Take a two-liter bottle, and fill it with water. Now, turn it upside down. Did you see a lot of bubbles as the liquid drains rapidly from the top? That's because air is trying to get back in the bottle due to the low pressure created in the space above the liquid as it runs out.

When it comes to liquid propellant tanks for rockets, the same scenario applies. While the engines are running, fuel drains at a rapid speed from the tanks.

However, to prevent the low pressure from reducing the controlled flow of the propellant, a pressurization system is required to maintain the density and required flow rate of the rocket fuel. But when you're talking about millions of pounds of fuel, you don't want to just shoot in gas that impinges directly on the liquid surface.

Instead, that's what a special piece of hardware is for -- to "diffuse the situation" in the tank and allow gas to flow uniformly at the lowest velocity possible and not stir up to the surface of the liquid.

That hardware, called a diffuser, is no stranger to rocketry. It was used during the space shuttle era. But these days, engineers at NASA's Marshall Space Flight Center in Huntsville, Alabama, have taken that proven design and cut it down to a much smaller size.

"Typical diffuser designs generally have long cylinders," said Mike Martin, lead on the low-profile diffuser project at Marshall.

"A lot of times, those diffusers don't make full use of the area in the tank. Our idea was to create a diffuser that makes a much smaller footprint without it severely impacting the performance of the pressurization system. That's how we came up with the low-profile diffuser, which is only about 10 inches tall."

"Using a smaller diffuser can allow us to raise the liquid level up higher and add more rocket fuel," Martin added. "When you do that, you have the potential to increase the amount of payload that you can carry on future launch vehicles, like the Space Launch System."

NASA's Space Launch System (SLS) will be the biggest, most powerful rocket in history making it possible for future explorers to travel on deep space missions to an asteroid and ultimately to Mars.

The Boeing Co. is the prime contractor for the SLS core stage, and is designing and building the flight diffusers for the rocket's liquid oxygen and liquid hydrogen tanks. Boeing is using the same Marshall facility where the low-profile diffuser is being tested, which is a "win-win" according to Keith Higginbotham, task lead for Marshall's Spacecraft Payload Integration and Evolution Office.

"Having Boeing and our team use the same testing facility not only has reduced costs, but we've been able to help Boeing gather additional data using our instrumentation for their flight diffusers. We also can conduct comparative tests to see if our low-profile diffuser may be a better option than the current flight diffusers for later SLS models."

The low-profile diffuser has already finished phase one of its trial series, which included about 30 different tests.

For the next round of testing, it will be mounted to a test rig, and run for two to three minutes to gather velocity data and validate computational fluid models used to design it.

Testing is scheduled through July. The design, production and testing of the hardware is a collaborative effort between Marshall's Engineering Directorate and Spacecraft Payload Integration and Evolution Office, within the SLS Program.

The first flight test of the SLS will feature a configuration for a 70-metric-ton (77-ton) lift capacity and carry an uncrewed Orion spacecraft beyond low-Earth orbit to test the performance of the integrated system.

As the SLS evolves, it will provide an unprecedented lift capability of 130 metric tons (143 tons) to enable missions even farther into our solar system.