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For some reason, this rocket is a full pound lighter than the last one. Some of the weight loss can be attributed to several things. A shorter motor mount tube, the tubular nylon is lighter, one less bulkhead in the upper body tube and the new parachutes are somewhat lighter. But I think the real weight savings came from the upper body tube itself. It was by far the best body tube I have made to date. I think I did a lot better job of minimizing the amount of epoxy resin in the composite, resulting in a thinner and stronger body tube.
To keep the altitude down, I'm using a much smaller load in the motor. I'm using the same LW-1 motor, only I'm using two 3" grains rather than 4 grains. The grains also have a 1" core to keep the Kn up. The result is a propellant load of .81 pounds of KN/SU. Assuming an Isp of 120, the motor should have a total impulse of about 97 pounds seconds (mid "I" class).
I set my apogee timer at 8.7 seconds, and the back up apogee timer at 9.5 seconds. 2.4 grams of black powder was used, that might be a bit much using a piston... The main timer to the PIRM2 is set to release the main chute 10 seconds later. Software predicts apogee at 9.5 seconds, but from experience I've found the rocket generally reaches apogee slightly before the prediction. Looking at the software predicted velocity, there is about a three second "window" between 8 and 11 seconds into the flight where the rocket is traveling below 50 feet per second. In reality, the timer setting only needs to be reasonably close.
Looking at the weather forecast, it appears I may only have one day in the next week to get the launch in. So I spent most of the day getting the rocket ready for the next days flight, one thing I didn't get done was to paint the new upper body tube. Oh well, it'll fly just fine in the raw.
Launch Day:
As predicted by the weather man, it was a gorgeous day to launch, clear skies and light wind. The rocket had been prepped in the shop, so all I had to do was set up the controller and launch pad. I seal the motor nozzle with foil tape, and use the tape off the nozzle to hold the igniter when I set up on the pad. Once the igniter was installed, I turned on the electronics module and went to the controller.
5, 4, 3, 2, 1 and ignition.
Liftoff!
Here is motor burn out. A nice straight flight.
Considering I have never used a light propellant load like this in the motor, I wasn't sure what to expect. But it had a very nice kick off the pad and boosted beautifully. I watched the flight and counted the time... at about 9 seconds I could see the cloud of smoke from the apogee deployment charge. So far so good.
The little white dot in the center left is the apogee charge going off. Interesting picture isn't it? Sorry.
I watched for the drogue chute, it appeared and quickly inflated. Then the main chute appeared and inflated. Hhmm, a little quick for the main. But better that than no main.
Here you can see the rocket coming in under both chutes
As soon as the rocket landed I went out to check on its condition. It was a quick walk, no half mile walk today. It is nice flying to a lower altitude...
The rocket was in perfect condition, no damage anywhere. I quickly checked the PIRM2 release pin, it was evident it had fired by the black powder residue on the pin. There were really only two reasons for the PIRM2 to fire prematurely. 1) I had the timers switched so that one of apogee timers went to the PIRM2. Or, 2) The apogee charge caused a sympathetic firing of the black powder charge in the PIRM2.
Once I got home I checked the electronics just to make sure I hadn't wired it wrong, I hadn't. So the PIRM2 would have to have been set off by the apogee charge. I wondered if 2.4 grams was a bit much for a piston deployment. I'll have to cut down on the apogee charge for the next test, I could also seal the igniter wire in the PIRM2 with a little RTV to help isolate it.
Even with the PIRM2 firing prematurely, the flight confirmed that everything worked as planned. As a side note: this is the third flight using the magnetic reed switches to start the timers, and it has worked flawlessly in each flight. I like the magnetic switches, as no outside wires or switches are needed and they are proving to be dependable.