Verification of the throttling characteristics of axial-injection end-burning type hybrid rockets

Yuji Saito, Toshiki Yokoi, Hiroyuki Yasukochi, Kentaro Soeda, Tsuyoshi Totani, Masashi Wakita, Harunori Nagata

Research output: Contribution to journalConference articlepeer-review


The axial-injection end-burning type hybrid rocket originally proposed twenty years ago by Nagata and Hashimoto et al. recently recaptured the attention of researchers for its virtues such as constant ξ (oxidizer to fuel mass ratio) during firing and throttling operations. Previous studies revealed that, for combustion in a single-port fuel grain, the end-face regression rate in the axial direction is proportional to pressure, with a pressure exponent of 0.95. Accordingly, these rockets were expected to display good throttling characteristics. Given that no ξ shift occurs, keeping the oxidizer mass flow rate within 1% of its initial design point ensures specific impulse will remain within 97% of its design point. There are several requirements for realizing this type of hybrid rocket: 1) high fuel filling rate for obtaining an optimal ξ 2) small port intervals for increasing port merging rate; 3) ports arrayed across the entire fuel section. Because common manufacturing methods were unable to produce a fuel that satisfied these requirements, no previous researchers have conducted experiments with this kind of hybrid rocket. Recent advances in high-accuracy 3D printing have enabled such fuels to be produced for the first time. The fuel grains used in this study were produced by a high-precision light polymerized 3D printer. Each grain consisted of an array of 0.3 mm diameter ports for a fuel filling rate of 98%. Last year, the authors reported the results of multiple firing tests of an axial-injection end-burning type hybrid rocket using 3D printed fuel grains and verified that solid fuel regression rate is linearly dependent on pressure. In this study, the authors conducted a unique set of experiments to verify the throttling characteristics of the axial-injection end-burning type hybrid rocket. Oxidizer mass flow rate and chamber pressure were throttled during firings by actuating valves in a fluid circuit consisting of four oxidizer supply lines. Chamber pressure and oxidizer mass flow rate were measured during each firing. These experimental data were analyzed by a reconstruction technique to obtain ξ history. The results show that ξ remains almost constant during firing, even during throttling operations. Therefore, this study verifies that the axial-injection end-burning type hybrid rocket has superb throttling characteristics. Additionally, the study supports findings in previous research that indicate the pressure exponent is close to unity.

Original languageEnglish
JournalProceedings of the International Astronautical Congress, IAC
Publication statusPublished - 2016
Event67th International Astronautical Congress, IAC 2016 - Guadalajara, Mexico
Duration: 2016 Sept 262016 Sept 30


  • Fuel regression characteristics
  • Hybrid rockets
  • Throttling characteristics


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