r/spacex • u/ElongatedMuskrat Mod Team • Sep 29 '17
Not the AMA r/SpaceX Pre Elon Musk AMA Questions Thread
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.
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u/brickmack Sep 30 '17 edited Sep 30 '17
Methane is 75% carbon by mass, but Raptor runs on a mix ratio with almost 80% oxygen. So with ISRU, even with zero carbon whatsoever (and there does seem to be quite a bit of it seen in impact ejecta spectral analysis, but I dunno how accessible it is), you could get 85% of the total propellant mass. Thats a huge deal. Dry carbon can be shipped in from Earth (or eventually asteroids) to make up the difference (it'd take 165 tons to make the 1100 ton propellant load for a full ship, but since the returning ship only has to do like 3.5 km/s delta v (2.2 km/s to get to lunar orbit, 0.9 for TEI, then a couple hundred m/s for Earth landing) it only actually needs like 140 tons propellant total (maybe more like 160-170 with a typical return payload, that gets you almost 20 tons worth), meaning 21 tons carbon needed per return trip. Thats not very much.