Advanced Hybrid Rocket Motor Propulsion Unit For CubeSats

Open Access
Mcknight, Brendan Robert
Graduate Program:
Mechanical Engineering
Master of Science
Document Type:
Master Thesis
Date of Defense:
Committee Members:
  • Richard A Yetter, Thesis Advisor
  • additive manufacturing
  • CubeSat
  • hybrid rocket motor
  • regression rate
  • combustion efficiency
CubeSats continue to become popular for universities and businesses to affordably conduct research in low Earth orbit. Falling within the nanosatellite category, CubeSats typically consist of a 1U, 1.5U, 2U, or 3U standard configuration. A 1U CubeSat is a 10 cm x 10 cm x 10 cm cube, while 1.5U, 2U, and 3U CubeSats consist of stacked U(s) with a 10 cm x 10 cm square footprint. More recently, 6U CubeSats have been developed in both 6U x 1U and 3U x 2U configurations. Larger CubeSats will continue to be developed as dispenser systems become larger to accommodate the satellites during launch and deployment. To date, CubeSats have been limited by the orbit into which they are deployed, either as an auxiliary payload on a rocket carrying a primary payload to its required orbit or from the International Space Station. With the development of a safe and reliable propulsion system, CubeSats will be capable of performing orbital control maneuvers, such as orbit raising to extend the lifespan of the satellite and provide onboard instruments with additional data collection time. The propulsion unit in the current work was developed in collaboration with The Aerospace Corporation to provide a 6U CubeSat with this ability. The 1U propulsion unit consists of an additively manufactured carbon-filled polyamide structure and integrated nitrous oxide tank with a cartridge-loaded, 3D-printed solid fuel grain of ABS or paraffin/acrylic composition and performance enhancing geometry. Strength testing of the structure material characterized its properties as-received and after exposure to nitrous oxide. A significant plasticizing effect was observed for the material exposed to nitrous oxide. Ultimate tensile strength decreased by over 20% and modulus of elasticity decreased by over 40%, while elongation to break was increased. For this reason, future work will investigate an additively manufactured metal propulsion unit of similar design. Hot-fire testing of various solid fuel grain compositions and geometries was conducted at the Pennsylvania State University’s High Pressure Combustion Laboratory using the Long-Grain Center-Perforated hybrid rocket motor with nitrous oxide. The results of the test series revealed interesting behavior for the additively manufactured paraffin, paraffin/acrylic, ABS, and Windform XT 2.0 fuel grains. Carbon-filled polyamide, Windform XT 2.0, fuel grains exhibited very slow regression rates, as was expected. ABS fuel grains with star-swirl port geometry were found to have increased regression rate over straight port ABS fuel grains, similar to what was found from previous oxygen testing of star-swirl geometry acrylic fuel grains. However, interesting to note was an observed increase in regression rate for white and black colored ABS fuel grains over pure ABS. This could be the result of increased radiation absorption with the added pigments, but due to a lack of provided information from the manufacture about the pigments this cannot be directly concluded. Combustion efficiency results show a decrease for star-swirl geometry fuel grains compared to straight port fuel grains for all ABS samples, while star-swirl geometry appeared to increase combustion efficiency for Windform XT 2.0 fuel grains. Increased regression rate was also observed for paraffin/acrylic diaphragm fuel grains over straight port paraffin. Examination of fired paraffin/acrylic 0.050” diaphragm fuel grains showed chipping of the acrylic diaphragms and resulting uneven regression in the downstream paraffin sections. The paraffin/acrylic 0.100” diaphragm fuel grains better maintained uniform regression due to the increased acrylic diaphragm thickness, however, combustion efficiency decreased significantly compared to that of pure paraffin. An iteration on the diaphragm fuel grain design included one 0.100” acrylic diaphragm with an acrylic mixing section downstream of the diaphragm. This particular fuel grain design was shown to increase combustion efficiency over the original paraffin/acrylic diaphragm fuel grain design and pure paraffin fuel grains.