Design and Testing of a Lab-Scale Hybrid Rocket Motor to Evaluate Swirl Injection Characteristics

Design and Testing of a Lab-Scale Hybrid Rocket Motor to Evaluate Swirl Injection Characteristics

Susane R. Gomes (Aeronautics Institute of Technology, Brazil) and Leopoldo J. Rocco (Flowtest Aerospace Research, Brazil)
Copyright: © 2018 |Pages: 26
DOI: 10.4018/978-1-5225-2903-3.ch009
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This research aims to provide a methodology for the project of labscale hybrid motors. This development began with the thermal analysis of the fuel grain using the Flynn, Wall and Ozawa method, generating simulation entry data to maximize the motor performance. The simulation was performed with the Chemical Equilibrium Specific Impulse Code. Based on the optimum oxidizer to fuel ratio, the literature was used to supply the mathematical background to calculate the motor geometrical parameters whose operating conditions were determined throughout the simulation. Finally, firing tests were conducted to verify the reliability of the project methodology. The firing tests were performed with three injectors: two swirling and one axial. The tests showed that the higher the operating pressure the more suitable is the project, meaning the methodology developed works best in hybrid rocket motors with high operating pressures. Additionally, it was shown that the swirling flow injectors produce higher efficiency.
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This section outlines the steps taken in the prototype project: selection of materials; thermal analysis of the grain; combustion simulation and geometry determination.

Selection of Materials

The selection of the oxidizer physical state can be done attempting to the complexity inserted by this decision. A swirling two phase flow is avoided when the oxidizer is gaseous, meaning that more accurate and resistant hydraulic valves, which are considered necessary to operate in cryogenic temperatures, are not needed; this choice decreases cost and engine weight and simplifies the systems design.

Gaseous oxygen was used for its good performance, ease of handling and cost. The desired fuel must present low burning rates, in such a way that small changes in regression rates can be easily determined, in other words, such attribute make comparisons between miscellaneous performance results readily feasible. Most polymers present high viscosity and high surface tension, which fulfills the features required. Polyethylene was found to be rather appealing, once it is cheap, easy to find in the market and to conform in the desired form.

Two branches of polyethylene were tested for fuel grains, Ultra High Molecular Weight (UHMW) and High-Density Polyethylene (HDPE). The first tests evidenced that UHMW has better combustion stability and did not produce excessive carbon black, see Figure 1a. Meanwhile, tests conducted with HDPE showed high concentration of carbon black deposited all over the grain surface, Figure 1b.

Figure 1.

A: Ultra High Molecular Weight fuel grain after test. B: High Density Polyethylene fuel grain after testing

The first nozzle used was made of stainless steel. But it could not handle a 15 s test without substantial throat erosion, see Figure 2. Therefore, graphite (20 µm grain length) was used. Graphite could endure each test with a maximum throat expansion of 0.5 mm, Figure 3.

Figure 2.

Stainless steel nozzle test (after 15s test)

Long tests were performed to address nozzle endurance in high temperature conditions. As a result it was observed that the stainless steel around the nozzle suffered melting before problems could be noticed on the graphite nozzle, which confirms the high quality of the graphite used, Figure 3.

Figure 3.

Graphite nozzles. A: Before test. B: After test, the nozzle is surrounded by melted stainless steel, 60s test

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