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Comment by beaner | Hacker News Reader

Genuine question, how will tankers get into space? If most of the ship's size is already dedicated to fuel that's burnt, that seems to leave little left for fuel payload. I also assume fuel payload would be much heavier than other types of payload, requiring more ship fuel to launch it, further taking up space.

It seems like for every full refill you could provide in space it'd take an additional 3-4 ships just to get it up there?

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The tyranny of the rocket equation, combined with Earth's deep gravity well, and the higher efficiency of rocket engines in vacuum makes it seem much worse than it is.

E.g. for a lunar mission: it takes 9.4 km/s Δv to get to low Earth orbit (LEO), but only 5.5 km/s Δv to get from there to the Moon [1]. Raptor (the engine used on Starship) has an exhaust velocity of 3.2 km/s on Earth, and 3.7 km/s in vacuum [2]. Plugging those numbers in the rocket equation, to get to LEO you need about 19x your payload mass in fuel. However, to get from LEO to the moon surface, you only need 4x your payload mass in fuel. So while a tanker can indeed only bring a small fraction of its launch mass as fuel to orbit, once you're in orbit you don't need that much fuel to get to the moon.

In reality the numbers to get to LEO are lower because of staging and the vacuum engines on Starship which can be used in the upper atmosphere, and you also need fuel to get back from the moon, but it probably won't add up to a full Starship.

[1] https://www.reddit.com/r/space/comments/29cxi6/i_made_a_delt... [2] https://en.wikipedia.org/wiki/SpaceX_Raptor [3] https://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation

The plan, at least as originally described is to have (at least) three variants of Starship: One for passengers, one for bulk cargo, and a tanker.

Each would be launched atop the common Super Heavy booster.

Depending on the trip, a passenger Starship would do an on-orbit refuelling from the tanker variant.

About that yes, but remember that a fully loaded starship in orbit can land on any solid surface in the solar system and return to earth (except for Venus)

Starship’s usable volume is over 100 times that of an Apollo lunar lander.

I don't get it, how does that answer the question about the economics of lifting fuel into orbit? Could you elaborate your point?

I think they are trying to imply that refueling a starship in orbit is worth it, whatever the cost - it gives a capability we didn't have before.

To land the volume of one starship on the moon takes 3 or 4 launches of a fully reusable craft - so just the fuel.

To land that with Apollo would take 100 launches of a single use craft.

You could also add 3-4 extra stages and put an even bigger tanker into space. But I guess there will be diminishing returns due to the crushing weight. It would probably be cheaper to make many smaller unmanned vessels and boost them via an electromagnetic rail, then assemble it in orbit.

Why not launch a bunch of larger payload tanker bays, empty, then link them together in orbit and then send Many smaller multiple regular trips to fill it up such that you have an orbital fueling station?

My layman’s understanding is that doubling the payload quadruples the fuel requirement. So it is much more efficient to launch multiple rockets with a small amount of fuel than one big one with all the fuel you need. It’s the difference between O(n) and O(n^2).

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