Relative motion of star systems in IMTU, what is means for the Express Boat Service Branch, and the legend (?) of Old Stinky and Mabel

[MontyLovering]

IMTU the different vector and velocity of star systems (relative motion) is a thing and Jumps are normally done after matching the ship's vector and velocity to the destination system to produce the desired vector and velocity on arrival there.

This means:

• Ships outbound to 100D normally do so on the appropriate vector and by doing so give a strong clue of their destination (IMTU you can't guess destination just by observing the Jump).
  • But you can Jump and adjust for relative motion on arrival, it just is more time consuming.
  • It also means if there are three systems within Jump range you can act like you're heading for System A and actually go to System B
• Ships inbound in a system with class C+ Starport will be aware of the standard approach vector and velocity to that world and the arrival box, which is basically a cube of space ships are expected to arrive in. No, jumping on top of another ship is not an appreciable risk because maths.
  • The approach velocity on emergence is no higher than a 2G drive could decelerate from into a geo-orbit to await clearance to land on-world or dock with the high port - this clearance may be given before entering geo-orbit but that is where you head for.
  • Any ships arriving too fast, or any ships arriving in the inner system outside of the Jump Box will get attention.
  • Ships arriving in the outer system operate on SabS (See and be Been); transponders on, standing watch on bridge, do what you like.
  • In-system traffic will follow approach protocols when nearing inhabited worlds which can be broken down to 'if it looks like you can't decelerate to a geo-orbit at 2Gs then you will get attention.
  • This allows safe inbound traffic and makes in-space inspections of ships far simpler
• Imperial ships (i.e. of the services) can operate under their own recognisance.

What it also means is the canon X-boat doesn't work... unless you assume that X-boat tenders work rather differently:

• The X-boat network will have their own Jump Box in system. This is typically near a source of fuel - a point 105D out from a gas giant in the direction of the mainworld of the system is normal:
  • Busy networks will have two tenders and attendant fuel shuttles (well, they have at least three as they have two on duty at any one time). They alternate between retrieval of inbound X-boats and 'pitching' outbound X-boats to match to the required velocity and vector of the X-boat Jump Box in the destination system. This is very simple; X-boat in bay, accelerate to required V&V, open bay doors and stand off 100D+ (yes ships can make ships misjump) and wait until the X-boat Jumps, return to Jump Box.
  • Quieter systems will have a single tender on duty at anytime (with two in each system).
  • Typical duty cycle is three weeks on, one week off, but off-duty X-boat tenders are subject to recall at any time.
  • Off-duty X-boat tenders will not normally go to the system's mainworld, as this would potentially mean they could not return to duty within a reasonable period. If the gas giant has any inhabited satellites they will be used to regular arrivals of Scout off-duty. If there is no possibility of planet fall within a reasonable recall time (24 hours), they will normally 'pick a pretty orbit' and
  • X-boat pilots (colloquially known as 'the great unwashed') have a similar duty cycle, three weeks on, one week off. If they are lucky, they will be picked up by an X-boat tender that is due to go off-duty. X-boat schedulers on busy routes can make sure Scouts they like have this happen far more often than chance, or ensure a Scout they dislike doesn't leaves a ship for the four years of a term.
  • The great unwashed are renowned for an adventurous approach to personal hygiene, mental health issues, being avid hobbyists, carrying at least a exabyte of encrypted data storage crystals and a collection of pot plants (in both senses of the word) with them. When they finally do get planet-side they can be a little excessive.
  • The expression 'what happens in the Express Boat Service Branch, stays in the Express Boat Service Branch' is well deserved. Comparatively low levels of work, and what work there is being repetitive means boredom and ennui are ever-present. Crews get to know each other... very well, often on multiple occasions. The understanding is that if your turn-around times are under four hours no one cares if you have a small herd of goats on an X-boat tender, if the ship has a 'no textiles' policy, or if its corridors get turned into hydroponic bays dedicated to the cultivation of recreational pharmaceuticals from around Charted Space. There is an unspoken understanding that an X-boat tender will be given adequate notice of any inspection and that the ships will look adequately normal when they come in for their annual servicing. There is also an understanding that extends beyond all branches of the Imperial services - if any member of the Scout Service gives you something to smoke, sniff, eat or drink, you make damn certain you know what it is before accepting it, and don't assume just because you did a bit of Terran Green back at university that you will not pull a major whitey if you are not very cautious to begin with.
  • X-boats themselves are taken out of service for maintenance (for 24 hours but it's actually closer to 8 hours of work) every three Jumps - this does not necessarily coincide with a X-boat pilot's duty cycle and would mean an X-boat pilot swaps ships on a regular basis. However, some pilots find a ship they like and synchnronise their duty cycles to the ship's maintenance cycle, finding some mysterious issue that prevents the ship returning to service until the end of their break or volunteering to return to duty early. For the week-long annual maintenance period (actually five days work) serious creativity on the X-boat pilots part, understanding maintenace chiefs, and the fact that there is a surplus of X-boats (every node has at least one kept powered down but fuelled and ready to Jump within one hour) means that some X-boat pilots stay on the same ship for years.
  • The current record is claimed to be held by 'Old Stinky' aka Terry Butcher who allegedly did not leave his X-boat 'Mabel' for 37 years - and as he disappeared in a mis-Jump he may still be on it, some where or some when. There are occasional claimed sightings in this system or that, and claims that X-boat tenders will refuel Mabel, send over supplies, and send her on her way with no questions asked (and no data transfer) and then delete all records of the occurance. Other stories are imply that the crew of the tender are compelled to lend assistance and have their own memories altered along with the ship's, If true then Old Stinky and Mabel have been moving around beyond the Claw spending c. 98% of their time in Jump for over sixty years since the alleged misjump, which would mean Old Stinky is around 120. Many people will say this is exactly the sort of bullshit you'd expect hearing from the Express Boat Service Branch, a bunch of social misfits with permanent ship fever. Others notice that Express Boat Service Branch members agree with this very quickly and enthusiastiucally.

