Note: The section below was written for NaNoWriMo 2018, giving some background on the jump drive technology being used in-universe. I started with the effects I wanted (jump points, jump drive used as radiator, purpose for having smaller ships, efficiency breakdowns at larger sizes) and then worked backwards from there. Just like I learned at Atomic Rockets!

Jump Rails

The jump rail has to be cold to initiate a jump. Smaller ships have correspondingly smaller rails, which cannot accept as much heat, and therefore can make only shorter jumps. The rail size has more to do with volume than mass; jumping a blimp, for example, would require a larger rail than a teaspoon of lead. There is an upper limit, based on current technology. A rail over a certain size creates so much background heat as to make it useless for a jump, while adding two rails to a ship cannot correctly synchronize and generally produces a ship whose halves jump separately.

Enterprising technicians discovered the rail could be used as a giant heat sump when not being used for transition to jump space, or in jump space. This gives larger ships an advantage in combat, allowing them to have a larger throw weight of beam weapons, coilguns or rail-launched missiles. However, if used as a heat sump, the ship cannot perform a jump until that heat has dissipated. This also means that ships who have just emerged from jump are at a disadvantage, as the rail has yet to cool from the trip to jump space. Ways of rapidly cooling a jump rail are expensive, and thus used only by military vessels, well-off mercs, and specialty courier ships. A handful of merchant ships have used these “cool shots” to gain comparative advantage against competitors, but unless the route is particularly lucrative, or the route can only be assigned to one company, this is an extravagant waste of money.

The smaller the jump rail, the shorter the jump taken, unless the commander and crew want to risk jump failure. Generally speaking, it’s controlled by the length of the rail, which is set by the volume of the ship. A 2 meter rail, the shortest possible under current technological conditions, allows for a 6 light year jump. The jump will take 12 hours, a ratio of 2 light years: 1 hour. A 64 meter rail, the largest ever constructed (currently undergoing trials in the Remus system) only allows for a jump of 36 light years, and a 1:3 ratio for light-year/hour. Both would cool down for another jump in 72 hours, excepting for combat or using an active coolant system. The larger rail still holds an advantage if one needed to go 36 light years, but breaks down near the 24 light year mark.
Custom dictates ships enter at a point perpendicular to the plane of the system, and pass through zenith to nadir or vice versa, allowing their rails to cool. An extremely talented astrogator, or a thrill seeker, might jump into the plane, having calculated the position of whatever planet or station pre-jump. The issue here is that the emerging ship appears in a given volume of space but where inside that volume is not up to the astrogator or pilot, strictly speaking.

Courier ships use both active cooling and flushing the rail compartment with coolant. This is a hazardous material that, after cooling the rail, can be stored, cooled and reused. An average courier could jump, cool and jump again in a matter of hours. The record is held by Zig “Stardust” Jant, having jump and rejumped in 38.3 minutes. However, as her ship, the “Odessa” was a victim of jump failure on the second jump, some sources dispute the record. The record without jump failure is held by the courier Malediction, at 44.8 minutes and a total of 24 light years in 48.8 hours.