Grain cracking test setup

Over the past week, the propulsion team has been busy preparing for the last full-scale test. During the previous test, we encountered another burn trough of the chamber (see the previous story). Investigation of the chamber and grain afterwards suggested that the grain cracked during operation resulting in the flame reaching the wall.

The hypothesis for the cause of this failure mode was that the sealing of the grain caused a pressure differential over the grain, which was too large for the grain to support. Before the next full scale test is performed, the team wanted to validate this hypotheses. In order to do this, a small pressure vessel was build. A short grain was placed in the vessel and pressurized using gaseous nitrogen. First the grain was only sealed at one side, to simulate the sealing at the nozzle side. This test resulted in an undamaged grain.

Grain before cracking

After inspection the grain was pressurized again, but now with sealing on both sided, to include the sealing that was present in the pre combustion chamber. During the pressurization, a loud cracking sound was observed. After the test, the grain was inspected visually and cracks, similar to those which were present after the last full scale test, were observed. The cracks spanned the entire web thickness and run along the full port length.

Grain with marked cracks on the top side

Both test were repeated with a second short grain, resulting in similar results. These test validated the hypotheses of the failure mode. As a result, the ablative protection ring in the pre-combustion chamber will no longer be sealed to the grain. By implementing this change the team feels confident that the next full scale test will result in a success.

Written by Jeroen Wink