Sunday, May 3, 2009

I need advice

So I'm moving right along with the Machnum Force. I've got all three fins finished and very smooth and strong, and the nose sone now has about half an ounce of weighting plus a nice strong eyehook in it. Now, though, I'm not exactly sure what to do. I've enever built a rocket to stand up to 1000 mph forces before - Mach My Day was slower and smaller, and Mozzie has a much stronger design and is bigger and slower. Here I have a half-millimeter thick cardboard tube that's going to be subjected to incredible lateral forces, and less than half a square inch each to attach thich fins and will carve out a slot of 65 Liters of air per second.

Currently, for the fins, I plan to attach them with wood glue, put CA into the cracks, then do 2 or 3 layers of fillets. I think that properly done it should stand up to anything I can put this rocket through. is this a)overkill, b) okay, or c) I need epoxy and fiberglass to have any chance of hitting Mach?

For the body tube, I have 13" of thin-walled kraft paper 29mm tubing from Apogee. Will this alone be sufficient, or should I slather a layer of wood glue or two on top, or spend 150 bucks on an otherwise cheap rocket to get fiberglass?

Please, respond. I need all the advice I can get.


Sascha Grant said...

I think that you'd do better with epoxy fillets rather than CA or Wood glue. The fins will be under quite a lot of stress. Epoxy can be a pain to work with, but having some isopropic alcohol on hand is a must. After you apply a fillet, you can dip your finder in the iso and wipe it over the fillet to smoth it out :)

Check out this :

This is Apogee's mach-breaking rocket. Uses balsa fins and standard body tube for the airframe.

Good luck!

Dick said...

The only possibly mach-capable small rockets I have built used waferglass fins and were built with epoxy.

I don't know if wood fins are suitable for mach at all (depends on the size) but it has been shown over and over that, for wood on paper, wood glue is as strong or stronger than epoxy. That being said, epoxy will form better fillets than wood glue, independent of the adhesion strength. Epoxy with microballoon fillers are best for fillets.

I say try the wood with wood glue and report back.

Dick said...

Another thought. The Rocketvision Mach Buster's fins were notched into the body. Basically the fin slots add a bit more strength. I wrapped a motor with wax paper to make sure the fins were positioned perfectly.

Granted this was a phenolic BT, but it may help on paper too.

The EGE said...

Thanks for all the advice. I'm going to try it with wood glue to attach the fins - they're very strong 1/8" plywood and their thin, long-rooted trapezoidal shape isn't likely to flutter. I'll use 2 layers of wood glue for the filets, unless I can get some dental epoxy from my dad. They're heavy and a bit draggy, but the G78G has plenty of impulse to spare.

The EGE said...

I'm still not sure whether to use the standard 0.7mm thick tube, or to apply a layer on wood glue on top. It'll add some weight, but strength too, and I don't know if I need that strength or not.

For attaching, I'll poke holes in the body tube and fin for glue to soak in for extra strength.

I'm not sure what I'll do for launch lugs. Since I'll have to fly it at Pine Island (the lcoal waivers for CATO launches are 4000' and it'll hit 4500), I'll see if I can borrow a tower launcher for less drag. If not, then I'll use a 3/16" lug, which I'll put on anyway after the Mach flight so I can use smaller motors.

Dick said...

Glue rivets are a good idea.

Wood glue will add no strength to the tube. Best = a thin veil of fiberglass. This adds lots of strength and very little weight. FAI competition rockets fly with one layer of veil-weight glass as the tube itself. 2nd best is probably to soak the tube in CA.

A tower is of course best and then pop-lugs.