Having done the introduction, the next logical step is to look at how the setting differs from today. And the main difference is - quite obviously - space. In itself, we can look at space technology in three different perspectives: Faster-than-light travel, STL travel and interface (or surface-to-orbit) travel. We’ll start with the FTL travel.

One might argue that one of the main setting decisions is which FTL drive to use. We need an FTL drive to get to other star systems in a useful timeframe (the Space in our Space Opera) - and any restriction and attribute of the FTL drive shapes our setting.

Do we even need consistent rules for the FTL drive? Of course we do. First of all, for convenience. It allows me to simply look up questions like “Can the PCs make an escape jump?” on a table. It also allows me to answer setting question - there’s quite a difference between a setting with the Star Wars and the Star Gate FTL mechanism.

Let’s start with discussing the relevant parameters of the drive. For this, I’m going to refer to GURPS Space (page 42ff). The parameters they discuss are

  • drive reliability
  • astrogation difficulty
  • drive speed
  • comparative speed (between different drives)
  • maximum range
  • fuel consumption
  • side effects

A few a-priori thoughts

  • Drive speed probably only matters in terms of time between interesting systems.
  • Maximum range and fuel consumption are linked.
  • In all of these, it is important to avoid both the ability to do an undetectable drive-by shooting as well as the reliance on decisive battles.
  • I want to concentrate on real-space, not on any hyperspace. Therefore, no combat during FTL travel.

Let’s think about some more things.

Drive Speed

Drive speed is one of the attributes shaping our world. It makes a huge difference whether it takes a at least a week to travel between neighbouring systems (c.f. Traveller) or a week to cross most of the galaxy (c.f. Star Wars). In our case, this is a question closely related to the number of systems a nation might span. Generally, we’d like travel times between a week and a month at most for in-nation transport. If we assume the larger nations in that part of space span ten systems, we can set the speed between a week to a month per interesting system (for compact nations) and one to three days per interesting system (for the most elongated nations).

I’m going to (somewhat-arbitrarily) pick five days of travel between interesting systems. This also means we can mostly ignore Sol assuming it’s more than forty or so interesting systems away - half a year of travel time leaves Sol powers with little influence on local things.

Maximum Range

Again, this defines several attributes of the settings. A less open FTL method (as in 2300AD or Traveller) defines certain bottleneck systems and makes war a series of highly-decisive battles between entrenched battle lines. An even more restricted system probably uses jump points. My experience playing Aurora makes me loath to include them: They are far too easily fortified, resulting in a bogged-down war. A completely open method (as in the Honorverse books) makes defending basically impossible, but provides freedom to each individual spacecraft.

In the end, what I’d like to achieve is a mixture: There are supposed to be battles, and no space force should be able to strike every enemy system without being interceptable. On the other hand, daring raids should be possible.

Accordingly, we can try and ensure there’s some way of sneaking past enemy systems but, at the same time, allow interception of resupply transports. This’d force a deep-striking force to guard their own supply lines.

To do so, we can remember that we’re only talking about interesting systems. If we assume there are several uninteresting systems between each interesting system - and you can’t really skip those - you should get the best of both worlds. Spacecraft aren’t restricted to a few routes except by economics and the logistics train of big task forces will be a huge issue to defend but you can skip systems, for example if you want to raid another system.

Hyper Limit

How far in can we go? Can we drop into a low orbit from light-years away (or even into atmosphere) or are we arriving near the edges of a system? The previous section already imposes a constraint on our drive: No emerging in open space. Otherwise, spacecraft still cannot be intercepted. But what’s the closest they can arrive? Clearly, arriving just over a planet makes space combat less likely and manoeuvring unnecessary. On the other hand, arriving at 10AUs out (roughly the orbital distance of Saturn) gives you a large travel time even at constant acceleration - a month at 0.1g or 9 days at 1g. This makes other systems more accessible than your own. If we do not assume reactionless drives (which I’m going to decide later on), those travel times quickly become far longer.

What can we do against that? First of all, we can decrease the emergence distance. If we keep assuming 0.1 or 1g constant acceleration, 5 AU are almost three weeks/6.3 days, while 1 AU is still 9 and 2.8 days respectively. The numbers for 1G look nice, but do require constant acceleration. Constant acceleration at 1g for days isn’t achievable with anything but reactionless drives, and I’m far from decided on including them. Clearly, we need an alternative.

How about including in-system jumps? Those would avoid the whole issue of long in-system travel. We still require a limit for in-system jumps, though. Traveller assumes a 100 * diameter radius for jump drive departure and arrival. For Earth, that would be 1.2 million kilometres; roughly three times the moon’s orbital distance. For Saturn, it’s about twelve million kilometres. Additionally, the sun’s imposing a 1 AU exclusion radius around it. A small-ish asteroid, on the other hand, will have an exclusion zone of about a hundred kilometres. At first thought, the values for both Earth and Saturn sound good, but the Sun’s exclusion zone is too big, and the asteroid’s seems too small.

