FDM 3D Printing of Rocket Fuel Grains

In ‘Small-Scale Static Fire Tests of 3D Printing Hybrid Rocket Fuel Grains Produced from Different Materials,’ authors Mitchell McFarland and Elsa Antunes create a device to evaluate small-scale fuel grains, focusing on material regression rates, as well as comparing them to other hybrid fuels in experiments that are important to the space industry as new rockets and motors are created, using a variety of technologies and new materials.

McFarland and Antunes explain that 3D printing has been the ‘most impressive advancement,’ and especially with FDM 3D printing as it offers all the benefits that are attractive to investors and manufacturers, from speed in production to substantial savings on the bottom line.

“FDM has enabled designers to incorporate complex combustion ports into HRMs and has opened up an entirely new set of materials for the fabrication thereof,” state the researchers.

Typical casting and curing techniques can be more difficult—and despite improvements in such methods—designs are still limited in innovation and must account for post-processing in removing the tooling. Supports are not required for fuel grain fabrication very often, but even if they are, the structures are easily removed due to water-soluble materials.

To complete their goal of analyzing regression, the authors performed several small-scale static fire tests with 3D printed materials, including:

• ABS – performs as well as HTBP
• ASA – beneficial in similarities to ABS
• PLA – benchmark for testing Al
• PP – low price/high crystalline material
• PETG – excellent mechanical properties
• Nylon – excellent mechanical properties
• Al – PLA with aluminum particles

Structural, thermal, and mechanical properties of test materials

Each grain was created 100 mm long and 20 mm in diameter, with a 6 mm diameter combustion port. The team 3D printed a series of ABS grains using a Prusa i3 MK2 FDM 3D printer, with the materials subjected to three-minute burns, tested, and measured.

Small-scale fuel grains, left to right: ABS, PLA, PETG (Polyethylene terephthalate glycol), PP, ASA, Nylon, and AL (PLA with aluminum particles).

“Once the burning time was reduced to three seconds, the structural integrity of the fuel grain was far better preserved with combustion of fuel being limited to within the combustion port,” noted the researchers. “Three further tests were then conducted with 100 mm × 20 mm ABS fuel grains to ensure the repeatability of the test stand chamber pressure measurement. The results of the thrust validation show excellent consistency across the three burns, with very similar profiles demonstrated in each run.”

Some of the settings were modified to avoid any defects that could affect the performance of the materials. The team noted ‘zits’ on the outer surfaces of some prints—describing them as ‘oozing’ in some cases, or ‘stringing’ across the chamber. For future projects and manufacturing of components, such issues would have to be resolved. For this experiment, the researchers emphasized that they were concerned with material selection over port geometry.

“Visual inspection of each burn suggested that the ABS and ASA performed very well, and it was also noted that the PETG burn was significantly hotter than any of the other burns,” stated the researchers. “Of all the materials tested, the AL appeared to have performed extremely well, with an incredibly energetic combustion. However, inspection of the regression rate data shows that it was, in fact, one of the worst performing materials.”

ASA had the highest average regression rate, followed by nylon, and then PEETG.

“It was observed that the fuel port radius of the ASA grain increased by the greatest amount. Despite the expected energetic combustion of the AL fuel grain, it was found to have a regression rate like the PLA without the addition of aluminum powder,” stated the researchers. “This shows that the aluminum powder did not impact on the regression rate, perhaps due to its particle size and surface area. The regression rate of ABS fuel grain was one of the lowest of the materials tested. A similar value to ASA was expected due to their similar chemical and mechanical properties. This unexpected value can be explained by the low oxidizer mass flux for ABS.

“It was speculated that the poor performance of the AL fuel grain was largely due to the size, shape and surface area of aluminum particles. Despite this poor performance in this research, the impact of Al particles on fuel grains performance should be analyzed, especially the contribution of different variables, such as, size, shape, and surface area of aluminum particles.”

While the world of 3D design and 3D printing is full of wonders that are wide-ranging—and from around the world—the study of materials continues to grow more fascinating. 3D printing and additive manufacturing processes with metal are involving impressively, along with polymers, and many other alternative forms such as concrete, wood, and more. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.

Combustion port comparisons, left to right: ABS, PLA, PP, ASA, PTEG, and AL.