Aerospace Prints Rocket Motors in 3-D

Jerry Fuller, a senior research associate, holds a helical star grain, printed in acrylic with a 3-D printer. (Photo: Elisa Haber)
Jerry Fuller, a senior research associate, holds a helical star grain, printed in acrylic with a 3-D printer. (Photo: Elisa Haber)

Over the past few years, Aerospace’s Jerry Fuller has worked as a developer of rocket fuel grains. His primary responsibilities include developing mechanisms and technologies for the corporation’s picosatellite program, and other mechanics research. But his work with fuel grains is gaining increasing attention, primarily because it utilizes a versatile method of production that is just beginning to come into its own — rapid prototyping.

Rapid prototyping — also referred to as Additive Manufacturing — is a group of technologies that use computers to make 3-dimensional shapes. This form of 3-D printing transforms 2-dimensional layers of raw material into a finished product that is capable of structural nuance and complexity that might otherwise be impossible with traditional production techniques. Currently, scientists and hobbyists alike are quickly introducing new materials and products to the technology. Everything, from tennis shoes to chocolate frosting to pieces of art and even cell-based, lifelike prosthetics, is being explored as a potential product of rapid prototyping. The technology seems primed to take off in a paradigm-shifting way that could influence any number of fields including manufacturing, retail sales, scientific research, and healthcare.

Primarily, Fuller has been utilizing rapid prototyping to develop efficient fuel grains for hybrid rockets. A hybrid rocket uses a motor composed of propellants that are in two different physical states — the fuel is typically a solid while the oxidizer is either a liquid or gas. Sometimes humorously referred to as “high-pressure tire fires,” hybrid rockets will often use rubber, plastic, or paraffin wax for fuel and oxygen or nitrous oxide as oxidizer. Fuller and his colleagues at Aerospace have been printing their own fuel grains, which employ a couple of interesting design adjustments, aimed at increasing efficiency.

A selection of burned and unburned fuel grains manufactured with rapid prototyping technology. Photo by Elisa Haber

A selection of burned and unburned fuel grains manufactured with rapid prototyping technology. (Photo: Elisa Haber)

“The standard techniques of production make it difficult to force the oxidizer and the fuel to mix together,” says Fuller. “But, if you can print the fuel, you can make any internal shape you want. And that gives you two things: It gives you an opportunity to create shapes that force the oxidizer and the fuel to interact. And it gives you a third dimension in which to create surface area. Surface area equals thrust.”

In the fuel grains that Fuller and his Aerospace team have developed, there are a selection of different shapes that have been explored, including a number of variations on the helix structure, a few star-grain designs and a coaxial grain with a combination of port shapes. The helix structure is used to increase surface area and enhance the mixing of oxidizer and fuel without increasing the physical size of the grain. The increased surface area creates more time for the fuel and oxidizer to interact.  A number of these fuel grain designs have been successfully tested in the Aerospace labs and the results confirm that rapid prototyping is a feasible method of production for fuel grains.

With rapid prototyping, many of the inefficiencies associated with hybrid rockets can be mitigated. As the designs are refined and tested further, it is likely that hybrid rockets will be used in situations beyond the launch. “People would like to use hybrid motors more for on-orbit applications,” says Fuller. “And that gets to one of the advantages of hybrids. Hybrids are motors that you can throttle and turn off and turn back on, so that’s really good for on-orbit applications because you can precisely tailor exactly where you’re putting the thrust and when. There are people currently developing small launch vehicles using hybrid rockets.”

A basic 3-D printer, located in the PicoSat laboratory at Aerospace. Photo by Elisa Haber

A basic 3-D printer, located in the PicoSat laboratory at Aerospace. (Photo: Elisa Haber)

In its infancy, rapid prototyping was used to build models and approximations of products for reference and marketing purposes. As the technology improved, it became easier to make a fully functional part or product. For years, Aerospace has been pushing the boundaries of what rapid prototyping parts can do in a real world context. In 2006, Aerospace’s David Hinkley, senior projects leader, Mechanics Research Department, flew an elaborate, cold gas propulsion system on a MEPSI PicoSat, which marked the first time a rapid prototyping part had ever flown in space. Since then, the use of rapid prototyping parts in Aerospace’s PicoSat missions has become more and more prevalent.

In the coming years, Fuller believes that the aerospace industry will benefit greatly from rapid prototyping technology. “Spacecraft manufacturers often make very expensive things in small numbers of units,” says Fuller. “When you’re making small numbers of things and you don’t want to pay for tooling that you will never use again, rapid prototyping is perfect.” This need for complex, one-of-a-kind manufacturing exhibits a clear synergy with the rapid prototyping technology and bodes well for its widespread application in the near future.

Currently, rapid prototyping is at a critical juncture where it seems poised to leap into the mainstream as a major force in manufacturing. Websites currently exist where consumers can download design plans for a product and then print it in 3-D on a personal or rented printer. The scope of the technology seems almost limitless, but for now, it will take the foresight and ingenuity of those engineers and scientists on the front lines to push things forward in an effective way.

—Matthew Kivel
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