I bent my Interceptor

I flew my interceptor a year ago and it flew great. Straight up. Great flight. This last time however it squirreled the second it came off the rail. Luckily it pancaked into the ground. Two seconds later the ejection charge fired the parachute out across the desert floor. Comical.

That leaves me with a few problems.
1) How to fix the slightly bent fuselage
2) Figuring out how much weight I need to add to the nose to guarantee stability
3) How to go about filling the nose with lead.

Here is the bird in question.

You can see where the fuselage is wrinkled just ahead of the strakes. I have an idea that I can cut the fuselage just ahead of the strakes, insert a coupler, glue it back together, and put clear Scotch tape over the boo-boo. Seems simple. That addresses problem 1. What do you think?

Now for problem 2. I weighed it and measured the CG. I simmed it in OpenRocket and sure enough it is unstable. I then parametrically varied the a nose weight and the new CG that goes with it. It seems I need about four ounces of lead in the nose. That seems like an awful lot to me. What do you think?

As for problem 3 I am pondering the best way way to put lead in the nose. I think that lead shot and epoxy would do it. I could dribble it in. I am worried however that a large volume of epoxy would exotherm and melt my nosecone. I am thinking of doing it a bit at a time. What do you think?

1) Insert a coupler into the bent area, it may be necessary to cut the airframe to do so, depending on shock cord mount and other issues. Thats really not too hard a bend to fix.

2) why add more weight? What motor and wind conditions was it being flown in? How long was the launch rod? More thrust might be better than adding weight, and a longer launch rod/rail allows more velocity before leaving the rail hence better stability. Flying a marginal motor in windy conditions is a recipe for instability.

3) if nose weight is the only solution, pack some plasticine clay into the nose, really I just believe you need a bit more thrust/ speed off the rod.

As you said, cut the tube and insert a coupler. I have done it a few times. However, you may create a potential bend point where the coupler ends. Notice that your tube bent just past the strake on the one side. You may create a similar stress point where the coupler ends; especially if the tube has been weakened during the crash. This is especially a problem if your shock cord is attached at the top of the tube like on many Estes kits. You have a large, somewhat heavy tail with supporting structure part of the way up the body tube. Then an unsupported section of tube with a shock cord at the top. The tube will always be prone to bending where the support structure stops. Nothing you can do about it other than avoid stressing that joint. So launch as straight up as possible and time the ejection to occur at apogee.

I see this is an Interceptor-E. If you flew it on an E9, that's the problem (even though that's the recommended motor). I've flown mine once on an E9, and I wouldn't do it again. There's really insufficient thrust to get it moving fast enough for stability, unless perhaps using a 6' rail. Adding nose weight will make things worse, not better. All it needs is a faster burning motor, it flies really well on an E18 RMS.

I neglected to mention I upgraded it to a 29mm. I 3D printed a custom nozzle and custom bulkheads. I was using a G79. The heavier motor and nozzle moved the CG aft. Also the vinyl skins add weight to all surfaces and probably move the CG aft a bit as well.

I ran it in Open Rocket and it was unstable. What was weird is the first time I flew it I used a G80 and it was fine. I used my own rod and not the club launch rod.


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I'm gonna go along with the tube fix by using a coupler. Make it a long one since the entire area is weakened. As far as adding weight goes, If you are going to epoxy the weight in the nose, the best bet is to set the tip of the nose in a glass of water to provide cooling as the epoxy sets--I'd use 5 minute epoxy. One thing I've done with all my Interceptors to add weight was to use a threaded eye at the base of the cone. You'll have to epoxy in a nut and washer on the back side and that requires a small hole to access the area. then simply put on as many fender washers as you need for weight with a set nut. Most times the weight of the eye is enough alone. The down side to this method is you need a little more weight since it's set back from the tip quite a ways. But you can adjust the weight as needed.--H