[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.