Re: [Spaceships] Martian “Gas” Station
 

Generally not. Even if you can net gain a little more Delta-V from decelrating, refuelling and then accelerating again it won't be worth the time and/or the wear and tear.

Hard science ships can't just "stop". They have to spend Delta-V. So the efficient thing is usually to accelerate once and then decelerate once while you coast at maximum speed in between.

Re: [Spaceships] Martian “Gas” Station
 

Well, it depends on how you set up the case. For example, say we have rocket with a dry mass of 1 ton and ISP of 1 km/s. It can travel from A to B for a ΔV of 4 km/s, or from A to M for 3 km/s then M to B for another 3 km/s. While the A to B via M route appears less efficient (taking 2 more km/s), if you can refill propellant tanks at M, it would save 15 tons of propellant compared to the A-B route.

Re: [Spaceships] Martian “Gas” Station
 

An exhausted velocity of 1 km/s is a weak drive. Assuming that the drive tosses out 3 kg/s/metric ton of mass, that would only allow an object with a dry mass of 1 metric ton and a wet mass of 2 metric tons an acceleration of 0.1 g for 1000 seconds (a delta-v of only 980 m/s).

Re: [Spaceships] Martian “Gas” Station
 

If A to B is 20 million kilometers and you can get there on 4 m/s of Delta-V then you are accelerating to 2 km/s then drifting for 5 million seconds and then decelerating for a total trip time of c.58 days.

If M is exactly between A and B you're doing 5 million kilometers on each leg at an average velocity of 1.5 million km/s and it will take you 3,333,333 seconds on each leg or about 77 days ignoring any time you spend finding and refining.

Note that these are the maximally favorable assuptions for the relative positions of A,B and M. Zig-zagging will take even longer.

Spending almost 3 weeks longer and putting 50% more wear on your engines will be less efficient by most standards.

Re: [Spaceships] Martian “Gas” Station
 

I'll admit I'm a bit biased towards "Time is Cheap, Mass is Expensive", though looking at near-future space travel that seems plausible enough (traveling outward would take months if not years). I'm also a little biased towards one-way flights.

Re: [Spaceships] Martian “Gas” Station
 

Okay, we are getting a little off focus here. If there was a reliable source of reaction mass, oxygen, food and water in Mars orbit incoming ships from Earth could re-provision themselves there for the return trip and reserve more room for cargo and passengers. Note that without a CO2 atmosphere there would be no methane production so that capacity on the moons would be dedicated to processing ice. A big import from Earth would probably be certain organic compounds to promote food production.
Even hydroponics needs something to work with. So do humans, making food from Earth another big import item (at least initially). At least everyone (no matter how useless) would have something to contribute. ;D

Re: [Spaceships] Martian “Gas” Station
 

I would note that you may want to rethink ratios of production to storage.

A basic refinery complex is 3 mining modules feeding one refinery, plus power plants capable of providing 4 energy points. At SM+10, that costs $40M for the mining modules, and $120M for fission reactors (solar power is vastly more expensive -- it starts at $200M, but because of martian sunlight and the fact that it only operates during the daytime, you actually need more than 5x as much. Note that both reactors and solar panels have options that are cheaper but heavier). It produces 150 tons per hour.

15,000 tons of fuel tanks costs $100M, so if your normal business cycle is 100 hours of business and then 10,000 hours of inactivity, you can drop down to SM+8 mining modules (total cost is $116M) and still refill your tanks in six weeks.

Re: [Spaceships] Martian “Gas” Station
 

Looks like I may have to redesign the whole thing. My problem is the first step: breaking down water into hydrogen and oxygen. The result is eight tons of oxygen for every ton of hydrogen. I see this as two standard and two small tanks for oxygen, one small tank for hydrogen, a standard mining system, two small refineries (LOH, CH4), a small fission reactor (powers one refinery at a time), a regular methane tank and a regular fission reactor (powers the mine). The H2 and O2 tanks take 30 hours to fill while the methane tank takes ten.
That takes care of the bottom section. I think I'll leave the top level much the same and work on the middle one tomorrow.

Re: [Spaceships] Martian “Gas” Station
 

The mass and cost of the gasbag envelope go with volume. Where the air is very thin you would need a vast, massive, costly envelope, perhaps impractical or uneconomic.

Re: [Spaceships] Martian “Gas” Station
 

True, but I have no idea how to model that. Sadly the "Gasbag" description in Spaceships 7 seems rather generic. I understand your concern about the expanded volume (I'd add 3 to the SM of the gasbag based on my first quote from GURPS Mars) but I think the increased area of the envelope would be amply countered by Mars lower gravity and Hydrogen's increased efficiency as a lifting gas there. The cost issue I'll grant (what sort of premium should I tack on?) but impracticality is a tech issue I'm not qualified to comment on. I stand by my preference for Hydrogen as a lifting gas on Mars for reasons I've previously stated and consider it an intrinsic part of my "Domed Mars" setting.