Demonstration of dual-thrust capability in hybrid rockets using multi-layered tubular fuel grains

This study demonstrates the thrust modulation capabilities of a hybrid rocket motor employing a Multi-Layered Tubular (MLT) fuel grain. The MLT configuration involves arranging the fuel grains with distinct regression rates as layers. As combustion progresses, the regression rate changes based on the burning of each fuel layer, leading to variable thrust while maintaining a constant oxidizer flow rate. Variable-thrust fuel grains are suitable for diverse mission types, including employment in sounding rockets and cruise missiles, particularly those designed for anti-ship operations and missions wherein a boost-sustain thrust profile is required. The MLT fuel grains were designed for a boost-sustain phase thrust profile generally observed in missiles. Wax was used as a booster fuel, and wax with 20%EVA was used as a sustainer fuel. Two MLT configurations with different durations for the boost phase were demonstrated. During the tests, the boost-sustain phase was achieved successfully. However, a transition phase was observed while shifting from the boost to the sustain phase. The delay of the transition phase was observed to increase when the duration of the boost phase increased. The combined effect of axial variation of fuel regression rate and local mass flux is the reason for the transition phase. The Thrust Turn-Down Ratio (TDR) for Multi-Layered Tubular (MLT) fuel grains was calculated. At a web thickness (for booster fuel) of 4 mm, the TDR was approximately 1.23:1, and at a 5.5 mm web thickness, it was 1.28:1. Further, 20%Mg was added to the wax to increase the regression rate of booster fuel. The Wax/20%Mg fuel showed 41% and 14% improvement in regression rate and thrust, respectively. The TDR was improved marginally compared to pure wax-based MLT grain.