I just realized that Compressed Air Rocket Launcher can be shortened to CARL. I like that. I think I shall use this name from now on to refer to my launching platform.
Anyhow, we took CARL out again on Friday to test a few new rockets we had made since every one of them blew up last time. Since CARL had some sensitivity in the air tube leading into the main tank last time, I got some zip ties of my own to add a second tie. It seemed to make the seal a lot more secure up until about 85 psi, which is plenty of pressure for some awesome launches.
CARL’s battery box received just one more upgrade as well. If you recall from last time, the only thing I had left to do in the battery box was to secure the 9v battery clips to the inside so they wouldn’t rattle around.
We went to Target and got a cheap hot glue gun, which I thought would be good because the metal battery clips had several holes that the glue could rise above and spread out, securing the metal to the plastic base. Since the holes were right by where the battery sits inside the clip, I wrapped the 9v batteries in parchment paper to prevent the glue from sticking to them, then I applied glue to each of the holes from the bottom, and pressed them into position inside the box. After the glue hardened I removed the batteries, unwrapped the parchment paper, and replaced the batteries. It worked perfectly, and those clips are very secure.
During the operation one of the wires snapped. That’s where a nice wire stripper comes in handy. The wire was weakened when I used a knife to strip the wire and accidentally nicked the copper in the center. Anyhow, I just re-soldered a new wire into the clips (it was the wire connecting the two clips to combine the batteries into a single power source) and made sure I remembered the heat shrink this time.
In all, the battery clips ended up looking very nice, they are extremely secure in there, and the system still functions wonderfully. I only wish I had taken some photos of the process.
As for the rockets, we wanted to try a few different approaches this time. Here are the three models we came up with:
Starting from the right, my first attempt at a tougher body didn’t go very well. I went ahead and threw a couple of fins and a nose cone on it, but it was fatally flawed – I had wrapped it around the 1/2″ pipe a little too tightly. Even my attempts at removing some of the inner paper couldn’t remedy the situation.
My approach with this rocket was to use clear packing tape in such a way as to prevent air from escaping through the seams. I think the best approach was to cover the entire length in overlapping rings (overlapping by at least half the width of the tape), then start at the top and go down the entire length in a spiral pattern, again overlapping as much as possible. If the first spiral descended in a counter-clockwise spiral, I then did a second spiral clockwise. Then I reinforced each end of the spirals (top and bottom) with duct tape.
This rocket proved tough enough for the pressure, but because it was too tight I couldn’t get it all the way onto the pipe, causing poor performance (not much altitude, crooked flight path).
The middle rocket was built by my friend James. He showed up in the last post helping me with some of the soldering. You can see in the photo that his rocket is about 10% thicker than the other two. His technique for preventing a blowout was to put several thick layers on. Some of his layers were spiraled, some weren’t. I think he spend a full hour applying tape to his rocket’s body. Keeping with the thick build theme, he decided to use cardboard for the fins. For that we brought out the hot glue gun again.
Seeing how fun the hot glue behaved and how hard it hardened, he decided to “armor plate” his nose cone and the leading edges of his fins with the glue. Below you can see a video of all three rockets and their performance. Make sure you pay special attention to how the armored nose cone looked after the landing.
The final rocket, on the left, was my follow-up attempt after getting the other one too tight. To prevent myself from getting it too tight, I wrapped a whole sheet of paper around the length of the pipe before even starting my rocket. That way, when I finished the rocket and the body was on there real tight, I could pull it all off and remove the inner layer of paper. This resulted in a nice, loose fit on the pipe.
I ended up putting a tad too much tape on there, I think. At first I was only going to put packing tape on, so there are several layers of that, but then I decided to put duct tape on as well. I ended up putting about three or four layers of duct tape on (following the opposing spirals rule and finishing with a neat overlapping rings pattern).
Finally, I put three fins on it that had a slight angle to them that caused a really great spiraling motion after launch. The spin appeared to be just right to keep the rocket on a relatively straight flight path.
Check out the video of the launches below.
We finished up the weekend with a camping trip (CARL was not invited) which I will write about on our family blog in a couple of weeks.
- Compressed Air Rocket Launcher (mereman.wordpress.com)
- How-to: Building a compressed air rocket launcher (makezine.com)