3 ms·

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.

by avmich 2y ago

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.

JumpCrisscross 2y ago
> 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 https://en.wikipedia.org/wiki/Pressure-fed_engine
- avmich 2y ago
  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 https://en.wikipedia.org/wiki/Vacuum_ejector#Water_aspirator)
  - JumpCrisscross 2y ago
    > 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.
    - avmich 2y ago
      > 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.
      - JumpCrisscross 2y ago
        > 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.
        rbanffy 2y ago
        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.

by avmich 2y ago

JumpCrisscross 2y ago

> 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 https://en.wikipedia.org/wiki/Pressure-fed_engine

avmich 2y ago
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 https://en.wikipedia.org/wiki/Vacuum_ejector#Water_aspirator)
- JumpCrisscross 2y ago
  > 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.
  - avmich 2y ago
    > 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.
    - JumpCrisscross 2y ago
      > 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.
      - rbanffy 2y ago
        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.