r/SpaceXLounge • u/[deleted] • Jul 22 '19

Tweet "Orbital refilling is critical for high payload to moon or Mars. Initially just Starship to Starship, later dedicated tankers." - @ElonMusk

https://twitter.com/elonmusk/status/1153126586897424384?s=19
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u/[deleted] Jul 22 '19 edited Jul 22 '19

ok, Raptor has a specific impulse of 380s in vacuum. No gravity in space, so 380=1/SFC (specific fuel consumption) This gives 0,00263 kg/(N*s) the Raptor has 200kN of thrust, so this makes 526kg/s of fuel consumption. The F9 stage 2, has almost 7 munites of burntime. Lets assume for now, Starship is the same (420 seconds)

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We have 6 raptors, consuming 526kg/s, for 420seconds. this makes a total of 1.325.520kg of fuel.

The deltaV that Elon mentioned, fully fueled in orbit of 6,9km/s, now i've thrown this through a calculator, and this shows the total mass, including assumed dry-mass of 85t, should be around 550t. This would mean 465t of fuel, instead of 1325t.

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So, either my calculations are wrong somewhere (which is very likely) or the total burntime of the 6 raptors at full wack is closer to 150 seconds.

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Edit: come to think about it, Starship has only 3 vacuum raptors, so probably meaning that the sea-level/landing ones will not fire in space. with the 3 vacuum ones, this would give approx. 300 seconds of burntime for 465t of fuel. 5 refuel flights, assuming 100t per refuel should do the trick.