orbital mechanics

While tinkering with the technology levels in Diaspora Anabasis, we had an interesting opportunity disguised as a problem.

Technology level 0 means no space travel. There are six different oracles to explain this (and of course you can invent your own) but only the space travel bit is interesting here.

Technology level 1 is early space travel requiring huge resources if there’s a gravity well. Chemical rockets.

Technology level 2 is commercial space travel, more advanced rockets.

Technology level 3 introduces the slip drive but not much more.

Technology level 4 is even better drives but so what?

Technology level 5 is magical tech. Crazy off the wall unpredictable tech. It’s peak technology just before a civilization disappears.

So the problem with this is that level 4 is not actually differentiated much from 3. So here’s my idea. From tech level 1-3 rockets have efficiencies such that you can only practically travel most places inside a system using orbital mechanics. At tech level 4 you get so efficient that you can just do a Traveller-style constant burn, turn over, constant burn pattern to anywhere you want to go.

I’m pretty sure most of my audience knows what this means but let’s spell it out.

Right now in the real world we are bound by orbital mechanics when we go into space because we don’t have rockets efficient enough to just point at our destination and burn. Instead we steal energy from the orbits of planets and pay for it in time.

you orbiting home
Orbiting your home, A, you are moving really fast along A’s orbit and also pretty fast around A to stay in orbit.

Any body in orbit is travelling at some huge velocity at right angles to the sun at any given time. It’s basically falling on a ballistic trajectory forever, continuously missing the sun. So as soon as you get out of the gravity well and into orbit around your starting point, you’re already travelling super fast with respect to the sun.

you orbiting the sun
Escaping A and now just in orbit around the sun. Relatively cheap on fuel.

If you accelerate a bit in the direction of orbit or away from the direction of orbit, you’ll escape from your planet and be orbiting the sun instead of the world. You’ll be pretty much travelling alongside the world, but you’re now revolving the sun alone instead of the world and the sun. This is a relatively cheap burn.

Now you can spend energy to slow down or speed up. If you speed up, your orbit will descend towards the sun, allowing you to intersect (if you time it well) with an inner planet and be captured by its gravity. If you slow down, your orbit will ascend outwards from the sun, allowing you to intersect with an outer planet and be captured by its gravity.

you slowing down
Speeding up to go visit B in a closer orbit. Your new orbit is elliptical.
you later
So you wait until you get around here. Notice everything else is moving too.
Canvas 5
Then you burn a little here, slowing a little, to make your orbit closer to circular.
Canvas 6
Then you coast a little, burning no fuel, until you get inside B’s capture radius.
Canvas 7
And make another little burn to start orbiting B. Now you can launch your interface vessels!


This is a cheap way to travel but since you are coasting most of the time the following things are true:

  • Your travel times are much longer than if you can burn directly
  • You are at the mercy of the orbits — if you orbit doesn’t intersect your target right away you might have to go around the sun a couple of times. This could get old fast. So could you (though at the usual rate).
  • Most of your time is spent in micro-gravity

This is the sense in which you buy energy for time: you don’t need a whole lot of delta-v (your capacity to change your velocity, mostly measured in how much reaction mass you have to spend) to use orbital mechanics, but you do need to spend some time in transit and you have to plan if you want to minimize that time. Of course what I’m thinking about now is how to mechanize this so that it’s at least roughly realistic but also simple and fun. It’s a bonus for me that this introduces a lot of meaningful downtime — this can then be a phase of play, introducing projects and healing opportunities as a feature of simply travelling from A to B.

Generally your pattern is this: you decide you want to go to B from A. You sit around at A and check your charts and computers and determine when the best time to leave is in order to minimize your total wait time (sitting around before launch + coasting between the planets) and also get a course that’s within your ship’s delta-v limits. Then you wait your WAIT TIME (a downtime period that takes place on planet). Then you launch and do your initial burns. Then you wait your COAST TIME (another downtime period but it takes place on your ship). Then you do your terminal burns and arrive at B.

And now that we have a pattern, we can start to dream up ways to mechanize it and keep it fun. Well I can anyway, in a vague sort of way. Watch this space and look for actual rules to start showing up at our Patreon page.

Now this also implies that using slipknots is pretty expensive and maybe requiring very specific ship designs at T3 — you just don’t get any benefit from orbital velocities when you need to stop at a stationary point in space 2500 light seconds above the star. You are going to have to pretty much cancel your orbital velocity by the time you hit the slipknot and also accelerate there and decelerate before arrival — I think you’ll be taking a kind of rising spiral to the knot (please correct me if I’m wrong) and it’s going to chew up a lot of delta-v. Tech level 3 might be really very interesting indeed as it deviates from tech level 4! Slip capable ships might be defined as having very high delta-v capabilities, sacrificing other capabilities. If you want to blockade a slipknot at T3, your optimum strategy might be to build warships at the slipknot and jump them through where they stay, not having enough delta-v to endanger the system proper but rather just able to station-keep and act as a massive weapons platform. This introduces a whole new set of stories — the old Diaspora stories were all the same when slip is introduced but now we have two distinct narratives around slipknot travel, the orbital mechanics story (T3) and the direct burn story (T4+). I think that’s pretty cool.