On Friday, March 30th, the first BEM, or Ballistic Evaluation Motor (see the previous solid six post: from desktop to tabletop) has experienced its inaugural firing! An 8-second burn with a roaring sound and a huge flame marked the start of a new test campaign. The concept of the BEM seems to work as we hoped.
For the first test, a conservative nozzle and propellant configuration was used, to ensure that during the first test the pressure inside the combustion chamber would not reach a very high level. The purpose of this test was to verify the working of the BEM and to check whether the thermal design has produced a reusable motor, so that we can do a lot of tests during a short period, at minimal cost.
We started the day at the workshop, where we did a pre-calibration of the pressure sensor and packed everything we needed on a cart. Then we headed out for the test field. After a long period of nice weather, today a stiff wind was blowing and the sky was heavily overcast. However, this did not diminish our good mood and without further hesitation we started to work. We put the test bench with the motor firmly on the ground and fortified the setup with sand bags, to ensure that the surroundings of the test setup remain safe, even if something unexpected, such as a motor explosion, would happen. The firing line was unrolled to the command post, 60 meters away. And finally, the cameras were set up to provide the highly desired video footage of the test. All preparations were finished at 10:30, just in time for the people that had morning lectures to join the spectacle.
When the countdown reached T minus 0 seconds, the motor burst to life. A bright, two meter long flame erupted from the exhaust and lasted for a full eight seconds, spreading sparks towards the sandbag shield that we set up 5 meters away. Then the motor noise died out as quickly as it had started, leaving the crowd with big smiles on their faces. After every sign of smoldering had disappeared, the site was declared safe by the safety officers and the crowd came in to take a peek at the scorched grass. No signs of leakage or other kinds of damage were visible and it could be concluded that the test was a success. To celebrate this, cigars were shared with the public in true rocketry tradition.

A close inspection during dismantling of the motor revealed that indeed very little damage occurred on the reusable parts: the liner that protected the casing from the heat in the combustion chamber was largely intact. The casing showed no signs of overheating, however the next test will prove if this is actually the case: aluminum may undergo a heat treatment during a burn, which may weaken the material. However, looking at the condition of the thermal liner, it is believed that this did not happen. The endcap and the igniter assembly were in pristine condition after cleaning and also the nozzle and nozzle retainer, the parts that have to withstand the highest heat load during the burn, were in great condition. A lot of hard residue was found on the divergent part of the nozzle: a sign that the combustion may not have been as complete as we hoped for. However, overall it was concluded that this motor could easily be used a second time.
From the recorded pressure data, it was found that the pressure inside the combustion chamber was indeed very low: even lower than we had expected on beforehand. As a result, the burn duration was long, as we found from the video footage: an impressive 8 seconds. A good, steady pressure curve was obtained, showing that we had a complete and fairly quick ignition and no combustion instabilities. A characteristic exhaust velocity of only 1172 m/s was deduced, considerably less than the 1450 m/s which is theoretically achievable. This shows that the combustion efficiency was low, only 80%. However, it is believed that this increases dramatically at higher pressures.
Overall, we can conclude that this experiment was a very good start of a new test campaign, and we are eager to perform more so that we can obtain more data and present more and better pictures. A quick impression of the test is given by the photos below. For a video compilation of the test, I would like to direct you to the DARE youtube channel: .