We observe a peculiar behaviour on the LOX side not experienced before during static test. The stable pressure at around 13 bar from T + 10 to T + 40 seconds more or less reveals what has gone awry. The stable pressure can have one of two causes, either the main valve is closed, or the pressure is maintained by rapid evaporation of LOX forming GOX (Gaseous OXygen). The former can be excluded as the position of the main valves is logged by the ECU (Engine Control Unit), and these data show both valves open until splashdown. The other cause is better known as VaPak or autogenous pressurisation.
Having pressurisation through VaPak from T + 10 seconds is an obvious indication that the LOX has been warm, is has only been held at sub-critical state by a high pressure. In this state the slightest fall in pressure will result in the LOX shifting to super critical state where it evaporates at a rate high enough to maintain a stable pressure. This may sound like a fantastic feature, but for Nexø I it proved fatal. In its present design, the BPM-5 engine injector isn’t capable of working correctly with super critical LOX, the injector is designed to inject fluid, LOX, and not GOX or some obscure combination of LOX and GOX for that matter. Had we just had a thermal sensor in the LOX dome, we would now know the exact conditions …
Based on the pressure charts we can conclude that the LOX has been close to going critical from the very start. From this we can assume that we have had GOX flow through the injector from the very start, resulting in a lowered mass flow. This has again resulted in lowered chamber pressure and thus a higher than expected fuel flow. This explains both the low engine thrust and the relatively fast depletion of the fuel tank, resulting in the tank being depleted at T + 24.4 seconds.
Video Caption:InterStellar Technologies Inc.
The 4th flight of experimental rocket for attitude control at Taiki-cho, Hokkaido, Japan.
This rocket name is “LEAP”.
This engine is 1500N force Ethanol/LOX rocket engine.
This rocket is controlled by gimbal and cold gas jet reaction control system.
Video Caption: Test 16 of our 10000 N Hybrid Rocket Engine “HyRES”. This was done with the configuration and conditions during our launch in October 2015. As we expected from theory, the combustion is clearly unstable under these conditions and we modified these conditions for the next test 17, which shows excellently stable behavior.
From the teams Facebook page:
A further important step of our analyses and tests was, to get to the bottom of the launch failure last year. With test 16, as in the video below, we could show, that a too low nitrous oxide temperature at launch was responsible for instable combustion in the engine. This increase thermal and mechanical loads and lead to the failure of the engine.The configuration and conditions of test 16 represented the same as the launch last year. Compared to test 17 the differences in stability can be recognized easily.