iPhone rocket data quicklook
3 November 2008
by
lolife
Filed under
Journal + Rockets
It’s taken me awhile to understand the data I got from the first launch. Using the video, the GPS data, the accelerometer data and my own recollections, I’ve put together a rough timeline of events. I’ve notated it on the plot of the altitude. A couple of weird things happened.
First of all, with the engine I used, it should have gone to 1200 feet or so. It actually went 670 feet. This was an Aerotech G80-13 engine. Every calculation I ran using various software packages for this purpose had the altitude between 1200-1500 feet. So why did it only go to 55% of the predicted height? I noticed it says in the Aerotech instructions that if the igniter doesn’t penetrate all the way up the tube you can get a reduced thrust situation. That is apparently what happened.
But why did it go up for so long then? The 13 second delay wasn’t nearly enough, apogee wasn’t until almost 30 seconds into the flight. I’ll note that I don’t have error bars on my altitude data because I didn’t log it, which was stupid. Is it possible that GPS is under-reporting the altitude? The calculated velocity is quite slow from this plot — about 11 m/s or about 25 mph. Did the motor burn with less thrust but longer?
As I watched it, you could see the rocket turn over at apogee before the chute came out. So I think the ejection charge went off at 15 seconds or so but the payload bay didn’t come all the way out. Things get really turbulent at 22 seconds and when the rocket tips over I think the payload bay finally comes loose. On the video you can hear me say that I see the chute at 31 seconds. I’m not sure if it was fully deployed or if that is the anomaly just before 40 seconds.
The descent looks like a constant, terminal velocity of 3.8 m/s (8.5 mph).
I’ll show you the accelerometer data next!
4 November 2008 @ 6:18 am
Unless your GPS has a calibrated barometric altimeter, then your altitude readings will be off, potentially by tens of meters. Based on the 3DGPS readings I’ve gotten on circuit hikes, I’ve had the altitude of my (parked) car change up to 100m from when I’ve left it to when I got back.
Surely it’s not enough to explain your data entirely, but it’s something to factor in. Also, in terms of error bars, you should be able to get the (horizontal) relative accuracy from the GPS, and you can roughly calculate your vertical error from that…
Some good reading: http://gpsinformation.net/main/altitude.htm
4 November 2008 @ 9:03 am
You’re right, your altitude and thrust numbers don’t quite make sense. Do you have a way to calibrate the elevation data you’re getting?
4 November 2008 @ 9:37 pm
Yeah, either the motor didn’t go to full thrust or the altitude has huge error bars. In the latter case, I’d think that there would be more scatter in the data, but maybe the GPS altitude calculation has a low-pass filter on it or something. I expected the altitude data to suck way more than it looks like it does.