It's reasonable to assume interstellar supply chains.
It might, but even then there ought to be limits (and yes, I know that the LBBs do NOT even head fake in the direction of what those limits ought to be).
So if we take the example of
Kinorb/Regina/Spinward Marches ... yes the world has a type A starport and is TL=8.
Does that mean it is reasonable to assume there are unlimited quantities of standard Z-drives manufactured to TL=15 standard just lying around the starport waiting for someone to buy them?
No, that's stupid.
The closest TL=15 world to
Kinorb would be
Rhylanor/Rhylanor/Spinward Marches ... a full 16 parsecs (basically 2 subsectors)/10 jumps by XBoat away.
However, TL=13 industrialized
Efate is only 5 parsecs/2 jumps by XBoat away, so importation of standard drives up to Q (the TL=13 maximum for standard drives) could be reasonably assumed to be deliverable within 1-2 months should the need arise if the specific drive letter desired is not in stock at the time. TL=9-10 drives could reasonably be assumed to be in stock (Express Tenders use all H drives, for example) but not necessarily (Referee rolls dice...).
That's a more nuanced view of how the interstellar trade for such items could be assumed to operate, but it's not something that easily lends itself to explanation by a "simple rule" that requires a paragraph to lay out clearly. Additionally, there are going to be places where the logistics of importing high tech starship components will simply take a REALLY LONG TIME for an order to go out and come back with a delivery (I'm thinking Five Sisters and Vilis subsectors would be prime examples of this). Aramis subsector would be another location where "getting stranded" waiting for an interstellar delivery could take quite a long time if the specific (standard) item you need is not currently in stock.
When it comes to custom ship components built using LBB5.80 ... you're basically STUCK relying on whatever the local tech support base can provide for you. The starports aren't automatically going to have spare parts on hand for your
custom drives (and/or weapons, let's be real) unless the local tech level equals or exceeds the tech level of your ship.
So if you've got, for example, a TL=13 Beam Laser that needs repairing ... if it's a custom mount (LBB5.80 construction rules, hardpoint has no cost, turret to put the weapon into has no cost) that is basically integrated "too tightly" into the ship design ... TL=12- shipyards won't be able to repair that weapon for you without scheduling an import from a TL=13+ world (if they don't already have one in stock, which they probably won't ... Referee rolls dice ...).
Conversely, if you had a standard "off the shelf" weapon mount (LBB2.81 construction rules, hardpoints cost MCr0.1 each, single/dual/triple turrets cost MC0.2/0.5/1 each) for a TL=13 Beam Laser that needs repairing, a TL=12- shipyard would be able to "swap out" your (broken) TL=13 Beam Laser for (say) a TL=12 one (downgrading the factor by -1) until you can get your ship back to a TL=13 world and swap out the laser again. So LBB2.81 turrets and hardpoints "cost more" as standard off the shelf weapon mounts, but they can be exchanged/repaired/replaced in the field much more easily than "cheaper to design" custom weapon mounts can.
This is one of those things that involves giving the Traveller universe more ... local texture ... than simply making bland sweeping universal statements, such as that type A starports can handle maintenance and repairs of ANY starship, regardless of tech level (or polity that constructed it) no matter what because
everything is the same everywhere no matter what (ho hum, if you've seen one you've seen them all!).
Getting the map involved in thinking about interstellar trade and supply chains can make the setting of any Traveller campaign at lot more nuanced and interesting in a hurry. When different places ARE DIFFERENT ... suddenly you have a reason to travel from place to place.
Funny how that works ...
Is it a Rule written down somewhere? No, not really.
Does such a concept make for a more immersive and richer gameplay experience? Why yes ... yes it does ...
Alphabet drives and parts should be in stock for grade alpha starports.
Or at least, the first half of the alphabet.
More like the lower end letters (A-H for TL=9-10) will tend to have the highest demand.
- Scout/Couriers use A drives.
- Free Traders use A drives.
- XBoats use B jump drives.
- Far Traders use A and B drives.
- Fat Traders use C drives.
- Subsidized Liners use C and J drives (J requires TL=11).
- Patrol Cruisers use F and H drives.
- Express Tenders use H drives.
- Mercenary Cruisers use M drives (M requires TL=12).
So there isn't going the an equal distribution of demand for the different drive letters (if using stock LBB2.81 starship designs to baseline demand). But if your ship is using a drive letter that is also used in common for other standard ship designs, the odds of spare parts for your standard letter drive being in stock ought to go up (Referee applies +DM when rolling dice...).