 

[Spinward Flow]

The X-boat network will have their own Jump Box in system.

Primarily for the purposes of deconfliction with other inbound/outbound traffic. By operating "away from" other system traffic (tethered to the mainworld), any craft that approaches an Express Tender without an authorized IFF signal can be default assumed hostile (because they're intruding on ... space space ...?).

Busy networks will have two tenders and attendant fuel shuttles (well, they have at least three as they have two on duty at any one time).

For the Imperial Express Network, my headcannon assumption is that Type-S Scout/Couriers do a LOT of the "runabout shuttle" work on a rotating basis. So if there are 2 Tenders on station (wherever that might be) then there would be 3 Scout/Couriers making fuel shuttle runs (2 active, 1 reserve). The Scout/Couriers have "sufficient" fuel capacity to be able to "top up" a Tender with a few shuttle cycles.

When a Scout/Courier that has been "out on deliveries" to other star systems (off the Express Network) returns to the system (on the Express Network), it takes up a place in the "Tender support services" queue, releasing another Scout/Courier from fuel shuttle/runabout duty so it can be dispatched to run communications to star systems off the Express Network. Sort of a "cab rank" type system that keeps the Scout/Couriers (and their crews) busy with logistics duties in between dispatches to worlds off the Express Network.

They alternate between retrieval of inbound X-boats and 'pitching' outbound X-boats to match to the required velocity and vector of the X-boat Jump Box in the destination system. This is very simple; X-boat in bay, accelerate to required V&V, open bay doors and stand off 100D+ (yes ships can make ships misjump) and wait until the X-boat Jumps, return to Jump Box.

This is where external loading of small craft would actually work out a lot better, I'm thinking.

A 15 ton small craft with a (17%) 6G maneuver drive would require 15*0.17=2.55 tons.
That same displacement of maneuver drive would be capable of (2%)1G @ 2.55/0.02=127.5 tons ≈ 127 tons.
127 - 15 = 112 tons external load capacity
And as we all know, 110% of 100 tons is ... 110 tons.
So a 15 ton small craft with a 6G (LBB5.80) custom drive would be capable of externally docking with and providing 1G maneuver services to Express Boats that have no maneuver capability.

