So, what would be different if that 4 weeks across the subsector took, say, 7 days? Spinward to Core in 2 months (maybe even 1 month)?
I forgot to say another important change: microjumps would be more usual. If jump takes only 24 hours, then in system movement (that often takes quite more) will be better achieved with microjumps, as the cost of the JD and the 10% tonnage needed for fuel can be easily offset if the trip takes less time.
You miss a key point. The Zho historically begin these wars, which means they choose when and with what resources. With the resources of 9 sectors available at the outset, they can overwhelm the Marches in short order. There are no minor delays when you are outnumbered nine to one at the outset: Jewell falls in the first week, Efate a week or two after that. With what were once their jump-4 units, they could leap from Cronor and be attacking Regina on the first week of war, or establish a forward base at Tremous Dex the first week and, refueling with tankers, be at Rhylanor the second week.
And don't forget that the Zho supply line has the same quadrupled range. They can leap from protected ports to supply forces over a 12-parsec range, which means they can operate from fewer and therefore better protected positions.
I still fully disagree with your analysis. All those points are the same regardless the jump takes 24 hours or 168 hours. The Zhodani have been preparing for war from long before it began, and so forces are already marshaled from the whole consulate, and the relation of jumps needed for the Zhodani to advance and for the Imperials to ask for reinforcements and receive them is unchanged. What really changes is the percentage of time this represents, and the shorter time in jump, the most important this is:
For strategical pourposes, I'll divide the jumps into several categories, according the time needed between jumps (numbers are quite simplified):
- Non refuelling jump: the ship arrives at system and jumps again without refuelling. This would apply for the pony-express-like courriers (though in this case what jumps again is the information, not the ship proper), ships with enough fuel for 2 consecutive jumps and ships using drop tanks, saving integral fuel for the second jump. Let's say 2 hours between jumps (infromation transmiting, calculating vectors, conducting checks, etc).
- Planned refueling jump: the ship exits jumpspace and is met by refuelling ships, that refuel it so that it can jump again ASAP. This includes refuelling at bases that expect them or tanker support. Let's say 4 hours between jumps
(docking refuelling ships, transfering fuel, etc)
- Unplanned refuelling jump: mostly are ships that arrive at a starport capable to refuel them but not expecting them, so needing to ready their ressources to do so (or sips docking at refuelling points in the highport). Let's say 12 hours between jumps (Reading refuelling ships, inbound/outbound trips, etc).
- Frontier refuelling: ships have no refuelling facilities usable, having to conduct frontier refuelling. As specified before, about 48 hours between jumps.
See that this difference among 2 hours or 48 has little importance if the jump takes 168 hours, as it represents at most just over 25% of time so lost, but when the jump takes only 24 hours, this means nearly contiuous jump or tripling the time needed.
Now let's see how does that affect the Zhodani invasión:
As the Imperials detect the Zhodani, courriers are sent to Core. Those ships, using class 1 jump above, manage to make about 6.5 jumps a week along preplanned routes. At each system they stop, news spread and the preplaned paths are reading so that the incoming reinforcements can use class 2 jumps in their way frontlines bound. Due to this preplanning and warning, reinforcements transiting to front lines can use class 2 jumps (perhaps an occasional class 3 due to unpredicted problems), achieving about 6 jumps per week. As they are moving through friendly space, they don't need to care about anything else than moving forward.
Meanwhile, the Zhodani make their initial jumps using drop tanks, so that they can use class 1 jumps in their first stop, and are supported by tankers so that they can do class 2 jump in their seccond. After that, with the tankers empty and in hostile territory, they will have to ressort to class 4 jump, stoping to just over 2 jumps per week, assuming they are just moving forward (something unlikely if any defenses have to be overcome).
As you can see, the speed raising does not affect everyone equally...
With this much force available to the Zho at the beginning, there is simply no stopping them from taking the Marches before news reaches Capital - and from being in Deneb before the Imperial forces arrive to start shoving them back - unless the Imperials drastically reinforce before the war ever starts, or unless the Impies respond the first time the Zho's try it by marshalling everything and crushing them to prevent future wars. The sole Imp advantage is, being a larger empire, they can marshal a much larger force to respond with - but that response is likely to occur in Deneb sector.
No need to stop them, just delaying them, and most of it is already done due to refuelling needs (as said above).
This aside, you assume Jewell and Efate fall in 2 weeks, This is quite unlikely unless they ressort to WMD, and no such attrocity is ever seen in Frontier Wars. Taking planets would be a major delay (AFAIK, no such quick taking of a high pop world has occured in the whole Traveller canon history, except the coup at Menorb explained in the 4518
th history, but that was not invasion, but a coup), and just besieging the world while leaving the rest of the fleet advance will seriously weaken your advancing forces.
And it's likely that most attacked systems' SDBs, seeing the huge fleet attacking them, just disperse and hide instead of accepting battle (as the ones in Louzy did in FFW), forcing the Zhodany to either stop chasing them (long delay, more critical if jumps are just 24 hours than with 168 hour jumps), dividing their forces or put their supply lines in jeopardy.
If you multiply speed by a factor of four, you multiply EVERYTHING by a factor of four, and in some cases four squared, because the Zho are drawing forces from a broader area to make a much more powerful initial thrust to quickly reach places that under the old model were considered rear areas. In essence, the entirety of the Marches is cast in the previous role of Jewell subsector as a forward combat front, while the previous role of Rhylanor as a rear base is taken on by bases in the trailing region of Deneb.
No, you don't multiply everything by a factor of four (or seven), just the time in jumpspace, so the time in normal space takes more importance, and in the OP there's no hint that this is shortened too.
It's like the difference between battling with Napoleonic infantry and battling with WW-II armor: they'll assemble more quickly, move faster and strike deeper than what you're accustomed to.
Or more like infantry and battling with Twilight 2000 tanks. They assemble more quickly, move faster while their fuel lasts... and they stop for several days to brew more alcohol to refuel before another move, while enemy forces, with fuel depots ready on their way, are assembling ahead to stop you.
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