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Comment by avmich | Hacker News Reader

Do they describe somewhere how they achieved the apparently successful propellant transfer in microgravity in flight 3? Having pressure difference might not be enough if the fuel is a mix of liquid and gaseous phases.

Edit: aha, found - "Then, SpaceX will fine-tune tank pressures and fire propellant settling thrusters." And they are using free flow. Wonder how long the transfer will take and how much of a propellant settling acceleration is needed?.. Also :) the relative positions of ships become important if no pumps are used...

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I understand their desires to eliminate parts and reduce weight in the second stage (Starship), but I foresee them eventually adding in pumps to aide in transfer of propellants.

Maybe not in all variants, but definitely in the crew version, as having the ability to transfer propellant even if the pressure differences and settling cannot be achieved seems worth the lost performance.

> having the ability to transfer propellant even if the pressure differences and settling cannot be achieved seems worth the lost performance

Pumps are way more complicated than pressure feeding.

> the relative positions of ships become important if no pumps are used

It shouldn’t if they’re using pressure to feed propellant. Do you mean relative to the Sun in respect of boil off?

No, I'm confused between "free flow", "no pumps" and "pressure difference", which to me a sort of a pump - in some sense, active forces applied to propellants. So I was thinking about flowing from a point of higher potential energy to point of lower one, on Earth you put the empty glass below the full when you need to pour water from the full to the empty. If Starships actually use pressure difference, then it's not quite free flow, and geometrical positions aren't that important.

> If Starships actually use pressure difference, then it's not quite free flow, and geometrical positions aren't that important

That's how I'm reading it, analogous to a pressure-fed engine [1]. A separate, expendable gas supply pressurises the donor tank to force the propellant out and over.

[1] https://en.wikipedia.org/wiki/Pressure-fed_engine

Not sure if it's too similar to pressure-fed engine, that one works with a clear acceleration direction, which really helps to keep fuel at the local bottom.

> A separate, expendable gas supply pressurises the donor tank to force the propellant out and over.

But what if the fuel is a mix of liquid bubbles and the gas freely floating in the tank?

(BTW, on the different point, some pumps applicable here could be really simple, like analogous to https://en.wikipedia.org/wiki/Vacuum_ejector#Water_aspirator)

> that one works with a clear acceleration direction, which really helps to keep fuel at the local bottom

I believe the plan is to impart a gentle spin.

> some pumps applicable here could be really simple

The Venturi effect reduces pressure. It's definitely worth exploring. But you'd need to test it to explore where it causes volumetric effects, e.g. by exacerbating boil off.

> I believe the plan is to impart a gentle spin.

That would be something new. And for a partially refueled stage, that's a lot of mass to spin. How fast? the speed of spinning could be important too.

> that's a lot of mass to spin. How fast?

No clue! But I think they were targeting mili-g acceleration--could probably be achieved by the boil off alone.

Heating the donor by exposing it to sunlight while shading the receiver should provide some pressure differential for no added power. Gentle rotation would start with the center of gravity wihin the donor, but as the transfer goes on, it'd move to the middle of the two rockets and stop the transfer at that point.

Ohhhh, is that why it was spinning going into the reentry attempt?

deepcut :D

It wasn't intended as a jab, spinning it up to settle the propellant for transfer sounds like a good idea to me. Of course, failing to exit the spin is still mission-ending but they'll figure it out.

Now we just need the Attenborough voiceover about the mating habits of starships as they spin through space.

you would but you need to stop the spin and then dock the two together.

That would send fluids to the back of the tanks.

At this stage it might be easier to build fueling station in the orbit with actual transfer pumps.

I'd definitely prefer to decouple the carrying fuel to the orbital depot and the refueling at the said depot, when it already does have the fuel amount needed.

As for spinning two Starships of significantly different mass... that should be interesting to see. Doable, yes :) but interesting nevertheless.

Spin after docking, of course.

Pushing the liquids is not the hard part. Settling the liquids in 0g is the hard part.

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