I decided to try my hand at going after my local club's A and B records (300' and 865 feet, respectively) because I have a new, tiny altimeter that's going into production this month and I wanted to see what I could do with it. See My little Altimeter in the support and recovery section for more information. This alt is designed to fit, with its battery, almost completely within a BT-20 nosecone, with only a mode selection/power button and a USB port. It weighs 7.6 grams. So after designing my rocket in RockSim, I made a nice, tiny rocket with really small, contoured fins that sims at 580' on an A8-5, and has 1.67 calibers of stability margin. At the field, the thing lept off the pad, and then made a hard left turn, spiralled mightily, and didn't get very high off the ground.
:trytofly:
Bummer. I thought maybe my fins weren't big enough or the alt was throwing off the CG too much radially (though I did a roll test before hand and it seemed almost perfect.) I get the rocket back, and hmm, that's funny, the tube is all bubbled just forward of the motor. I've never seen that before. It's a brand new, unfinished tube, and the nosecone pops off easily Then I checked the alt data, and saw something I've never seen before. The X-axis acceleration was shooting up toward 30 Gs as expected, and suddenly, it goes to negative 30 G's for a fraction of a second, and then the Y-axis (lateral) acceleration goes all over the place. Hmm. How could the rocket see a net 30-G acceleration backwards while the motor is thrusting? Maybe it was thrusting backwards because of a failure in the forward closure?
I pull out the engine, and sure enough, it has a skid mark on one side of the forward edge. There's one piece of the puzzle that doesn't fit, though. At the expected time of the ejection at about 4 seconds, I see a pressure spike and a moderate acceleration in the X axis, which would be consistent with a normal ejection and the nosecone popping off. But if the forward closure had failed and it was thrusting backwards, the nosecone would have already have been off. And the ejection charge wouldn't have still been intact either, would it?
I guess I'll have to fly it again to see if the problem repeats itself.
Attached is a photo showing the rocket with bubbles, the altimeter sticking out of the nosecone, and two A8-3 engines. The one on the right is the suspect one, and the one on the left is one I flew on a different rocket. I'll add in some graphs from the accel in a little while, but I gotta go for now.
:trytofly:
Bummer. I thought maybe my fins weren't big enough or the alt was throwing off the CG too much radially (though I did a roll test before hand and it seemed almost perfect.) I get the rocket back, and hmm, that's funny, the tube is all bubbled just forward of the motor. I've never seen that before. It's a brand new, unfinished tube, and the nosecone pops off easily Then I checked the alt data, and saw something I've never seen before. The X-axis acceleration was shooting up toward 30 Gs as expected, and suddenly, it goes to negative 30 G's for a fraction of a second, and then the Y-axis (lateral) acceleration goes all over the place. Hmm. How could the rocket see a net 30-G acceleration backwards while the motor is thrusting? Maybe it was thrusting backwards because of a failure in the forward closure?
I pull out the engine, and sure enough, it has a skid mark on one side of the forward edge. There's one piece of the puzzle that doesn't fit, though. At the expected time of the ejection at about 4 seconds, I see a pressure spike and a moderate acceleration in the X axis, which would be consistent with a normal ejection and the nosecone popping off. But if the forward closure had failed and it was thrusting backwards, the nosecone would have already have been off. And the ejection charge wouldn't have still been intact either, would it?
I guess I'll have to fly it again to see if the problem repeats itself.
Attached is a photo showing the rocket with bubbles, the altimeter sticking out of the nosecone, and two A8-3 engines. The one on the right is the suspect one, and the one on the left is one I flew on a different rocket. I'll add in some graphs from the accel in a little while, but I gotta go for now.