Such a small craft could be armed with a turret and a bridge plus model/2 computer so it can operate as a "mobile turret" that patrols around the Express Tender, offering both interception (vs hostiles) and towing (of Express Boats) services in the vicinity of the Express Tender. Could do something like a 6x fighters per Tender (4x shifts of 1 deployed, rotating 1 on reserve during each shift, 1 undergoing routine maintenance each shift). Increase that to 10x fighters per Tender and you get (4x shifts of 2 deployed, rotating 1 on reserve during each shift, 1 undergoing routine maintenance each shift). The downside to having such a large complement of fighters per Tender is the quantity of crews needed to maintain a 24/7 patrol deployment, which then becomes a pretty significant investment (both construction and operational overhead) for the purposes of mission security.

Your mileage may vary, of course.

For the week-long annual maintenance period (actually five days work)

In CT, annual overhaul maintenance takes 2 weeks.

 

[MontyLovering]

Primarily for the purposes of deconfliction with other inbound/outbound traffic. By operating "away from" other system traffic (tethered to the mainworld), any craft that approaches an Express Tender without an authorized IFF signal can be default assumed hostile (because they're intruding on ... space space ...?).

IMTU a mainworld's Jump Box is 100D out, ahead of the planet's position in the orbit and somewhat closer to the primary. This would allow system entry to be at vector and velocity that would result in a deceleration burn result in insertion into the desired orbit, as the deceleration will also move it into the destination world's orbital path.

The X-boat Jump Box is where the tender is which I assume is in orbit around a gas giant as they need fuel and lots of it.

So seperate Jump Boxes as they are in different places.

For the Imperial Express Network, my headcannon assumption is that Type-S Scout/Couriers do a LOT of the "runabout shuttle" work on a rotating basis. So if there are 2 Tenders on station (wherever that might be) then there would be 3 Scout/Couriers making fuel shuttle runs (2 active, 1 reserve). The Scout/Couriers have "sufficient" fuel capacity to be able to "top up" a Tender with a few shuttle cycles.

When a Scout/Courier that has been "out on deliveries" to other star systems (off the Express Network) returns to the system (on the Express Network), it takes up a place in the "Tender support services" queue, releasing another Scout/Courier from fuel shuttle/runabout duty so it can be dispatched to run communications to star systems off the Express Network. Sort of a "cab rank" type system that keeps the Scout/Couriers (and their crews) busy with logistics duties in between dispatches to worlds off the Express Network.

So imagine a tender 100 and a bit D out of a typical gas giant (130k D).

• If we have a fuel shuttle working to keep the tender supplied, a round trip to skim takes 23 hours at 3G, skimming 60 tons fuel takes 7 hours, transferring 60 tons fuel takes 1.5 hours so 31.5 hours a round trips and five round trips a week (rounding down) providing 300 tons. Cost of fuel shuttle is 16 MCr
  
  
• If we have a Type-S working to keep the tender supplied, a round trip to skim takes 28.5 hours at 2G, skimming 20 tons fuel takes 2 hours, unloading 20 tons fuels take 30 minutes so 31 hours a round trip, so 5 round trips a week (rounding down) providing 100 tons. Cost of Scout is 41 MCr
  
  
• A X-boat tender will need to refuel c. 7 X-boats a week, requiring 280 tons of fuel
  
  
• This would require 1 fuel shuttle costing 16 MCr, or 14 Scouts costing 574 MCr
  
  
• So at least IMTU, tenders have a fuel shuttle, and due to the costs of small craft pitchers and crew you outline, the tenders do the pitching.

 

[Badenov]

So, we ran this calculation, that is, the relative motion of stars, last year in another thread that I can't currently find. Typically, we mathed out, that it takes them a couple thousand years to cross a hex, given typical stellar velocities. Rogue planets, which had been the topic of that thread, had been found moving at about twice the pace of typical stars, though it wasn't stated whether that was a typical pace.

That's not to say stars don't move inside a hex, they do. A hex is just really huge. So accurare mapping of where in a hex the star is would be super important, because if you arrive in the wrong part of the hex, you can run out of fuel before you make it insystem. At thrust 1, from the edge of the hex to the center is about 2 months and 6 days, using the standard accelerate halfway, turn, decel halfway.

As far as X-Boats, good grief, put a min rating thruster on it so it's not stranded in space if something bad happens.

