Testing Different Grain Geometries for Solid Rocket Boosters

     Back in February, my friend and I decided to see if we could measure a difference between different grain geometries in small solid rocket boosters. Grain geometry refers to the shape of the hole running down the center of the fuel: the hole changes shape as the fuel burns, resulting in a different thrust pattern depending on the shape of the hole. We tested 2 different grain geometries: a circular hole and a cross-shaped hole.

     We made some SRBs from PVC tubes and fueled them with a potassium nitrate - sugar mix (we found out later that using PVC for rockets is a terrible idea: if it explodes and shatters, the pieces of PVC lodged into your skin will not show up on a x-ray). We attached the boosters to a force sensor to collect thrust data.

     Our tests for the circular grain geometry both failed spectacularly:

   

    However, our test for the cross-shaped geometry went beautifully:

     In fact, in the data, you can clearly see two peaks in thrust:

     This thrust pattern is due to the changing surface area of the hole: as the fuel begins to burn, the cross begins to morph into a circle, reducing the surface area and the overall thrust. As it continues to burn, the circle expands, increasing the surface area, resulting in a second peak.

     Overall, this project was a great learning experience and a great excuse to explode things. If you choose to try this yourself, be safe, and don't use PVC like we did.