In the previous post you read on our website you will have been able to see that the last tests at the TNO laboratory a few weeks ago did not end so well for our hybrid rocket engine (article). Just like the last tests in May the combustion chamber failed a few seconds into the burn. The propulsion team is working hard on finding a solution for this issue.

For the analysis the team turned to Aerospace Propulsion Products bv (APP). This small company in the Netherlands is the producer of the igniters for the big rocket motors of the ESA rockets such as the VEGA, and they harbour a large amount of knowledge on rocket motor development and rocket motor production.

Root Cause Analysis
The propulsion team performed together with the experts from APP a so called Root Cause Analysis, a systematic approach to determine the ultimate cause that lead to the failure of the combustion chamber. The analysis is structured in such a way that all possible causes are looked at and to make sure that you do not jump to conclusions too fast. This is important because although when looking at a problem some issues might immediately seem to be the logical cause of the failure, but then afterwards you might find out it actually was not thé problem (or maybe; not the only problem).

 
Cause of the failure
As was concluded from the inspection on the grain, after the last tests, two large cracks had formed along the length of the fuel grain. The rupture of the chamber occurred at one of these cracks. The root cause analysis performed also looked into the underlying reason why these cracks formed. Were the cracks caused by imperfections in the grain? Were they caused by shocks or hits before the burn? Did the ignition cause the crack?

 
A lot of possible causes were discussed. Among these was the reasoning that there was a small air gap between the fuel grain and the aluminium chamber wall. The fuel grain is casted separately and only later slid into the chamber wall, due to differences in diameter there are always small air pockets trapped between there. During the two tests were the engine failed, these air pockets were sealed in, so no combustion gasses could get between the grain and the chamber wall. Although this sounds good, it actually means that as soon as the burn starts, the pressure in the combustion chamber presses the grain outwards with 30 bar of pressure, while these air pockets are only pushing back at 1 bar. The grain cannot handle this pressure difference and cracks.

 
This is only one of the possible causes that came up during the meeting, and all of the causes will have to be investigated thoroughly to determine what the real cause of the failure was. Only after that we can again go out and test our rocket engine.