Data from Nexø I

Nexø I launch (Credit: Copenhagen Suborbitals)

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.

Read the full update here.

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