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So, the current plan for the BFR has the spaceship massing 85 tons and carrying 50 tons of cargo plus around 1000 tons of fuel and oxidiser (according to wikipedia), most of which is presumably expended getting into orbit. Then, for deep space missions, the spaceship is supposed to be refueled by the BFR tanker. This is usually shown on the animations etc. as just one tanker flight per spaceship, but the numbers don't seem to add up. Even if the dry mass of the tanker is less, say 30 tons, it will only deliver about 100 tons of fuel/oxidiser to LEO, so it looks like it would take about 10 tanker flights to fully refuel the spaceship.

Does anyone know if that is actually the plan? or have I miscalculated somewhere? or does the spaceship not need to be fully refueled for (say) a Mars mission?

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The presentations Musk made at the IAC suggested 6 tankers to one spaceship, for a Mars mission. Two tankers to one spaceship for a lunar mission.

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  • $\begingroup$ Would depend on the lunar mission... It is supposed to take more than a fully refueled BFR to get from LEO to the Moon, although it can be done with an elliptical orbit around Earth. $\endgroup$ – PearsonArtPhoto♦ Aug 29 '18 at 21:26
  • $\begingroup$ That seems to be a mission where the whole BFS lands on the moon and returns to earth. It's really hard to see how some version of lunar orbit rendezvous apollo style isn't going to be better $\endgroup$ – Steve Linton Aug 30 '18 at 19:12
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I came to a similar conclusion. One launch to put the spaceship into orbit, 10 or more tankers to "fill the tanks". Maybe a dozen launches all together, each launch the equivalent of a Saturn V. There were only 13 Saturn V launches in all before they pulled the plug on the whole program. Direct ascent and Earth Orbit Rendezvous (EOR) were rejected in favor of Lunar Orbit Rendezvous (LOR), and for good reason. Weight. The Lunar Lander Weighed between 12-13 tons, the spacex vehicle weighs 10 times that - empty! That means a lot of fuel needed to go to the moon, slow down, land, take off, go back to earth, re enter, and land tail first. Well, good luck to them, and I hope they succeed. Sure makes me wish we had stuck with Apollo however. We'd be on Mars by now.

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The requisite number of refueling flights would depend on how much payload the spaceship is bringing to the surface of Mars. If you are sending less payload then you might not need as many refueling flights. Graphs showing the delta-v to payload curve can be found here:

https://gfycat.com/gifs/detail/embarrassedcommoncoyote

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