned at the destination system.
definitely
Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
4. The major variable, assuming pursuer starship performance matched that of the pursuee, would be time spent in transit.
agreed, both system transit and jump transit
Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
5. You could viably transit after ten diameters, especially if you minimize any other penalties, or add bonuses.
yes early jump are possible, but risky. that's noted in the post.
Condottiere wrote:
Tue Feb 09, 2021 12:03 pm
6. One way to throw off pursuit would be an insystem jump, which would register the same amount of energy as a factor one.
totally, a very good idea i'll have to note it thanks!
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 4:00 pm

Old School wrote:
Tue Feb 09, 2021 12:34 pm
The companion also has more detailed rules for time in jump and also distance from destination when exiting jump. Have used them a couple of times and found they didn’t make a huge difference.
yup that's in the post already
Old School wrote:
Tue Feb 09, 2021 12:34 pm
There are also some rules for detecting/guessing the destination of another ship when it jumps out. It’s mentioned in POD, but I don’t remember if they are pulled from somewhere else.
yup from PoD i'll add that in
Old School wrote:
Tue Feb 09, 2021 12:34 pm
Ships carry their velocity into jumpspace with them. IMTU, ships are expected to arrive at rest relative to their destination world. Emerging from jumpspace moving at several hundred km/second is bad for several reasons. We abstract that by having ships be at rest relative to their departure world prior to jumping. In other words accelerate halfway to the jump point, then decelerate the second half. Ships in a hurry can skip this, but traffic control in civilized space frowns heavily upon commercial traffic emerging at speed. This has the effect of reducing the importance of M rsting on time to get to the jumppoint. 4G acceleration gets you there in half the time that 1G does.
yes we recognized the same problem you note: if mg2e jump rules follow what the traveller wiki notes (conservation of momentum) then starships are now the best jump capable planet killers due to the massive speed you can get them up to and the short 100d distance before they impact a planet.

any idea on how to control/ mitigate that in-world?

otherwise planet killing becomes ridiculously trivial...

in our own game we have ruled that you can enter at any speed but you always emerge motionless (the energy from your entry velocity is used to help form the jump bubble).
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Re: Adzling’s Guide to Chasing Starships

Postby AnotherDilbert » Tue Feb 09, 2021 5:01 pm

adzling wrote:
AnotherDilbert wrote: Yes, but note momentum is preserved in jump. You maintain the same velocity after jump as you had before jump. The relative motion of the star systems involved can be added or subtracted from that.

Due to jump masking you may not be able to plot a straight direct course from planet of origin to destination planet (with a jump in the middle). So, you may arrive with an inconvenient velocity vector pointing in the wrong direction, affecting Point 1 above.
Do you have a reference for speed being preserved on jump exit?
I'm not sure it's in the rules, but:
JTAS vol3 p12 wrote: The laws of conservation of mass and energy continue to operate on ships that have jumped; when a ship exits jump space it retains the speed and direction that it had when it jumped.
This is an exact copy from CT JTAS #24 p35.

adzling wrote: This does create a huge in-universe issue however; starships as planet killers.
Agreed, this is a well-known and much discussed consequence. Basically the Manoeuvre Drive with constant acceleration for months on end leads there.

adzling wrote: Regarding your second point there is no rules or mention of this in mg2e anywhere i can find, do you have a citation?
See Companion p140. Basically you can't jump through a 100 diameter volume around any gravity source, so you can't jump through a planet or star to appear on the other side.

This means that the star blocks a lot of jump directions. This is often too bothersome to keep track of, so is generally ignored.
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 5:32 pm

adzling wrote: Do you have a reference for speed being preserved on jump exit?
AnotherDilbert wrote:
Tue Feb 09, 2021 5:01 pm
I'm not sure it's in the rules, but:
JTAS vol3 p12 wrote: The laws of conservation of mass and energy continue to operate on ships that have jumped; when a ship exits jump space it retains the speed and direction that it had when it jumped.
This is an exact copy from CT JTAS #24 p35.
yeah its in the traveller wiki as well but not anywhere i can find in any of the mg2e rules
adzling wrote: This does create a huge in-universe issue however; starships as planet killers.
AnotherDilbert wrote:
Tue Feb 09, 2021 5:01 pm
Agreed, this is a well-known and much discussed consequence. Basically the Manoeuvre Drive with constant acceleration for months on end leads there.
yeah so what do you think about that?
in our game we have ruled you always emerge from jump at zero velocity, regardless of whatever speed you entered.
adzling wrote: Regarding your second point there is no rules or mention of this in mg2e anywhere i can find, do you have a citation?
AnotherDilbert wrote:
Tue Feb 09, 2021 5:01 pm
See Companion p140. Basically you can't jump through a 100 diameter volume around any gravity source, so you can't jump through a planet or star to appear on the other side.
This means that the star blocks a lot of jump directions. This is often too bothersome to keep track of, so is generally ignored.
yeah i know that's the theory/ lore but there are no mechanics to determine if a different system has an intervening jump shadow so it gets ignored.
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Re: Adzling’s Guide to Chasing Starships

Postby AnotherDilbert » Tue Feb 09, 2021 6:53 pm

adzling wrote: yeah so what do you think about that?
It has been that way since I started playing in the early 80s.

There are many ways to sterilise worlds. Navies regularly throws around thousands of nukes.

It does not bother me.
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Re: Adzling’s Guide to Chasing Starships

Postby Old School » Tue Feb 09, 2021 7:03 pm

Traveller has always essentially ignored the the "spaceships as kinetic energy planet killers" angle. Its slightly ridiculous, but I suppose every science fiction universe has its holes. Nukes seem silly given the technology to sling mass around.

