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Hybrid Rocket Underwater Propulsion: A Preliminary Assessment

Aerospace and Mechanical Engineering, Korea Aerospace University, Goyang 10540, Korea
Agency for Defense Development, Daejeon 34186, Korea
Author to whom correspondence should be addressed.
Aerospace 2019, 6(3), 28;
Received: 28 January 2019 / Revised: 17 February 2019 / Accepted: 22 February 2019 / Published: 6 March 2019
PDF [9458 KB, uploaded 6 March 2019]


This paper presents an attempt to use the hybrid rocket for marine applications with a 500 N class hybrid motor. A 5-port high density polyethylene (HDPE) fuel grain was used as a test-bed for the preliminary assessment of the underwater boosting device. A rupture disc preset to burst at a given pressure was attached to the nozzle exit to prevent water intrusion where a careful hot-firing sequence was unconditionally required to avoid the wet environment within the chamber. The average thrust level around 450 N was delivered by both a ground test and an underwater test using a water-proof load cell. However, it was found that instantaneous underwater thrusts were prone to vibration, which was due in part to the wake structure downstream of the nozzle exit. Distinctive ignition curves depending on the rupture disc bursting pressure and oxidizer mass flow rate were also investigated. To assess the soft-start capability of the hybrid motor, the minimum power thrust, viewed as the idle test case, was evaluated by modulating the flow controlling valve. It was found that an optimum valve angle, delivering 16.3% of the full throttle test case, sustained the minimum thrust level. This preliminary study suggests that the throttable hybrid propulsion system can be a justifiable candidate for a short-duration, high-speed marine boosting system as an alternative to the solid underwater propulsion system. View Full-Text
Keywords: hybrid rocket; marine propulsion; rupture disc; idling operation; underwater environment hybrid rocket; marine propulsion; rupture disc; idling operation; underwater environment

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Moon, H.; Han, S.; You, Y.; Kwon, M. Hybrid Rocket Underwater Propulsion: A Preliminary Assessment. Aerospace 2019, 6, 28.

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