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It has been announced that SpaceX Starship SuperHeavy will need 16 launches with the tankers to refuel Lunar Starship. Why is the Starship going to be empty after reaching orbit, since the booster is going to exert most if not all of the thrust? In other words, isn't the booster powerful enough to carry a fully loaded Starship into Space?

Thank you

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  • $\begingroup$ The booster exerts precisely zero thrust for the entire portion of the launch that occurs after staging. The Starship's tanks are empty when it reaches orbit because it needed that propellant to reach orbit. What would be the point in carrying all that propellant if it wasn't needed? $\endgroup$ Aug 12, 2021 at 20:33

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It has been announced that SpaceX Starship SuperHeavy will need 16 launches with the tankers to refuel Lunar Starship.

16 refueling launches is an extremely high number. According to NASA's own analysis in the updated Artemis III Concept of Operations, only four tanker launches are planned.

Elon Musk was talking about 8 launches, maybe even as few as 4, depending on the exact mission profile and the actual capabilities of the final vehicles.

The highest number that I could find is 14 tanker launches. This comes from the GAO report on NASA's selection process after Blue Origin's protest. Note that even Blue Origin, SpaceX's main competitor who have an incentive to make SpaceX's proposal look as bad, complex, risky, and expensive as possible has only claimed 14 tanker launches.

Why is the Starship going to be empty after reaching orbit

Because it burns the fuel to get to orbit.

If it had fuel leftover after getting to orbit that means that you could instead remove that fuel and add more cargo. In other words: if the Starship weren't out of fuel after reaching orbit, it would be wasting cargo capacity.

In other words, isn't the booster powerful enough to carry a fully loaded Starship into Space?

It is. It can either carry a fully loaded Starship full of fuel but very little cargo, or it can carry a fully loaded Starship full of cargo but very little fuel.

Both of those are useless on their own. The best combination is to carry both Starships full of cargo and Starships full of fuel and let the ones full of fuel refuel the ones full of cargo.

It's simple: every single kilogram you carry to orbit can only be one of three things: structure of the rocket itself (tanks, engines, piping, electronics, antennas, etc.), consumables (fuel, oxidizer, pressurant), or useful cargo. If you want more of one, you have to carry less of something else, and you cannot really choose to remove stuff from the rocket, so you can only decide between consumables and cargo.

why will the tankers have fuel whereas the Starships to reach moon or mars will not

Because the Lunar Starship which goes to the Moon has 100 tons of stuff in it that needs to go to the Moon. The Tanker Starship has 100 tons of fuel in it that goes to the Lunar Starship.

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    $\begingroup$ @Cris: The booster does not get the ship anywhere close to orbital velocity. $\endgroup$ Aug 12, 2021 at 18:24
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    $\begingroup$ @BrendanLuke15: There is only one single place where I have seen this extremely high number of 16 launches mentioned. Elon Musk has never talked about 16 launches, SpaceX has never talked about 16 launches, NASA has never talked about 16 launches, Congress has never talked about 16 launches, no space news organization has ever talked about 16 launches, even Blue Origin, when they tried very hard to make SpaceX look as bad, as expensive, as complex as possible, has not claimed a number this high. The only place where this number appears is in a report by the Government Accounting Office. $\endgroup$ Aug 12, 2021 at 19:44
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    $\begingroup$ 16 launches would mean 15 tankers or 14 tankers and a depot variant with some extra mass in insulation, for ~80-85 t per tanker load. Actual initial payload is expected to be more like 100 t, with an eventual optimized version possibly reaching 150 t. ~80 t seems plausible for an early prototype, and they might have used such a number in their proposal as a conservative worst case scenario. It was probably for the best, if they used any other number and BO found out about it, they'd be screaming even more about unfair treatment... $\endgroup$ Aug 13, 2021 at 0:32
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    $\begingroup$ @JörgWMittag there's no evidence that is the case, it is inconsistent with the tone and content of the GAO report, and it's questionable whether they would even perform such an analysis themselves. It is likely SpaceX would have given an upper bound in their proposal, and the 16 launches figure is consistent with an upper bound based on their current prototypes. BO likely wouldn't have any legitimate access to such information (which doesn't exclude the possibility of accidental or intentional leaks), but the GAO would, and the worst case scenario is what they're most interested in. $\endgroup$ Aug 13, 2021 at 15:15
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    $\begingroup$ The "16 launches" figure is actually mentioned in the GAO report (page 27, gao.gov/assets/b-419783.pdf) as being part of Blue Origin's protest, it's not something the GAO invented. BO's objection in the protest was that NASA waived a requirement of a flight readiness review for every flight. Of course, the tanker flights are all identical, interchangeable, and replaceable, so a FRR for each one really wouldn't make sense. Probably why they've shifted from "unfair!" to "it's too complicated!" in this...whatever you want to call this little display after their protest failed. $\endgroup$ Aug 14, 2021 at 19:20

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