 

[MontyLovering]

At thrust 1, from the edge of the hex to the center is about 2 months and 6 days, using the standard accelerate halfway, turn, decel halfway

Mybe you misrember?

A parsec is 3.086e+16 metres, so from edge to centre is 1.043e+16 metres.

Using s = ut + 1/2at^2, in 5,702,400 seconds (66 days) at 1g a ship would travel 1.625868288e+14 metres, no turnaround.

Using this it says 909.24 days, but that doesn't take relativistic factors into account. Chat GPT says a ship accelerating at 1g for half that time would get to 78.8% of c.

But whether the drives work that far out depend on Referee diktat and version of Traveller. I think MgT2 says 1,000 D of star but don't quote me on that as I ignore that rule.

And diktat or not, a ship at 50% of c is 1.15 x heavier and has time running 15% slower, but 75% it is 1.5 x heavier and time is 44% slower.

But whatever, I got distracted, lol... it doesn't take 2 months and 6 days to travel half a parsec from rest to rest.

Assuming one allows M-Drives to work in deep space, then I think a safe rule of thumb is a ship can accelerate to 50% of c in 5.5 months in which time it travels 0.1 light-years (rounded up) but after that it coasts until it needs to decelerate.

So a ship's travel time would be

Distance in parsecs x 3.26 to get distance in light years
- 0.2 to remove distance covered in acceleration/deceleration
Divide this by 0.5 to get years coasting
Add 5.5 months (0.46 years) to get time in acceleration/deceleration

So 0.5 x 3.26 =1.63 light years
1.63 - 0.2=1.43 light years coasting
1.43/0.5=2.86 years coasting
2.86+0.46=3.32 years total
So 3 years 3 months to travel half a parsec

 

[Badenov]

Mybe you misrember?

A parsec is 3.086e+16 metres, so from edge to centre is 1.043e+16 metres.

Using s = ut + 1/2at^2, in 5,702,400 seconds (66 days) at 1g a ship would travel 1.625868288e+14 metres, no turnaround.

Using this it says 909.24 days, but that doesn't take relativistic factors into account. Chat GPT says a ship accelerating at 1g for half that time would get to 78.8% of c.

But whether the drives work that far out depend on Referee diktat and version of Traveller. I think MgT2 says 1,000 D of star but don't quote me on that as I ignore that rule.

And diktat or not, a ship at 50% of c is 1.15 x heavier and has time running 15% slower, but 75% it is 1.5 x heavier and time is 44% slower.

But whatever, I got distracted, lol... it doesn't take 2 months and 6 days to travel half a parsec from rest to rest.

Assuming one allows M-Drives to work in deep space, then I think a safe rule of thumb is a ship can accelerate to 50% of c in 5.5 months in which time it travels 0.1 light-years (rounded up) but after that it coasts until it needs to decelerate.

So a ship's travel time would be

Distance in parsecs x 3.26 to get distance in light years
- 0.2 to remove distance covered in acceleration/deceleration
Divide this by 0.5 to get years coasting
Add 5.5 months (0.46 years) to get time in acceleration/deceleration

So 0.5 x 3.26 =1.63 light years
1.63 - 0.2=1.43 light years coasting
1.43/0.5=2.86 years coasting
2.86+0.46=3.32 years total
So 3 years 3 months to travel half a parsec

I must have mathed wrong, I simply ran the equation 2* sqrt((distance in meters)/10m/s^2), which is how the rest of the travel times table in MgT1 (p 145) and CT Book 2 (p 10) are calculated at thrust 1, out to 1/2 parsec. Admittedly, it ignores relativistic speeds and effects, so is pretty wrong as it is, presuming we're imagining the other laws of physics are holding true, which they're not because we're already accelerating volumes independent of mass, so I guess we're not really going with physics as we currently understand it. But if we were...

Anyhow, 3 years 3 months is even more past where a ship's onboard supplies and life support run out, which was the original point.

 

[Badenov]

Edit, I read my spreadsheet wrong. D'oh. Instead of 2 months 6 days, it was 2 years 6 months and some days, which winds up being 934 days, which is pretty close to what cyborgprime.com gets.