The Aerospace Corp. team that tested a 3D-printed liquid-fuel rocket motor: Hila Wright, safety staff; Andrew Cortopassi, propulsion science department senior technical member; John McVey, astrodynamics department systems engineer; Jerry Fuller, space science applications lab researcher; and Chris Kobel, astrodynamics department flight operations.

Solid, liquid, or gas might sound like a question on a high school physics test, but it’s important to consider when powering a rocket into space. Aerospace Corp. in El Segundo, California, is betting that a 3D-printed, hybrid liquid rocket motor will make rocket propellants less expensive, supporting more trips into space.

“The Aerospace Corp. created and has led the field of 3D-printed propellant from its beginning, but we’ve never flown any of the technologies we created in this area,” says Jerry Fuller, a researcher at the Space Science Applications Laboratory at the Aerospace Corp., who came up with the idea.

Propulsion options

There are several types of rocket propulsion systems. Liquid propellants are the most capable, but the most expensive. Solid propellants are simple, inexpensive, and common. Hybrid propellants that combine solid fuel with a liquid or gas oxidizer are reasonably inexpensive but are rarely used since they burn too slowly and are not efficient.

Finding a new way to deliver liquid fuel through 3D printing is a significant advance in rocket motor technology. Fuller says 3D-printed helical shapes enable the liquid or gas oxidizer to interact with the solid fuel more effectively, dramatically improving performance.

Normally, liquid motors require expensive turbopumps to deliver fuel and oxidizer from low pressure tanks into a thrust chamber. Fuller rearranged that traditional structure to look more like that of a hybrid rocket, where liquid oxidizer flows through a chamber that contains solid fuel. A printed liquid-fuel grain stores and meters liquid fuel flow as the oxidizer passes through. This arrangement eliminates the plumbing and turbomachinery needed in liquid motors, allowing them to be very small, simple, and inexpensive.

Put to the test

Fuller and systems engineer John McVey decided to test the motors and recruited propulsion science engineer Andrew Cortopassi and amateur high-power rocketry expert Chris Kobel to complete the task. The team first tested the motors in Aerospace’s propulsion research facility, then took them to California’s Mojave Desert. There, they launched four of the liquid motors and one hybrid motor on high-power hobby rockets. They expected to reach an altitude of 1,000ft to 4,000ft, but one of the liquid motors exceeded their expectations, reaching an altitude of more than 5,000ft, with a maximum velocity of almost 600mph.

CAD drawing and a 3D-printed fuel grain ready to be filled with liquid fuel.

Fuller says the first flight test series was simply to prove the liquid fuel grain was viable, to show the benefits of printing hybrid fuel grains, and claim a first flight of the liquid motor technology.

“The next series will tell us about scaling our motor technologies and allow us to do more thorough engineering than the first series of proof-of-concept tests,” Fuller explains. “By the end of this round, we expect to have enough information to begin designing a space-capable sounding rocket. It will also let us investigate refinements like throttling.”

Looking forward

Fuller emphasizes that the research so far has been for small motors. Larger motors still need testing, but current 3D printers could create a fuel grain for a sounding rocket or a small CubeSat launcher.

“Ultimately, we would like to provide the Aerospace Corp.’s Space Science Applications Laboratory with a unique space science capability, enhancing existing and enabling new space science efforts by making sounding rockets with inexpensive, nimble, and effective hybrid and liquid rocket motors, while at the same time, testing concepts that might scale up to much larger vehicles,” he says.

The potential for a less-expensive, high-performance motor is attractive. As Fuller puts it, “A simple liquid motor is something that doesn’t really exist now, and if we can find ways, probably through 3D printing, to manage the delivery of liquid fuel, then we will have made a significant advance in rocket motor technology. It looks like we have found at least one of these ways.”

Next on the agenda, according to Fuller, “We will test fuel grains that target about a 4x increase in thrust and total impulse, using a larger motor. We will also work on improved instrumentation for the experiments, and more careful design.”

Aerospace Corp.
www.aerospace.org