The starship tankers that will refuel starships in orbit will only carry fuel. I’ve heard it described that at first they will simply be normal starships, but with more fuel leftover when they reach orbit because there is no cargo. That sounds nuts to me. It seems obvious that they would soon take the missing cargo weight and increase the fuel tank capacity by that amount.

(Speaking with methane and oxygen considered together as fuel.) The cargo capacity we’ve heard lately is 130 tons. Fuel at take off is stated as 1200 tons. So together thats 1330 tons of fuel. However I can’t find a figure on how much fuel will be left when it reaches LEO. I also would presume these figures include fuel in the header tanks that is needed for landing.

To know how much fuel a tanker would have available to transfer, we would first need to know how much would be left once in orbit. (starting with 1330 tons) and then subtract the landing fuel. Obviously I don’t have all the parts. So here’s my question.

How much fuel, (in tons) will a dedicated tanker be able to deliver to a starship in LEO?

A similar question was asked 2 years ago when the numbers were really different (50 tons of cargo and 1000 tons of fuel and oxygen) Also, it was asked in terms of how many tanker loads needed. Of course the number of tanker trips needed depends on the destination and the weight of cargo on the trip. So I am asking how many tons of fuel a tanker will have to contribute once in LEO. I don’t know if you folks think this question is different enough, but the answer I seek was not in the earlier question.

  • $\begingroup$ It seems this could be figured out with the rocket equation. Given the specific impulse of the raptor (330 seconds) The dry weight and fuel weight of both the super heavy and Starship from wikipedia. Super Heavy rocket 180,000 Kg. fuel 3,4000.000 kg. For Starship the dry weight is 120,000 Kg Fuel weight (interestingly enough) also 120,000 Kg (is that right?) But of course I would suggest adding the 130 Metric tons of cargo on to that. It seems figuring this out with staging must be hard. It's beyond me. $\endgroup$ – Johnny Robinson Sep 15 '20 at 0:11

This is equivalent to asking "how much payload mass can Starship + Superheavy put into LEO per launch". It's just that in the case of a tanker the "payload" is fuel rather than cargo.

At the moment that's actually a rather tricky question to answer, since the design of Starship and Superheavy are still constantly changing as the development process continues. The most recent revision of the Starship users guide on SpaceX's website is rather vague on this topic, promising only "100+" metric tons of payload mass to LEO.

The most recent estimate I could find from Elon Musk is only slightly more specific, suggesting 100-125 metric tons of useful payload to a useful orbit:

@elonmusk: 100mT to 125mT for true useful load to useful orbit (eg Starlink mission), including propellant reserves. 150mT for reference payload compared to other rockets. This is in fully reusable config. About double in fully expendable config, which is hopefully never.

It's possible a dedicated tanker might also have slightly more capacity than cargo Starship depending on the design; every gram of weight they can shave off the ship's dry mass is another gram of fuel they can deliver to LEO. At this point however the design isn't nearly far enough along for us to be speculating about such small differences.

  • $\begingroup$ It not really equivalent to asking "how much payload mass can Starship + Superheavy put into LEO per launch" because any payload besides fuel needs a different containment (faring etc). In the case of a tanker, they could increase the size of the same tanks used for launch and simply deliver whatever is left as cargo. So no additional tanks in the nosecone / cargo area. I am picturing a ship designed as if there is no cargo at all, (other than the header tanks for landing.) $\endgroup$ – Johnny Robinson Nov 23 '20 at 23:30
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    $\begingroup$ @JohnnyRobinson I covered that in my last paragraph. Admittedly a bit of a handwavy explanation, but Starship's design isn't yet finalized to the point where small optimizations like that make a huge difference to the answer. "100-125 Mg" encompases a pretty big margin of error. $\endgroup$ – Ajedi32 Nov 24 '20 at 1:18
  • $\begingroup$ Yeah, too soon to tell. $\endgroup$ – Johnny Robinson Nov 24 '20 at 3:50

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