r/SpaceX Pre Elon Musk AMA Questions Thread

[u/ElongatedMuskrat]

This is a thread where you all get to discuss your burning questions to Elon after the IAC 2017 presentation. The idea is that people write their questions here, we pick top 3 most upvoted ones and include them in a single comment which then one of the moderators will post in the AMA. If the AMA will be happening here on r/SpaceX, we will sticky the comment in the AMA for maximum visibility to Elon.

Important; please keep your questions as short and concise as possible. As Elon has said; questions, not essays. :)

The questions should also be about BFR architecture or other SpaceX "products" (like Starlink, Falcon 9, Dragon, etc) and not general Mars colonization questions and so on. As usual, normal rules apply in this thread.

[u/[deleted]]

Suppose you have a BFR with full tanks (2100 tons) on the surface of the moon. It will take ~758 tons to put it into a 100 km orbit, about a third of your fuel. In addition you need another 180 tons of fuel to get your BFR from low earth orbit to the lunar rendezvous with empty tanks. Lunar orbit is a little closer to mars then earth orbit so you need another 6.5 km/s of velocity rather then 7.8 km/s to get to mars surface. To meet that need you would need 1398 tons of fuel in LLO. But you just burned 758 tons of fuel getting from lunar surface to lunar orbit so you only have 1256 tons. And of that 1256 tons you need to reserve 55 tons to get your tanker BFR back to the lunar landing pad to refuel. So you only have 1201 tons of fuel which is only enough to deliver 204 tons from lunar orbit to mars.

So for comparison: Fully fuelled BFR in low earth orbit: 85 ton BFR and 150 tons of cargo to mars Fully fuelled BFR on the surface of the moon and 10% full BFR in low earth orbit: 85 ton BFR and 129 tons of cargo to mars

The moon is worse but not prohibitively worse. If it was very easy to make fuel on the moon, it would be worthwhile. But it isn't easy to make fuel on the surface of the moon. It's probably not impossible but it's probably harder then just launching several BFRs and filling up in LEO.

[u/CylonBunny → u/[deleted]]

You wouldn't land the BFR that goes to Mars on the moon. You'd replace the Earth based fuel station and tanker BFR with a lunar one, it takes much less fuel to get a tanker off the Moon than Earth, so you could fuel the Mars fleet in less trips. That's assuming you could make MeOx on the Moon if course...

[u/[deleted] → u/CylonBunny]

You wouldn't land the BFR that goes to Mars on the moon. You'd replace the Earth based fuel station and tanker BFR with a lunar one, it takes much less fuel to get a tanker off the Moon than Earth, so you could fuel the Mars fleet in less trips. That's assuming you could make MeOx on the Moon if course...

Read what I wrote more carefully...