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The first test worked well. The team used a less energetic propellant (easier on the motor case, less heat and lower pressure) to start with. The rocket motor achieved supersonic flow and produced thrust. The case survived the test in decent shape. A few millimeters of plastic eroded from the throat.
Although the motor case was only designed for a single use, the team decided to see what would happen if fired again. In the second test, they used a more energetic propellant. The throat was already worn from the first test and it quickly eroded and became too large to maintain supersonic flow. The rocket’s combustion became unstable, the flow through the nozzle was subsonic, and little thrust was produced. While this unstable combustion is not useful for propulsion, it looks really cool in 240 fps slow motion!
Only the first test yielded clean pressure data:
The erosion of the throat is readily apparent in the falling pressure. Future designs will account for this by varying the propellant regression rate to match the nozzle erosion.
Printing rocket motors from plastic is a unique accomplishment. Several groups, including SpaceX and NASA, print rocket engines from metal. But metal printers are expensive, costing north of six figures. MIT’s plastic motor is produced on the more affordable Markforged composite plastic based system, which has a price accessible to hobbyists and small teams. The case was also designed to work with modern composite propellants.
The team has already begun follow up work to explore larger motors, more resilient motors, and potentially even flight hardware!