To Vesper Skies VII: Propulsion

Distance is a bit thing in space. Everything is so far away from each other. So when making a sci fi game, you have to ask yourself how fast all of the spaceships can travel outside of combat. To answer that, you first have to figure out what kind of scale you want for your game.

From skymaps.com

Distances

First things first, we have to discuss different measurements used for space movement. There are three measurements used. Astronomical Units (AU), Light Year (ly), and Parsec (pc). We've heard these before but some probably don't know the definition of the terms.

AU is the smallest measurement used in space and is very useful for solar system travelling. 1AU is the distance from the Sun to the Earth, about 93 million miles (or 150 million km). A ly is the distance light can travel in one year. This turns out to be about 186,000 miles per sec (300,000 km/s) or roughly 5,880,000,000,000 miles (9,460,000,000,000 km or 63,240 AU) in a year. That's a lot of distance and is great for going between stars in the same sector. Of course, sometimes even that's too small of a scale. So we get into parsecs, which are 3.26 ly in distance. So each unit of measurement here will have its usage when figuring out the scale of your game.

Propulsion Technology

In science fiction, you can categorize movement into subluminal and superluminal movement (slower and faster than light, respectively). Subluminal can cover anything from basic rockets to solar sails and any kind of drive that simply can't reach light speeds. There are all kinds of examples of superluminal drives, but they can be generalized into three categories.

• Faster Than Light Drive: Simply put, FTL drive gets you from point A to point B at faster than the speed of light. You see this kind of warp drive in Star Trek, where Warp # is essentially used like the Mach number system, but for FTL speeds.
• Hyperspace Drive: Seen in Star Wars, hyperspace is a higher dimension that can get you to where you need faster. By imputing coordinates and doing the calculations, hyperdrives can cut down the distance between two points and open up more of the galaxy.
• Wormholes: This technically isn't superluminal, as you aren't really moving. Instead, you open a hole in space-time that folds two points together. This bridges the points and allows for instantaneous travel. Wormholes can be stuck to just gates, or more advanced ships can simply do it.

If you want to limit these methods of propulsion so your players aren't zipping to the other side of the galaxy, there are several tried and true methods. Wormholes can be limited to gates and treated like a turnpike system. A common trope for hyperspace and FTL are that gravity wells interfere with the drive and make it difficult to leave. "Tachyon inhibitors" or any other techno-babble thing could prevent warping out. I'm generally okay with players zipping around a little bit, so I'm fairly cool with FTL in star systems until they approach a planet.

Solar System Focus

The scale of your sci fi game will decide on how fast your ships can go. If it's going to be centered on a single solar system, then chances are most ships will be going at subluminal speeds. Movement will be based on AUs or even miles/km for early technology. If you want your game to focus on solar system exploration or a race just getting into space, miles and km or fractional AUs will be useful. Original Gundam is a good example of this, where only the moon is really colonized. If you want the inner system explored but have a focus on exploration of the outer planets and Oort Cloud, then AUs will be more useful. Remember that Pluto is on average 39 AUs from the Sun. 

Your propulsion tech will also be much more early. Chances are that you'll probably want to keep it at subluminal drives. This makes exploration to the outer system and back a much bigger deal. Depending on how much of the system is colonized will depend on how fast your ships can go. Something like Gundam or Total Recall, for example, only focuses on Earth, the Moon, Mars, and orbiting stations. On the other end of the spectrum, Eclipse Phase and Cowboy Bebop has a large focus on the outer ring as well as the inner system.

You can actually have wormholes if you want if they are stuck to gates. These wormholes act as kind of a Panama Canal in space. It'll take you to between two areas only, so that makes them prized and limited, but still opening up an entire solar system to your players.

Star Focus

If the focus is going to be on travelling between the stars, then you probably want to go into the superluminal propulsion. Everything is measured in lys and if you want to get to the stars in your lifetime, you'll probably want ships that can go lightspeed. Like before, this is dependent on the story you are looking to tell. If you want to emphasize the journey and exploration, then maybe lean on the slower end of things. This is especially true if the focus is on a singular, alien planet like the movie Avatar, or the video games Alpha Centauri, Pandora: First Contact, and Beyond Earth (see the previous section about focus on a singular system).

If you'd rather the focus be on the destination of exploration and seeing more in a star cluster, then lean more towards the faster side of travel. Wormhole gates would fit pretty well with this, as would hyperspace for covering the large distances involved. FTL would have to be fairly high if you want to get to the nearest star in a game session within the lifespan of the PCs.

Galaxy and Beyond Focus

If you want your players in a galaxy spanning adventure, zooming around the Milky Way or beyond, then you pretty much want the fastest propulsion available. Unlimited wormholes to get from one galactic arm to the other if the focus is more on the destination. Super FTL  or Hyperspace drives if you want the focus on the journey (I'm talking severaly ly/pcs per hour). Parsecs are going to be your main unit of measurement going between star clusters and galactic clusters. This would be interesting if your players end up in another galaxy or even an completely different galactic cluster.

Remember, space is big, but it's only as big as you want it to be. Whether the focus is on a single colony or a massive galactic empire, choose the propulsion that will fit your story.