But, with the existence of in-system jumps, we no longer have to restrict ourselves to in-system jumps. Time to look for a new model. A model which makes sense is to say our emergence distance is defined by gravity. If we, for example, say that we emerge right on the edge of 0.01g gravity, our exclusion zone scales with the square root of the mass.

For Earth, this would be at an altitude of about 58 000km (64 000km orbital distance). For Saturn, it would be 200 000km orbital distance, and about a quarter of an AU for the sun. However, smaller bodies have a small issue: 67P/Churyumov–Gerasimenko (the comet the Rosetta mission visited), wouldn’t stop our FTL drive at all.

Therefore, let’s say it’s 0.01g or double the body’s radius.

Cost of FTL Travel

Our next question is an economic one: How much does it cost to travel between systems?

One of our examples, Traveller, posits a flat charge of 10% of spacecraft mass in fuel per parsec of travel. This greatly influences spacecraft design - a usual military design might carry as much as half of its mass as fuel - and this makes local designs stronger ton for ton. It also implies a question - where exactly does that fuel disappear to?

Other settings (Mass Effect and Star Wars come to mind) assume one FTL jump costs almost nothing. This means a spacecraft could continue operating behind enemy lines until too many things break down.

One inspiration for solving this comes from the Night’s Dawn trilogy. There, spacecraft jumping to another system keep the velocity vector from their old system, and have to spend fuel to correct for that. This means the cost for FTL jumps scales with the velocity difference between the star systems. In the books themselves, this remains in the background, since it requires computation of that velocity difference.

Inspired by that idea, let’s assume that spacecraft have to use their engines to change their orbit after emerging from an FTL jump - but let’s simplify that by assuming they’re arriving at rest with respect to their target body. This means they have to spend dV to establish a stable orbit.

Thinking back to last section, we established that we’ll arrive at - for Earth - 58 000km altitude. This would cost 2.5km/s delta v to establish a circular orbit. However, we want to avoid combat near the edge of the FTL barrier to avoid having our tactics defined by FTL jumps. Therefore, let’s posit a limit of at least 0.1g needed for recharging a jump drive. This means, for Earth, that you have to close in to about 14 000 kilometres of altitude. Circularizing there costs you almost 4.5km/s delta v.

Of course, you don’t want too much time falling towards Earth - either enemies are to be vanquished, or money is to be made. By spending more delta v you can arrive in the recharging orbit faster, and therefore spend less time in total. By spending 5 km/s delta v in total, you’ll arrive after 8 hours of coasting. We’ll call it another 8 hours to recharge the drive, and another 8 hours off - which keeps us with our assumed time of a day per unimportant system. You could also spend more delta v for a shorter transit time - 10 km/s would give you 4 hours each way, meaning only 16 hours stopover time. It doesn’t really scale, though - 50km/s total would get you to your orbit in less than an hour, but your total stopover time would still be 9 hours.

This, by the way, makes earthlike planets much more valuable - you don’t want to recharge at gas giants due to the delta v cost, and you can’t recharge at asteroids (even Ceres lacks the mass).

Other Concerns

Coming back to GURPS Space’s list, there are a few more things to think about, though those should be fairly fast.

  • Drive Reliability I’m strongly leaning towards the “only needs to order a checkup every few years” - it may be fairly expensive, and maintenance expensive, but it won’t be difficult.

  • Astrogation Difficulty Not quite point-and-click, but almost.

  • Drive Speed We’ve already talked about this in the previous sections, but the idea is roughly five days between interesting systems. Let’s call it one day per uninteresting system, most (or all) of that spent in real space.

  • Comparative Speed There’s no advantage in using multiple drives to be faster (since a jump is almost instantaneous anyways). There may be a use for them in allowing multiple consecutive jumps. Hm. Or maybe the FTL jump disrupts every drive on board?

  • Maximum Range About one to three unimportant systems per jump.

Summary

All in all, our FTL drive has the following characteristics: It has a limited range and requires about eight hours to recharge after each jump. Since it arrives in rest (relative to the target), it requires no fuel directly, but needs a transfer to the recharging orbit plus circularization, for at least 5km/s. The range of the drive is sufficiently large that there are no (or few) effective bottlenecks, but since you’re always going to have to recharge in real-space, frontlines in fact do exist. This implies scouting, raiding, and supply lines.

In-system travel is accomplished by intra-system jumps which arrive at a close (but not too close) distance to the target, again allowing for real-space combat and interception - and, crucially, makes high-fractional c kinetic attacks essentially impossible.