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Starship uses Raptor engines which in turn uses liquid methane as fuel. The idea for using methane as fuel being - methane can be produced on Mars from CO2 and H2O. And using the onsite produced methane, Starship can lift-off from Mars and head back to the Blue Planet.

But what about Moon Missions? I don't think methane can be produced on the Moon (but I guess hydrogen can be). So how would starship lift-off from the Moon? Does it have to carry enough Methane considering the lift-off too, or what else?

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    $\begingroup$ google search did not help - spacex starship how will methane be produced on moon - no relevant results. $\endgroup$
    – samshers
    Jan 13, 2021 at 4:51
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    $\begingroup$ I think some carbonate minerals some m deep in the soil is quite likely. However, we have only some surface samples yet. CO2 could be extracted from the carbonates by heating, and then processed to methane (requires H2, 400C and ruthenium catalyst). $\endgroup$
    – peterh
    Jan 13, 2021 at 13:34

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The Lunar Starship will have to be refueled from Earth, for each mission, and will brings its fuel for return with it, when it lands.

As noted, while there may be Hydrogen available in ice in craters, there is no expectation of easily available carbon in a format appropriate for methane manufacture on the moon.

Thus ISRU (In-Situ Resource Utilization aka - make your own fuel from local resources. Like CO2 + H20 to make CH4 on Mars.) will not be on the table for the first launches to the Moon, in contrast to the Mars missions which are dependent on ISRU for return flights.

The propellant required to launch from the moon is low enough that it can be carried down and used to return. On the other hand, the absence of an atmosphere means aerobraking, as planned at Mars/Earth is not available and all the velocity needs to be scrubbed via thrust which means more fuel needed for landing.

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  • $\begingroup$ Cool. Then a Methane starship for Mars and Hydrogen starship for moon should have been developed. Ofcourse moon is near so possiblly saving some fuel for return is easier. But for how long can we keep methane at cryo temps.. few months may be like with mars voyage.. but if we have to keep the lunar lander on moon for years.. keeping methane at cryo could be bottle neck. $\endgroup$
    – samshers
    Jan 13, 2021 at 12:18
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    $\begingroup$ @samshers No, both would fly on methane. The moon has considerably lower delta-v requirements. It won't provide aerobraking but the gravity is much lower. $\endgroup$
    – SF.
    Jan 13, 2021 at 12:59
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    $\begingroup$ This post could be improved by providing sources. $\endgroup$
    – user20636
    Jan 13, 2021 at 13:10
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    $\begingroup$ @JCRM, there is no expectation of a carbon source on the moon. this can be debated. See the comment on Question from Peterh $\endgroup$
    – samshers
    Jan 13, 2021 at 13:45
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    $\begingroup$ this isn't a site for debating @samshers, nor is one for assuming. $\endgroup$
    – user20636
    Jan 13, 2021 at 14:59
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The whole Starship system is built around two fundamental ideas:

  • Robotic refueling and
  • A large fleet of cheap, aggressively simple, mass-produced, mostly identical vehicles driving cost down. Even Super Heavy is essentially just a strengthened, lengthened Starship with more engines and no nosecone.

So, yes, the Starship will need to bring enough fuel to both land (no aerobraking on the Moon!) and launch. But, remember that the Artemis architecture only has the lunar landers going from the Lunar Gateway to the surface of the Moon and back. They don't have to get there all the way from Earth (except for the first time, of course), and they never have to get back to Earth. They only go up and down from lunar orbit to lunar surface and back.

So, the Lunar Starship (Moonship?) only needs enough to fuel to de-orbit and land, and then launch and get back to orbit. And it does all of this in the Moon's much lower gravity. And while the lack of aerobraking means spending more fuel on landing, it also means spending less fuel due to no air resistance on launch.

If they have to refuel every time before they go back down, then so be it. Refueling is what Starship does. Remember, for a Mars mission, they already expect to refuel 5–10 times in LEO, so clearly, refueling is something they're not scared of.

Using hydrogen makes no sense because of fundamental idea #2. A vehicle like Starship is essentially just two things: a giant tank with engines attached at the bottom. For hydrogen, you obviously need completely new, completely different engines. And you need completely different tanks. Not only do the sizes of the tanks need to be different because the volume rations are going to be different for hydrolox vs. methalox, but probably also the materials need to be different, since liquid hydrogen is much colder than liquid oxygen or methane.

So, if a vehicle essentially consists of tanks and engines, and you need to design, develop, manufacture, test, qualify, human-rate completely new engines and completely new tanks from scratch, the answer to "why don't they just make a methalox and a hydrolox Starship" should be clear: it simply wouldn't be a Starship, it would be a new vehicle with the same amount of development resources, costs, engineers, and the same development time as the existing Starship.

This is the exact opposite of "a large fleet of mass-produced, mostly identical vehicles driving cost down".

Now, you might say: "Wait, isn't the Lunar Starship already very different?" Well, not really. It will have no aerodynamic control surfaces and no heat shields, but those are externally mounted "appendages". You can just … not mount them. And it will be painted white, but that is literally just a paint job.

It will have the nosecone with the airlock(s) and the crane in it, but the nosecone is a fairly simple piece that is mostly separate. (They will have to build different nosecones anyway, even if they don't win the HLS contract, since those are more or less the only difference between Cargo Starship, Crew Starship, and Tanker Starship anyway) And it will have the additional landing thrusters, but those are going to be "Raptor-derived", so they will still share some commonality with the Raptor as well as with the hot-gas thrusters, which will also be "Raptor-derived".

So, the Lunar Starship is more a straightforward modification of the Crew Starship than a completely new vehicle.

So, in the end there are two major reasons for using liquid methane:

  • Designing, developing, and building a one-off completely different vehicle just for HLS makes no economic sense.
  • Refueling is in Starship's DNA, so who cares if you have to refuel every time?
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  • $\begingroup$ It's also far from certain that we can even mine enough water to make propellant...it may be too precious a resource to use for such things, and even if possible may take a long time to set up. Oxygen can be produced anywhere you can find rock, and Starship's propellant needs are 78% oxygen. And if you're importing the fuel part, it's a lot easier to import and store methane than hydrogen. Starship could have a big advantage over hydrogen-burning craft for a long time. $\endgroup$ Jan 15, 2021 at 2:12

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