Rocketry

I’ll provide an update on the dual deployment system after one more test, probably tomorrow, but it looks like it works now!

After working on that today I decided to try out the parachute. I have never been successful with a parachute before so I wanted to make sure that I was folding it in a manner that would work. One time when I was a kid I tried a parachute without enough wadding and it melted. Since then I’ve focused on other types of recovery systems. Now I’m building a huge rocket that is going to rely on not one but two parachutes!

This first video is a super slow motion capture, about 1/16th normal speed (3.75fps versus 60fps), of the parachute unfolding. I folded it exactly the way I’m planning too in the rocket and threw it off my deck with a five pound weight attached to it. You can see that it finishes opening right as the weight hits the ground. It’s only fully open for one frame, but it clearly worked.

Having seen that the folding technique works, I wanted to see what this parachute would look like flying through the air fully open. This time I left it unfolded when I threw it. It looked very good floating quickly to the ground.

Here it is at full speed in case you want to see it. Just don’t blink…

I saw this article about building a dual deployment device without black powder and thought that would be perfect for Project Overkill.

I didn’t build it exactly as explained in that article, but close enough. After building it I put in an igniter and tied it together with fishing line. Then I grabbed the shock cord and shook it like crazy. I wanted to see how it would react. The fishing line broke off all the pyrogen from the igniter. Were this a real flight, the main would not have deployed. I need to think about this design and rework it.

One thing I’ve done since that test that may help is build a guide for the fishing line that extends right up to and across the break between the two halves of the container. I’ve also added a tab near the bottom of one of the halves to try to keep them aligned a little better. Finally, I added some screws to hold the fishing line instead of just tying it on the back side. I’ll give it a go tomorrow night and see how it works out. I can’t try it right now because the epoxy is still wet.

I read somewhere that if you are going to use an eye bolt for a shock cord mount in a high powered rocket you should use one that is welded shut. The person who wrote that article was claiming that the strain on the bolt can open the eye if it isn’t welded shut.

That seemed like a bit of overkill to me, but then again this is Project Overkill so… I definitely considered it. All of the sources I know about don’t have the type that is welded shut, but I came up with another idea. Take a look at these rope clamps.

If you take them apart they look like this.

If you take one of those U shaped bolts, four of the nuts, and add a couple of washers, I bet that’s pretty strong and probably over kill. So, that’s what I did.

You can’t see the washers, but I did put a couple on the bottom of the centering ring. I figure that’s where they are needed.

I’ve worked on the fins on and off over the last few days. I started out making 1/8 inch birch plywood fins with tabs long enough to pass through the body tube all the way to the motor mount. I figure that’ll add much more strength than the fins that came with the LOC IV kit I started with. The tabs on the LOC fins were just 1/8 of an inch deep or so. They were enough to pass through the body tube and get a glue fillet on the inside, but no where near long enough to attach to the motor tube and centering rings.

Once I had three of those plywood fins cut, I cut six 1/8 inch balsa fins slightly larger but without the through the tube tabs. I then attached one balsa fin to each side of each of the three plywood fins. This gave me the ability to shape the fins without sacrificing much of the plywood.

[In this photo only the leftmost fin is seen straight on. The others are seen at a bit of an angle. When viewed straight on they appear more uniform.]

I decided this time around to try to shape the fins into an airfoil. I’ve never done that before and I know it is fairly exacting, so I was pretty nervous about getting it right. I started by laying down a piece of tape one inch from the rear edge of the fin to use as a guide while sanding the initial bevel. The tip is only two inches long so this is half the length of the tip. I then beveled the rear end of the fin. I did that on both sides. At this point the fin was 3/8 inch thick one inch from the rear edge, but only 1/16 inch along the rear edge. This was just a plain, straight bevel. I did not put any curve into that bevel yet. Once I had done both sides of the rear edge, I moved to the forward edge. Here I started by adding a bevel, but quickly turned it into a blunt rounded curve. Once both sides of the forward edge were rounded I added a curve from the rear bevel to the front rounded edge. That gave me a nice airfoil shape. I don’t know if they are good enough, but they look decent to my untrained eye.

I know that I should have tapered the fins so that the chord to thickness ratio remained constant for the entire span, but I just couldn’t bring myself to attempt that type of complexity yet. Maybe on the next big rocket.

I also added a coat of cyanoacrylate (CA or super glue). I wanted to make the balsa a little more resistant to dings and dents. The CA seemed to wick into the fibers and add some strength. Once it was dry I gave them a good sanding.

I’ve decided that I’d like to get my high power rocketry level 1 certification. I do have a LOC IV kit down in the cellar I could slap together and fire off with a small H motor for that purpose. Of course, being me, I just can’t do something that simple. Why follow the KISS principle when you can get overly complex?

With this in mind I’ve decided to start Project Overkill. This rocket with be much stronger than it needs to be. It will be much larger than it needs to be. It will be much heavier than it needs to be. It will be more complex than it needs to be. It will be daunting and intimidating to build, but it will be fun.

The basic idea is simple, three fins and a nose cone. However, the body tube will be four inches in diameter and the rocket will be about six and a half feet tall. It will fly on an H, a small I, or a big I. The H will drive it about 900 feet and the small I will push it up to 1,500 feet. I won’t use the big I for now since I don’t have access to a field big enough.

Since I’m concerned about recovering the rocket within the field I’m going to use a dual deployment system, even though that isn’t strictly needed. I decided I didn’t want to have to go through the hassle of dealing with black powder so I’m using one based on an igniter melting through fishing line similar to this one.

I also plan to fly a video camera in the rocket. I picked up a Muvi video camera similar to this one on sale with that in mind. I haven’t figured out how I’m going to mount it, but I guess I will.

I’ve simulated this rocket in RockSim. It looks like it will fly just fine. When I first simulated it RockSim predicted that it would be heavy enough that the best engine would be an I212, so I ordered one. However, as the parts started to arrive it became clear that the sample parts I had picked from the RockSim database were heavier than the actual parts. For example, I had picked a 60″ parachute and it was listed as 14 ounces. The actual 72″ parachute that I purchased only weighs 6 1/2 ounces. I decided that I should use a smaller engine for the first flight and ordered an H125. Of course, that means that if it survives the first flight, I have a second, larger motor to fly it a second time on the same day.

The current plan is to launch Project Overkill on July 17th at Woodsom Farm Park in Amesbury, MA if it is finished in time. If I don’t finish it in time then I’ll launch it in September (the next HPR launch in Amesbury). I’m not going to stress out and push to finish by July 17th. If I finish by then, great. If not, so be it.