# Demonstrating Rocket Fuel Transfer in Space: Challenges and Solutions

> An astronaut aboard the ISS explores the complexities of propellant transfer in microgravity using hands-on demonstrations.

[Watch on YouTube](https://www.youtube.com/watch?v=m4hvv2AfIhM)

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## The Challenge of Refueling in Space

![Astronaut speaking from the International Space Station interior](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t005.jpg)
*[0:05] Astronaut speaking from the International Space Station interior*

For decades, rockets have been launching into space, taking payloads, satellites, and humans. Each time that happens, the rocket launches and uses almost all of its fuel to get the payload into orbit. The rocket then falls into the ocean. Some new companies are now making rockets that save a little bit of fuel to come back and land on Earth to be reused again. But for the most part, rockets use all of their fuel, go to space, and the only place that rocket could get fuel is on Earth.

There are some small exceptions. The International Space Station receives a Progress cargo vehicle that launches and docks with the station, transferring some of its fuel so the station can adjust its orbit or maintain attitude control. But that's a very small amount of fuel transferred. We're not doing big, big things—by big things, I mean going to the moon or Mars.

## The Artemis Architecture: Orbital Refueling at Scale

As part of the Artemis program, we have an architecture whereby a large Starship is going to launch, go to space, and meet another Starship. They're going to connect and transfer fuel from one Starship to the other. Then one of those Starships will disconnect, go back to Earth, and continue going back and forth each time up into space to refuel this mothership full of fuel.

![A Starship booster landing surrounded by exhaust plumes](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t030.jpg)
*[0:30] A Starship booster landing surrounded by exhaust plumes*

Once that mothership is full, it'll transfer its fuel to another Starship—a special Starship. And this Starship goes to the moon. It will launch from Earth and come up with all that fuel out of the mothership, then go off to the moon to be met by the Orion spacecraft. They can then take that Starship down to the surface.

> **KEY** — This orbital refueling architecture is essential for the Artemis lunar missions, allowing spacecraft to carry sufficient propellant to reach the moon and return safely.

## Understanding the Physics: Gas and Liquid Separation

Before launch, I was thinking quite a bit about this and decided to bring up some demos and experiments that I can use to explain some of the difficulties in transferring a fluid from one place to another. As you see us go through these experiments with the help of Don, you'll see some of our struggles making these happen. Imagine how you would do this on the scale to go to the moon and Mars.

![Astronaut holding a small white object inside the ISS](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t060.jpg)
*[1:00] Astronaut holding a small white object inside the ISS*

We're going to simulate our rocket ship with a bottle of water. The water in there is going to simulate our rocket fluid, and you can see the gas bubbles in there. If you had a rocket engine fueling it with rocket fuel, you don't want a gas bubble to go into your rocket engine. To prevent that in space, different spaceships handle that in different ways. It's pretty common for spaceships to do something called a settling burn.

They take the fuel that's near where it's going to go into the rocket engine and do a little burn. They push it a little bit, and that little push moves the rocket fuel back towards the nozzle. So we're going to simulate that today with water as our rocket fuel and our gas bubble, letting this pressurize.

## Experiment 1: The Settling Burn Demonstration

The first thing we're going to do is just let it shoot a little bit. You'll see a little bit of gas comes out and it won't really accelerate a whole lot. Then maybe we'll do a settling burn—simulate a settling burn by pushing a little bit to get all the gas out of this side—and watch our rocket take off.

![Two astronauts demonstrating with water bottles and a towel in microgravity](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t120.jpg)
*[2:00] Two astronauts demonstrating with water bottles and a towel in microgravity*

After setting up the experiment and pressurizing the bottle with Alka-Seltzer, the demonstration begins. Initially, with no fluid near the nozzle and the Alka-Seltzer just shooting gas, you can hear it just puttering along—not generating much thrust. The gas alone provides minimal propulsion, illustrating why you can't effectively operate a rocket engine with gas bubbles in the fuel line.

![Astronaut holding a clear water bottle with visible bubbles](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t130.jpg)
*[2:10] Astronaut holding a clear water bottle with visible bubbles*

Now we're going to give it a settling burn to get some of that fluid there. By pushing the bottle to move the liquid toward one end, the experiment simulates how a small rocket burn settles propellant in the tank. Once the water is positioned near the opening, the thrust dramatically increases as liquid is expelled instead of just gas.

> **KEY** — A settling burn uses a small amount of propellant to create acceleration that moves fuel to the tank outlet, preventing gas ingestion and ensuring consistent engine performance.

