Trying to Decide on the "Summer Drop-in / Drop-Out Campaign"

I can see the signs already, and I suspect we'll have a hard time getting the full Saturday Night group together over the next few months with vacations and family visits and everything lese that the summer entails in regards to "real life".

So, i want to put together a "drop-in / drop out" styled campaign for the gaming nites that the regular group is going to be running short. It's easier to drop folks into a campaign that is designed with that in mind as opposed to say, I don't know, maybe a party 5 levels deep into Rappan Athuk ;)

I'm toying with the following ideas:

Stars Without Number - not sure what exactly I would run with it, but it's a nice system, and I never seem to run Sci-Fi these days.

Starships & Spacement 2e - players would be officers on the ship, and whoever was playing that session would be the "Away Team" - this would probably be loose plots and very sandboxie in nature. Oh, and Sci-Fi.

Razor Coast / S&W - it would be more the setting and less the underlying plots I suspect, although that would depend on the players' actions I suspect.

Any other thoughts on systems / settings to use? Any feedback on the above?

I'm not recruiting yet, but I will when this get's closer.