A couple tons of of mass accelerated for 40 minutes at 10G will hit you with the energy of the Himoshima explosion. For reference, that's a 2m long, 30 cm diameter tungsten carbide rod. Does the warhead matter if that hits your ship? Give it a few hours, and you're up to the energy of a modern United States nuclear warhead fired from a submarine (yes, I know ground strikes vs airburst aren't really comparable levels of destruction). Give it a day and you've got Mt. St. Helens.

Ships coming out of jump space at rest relative to whatever gravity well they emerged at makes more sense. Planet killers can still exist, of course, but they have to accelerate locally.

Of course, I always thought the universe would be much more interesting and playable if jump took a day but the jump shadow was 1,000D instead of 100D, but what do I know.
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 7:06 pm

AnotherDilbert wrote:
Tue Feb 09, 2021 6:53 pm
adzling wrote: yeah so what do you think about that?
It has been that way since I started playing in the early 80s.

There are many ways to sterilise worlds. Navies regularly throws around thousands of nukes.

It does not bother me.
yeah my memory from my first traveller experience in the early 80's is a bit vague...har

but really this narcs me because it means any random scrub with a 100 ton scout ship can wipe out almost any planet with very little chance of failure.

that sort of power imbalance would wreck the entire setting as now planetary populations can be wiped out by anyone at anytime for almost no risk and very little cost.
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 7:09 pm

Old School wrote:
Tue Feb 09, 2021 7:03 pm
Traveller has always essentially ignored the the "spaceships as kinetic energy planet killers" angle. Its slightly ridiculous, but I suppose every science fiction universe has its holes. Nukes seem silly given the technology to sling mass around.

A couple tons of of mass accelerated for 40 minutes at 10G will hit you with the energy of the Himoshima explosion. For reference, that's a 2m long, 30 cm diameter tungsten carbide rod. Does the warhead matter if that hits your ship? Give it a few hours, and you're up to the energy of a modern United States nuclear warhead fired from a submarine (yes, I know ground strikes vs airburst aren't really comparable levels of destruction). Give it a day and you've got Mt. St. Helens.

Ships coming out of jump space at rest relative to whatever gravity well they emerged at makes more sense. Planet killers can still exist, of course, but they have to accelerate locally.

Of course, I always thought the universe would be much more interesting and playable if jump took a day but the jump shadow was 1,000D instead of 100D, but what do I know.
yeah i am going to amend my post to note both the RAW and our houserule to forestall unstoppable terrorists wiping out planets whenever they want
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Re: Adzling’s Guide to Chasing Starships

Postby ruhalla » Tue Feb 09, 2021 9:22 pm

I think ill be using the same ship emerges from jump at no or low velocity..

Btw shouldn't you rename this to Adzlings guide to Planetary Extermination ;)
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Re: Adzling’s Guide to Chasing Starships

Postby AnotherDilbert » Tue Feb 09, 2021 10:31 pm

Doesn't matter much, just jump in a light-month or so out and accelerate locally. You will come in at nearly light-speed not leaving the defenders time to react.

The problem is M-drives, not jump drives.


If you don't like it use TNE-style HEPLAR instead of M-drives?
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 11:28 pm

ruhalla wrote:
Tue Feb 09, 2021 9:22 pm
I think ill be using the same ship emerges from jump at no or low velocity..

Btw shouldn't you rename this to Adzlings guide to Planetary Extermination ;)
haha good one!
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Tue Feb 09, 2021 11:35 pm

AnotherDilbert wrote:
Tue Feb 09, 2021 10:31 pm
Doesn't matter much, just jump in a light-month or so out and accelerate locally. You will come in at nearly light-speed not leaving the defenders time to react.

The problem is M-drives, not jump drives.


If you don't like it use TNE-style HEPLAR instead of M-drives?
yeah that's a good point AD!

EXCEPT i believe it's states that M drives don't work in deep space due to there being no gravity wells to interact with..

However I think that you can get up to some pretty cray-cray speeds within a solar system so that may not be relevant at all..
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Re: Adzling’s Guide to Chasing Starships

Postby AnotherDilbert » Wed Feb 10, 2021 1:53 pm

adzling wrote: EXCEPT i believe it's states that M drives don't work in deep space due to there being no gravity wells to interact with.
Quite, but there are drives that work in lower gravity, e.g. T5 has NAFAL drives that work out to 1/8 ly or so.

adzling wrote: However I think that you can get up to some pretty cray-cray speeds within a solar system so that may not be relevant at all.
A billion km at 2 G will get you to ~6000 km/s. Enough to destroy things, not close enough to lightspeed to deny the defender time to react.
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Re: Adzling’s Guide to Chasing Starships

Postby adzling » Wed Feb 10, 2021 3:56 pm

AnotherDilbert wrote:
Wed Feb 10, 2021 1:53 pm
adzling wrote: EXCEPT i believe it's states that M drives don't work in deep space due to there being no gravity wells to interact with.
Quite, but there are drives that work in lower gravity, e.g. T5 has NAFAL drives that work out to 1/8 ly or so.

adzling wrote: However I think that you can get up to some pretty cray-cray speeds within a solar system so that may not be relevant at all.
A billion km at 2 G will get you to ~6000 km/s. Enough to destroy things, not close enough to lightspeed to deny the defender time to react.
thanks for your thoughts they are very helpful in understanding the totality of the threat

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