## Experiment 2: Propellant Transfer via Rotation

I was at SpaceX in training and saw all these water bottles—the water bottles that come up in the Dragon, what we use to drink from on the Dragon when we're coming up to the space station or going back to Earth. I was looking at them and looking at a picture of Starship, which is going to launch as part of the Artemis program. The Starship is going to go into orbit, and we're going to send up a mothership for fuel. Then other Starships are going to go up and give it fuel repeatedly.

![Astronaut examining a water bottle filled with liquid and bubbles](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t138.jpg)
*[2:18] Astronaut examining a water bottle filled with liquid and bubbles*

I was thinking, well, how are we going to transfer fuel from one spaceship to another when the fluid—the rocket fuel—is inside, bouncing around? You kind of want to get it all to one end so your pipes can suck up that rocket fuel and transfer it someplace else. If you have one ship that just launched and is full of fuel, and it wants to get fuel to an empty rocket ship, you've got all these bubbles in there. Wouldn't it be cool if they docked like this, and then they could rotate about each other? The rotation puts all this fluid—all the rocket fuel—to one end, so your pipe picking up the fuel can access it.

![Astronaut demonstrating rotation concept with a water bottle](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t150.jpg)
*[2:30] Astronaut demonstrating rotation concept with a water bottle*

What we've got is—let's pretend this is one spaceship and this is another—we're going to transfer fuel from this one to the other one. We have a little pipe on the inside. Then we need to pressurize this side. Rockets move fuel or pressurize their fuel to get it to the rocket engine by pressurizing with gas, typically helium. We're going to use Alka-Seltzer in the water to pressurize this tank, push the fluid through the pipe, through this adapter that we 3D printed, into this side.

The extra gas in this side is going to come out a tiny little hole we have right here. Hopefully it doesn't shoot rocket fuel—hopefully not at Don, hopefully not at me. But the idea is to spin this and have it transfer, while we have this one pressurized. What'll be interesting is if we don't do that spin maneuver, we should just be pushing Alka-Seltzer gas through here. But if we do the spin maneuver like this, we should be able to get rocket fuel transferring.

> **WARNING** — Full disclosure: I tried this on the ground, not spinning and not in zero-G. So you might have a bit of a disaster here—watch Don and I make a mess of ourselves. But I think that's part of the fun.

## The Transfer in Action: Success and Challenges

![Two astronauts with connected water bottles and a towel in preparation](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t180.jpg)
*[3:00] Two astronauts with connected water bottles and a towel in preparation*

After connecting the two bottles and dropping Alka-Seltzer into the pressurized side, the experiment begins. The astronauts initiate a spin to force the liquid away from the gas vent port. As the connected bottles rotate, centrifugal force pushes the water outward in each bottle, keeping the liquid at one end of the pressurized tank where the transfer pipe can pick it up.

![Astronaut handling the experiment while colleague holds a towel nearby](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t203.jpg)
*[3:23] Astronaut handling the experiment while colleague holds a towel nearby*

We're already transferring fuel—I can see it! But what's interesting to me is the center of gravity might change as we transfer fuel from one to the other. It's tough to find which part to hit, because you can't hit the center of mass. As fluid moves from one bottle to the other, the balance shifts, making it harder to maintain a stable spin. Occasionally, fluid escapes from the gas port when the rotation slows.

![Astronaut holding combined water bottles up to the camera](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t240.jpg)
*[4:00] Astronaut holding combined water bottles up to the camera*

To counteract this, the astronaut spins it really fast to get the fluid away from the gas port. Now the center of mass is equalizing, and fluid is pumping either way. You can see the center of mass changing as it pumps fluid from one side to the other. The experiment demonstrates both the promise and the difficulty of propellant transfer in microgravity: rotation can separate gas from liquid, but managing the shifting mass and maintaining spin stability is challenging.

> **ASIDE** — One bottle should have been made visually distinct to better track the transfer. The one with more water now is the original pressurized tank.

## Lessons for Future Missions

![Astronauts laughing while handling the water bottle demonstration](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t270.jpg)
*[4:30] Astronauts laughing while handling the water bottle demonstration*

Despite some leakage from the gas port and the difficulty in maintaining a stable spin as mass shifted between tanks, the demonstration successfully showed that propellant can be transferred between two vessels in microgravity using rotation to manage fluid position. The experiment revealed several key challenges: maintaining rotational stability as the center of mass shifts, preventing gas from entering the transfer line, and managing pressure differentials.

