Sublight Travel

This one has bothered me for a while in my game setting and some others as well so I'll write up my thoughts here and try to sort it out.

So, let us assume that there is a way of working FTL which works with speeds of about 0,3-0,5 parseks an hour. It's drawback is that it doesn't work well in a gravity well. Previously I had specified that the limit a ship could approach an object in space was about 100 times the diameter of the object multiplied with it's surface gravity (a cheap ripoff from Traveller, I know). With average settled planets in this setting this would be about a million km.

Now then, concerning gravity wells, the largest in a solar system would naturally be the star(s). With our dear Sol this would be about a million kilometers, making it's base jump limit around 100 million kilometers, just short of Earth. However, when multiplied with the surface gravity of 27g this would create an almost 2,7 billion kilometer no-jump zone, about 18 AU, roughly around the orbit of Uranus.

Now, this makes interstellar travel a bit more slow. While it takes a few hours to make the jump from, say Alpha Centauri to Sol, it still would take quite a while to make the travel from Uranus to Earth on sublight. At light speed it would still make about two and a half hours to reach it.

Until now I've ignored the effect of the Sun in this, limiting myself only to planetary gravity wells but after doing a little study for another project of mine I realized that I cannot do this without proper explanation. It's paradoxical, isn't it? Why take some wells into account and not others? Are stars somehow different than planets, would this be a question of, say, densities? Star densities are way less than planetary densities, even Saturn is more dense than the Sun. It would make a more simple explanation to include the Sun gravity well into the accounts as well.

So, let us now consider the two models of gravity well jump limits with the stl technology available. Previously, starships were doing about 2-3 g tops, most ships around 0,5-1g. Courier ships and such might break for 8-10g but that would be it. That would mean that a 1g acceleration ship would accelerate to around 9,8 m/s squared. Let us say that fuel limits cap this acceleration to a delta-vee of 30 km/s. Travel from Uranus to Earth would take almost three years. Not really viable as I thought that the absolute travel time to a world 10 pc away would take about a day. Outer system colonies and stations would boom as trade hubs though, as merchant ships would want to minimize the travel time and just dump the cargo, get paid and leave. And faster in-system ships would then take the trade items further in-system.

Now then, ignoring the effect of the sun, a starship would appear around 1 million km from Earth. With 1g acceleration and a 30km/s delta-vee the ship would take about 9 h 15 min not including deceleration to reach Earth. A bit long still but more viable. In Earth's case, having stations in lunar orbit would be profitable since ships would then prefer to dock at the much nearer moon stations, leave the cargo there to the middle man and leave.

However, I thought a different solution. Increase the acceleration so that ships in-system would travel at around 0,1-0,3c. The solution I thought of is gravity drives which would be almost like warp drives in themselves. This would still make the travel from Uranus to Earth a long travel, around 12,5 hours on the average 0,2c speed, not including deceleration. With gravity compensators onboard (a technological item already included in the setting), inside the ship people would not notice the acceleration much. Which is good, since the planned 50-200g acceleration would squash a crew in an instant to tomato juice. I've planned that the grav drive would take it's power from fusion reactors onboard. This would however make smaller fighters less effective because of the space reactors require. Smaller ships would be mainly for maintenance, supply runs and crew transfer. On the other hand, ships built for battle would be huge.

This creates another problem. I've always liked the setting's emphasis on heroic fighter pilots fighting amongst large battleships and swarming the bigger ships with missiles. Emphasizing bigger ships would however make rebellions a much harder thing to do without orbital support and when the Imperial Navy kicks in, it's over. Then again, even a well-equipped fighter squadron defending a world would be doomed since the Imperial Navy has it's own fighter squadrons to deflect such attacks. And of course clever tactics will always be an advantage.

The appeal of having a large system to travel through before arriving at the inner system inhabited planets is largely of the chances of action along the way. While the journey of a million kilometers to a jump point might include some exitement, fast ships could intercept the pirate attacking around the jump limit and it is a little risky running that close to the planetary security. Not profitable. With a couple of light-hours to travel there might be "accidents" and the patrol is far away..

I'll have to think and see where this goes.