Developing and building a rocket is never smooth sailing, as has been shown again. Last Wednesday the Stratos II propulsion team was ready for another test of the Aurora engine at the test facility at TNO. Unfortunately again, just like the previous test, the rocket motor combustion chamber ruptured a few seconds into the burn.

In May this year the team also performed a number of tests with the rocket engine, starting off with a 5 seconds burn time, boosting that to 10 seconds and then trying to go for 15 seconds after the first two tests were successful. This was attempted in May as well but back then the combustion chamber ruptured after approximately 6 seconds. See the photo’s and footage of that test in the article of that time. The failure was then investigated and was attributed to a fault in the casting of the grain. CT-scans of the grain revealed back then that air bubbles may have been present in the grain, which allowed the hot gasses to reach the side of the combustion chamber and break through it.

The test
The test campaign was picked up this week after the investigations and new preparation during the summer months. In was made sure that the issue that was believed to be the main cause of the previous failure, the bubbles in the grain, were not present now. The grain was inspected thoroughly to make sure there were no imperfections.

The setup of the test was done already on Monday the 11^th of November but unfortunately we needed to wait to test until the weather was good for a test last Wednesday (the 20^th). The test could be conducted in the afternoon and all preparations went perfectly fine. The setup was made ready within an hour and although the temperature in the tunnel was quite low (around 5°C), the filling and heating of the oxidizer tanks was also prepared and ready within an hour.

At 15:23 the countdown for the engine test was started, intended to perform a 15 seconds burn. The first few seconds everything seemed fine. Unfortunately, in an almost exact repetition of the test in May, the combustion chamber ruptured around 6 seconds into the test. The rupture occurred near the top of the chamber, around one third of the combustion chamber length away from the injector.

Immediately the engine was shut down and the computer started a safety system that fed a flow of inert nitrogen gas into the chamber to extinguish possible flames. At 15:31 the first purge with N₂O from the oxidizer tanks was performed, followed by a number of purges to empty the oxidizer tanks between 15:31 and 15:36. The flow and expansion of this cold gas caused the ice-forming on the chamber and feed system that is visible in the pictures.

What went wrong?
The main question is now what the problem was with the engine, both now and last time. In May the conclusion was made that imperfections in the fuel grain likely caused the rupture of the chamber. It now turns out that was not the only or not the main reason as the failure appeared again. With the nitrogen purge system that was used the grain was preserved from further melting or deformations so that it now allows for a good investigation of the events.

The burn surfaces of the grain all look very smooth. The main port is still circular and in the middle so extensive melting and dripping of the grain due to flow in the chamber or gravity (as the chamber is horizontally placed) seems unlikely. In the grain two main cracks can be observed running straight all along the length of the chamber (see pictures). These cracks also have a smooth surface on the inside which shows that the cracks did burn a bit before the rupture.

No definitive answer on the question why this happened can be given now. Most theories the team has are looking into the material properties of the grain material. It might be that the fuel itself is to brittle so that it actually cracks as soon as the engine ignites or due to vibrations and shocks in the first few seconds of the burn. Further investigations will be conducted.

This is why we do testing
Of course this was not the result that we wanted to achieve with this test. But it is a good example of why we do these types of tests, and why we take extensive safety measures before going out and firing such a big engine. DARE prides itself with its dedication to maintaining safety during all its activities and doing these tests in a facility such as the one made availableby TNO allows us to do this and to explore the field of rocketry.

Although the test did not go as expected we still learn a lot from it. It shows some of the problems that rocket scientists face in the development of bigger rocketry systems. We hope to learn our lessons from this and to apply what we’ve learned in further iterations of our engine.

Implications for the project
What the exact consequences are for our project still needs to be investigated. With the whole Stratos II team we are keeping a tight schedule to finish our project. In the coming weeks we will together evaluate in what ways this and other events influence our planning and if needed we will adapt ourselves to the new situation. We will however of course keep on going. A test like this only shows what kind of challenges this scale of rocketry brings with it and will inspire us to bring out the best of us as students, to learn how we can become the best rocket scientists.

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