Ignition Improvements, November-2012


Work

This test is a repeat of October's static test (here) to make sure success with the segmented ignitor and pre-heater was not a fluke.

A few minor changes were made to the motor:
-Smaller nozzle throat to increase chamber pressure
-Shorter dip tube to allow for longer burn time

  • Stats:
  • *Temperature was cold on the day of the test and the Nitrous pressure never reached 400 PSI.

    Result: 7.5 Second burn, M-700



    Results and Video



    Analysis and comment

    The startup looks nearly identical to the October test. The fact that the motor lit is good news.

    The temperature was too cold to get a good estimate whether the nozzle throat is correct, however instinct tells me it is too small. The pressure difference between the tank and the chamber is only 75 psi towards the end of the burn (250 vs 175). A difference of 150 psi to 200 psi is preferable and safer.

    The startup transient at 8-10 seconds is UGLY. Viewing the video one frame at a time is informative. Annotated test stand graph is above and thoughts are below.

    (1) 0:19.15 sec Ignitor lights. No synchronization exists between the video and the test stand output. But the pop of the eMatch is distinct and can be seen in a plot of the audio channel.
    (2) 6 seconds of pre-heater burn
    (3) and (4) 0:26.23 sec

    The 4 capped injector orifices and the 1 center fill line melt and release about the same time. Looking at tank pressure (green line) the characteristic sudden drop in pressure which accompanies the opening of a UC valve is seen. The shape of the pressure curve is smooth and continuous, implying that the mass flow through the injector is well behaved. In other words, the oxidizer flow is good so combustion of the PVC solid grain is the likely problem.

    Also, the shape of the plume and sound of the motor is strange.

    (5) Good burn. Beginning about 2.5 seconds after the UC valves open, the motor burn stabilizes. This is seen as steady thrust at 250 lbf (blue line), smooth decrease in tank pressure (green line), and well behaved chamber pressure (red line).

    Theory:

    In previous tests the pre-heater extinguished when the UC valves opened and Nitrous began to flow. The same phenomenon may exist here, but since the new segmented pre-heater comes up to temperature quicker and is burning hotter when the vales open, the result may be decrease in thrust and sloppy burning until the system stabilizes a few seconds later.


    This is a photo of the injector after the burn. The 4 plugged orifices burned-through cleanly. The center UC fill tube also cut cleanly at the right location. While the ignition transient may be sloppy, at least he motor is lighting. This is an impovement.




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