Improving the Combustion Performance of a Hybrid Rocket Engine using a Novel Fuel Grain with a Nested Helical Structure

This method can help answer the question of how to improve the combustion performance of grains. The main advantages of this protocol is that the helical structure fuel grains will not disappear with the combustion process. This method can also be applied to grain formula with different material compilations such as EBS and PUX.

Begin by preparing the acrylonitrile butadiene styrene, or ABS substrate, using 3D software. Save the 3D substrate structure as an STL file. Then open the 3D slicing software and import the structure.

Click start slicing and select speed print mode from the main template. Double-click speed. Then change the infill density to 100%and select raft with skirt for the platform edition.

Click save and close and then click slice. Turn on the 3D printer and import the ABS substrate's slice file. Set the temperature of the heated bed and nozzle to 100 and 240 degrees Celsius respectively.

Click start to print after stabilization. To ensure successful printing, apply solid glue to the hot plate to increase the adhesion between the ABS substrate and the hot plate. For a paraffin-based fuel preparation prepare raw materials of paraffin, polyethylene wax, steric acid, ethylene vinyl acetate and carbon powder.

Configure the paraffin-based fuel according to manuscript directions and place the configured materials into the melt mixer. Then melt and stir them until completely mixed. Place the ABS substrate into the centrifuge and secure it with an end cap.

Plug in the powder and turn on the water cooling pump switch. Then turn on the centrifuge relay and increase the speed to 1, 400 RPM. Open the valve on the melt mixer and start casting.

Remove the fuel grain and trim the shape. Measure and record the weight, length and inner diameter of the complete fuel grain and photograph it. To assemble the hybrid rocket engine, fix the combustion chamber section on the slide rail, load the fuel grain, and install the post combustion chamber section.

Install the head and nozzle. Then install the torch igniter on the head of the hybrid rocket engine. Install the spark plug and connect the power supply.

Connect the nitrogen, oxidizer, ignition methane, and ignition oxygen gas supply lines between the test bench and the gas cylinder. Connect the industrial computer, the multifunction data acquisition card, the mass flow controller, and the control box of the test bench. Power on the test bench, the mass flow controller and the igniter.

Open the FlowDDE software and click on communication settings. Click the corresponding connection interface and click okay. Click open communication to establish communication with the flow controller.

Then open the measurement and control program, or MCP. Set the input and output channel of the multifunction data acquisition card and click run to establish communication with the entire system. Check the MCP running status and set it to manual control mode.

Check the working condition of the spark plug and perform a valve test. Test the data recording function. Next, open the setting interface and set test time, including valve opening and closing time, ignition time, and data recording duration.

Set safety requirements and clear personnel from the experimental area. Open the cylinder valve and adjust the output pressure of the regulating valve according to the different mass flow rate conditions. Open the setting interface and set the oxidizer mass flow rate.

Turn on the camera, then set the MCP to automatic control mode and wait for trigger. Click start on the MCP to start the experiment. After about one minute, click stop and turn off the camera.

Close the gas cylinder and open the valve in the pipeline to relieve the pressure. Power off the test bench and remove the fuel grain. Measure and photograph the fuel grain as previously demonstrated.

Changes in combustion chamber pressure and oxidizer mass flow rate are shown here. To provide the necessary time for flow regulation, the oxidizer enters the combustion chamber in advance. When the engine builds pressure in the combustion chamber, the oxygen mass flow rate drops rapidly and then maintains a relatively steady change.

During the combustion process, the pressure in the combustion chamber remains stable. A comparison of combustion chamber pressure oscillation frequency is presented here. The pressure fluctuation spectrum of the novel fuel grain contained three distinct peaks which were associated with the hybrid low-frequency, Helmholtz mode, and the acoustic half-wave in the combustion chamber.

The positions of the pressure peaks of the novel fuel grain were basically the same as that of the paraffin-based fuels, which indicates that the novel structure is not likely to introduce additional combustion oscillations. Regression rate as a function of oxidizer flux was compared between the fuel grains. At the same oxidizer mass flow rate, the regression rate of the novel fuel grain was higher than that of the paraffin-based fuel and the gap gradually widened as the oxidizer flux increased.

Characteristic velocity was used to compare combustion efficiency. The novel fuel grain exhibited a higher characteristic velocity than paraffin-based grains at various oxidizer and fuel ratios. This corresponds to an average increase of combustion efficiency of about 2%When attempting this protocol, remember that that the casting temperature of paraffin-based fuel cannot be higher than 120 degrees Celsius.