First build after being out of the hobby for a few years

[Neutronium95]

I'm getting back into the hobby after about 7 years away (you can check my post history to see just how much my youthful enthusiasm exceeded my capabilities and resources). For my first project, I am planning on rebuilding my SPEEEEAR, a 10 foot tall, quad E motor rocket built out of BT-80. The rocket flew well for several years, but it eventually had to be retired due to a few factors. My goal with this project is to rebuild it in a way to address those failures, and to allow it to fly on a larger variety of motors, possibly including some more gentle H and I motors.

The biggest weakness with the original was that the long length seemed to put a lot of bending force on the body tube, especially just above the fin can. This caused the body tube to start bulging around the spiral seams, and it necessitated at least one operation to cut out weakened tube sections. The second weakness in the design was that the surface of the fins collected a lot of dents and other minor damage. I used 1/4" balsa skinned on one side with very thin hobby plywood. This produced sufficiently strong fins, but the balsa surfaces collected a lot of minor cosmetic damage.


For the body tube, I am considering several different reinforcement options, and I will probably make a few test pieces of the most promising ones before going on to the main rocket construction. I might also only reinforce the lower third and just consider the upper body tubes as wear items that get replaced as necessary, but that kind of rubs me the wrong way. In no particular order, the options that I am considering include, but are not limited to:

• Just wetting the tubes out with epoxy. It won't provide a massive amount of support, but it should be pretty light, and is relatively simple.
• Using a carbon fiber or fiberglass sleeve. I've done it before, but it is very much overkill for my application, and might be too heavy for my tastes.
• Wrapping the tube in a layer or two of light (<4oz) fiberglass. It's probably the right balance of strength and weight, but I haven't done much work with big pieces of fiberglass cloth and am concerned that it might turn into a giant mess.
• Wrapping the tube in nice tissue paper or similar. Would definitely hit the weight requirement, could look cool if left unpainted. I'm not certain about how much it would improve the strength though.
• Nylon stocking "fiberglassing". Seems pretty easy, but I've seen some concerns about how much strength it would actually add.
• Running wooden stringers up the outside of the tube. Could look pretty cool, and provide a unique method of reinforcement. I'm not sure just how much mass they would add though.

For the fins, I'm planning on reusing the laminated balsa concept, but do it properly this time. I'm not sure whether I want to use the plywood again, or if I want to use paper or fiberglass instead this time. The previous fin fillets were done with epoxy clay, I'm interested in using something a bit stronger, but I would appreciate a good guide on how to get really clean fillets with regular epoxy.

For the general design, I am planning on essentially building a minimum diameter BT-80 rocket, and then having various motor mounts that can swap into the airframe. I figure using a piece of coupler tubing with a thrust ring at the back would work well enough to transfer the thrust. I'm planning on going up to 54mm motors, solely to make use of the 1 grain CTI casing that I have. To keep the adapter from kicking out, I'm planning on having a threaded insert that is used to secure the lower rail button built into the lower centering ring of the adapters.

I'm attaching some pictures of the problem areas on the original, as well as one of the current openrocket files if that would help. I'm not sure exactly how much everything will weigh, so I've been throwing some extra mass in to see how much it affects the performance.

 

[David Schwantz]

LOC tubes are great. Have you ever thought about a full length coupler inside what ever tube you use? If you were to choose thin wall fiberglass, no reinforcement would be needed. I would get the small Rocket Poxy, many places carry it. It work great for your fillets, and building rocket. After mixing, let it sit a bit to thicken up some. Spread in the joint, and pull the fillet. You can use your finger, spoon, pvc tubing and an array of other things. If you use the correct amount in the joint, no need for taping. But if you have not done it for a while, I would then tape off fillets. For the fins, I would glass them myself. You can use .75 oz glass, will weigh next to nothing but still give strength and durability. Just make sure they are shaped and sanded before glassing as you do not want to sand through the glass. When you glass them, if the coat is shinny, you have to much resin, squeegee some out. If it is white and dry looking, you need more resin.

 

[Speaknoevil]

Welcome back! Thanks for sharing.

 

[Neutronium95]

Just had some good news and some mildly annoying news.

Good news: I got my old certification back, and my Jr. L1 got upgraded to L1, so there's no need to deal with certification again.

Mildly annoying news: Local Hobby Shop was out of 1/4" balsa, so I'll have to wait a while before acquiring parts.

For now my plan is to buy a few extra BT-80 tubes, and try out a few different reinforcement methods to see what I like best. If things end up too weak for full up high power flight, that's alright. My main concern with this rocket is keeping it light enough so that it will fly comfortably on the 4x E motor cluster of the original.