Since some time the DHX-200 Aurora rocket motor for the Stratos II rocket was set ready in the TNO rocket test facility in Rijswijk. This rocket motor that will propel the Stratos II rocket to 50 kilometres altitude of course needs to be tested to make sure that it can perform as expected.
Stratos II is a rocket that is designed and built by students from Delft University of Technology in the Netherlands. The rocket is designed to launch a scientific payload to 50 kilometres altitude. To make sure the mission will succeed all different subsystems need to be tested. Therefore the propulsion segment was tested statically and all important parameters and the performance of the motor were measured.
Stratos II uses a so-called hybrid rocket motor. The rocket motor is comprised out of a combustion chamber containing solid rocket fuel that reacts with the pressurized liquid nitrous oxide that is being injected into the chamber. As fuel a combination of sorbitol, paraffin wax and aluminium particles is used. In combination with pressurized nitrous oxide as oxidiser this novel fuel combination delivers all the required thrust. The DHX-200 Aurora is designed to deliver a total impulse close to 200.000 Ns and a peak thrust of just over 10.000 N. Every second 5 kg of propellant are burned and expelled through the nozzle.
Test 1: 5 seconds burn time
The first test at the facility was performed on 28th May. This was the first time the DHX-200 Aurora motor was fully ignited. The motor was fired for 5 seconds, after which it was shut down in a controlled manner by closing the main oxidiser valve. The DHX-200 Aurora is designed to eventually fire for almost 20 seconds. This first time however, the oxidizer flow was cut off early so that the team could inspect the motor afterwards and evaluate the results.
Test 2: 10 seconds burn time
On Wednesday, 12th of June, the motor was prepared for a second test day. After the first 5 second test fire the burn time of the motor would be increased in steps. The next step was to fire the motor for 10 seconds continuously. It performed beautifully and delivered a good performance.
Test 3: 15 seconds burn time with rupture of the combustion chamber
The logical step after the successful 10 second test was to increase the burn time even further. The third test, which took place on the same Wednesday later during the day, was scheduled to take 15 seconds. After this the oxidizer flow would again be shut down. After about 5 seconds however a crack appeared in the wall of the combustion chamber and the motor was shut down by closing the oxidizer flow immediately, extinguishing the motor safely.
What went wrong?
The rupture of the combustion chamber is something that needs to be prevented. To analyse what went wrong during the test the team started to systematically check all parts of the engine to see what could have been the problem. The suspicion quickly fell on the fuel grain that sits in the combustion chamber. This fuel burns with the oxidizer to generate the thrust of the rocket. The fuel burns from the inside outwards and in that way it also protects the wall of the combustion chamber from the hot combustion gasses. If however the grain contained some production errors, it may not have done its job of protecting the chamber wall.
To analyse this, the team took a fuel grain that was prepared in a similar manner and with a similar technique as the one used during the test, and analysed this with an X-ray computed tomography (CT) scan. This particular fuel grain was not going to be used for a test because it had a clearly visible, large crack in the middle that that had already been observed beforehand. The team however wondered if there were also cracks and bubbles that could not be seen. Using the CT-scanner at TU Delft’s Faculty of Civil Engineering and Geosciences the inside of the grain could be inspected.
This was a grain that was deemed not usable during a test because of the big hole in the middle of the grain that was visible from the outside. The CT-scan however also revealed a number of smaller cracks and air bubbles in the interior of the grain.
As can be seen in the picture the grain contained, next to the big hole that could be seen from outside, also a number of smaller bubbles. Next to that parts of the grain appeared more porous than others. The team suspects that one of these bubbles allowed the hot gasses to reach the combustion chamber wall very quickly during the test. This weakened the combustion chamber and caused it to break.
The first test series at TNO is now completed and the team has learned a lot. The first two tests show that the DHX-200 “Aurora” can perform as intended. Test 3 shows that the employed safety measures function well and that the whole system is safe to work with. It however also shows that the team needs to rethink the production method for the fuel grain and for future tests needs to inspect the grain not only from the outside. For future tests the grains will probably all be inspected by scanning them beforehand. Near the end of the summer the team will continue with the test sequence to test the engine up to its nominal burn time of 20 seconds.
This first test campaign was very promising and very exciting for everyone and we are looking forward to the next opportunity to test the motor.