20 February 2020

Interplanetary or Intersphere Travel

 TL;DR- I faff about on the weird way to me the range bands in Engines of Babylon map to our star system and other shit.

As y'all should know, I've been working on Spelljammeresque material for Troika! Which for the most part is me taking a Troika spin on old TSR material. Everything I've written so far for Aetherjack's Almanac can be slotted directly into Spelljammer. But I, and I'm not the only one, thinks it's a bit sucky that Ship Rating, the speed of the ship, doesn't affect the long distance travel in SJ of 100 million miles per day.



So I turned to another osr game about space, Stars Without Number, specifically Engines of Babylon which has a section on system ships, in other words ships without spike drives/FTL. In the core rules, spike drive enabled ships use a kind of Speed of Plot interplanetary or in system speed. A system is divided into regions which are primarily based on how interesting it is, and the time is 48 hours divided by the Drive rating to travel between regions. Very speed of plot, but also directly acknowledges that it's a game for fun.

Crawford uses a similar speed of plot for system ships just with more details because interplanetary travel is now the focus rather than the interstellar travel of spike drive enabled ships. Instead of regions, a system is broken up by Range Bands detailing the distance from the primary star in a dimensionless unit of distance. Most habitable planets are within the first 10-12 bands, with most gas giants and other outer planetary bodies at 60-70 and the edge of the system being 100 for the average system. Travel time is difference in range bands times 2 divide by the sum of speed and one in days. I've probably written somewhere that for this instance I'd just use SR as written for the SPD+1 because SR can't be zero unlike SPD.

So why all this? Why not simply use the SWN rules and be done with it?

Well I did. I have notes written out in one of my SJ books on which range bands the planets in the Sol System would be in, but I didn't like it. It just felt off until I started drafting up my rules using real units, miles and now astronomical units, and noticed that the 10-12 range bands for habitable planets roughly corresponds with the Earth, approx 90 million miles or 1 au, and Mars, approx 140 million miles or 1.5 au. I wrote out how many range bands etc the rest of the planets in our system are, started calculating the travel times to see what felt good and made sense for a Troika style game, the TLDR of this is that I worked around to a straight 10 million miles a day divided by SR which is similar to the RB x2 divided by spd+1 and is also one tenth the speed of Spelljammer. That last bit is the problem. It would take about a year to get to Neptune and twice that to leave the crystal sphere, if you consider Neptune to be the last planet as Pluto is currently classed as dwarf planet I chose to disregard it. That feels good only if a game existed within a star system kind of set up, and barely at that bearing in mind that as it currently written most golden barges would travel at SR 2. When we get to taking 6 months to a year to exit a star system and travel to another it really feels bad.

But on the same note, the 100 million miles a day only feels, to me, to be reasonable as far as fantasy space travel, for the outer planets, in other words I'd rather it took more than 1-3 days to travel from Earth to Mars, but also not take a year to get to Neptune. I did some napkin math and thought about how the 10-12 and 60-70 range bands sort of lined up with what I was seeing and feeling. Crawford might have started with a one au equals 10 range bands, which would make sense for the inner habitable planets as that would put Earth at 10 and Mars at 15, and Mars is questionably habitable. But if he did, then he'd have noticed that past Jupiter, 5.2 au 52 range bands, the rest of our system, which makes sense to start with, just gets ridiculously far away such that it makes travel between the closer planets unreasonably fast or between the further planets unreasonably slow.

Side note: I did think about using some kind of logarithmic scale to dump real world data in and get range bands but MATHS.

So this is currently what I'm working with.

A star system or greater sphere is divided into two regions.

Travel in the inner region is 10 million miles per day divided by SR, and is from the primary to halfway (I must have been so tired to have 1.5 au be half of 30 au) 5-10% of the distance between it and the furthest planet or common sphere. In the Sol System that would be from the Sun to Mars as Mars is interestingly roughly halfway between the Sun and Neptune, the furthest planet thus far (Pluto can be a dwarf planet).
Travel in the outer region is 100 million miles per day divided by SR, and is from the halfway point to the edge of the greater sphere. So from Mars outwards until the Membrane or Crystal Shell or whatever is reached.
The edge of a system or greater sphere is twice the distance from the primary as the furthest planet or common sphere. This should sound familiar as it's how large a crystal sphere is.
And taking this I could turn the base days travel into range bands. Which are as follows:
  • Mercury 4
  • Venus 7
  • Earth 10
  • Mars 15
  • Ceres & asteroid belt 16
  • Jupiter 19
  • Saturn 23
  • Uranus 33
  • Neptune 45
  • Pluto 53
  • System edge 74
It does push the gas and ice giants closer to the primary than Engines of Babylon suggests, but it's just a game right.


Additional Notes: The heliopause is one of the 3 general points that can be considered the edge of our star syystem, and that is roughly 100-120 au. Crawford puts the edge of a system at 100 range bands. I was doing some regressions, badly because I can't remember how to log, and a couple of regressions that I got using 0=0, 1 au = 10 range bands, 100 au = 100 range bands, and there was some interesting correlation to my list and points on the curve; also, the first thing I got was a quadratic regression in which the curve looked good but formula plotted completely different from what the website showed.




Side note: I know I could totally ask Kevin Crawford on the whys he did in system travel the way he did.

Fun fact: I inadvertently wrote everything such that the Locksley Mk 9 Fighter-Bomber/Recon Plane can fly from the Earth to Mars or Venus, Mars to Jupiter, or Jupiter to Saturn on a single tank of fuel. Which is kind of funny and kind of fitting as it has a variant used as a demilitarized passenger craft.