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The rocket will use the altimeter for apogee and main deployment, with a single timer in the original electronics bay for back up. The timer was set for a long delay, well after apogee, that way I could be sure the altimeter was the device that actually set off the apogee charge. I used the MB-1 motor with a .5" diameter nozzle throat and 1.27 pounds of standard KNSU-PG propellant. That should give me about 153 pounds of total impulse (J class).
The rocket was prepped the night before, with the timer set and the altimeter programed and ready to go. All I had left to do before launch was load the propellant into the motor.
Bill showed up to help with the launch, so we loaded up and headed out to the launch site.
There was a little more wind than I would have liked, about 15 to 18 mph. A check of the rockets stability margin the night before had indicated about a 3 caliper margin of stability. While that's a good thing, it also does tend to increase the weather cocking of the rocket. Despite the wind, we quickly set up the pad and readied the rocket for flight. Once on the pad an igniter was inserted into the motor, the magnet set for magnetic launch detection, then the timer module was powered on, then the altimeter module was powered on.
Here's the rocket on the pad ready to fly. Notice the white string on the lower right side going to the ground. The string is tied to a weight on the ground, and to a small magnet on the rocket. When the rocket launches, the rocket pulls free of the magnet and starts the timer.
Bill and I took our positions at the launch controller, Bill was running the video camera, I decided to try to get some stills with my digital still camera. All clear, 5, 4, 3, 2, 1 and ignition!
Beautiful lift off!
We watched as the rocket made a fantastic boost to apogee overhead, unfortunately Bill lost the rocket on the way up, but I managed to keep it in sight and snap a few still images after apogee. It was quickly apparent both the drogue and main had deployed at apogee, now the question was; why? Either the main had just pulled out on its own, or the altimeter had fired prematurely.
Here is a highly magnified image from my still camera of deployment.
In the above picture you can see the drogue chute, and then the main chute above the rocket body. From the picture it appears the main chute came out on its own, rather than being deployed by the altimeter. The problem with the main chute is that it is so big, the shroud lines are long enough that it can come out of the rocket and deploy by just the shroud lines, even if the PIRM2 hasn't yet fired. That's apparently what happened here.
The drogue chute is 39" diameter, the main about 80" diameter. So, with the wind we had at launch, the rocket drifted a long way, a good 1/2 mile. So it was too distant to see at what point the PIRM2 fired. We drove out the landing site, easy to find in the open field and retrieved the rocket. Once back to the van I turned off the power to the timer and altimeter. Checking the PIRM2, it had fired and released, confirming that the altimeter had worked at both apogee and main deployment.
Once I got home I looked the rocket over, no damage was found. I quickly removed the altimeter module to download the flight data. I plugged the flight computer into my PC and ran the data retrieval program. The program returned the following data:
Ground Level at launch: 3171 data value
Maximum Altitude: 2909 data value
Main Deploy: 3107 data value
Translating the data values into feet:
Ground Level: 0' or 28.59" Hg
Maximum Altitude: 2,269'
Main Deploy: 587'
Running altitude software prediction, the predicted maximum flight altitude was 2,327'. Considering the flight was at a bit of an angle due to weather cocking, I'd say the altitude recorded by the altimeter was really close to dead on.
I do need to work on the PIRM2 main deployment, a little smaller main chute or shorten up the shroud lines to prevent the chute from deploying early. It seems the altimeter worked exactly as expected, nice to see something work on its first try. I think I'll fly a little lower on the next flight, so I can better see what's going on with deployment.
Enough said, it was a good day!