![Graphic showing SpaceX Starship tests for Artemis campaign at sunset](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t300.jpg)
*[5:00] Graphic showing SpaceX Starship tests for Artemis campaign at sunset*

These challenges scale up dramatically for actual spacecraft operations. When Starship vehicles dock in orbit to transfer tons of cryogenic propellant, they'll need robust systems to manage fluid behavior, control pressure, prevent contamination, and maintain structural integrity—all while potentially rotating to use centrifugal force as a propellant management tool.

> "I'm hoping our first Starship propellant transfer is a little bit more controlled than that." — at 8:36

Engineers are developing sophisticated systems including baffles, screens, vanes, and active propellant management devices to control fluid behavior. The rotation technique demonstrated here is one of several methods under consideration. Others include using surface tension devices, applying small accelerations (ullage burns), or employing bladders and diaphragms to separate liquid from gas.

## Why Orbital Refueling Matters

![Astronauts holding connected transparent bottles with liquid and connecting tube](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t330.jpg)
*[5:30] Astronauts holding connected transparent bottles with liquid and connecting tube*

The ability to transfer propellant in orbit is foundational to the Artemis program's lunar ambitions and future deep-space missions. By refueling spacecraft in low Earth orbit, missions can carry far more payload to their destination than would be possible with a single launch. A spacecraft doesn't need to carry all its fuel from Earth's surface; instead, it can be topped off in orbit where the gravitational penalty is much lower.

This architecture enables reusable spacecraft to make multiple trips, reduces the size and cost of individual launches, and opens the door to missions that would otherwise be impossible with current rocket technology. The technique scales from lunar missions to Mars expeditions and eventually to deep-space exploration, making propellant transfer one of the key enabling technologies for humanity's future in space.

> **KEY** — Mastering propellant transfer in microgravity transforms space logistics, enabling sustainable exploration of the moon, Mars, and beyond by allowing spacecraft to refuel in orbit rather than carrying all fuel from Earth.

## From Demonstration to Reality

![Close-up of astronaut holding connected water bottles showing fluid and gas separation](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t360.jpg)
*[6:00] Close-up of astronaut holding connected water bottles showing fluid and gas separation*

These simple demonstrations with water bottles and Alka-Seltzer tablets illustrate the fundamental physics that engineers must master for orbital propellant transfer. While the scale is vastly different—Starship will transfer hundreds of tons of liquid methane and liquid oxygen rather than milliliters of water—the underlying principles remain the same: separate gas from liquid, maintain pressure, control fluid motion, and account for shifting center of mass.

![Astronauts displaying the connected bottle system with visible liquid transfer](http://www.farzi.me/jobs/job-1779646830646-celc6w/screenshots/t390.jpg)
*[6:30] Astronauts displaying the connected bottle system with visible liquid transfer*

SpaceX and NASA are investing heavily in developing and testing these systems on the ground and in orbit. Upcoming Starship test flights will include propellant transfer demonstrations, gradually building confidence in the technology before it's needed for actual lunar missions. Each test will refine techniques, validate models, and prove that large-scale cryogenic propellant transfer is not just theoretically possible but practically achievable.

The success of the Artemis program hinges on solving these challenges. When astronauts return to the moon aboard a propellant-transfer-enabled Starship, it will represent not just a return to the lunar surface, but a fundamental shift in how humanity accesses space—one where spacecraft can refuel on orbit, enabling journeys far beyond what single-launch missions could ever achieve.

## Key takeaways

- Traditional rockets use nearly all their fuel to reach orbit, making refueling in space essential for deep-space missions to the moon and Mars.
- The Artemis program architecture relies on multiple Starship launches to refuel a 'mothership' in orbit, which then supplies the lunar lander.
- In microgravity, gas and liquid don't separate naturally, requiring techniques like settling burns or rotation to position propellant for transfer.
- A settling burn uses a small thrust to push fuel toward the tank outlet, preventing gas bubbles from entering rocket engines.
- Rotating docked spacecraft can use centrifugal force to push liquid propellant to one end of the tank, enabling controlled transfer through connecting pipes.
- Propellant transfer demonstrations reveal challenges including maintaining rotational stability, managing shifting center of mass, and preventing gas contamination.
- Orbital refueling enables spacecraft to carry more payload by topping off fuel in space rather than launching with all propellant from Earth's surface.
- Mastering cryogenic propellant transfer in orbit is a key enabling technology for sustainable lunar exploration and eventual missions to Mars and beyond.


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