[pigpop]

The inner solar system was what I was thinking, returning to Earth like you said is a bit pointless but even a slow trajectory with a constant supply would be useful to have fuel and feedstock already in orbit near Mars and Earth.

[credit_guy]

The Hohmann transfer time between Saturn and Mars/Earth is around 16 years. So, all the ships used for such a supply mission would reach their destination at least 32 years after they’ve been built, assuming we build them on Earth.

[pigpop]

That's not really a problem if the supply is continuous. Think of the last time you drank a 12-year scotch, that distillery had to be set up and start producing at least 12 years ago for them to label the product that way but they've continued production constantly since then which ensures there is a steady supply to be delivered to stores.

[credit_guy]

Spirits are not capital intensive. Building a rocket is very capital intensive. You'd like to be able to reuse it. But if one single mission takes 30 years, then you can reuse this rocket once, at most twice. Let's say you reuse it twice, you amortize the capital cost over a period of 90 years. Now, let's say someone builds the exact same rockets, but they do missions between the asteroid belt and Mars. Each trip takes about 2 years. Everything you can source on Titan, you can find an asteroid to source it from. By the time you reuse a Titan-bound rocket once, you reuse an asteroid-bound rocket many times.

[pigpop]

I'm not sure what you're talking about, my initial comment was a (sort of silly) idea about building a mass driver on Titan that uses native materials to launch payloads on an inward bound trajectory, not to ferry them back and forth with a rocket.

[credit_guy]

That's an interesting idea, but I don't think you can use a mass driver to shoot something from an higher orbit to a lower orbit.

[pigpop]

True, it would need to accelerate somehow. Maybe there could be some kind of on board propulsion that would be enough once it's in space. Solid rocket motors could be fabricated as part of the process but the timing and control systems would pose a problem. Maybe something clever could be done mechanically that leveraged temperature or pressure changes.

[perilunar]

Sure you can — you just fire it went pointing ‘backwards’. But either way, you still need a burn to circularise the orbit when you get there.