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If the booster attempts either land or sea landing, how much fuel (RP-1 and LOX) is left in its tanks right after stage separation?

Recently, I have heard numbers as high as 30% of its total tank capacity, but that seems unnecessarily much to me. Appreciate sources, accept calculations.

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  • $\begingroup$ You "THE" Everyday Astro? Anyway depends if they do a boost back burn (three burns) to RTLS or a downrange landing (two burns). But 30% was the number Musk quted over the years. (Not answering since i am not sure). $\endgroup$
    – geoffc
    Commented Dec 10, 2018 at 23:04
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    $\begingroup$ Was it 30% od the fuel mass or 30% of deltaV? $\endgroup$
    – jkavalik
    Commented Dec 11, 2018 at 7:57
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    $\begingroup$ Interesting question. And how much fuel is left after landing? If any. $\endgroup$
    – Joe Jobs
    Commented Aug 3, 2020 at 9:14
  • $\begingroup$ since there are videos of F9's from launch to landing, it should be possible to figure out simply from counting seconds of burntime and # of engines lit and mass fuel flowrates $\endgroup$
    – Alonda
    Commented Feb 24, 2023 at 22:24

1 Answer 1

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EDIT: I just realized that Elon Musk has repeatedly quoted 30% as the land landing value. That is almost certainly correct, meaning that Method 2 is probably correct.

Intro

I was actually going to ask something similar to this myself when I found it. I have came up with 2 ways to hopefully find the answer. First, looking at the average of some starlink mission numbers from Flightclub. Second, looking at the max payload capacity for each configuration.

Method 1

Short version: 13.75% for land, 4.4% for drone ship, and 2.27% for landed/expended

Long version: As you said it depends what they plan to do with it. So, to answer this question, I headed on over to Declan Murphy's flightclub.io. I went to a starlink mission simulation. First, lets talk about landed and expended boosters. Since you would ge the best cost per kg (or pound) ratio if you use the entire rocket, I am assuming that the falcon 9 launch a starlink only (no rideshare) mission uses every bit of fuel not in the safety margins. At the end of these missions (on avergae) there are around 9.3 tons remaining. Since Falcon 9 starts with 409.5 tons, this gives us about 2.27% of the fuel remaining. This also turns out to be the value for expend missions, which gives an insight into why keep this extra fuel. When say a merlin engine fails at certain portions of the flight, Falcon 9 is slightly less fuel efficent (due to having a smaller TWR). In order to still get the payload to the correct orbit, the first stage has a bit of extra fuel. Second, drone ship landings. These have (again on average) 27.1 tons of fuel left. Falcon 9's starting propellant is 409.5 tons so this gives us 4.4%. Finally, when landing on land Falcon 9 averages 56.3 tons. One quick note, Starlink missions never land on land, so I used transporter missions (as I assumed those are packed as well). Anyways 56.3/409.5 comes out to 13.75%.

Method 2

Short version: 30.2% on land, 9.2% on drone ship, and we can't find the landed/expended Long versionObviously, if SpaceX uses fuel to land a rocket, it can't be used to launch more payload. Therefore, based on the payload capacity of each configuration, we can find the fuel used for landing. Theoretically, if 50% of the fuel is used for landing, you have 25% of the payload. If 25% used, then you have 56.25% of the payload. In practice, falcon 9 has a dry mass of 30 tons and a safety margin (this is assuming that margin is 2.27%). So while landing on the drone ship it can only carry 80% of the payload. From this we get that 9.2% of the fuel is used for ocean landing. Now, from the same calculation using the land landing payload capacity of 22,500 pounds, we get that 30.2% is used for the land landing. God that is close to the 30% you heard, meaning that it isn't totally crazy.

Outro

So both methods produce pretty different results. Perhaps the best way to find the answer is averages. That gives 22% for land, 6.8% for drone ship, and 2.27% for landed/expended (last one is not an average as Method 2 cant produce this value).

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