I haven't gotten around to showing plots of the ignition of the last flight until now.
The last flight had the third and fourth Blue Raven channels connected to the sustainer igniter. The third channel is shown below. It was set up for the optimistic case where the flight was straight enough that the ignition could wait until the velocity drops down to velocity that optimizes altitude for a straight flight (based on RASAero sims). Knowing that there would be some ignition delay, I looked earlier in the sim to see what the velocity was 1.5 seconds before the start of the sim acceleration. In this case that was 300 feet/second, and so I used that for the third channel velocity trigger. In flight, the velocity did actually go down that low before the sustainer motor acceleration really started:
You can see that in the flight events for the third channel, when the "less than Vel1" goes true right about 15 seconds.
But the third channel wasn't what actually ignited the motor, because the fourth channel fired first, at 13.488 seconds:
I set up the fourth channel to fire if the expected tilt angle 3 seconds into the future exceeds a threshold of 13 degrees, as long as the vertical velocity is under 800 feet/second, altitude is over 500 feet, and the current angle is under 18 degrees. Here is what the tilt angle did during the flight:
Once the oscillations settled out, the future tilt angle did a pretty good job of predicting what the tilt would be three seconds into the future. Zooming into the ignition, the future tilt trigger was met at 13.468 seconds into the flight:
That's when the switch turned on. I'm pretty pleased with the plot above, because at the time of the ignition, the tilt was about 10 degrees. But the motor took about 2 seconds to come up to pressure, and 3 seconds after the future angle was triggered, the tilt angle was about 13 degrees. That put the sustainer apogee about a mile from the launch pad, (determined afterwards from the GPS data), which is consistent with what the simulations were predicting for the downrange distance.
The ignition itself is pretty interesting. Here are plots of the acceleration and the electrical data plotted on approximately the same timescale:
The fourth channel fired at 13.508 seconds, and at first there was wasn't much change to the measured electrical current or the acceleration. That output fired for 1 second, and presumably it did start the ignition process. Once the switch turned off, the voltage across the switch went back up to the battery voltage, and then some. I don't have a good explanation for why the measured voltage was briefly higher than the battery voltage. Maybe some electrochemical effect from the burning igniter? When the third channel met its criteria at 15.17 seconds, it turned on. This time, the current went up over an amp, because the third channel was firing into an igniter that was already ignited and was on its way to pressurizing the motor. As the motor started to come up to pressure, the current initially dropped and then went back up again. The motor fully pressurized at 15.59 seconds and there was relatively high current at the end of the 1-second firing duration, when the ignition wire stubs were connected together by the incandescent motor exhaust. Here's a closeup of the motor pressurization:
It took 2.0 seconds for the motor to go from electrical signal to first acceleration, and then another 0.1 seconds for it to get to max acceleration.
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