Orbital refilling is one of the key technologies that SpaceX needs to master to realize the full potential starship. The upcoming IFT-3 mission will see a small demonstration. What are the methods that could be used and what are the advantages and disadvantages? On a surface, being in a gravitational field and actually having a reaction force from the ground, the liquids are able to gather at the bottom. And therefore, you can simply use a pump. As long as you keep macrigenic temperatures, you can pump and provide the required pressure to move from one tank to the other. In orbit, there is gravity. But because both of them, and see the acts of the link here, because both the fluid and the ship are both falling together, they're in free-faults, so there's no effective gravity between them. So considering you're going to have mixed phase, particularly where it's partially filled, you're going to have liquid floating around with gas. This is going to be really difficult to use a pump to provide the amount of pressure required on the pressure difference to be able to transfer it. Now what are the possible methods? Now one of them I'm going to call static pressure differential. So in my transfer tank, the supply tank on the right, we need to get it hotter. When we get it hotter, you're going to have vaporization. You're going to increase the pressure in the right hand tank. The second side, we want to keep it cryogenically very cold. That will keep it at a low pressure. Now this can be done passively because space is hot and cold. Please see the attached link. When we have a pressure differential, we open the valve between them and we can begin to flow. Now we, difficulty here is, as you flow, you're going to equalize the pressure. So you need to maintain this differential. So one of the ways to do so is to actually vent out of the receiving tank. Problem is you may not be able to get a complete transfer or we may end up having to vent the significant proportion to be able to transfer most of the liquid. Now method number two, I'm going to call induced gravity and induced as a funny word here. We simply burn the engines. That creates an acceleration. So what happens, that sloshing liquid is then almost like in a gravitational field. It's able to move to and be collected at the bottom of the rocket or in this case, what we show is the left. Therefore you can then open a transfer tube and you should be able to get what we might call gravity feed. It may be pump assisted. What's the fall back with this? You should be able to transfer really all the fluid, but how much fuel will you use? If you use loads of fuel, it's not very useful. Method three, I'm going to describe as a syringe. It's a electrical mechanical equipment that you're going to actually have to get into space. Literally what you're going to do is you're going to attach it to the receiving tank. You literally pull it. As a force, that should pull most of the fluid into here. Then we swap the actual valves and then we add a pushing force to literally squeeze it into the receiving tank. Theoretically, this should work. I don't think it's ever been tried. The difficulties here are making sure that the pressures, we don't have issues with pressures, but secondly, how much energy is required to do this. Again, it's an energy balance. If I have to literally burn some of my fuel to do it, then it may not be very useful. Or perhaps another method that SpaceX or another agency knows. As we said, this is vital for them to actually master. In the upcoming IFT-3, my guess is to go for method one. That's the static pressurization. Though there is a possibility as they're going to have a relight of the engine, they could try the induce gravity method at the same time as it's a small amount of fluid transfer. That's wait and see. Now, if you've enjoyed this video, please hit the like button. If you're interested in similar content, please hit the subscribe button.