this post was submitted on 30 Sep 2025
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Tanking up a satellite to extend its lifetime is quite a different beast from tanking up a whole-ass second stage for a powered descent. You're looking at hundreds of tons of cryogenic liquid fuel and an enormous vehicle. For context, a Starship carrying no cargo has about 8km/s of delta-V.^1^ That's with 1500 tons of LOX and liquid methane. That's barely enough to lose the orbital velocity. Assuming that prop load doesn't include the header tanks (but it does), a fully tanked up Starship could maybe pull off a powered descent and landing from LEO. And that thing is huuuuge.
1: I ran the numbers for a block 2 Starship and came out with 11km/s of delta-V. That sounds like too much, so I'm going to assume I got the math wrong and go with the latest "official" numbers I could find.
Wonder if the calculus changes if they use a different type of fuel for descent vs ascent? That way a hypothetical tanker would only need a supply of descent fuel, whatever that is.
That would make things worse, because now you're carrying the extra mass of fuel, tanks, plumbing and engines for the descent. Can't run a rocket engine on two different types of fuel and oxidizer.
The rocket equation is a harsh mistress. As long as you're limited to chemical rockets, you're not gonna have enough spare propellant for a powered descent. The energy density just isn't there. We don't do direct burns to pretty much anywhere farther than Mars and a Mars Hohmann transfer (the most fuel-efficient trajectory) burn takes ~3.6km/s. Less than half of LEO velocity.