On 2020-01-25 12:38 PM, Jeff Findley wrote:


Elon's vision for Starship is to use Tesla solar as power for the same
Sabatier reaction that they plan to use on Mars to convert CO2 (from the
atmosphere) and H2O into O2 and CH4 (methane). In that way, Starship
propellant production would be completely carbon neutral. This is
important to him since he plans on Starship being capable of being
launched three times per day (no doubt most of those launches would be
tankers to refuel Starships in LEO).


I don't want to get into the weeds about carbon, but to call it carbon
neutral you need to take into account the carbon that was consumed
mining the elements and other processes used to manufacture those solar
panels. All of that would need to be offset by the savings over launch
with a traditional hydrocarbon fuel like kerosene along with the carbon
emitted when those solar cells are eventually disposed of at the end of
life, before the sheet balances. And given enough time it would. However
you have to realize it is not enough to have a net carbon zero process
to claim you are carbon neutral. The only way to reach that is if you
balance it off against the *potential* emission of carbon had you not
used the solar driven Sabatier process for fuel generation. Otherwise it
will still not be *net* zero. It also assumes a certain amount of time
of operation. If it's not in operation long enough the offset will not
get you to net zero either.

But strictly from an economics standpoint I agree with the point you
raise and that Elon makes. That being able to generate your own fuel via
solar is an essential cost savings if you want to have frequent launch
operations (and don't forget across multiple sites). Not having to
transport fuel and oxidizer to all those sites as opposed to being able
to generate fuel locally is another significant ongoing cost consideration.

Dave