In May, after weeks of preparations and never ending second thoughts, whether we forgot something, we managed to fire up the SL-1. We decided to schedule the test in the evening, when the flame would be more visible. After the 10-second countdown, which felt like an hour to us, we could hear the silent crack of the igniter. The whole test area instantaneously lit up with bright orange light and the motor produced such a roaring sound, we were left speechless. It reminded us of the raw power of rockets, which is exactly what makes them so attractive to us.
Figure 1 shows the SL-1 setup. It consists of three main parts – combustion chamber with paraffin fuel, liquid oxygen (LOX) tank and gas bottle for providing pressure to the LOX tank. The flow of oxidizer is controlled with solenoid valves, which allow us to start and stop the motor remotely and with an accurate computer-controlled sequence. Firstly, the computer opens the relief valves, which prevent the buildup of unnecessary pressure, and starts pumping the liquid oxygen into the tank. First 10 liters evaporate and cool the tank down to the boiling point of oxygen, which is at -183°C. Once the filling is complete, all valves close and the pressurization of the oxidizer tank begins. Due to high costs of suitable reducing valves to provide a constant pressure in the LOX tank, we have decided not to use them and allow the pressure to fall slightly during the operation as LOX flows out of the tank. However, we are already developing a regulation system for providing a constant tank pressure, which will be used with the SL-2 motor. The next step is start of the ignitor, followed by start of LOX flow and ignition of fuel.
The first static test we conducted was successful, however there were a lot of things to improve. First anomaly we can notice on Figure 2 is the massive pressure spike at the point of ignition, in rocket science also known as a hard start. It happens when there is a buildup of fuel/oxidizer mixture in the combustion chamber before the ignition, which can easily cause a catastrophic failure of a rocket motor. In our case the spike was estimated at 150 bar, whereas the operating pressure should be 30 bar. It was caused by the igniter being too powerful, which made a lot of paraffin evaporate before the ignition and fill the combustion chamber. The pressure spike cracked the fuel grain and pieces of unburnt fuel were ejected through the nozzle (other two pressure spikes).
The second anomaly we encountered was extreme oscillation of thrust measurements. The combustion chamber of the motor is mounted on the test stand, which can move horizontally and transfer force onto the load cell behind the stand, measuring the thrust of the motor. We found out the test stand bounces off the force sensor, which was solved by applying some initial horizontal preload on the test stand. Eventually, to our great satisfaction, we managed to achieve stable combustion and ignition of the motor (Figure 3). Although the motor exhibited a 20 Hz combustion instability (pressure oscillation), the amplitude stayed within the predicted limits.
Since May we have completed 7 successful static tests, where the SL-1 proved to be working as expected and that it can be used as a solid basis for building our future rocket motors. With the achieved efficiency of about 20% lower that predicted we however have quite a lot of details to perfect to get the most out of high-performance paraffin fuel.