I went out and flew Moment of Inertia last Saturday. It took off really fast and flew fine. The shock cord did break so it came down in two pieces, but I recovered both pieces. I did not hear a sonic boom separate from the noise of the motor, but the motor was really loud. I’ve heard that you might not hear a sonic boom since you are standing behind the direction of travel of the rocket. The max altitude beeped out by the altimeter in the field was 1059 meters.
When I downloaded the altimeter data it said the max speed was 195 m/s. The point of this rocket was to be a mach breaker, and 195 is well below the speed of sound. My original OpenRocket simulation said it should reach 399 m/s. I had added a little weight when I fixed the broken centering ring from the first flight, and I hadn’t re-weighed it to update the simulation. I was busy and I figured the altimeter would tell me how fast it went. But 195 seems really slow for this design so it’s time to dig into the data and find out what happened.
There are several possible things that could have happened:
1) OpenRocket doesn’t do transonic or supersonic drag right. There could have been a lot more drag causing the max speed to be slower, but 195 m/s is well below where even transonic effects should show up.
2) The thrust curve of this particular motor may have had slight variations from the average for this motor type. Just a longer burn time would have resulted in a slower max speed.
3) The altimeter is just a barometric sensor. It could have been fooled somehow. Maybe low-pass effects from air trapped in the body tube made the altitude change seem slower. I did make the vent holes a little oversize to try to avoid this, but everything happens really fast on this rocket so maybe it wasn’t enough.
4) The velocity reported by the altimeter has to be derived from the pressure/altitude data. Maybe there is some smoothing or aliasing occurring in the derivative algorithm.
And, of course, it could be some combination of multiple of these effects.
Here’s the overall graph from the altimeter. The height in black shows the ascent, a pressure spike when the ejection charge fired (labeled drogue), then linear falling at terminal velocity. It looks like a rocket flight.
Something unusual happens on ejection. The rocket seems to instantly gain ~75 meters of altitude. Was this an effect of trapped pressure in the body tube that got released on ejection? It’s possible that this resulted in a low-pass filter that made the max speed seem slower. Also, there’s some weird pressure readings on takeoff.
Zooming in on liftoff, this just doesn’t look like good data. Maybe shockwaves fooling the sensor? Starting at around 0.3 seconds at 100 meters in the air looks like good data. The altimeter is saying that the max speed occurred at 0.7 seconds on the graph, but this motor only has a 0.4 second burn time after which it should start decelerating rapidly from drag. That would be an unusually long burn time to be a random variation from the average for this motor type. So that points to low-pass effects slowing the pressure rise.
Another question is when did liftoff actually occur? The 0.0 time on the graph is when the altimeter detected liftoff because there was sufficient pressure change to arm the ejection charge. But the graph seems pretty flat before -0.35 seconds and then things start happening. That may have been the actual ignition and liftoff. If that’s the case, then the period of weird data corresponds approximately to when the rocket should have been supersonic.
So I re-weighed the rocket and went back to the simulation. The repairs only added two grams so it didn’t affect the simulation much. There’s one number from the altimeter that I’m pretty sure I can trust, and that’s the apogee altitude after ejection. Then the pressure should be a correct reading of ambient.
The altimeter beeped out 1059 meters, which appears to be the highest single data point excluding the ejection pressure spike. However, that data point looks like noise. 1025 meters looks like a better stable value.
I tried to adjust the surface finish settings in my simulation to get less drag so that the simulated apogee was 1025 meters. Even with the smoothest finish it wouldn’t go that high. It was a hot summer day so the density altitude was probably higher than the field elevation of 1556 meters, but I had to go to 4100 meters to get the apogee that high. That seems unlikely. So maybe the motor did have a slightly longer burn time than what’s in the simulation file. This flight profile has massive drag losses so a longer burn with a lower max speed would result in a higher altitude.
Overlaying the simulation on the altimeter graph gives good general agreement about the shape of the curve with the altimeter reading either being lower or later.
One final piece of evidence is that the supersonic phase in the simulation does correspond roughly to the period of bad data from the altimeter. The bad readings could have been caused by shockwaves.
Here’s the verdict on each of the hypothesized causes.
1) Extra supersonic drag lowered max speed
Evidence: Extra drag would have lowered the apogee altitude. Instead, the altimeter apogee exceeded the simulation apogee until I changed the simulation to lower the drag.
2) Longer burn time on motor
Verdict: Likely a contributing factor
Evidence: Trying to get the simulation apogee to match the altimeter apogee by reducing drag required an unrealistically high density altitude. A slightly longer burn time on this particular motor is a more realistic explanation for a raised apogee.
3) Barometric sensor was fooled
Verdict: Likely a contributing factor
Evidence: The sudden jump in altitude on ejection points to trapped air in the body tube. The bad data early in the flight corresponds to the time period when the simulation says the rocket should be supersonic. One lingering issue on this is why the discrepancy between the altimeter and simulation kept growing throughout the flight. If it’s a low-pass effect, one would expect that the discrepancy would grow when the rocket is moving fast and then shrink again when the rocket slows down near apogee. Possibly the altimeter line on the graph is not just low, but also late because of a longer burn time on the motor.
4) Calculation of velocity from altitude data by altimeter
Verdict: Possibly a contributing factor
Evidence: The altimeter software graphed a smooth curve for the velocity even through the time preiod when the altitude data was bad. So they must have some algorithm that interpolates over bad data.
And the overall conclusion is that I think I can confidently say that the rocket did break the sound barrier. Except for the period of bad data, the altimeter data matches pretty closely a simulation that says the rocket should have exceeded mach 1, and the period of bad data corresponds to the time when the simulation says the rocket should be supersonic.