Balancing and straightening longer rockets

[RebelRockets]

When I see photos of multiple bottle rockets, especially longer ones with several couplings and cemented joints, I wonder how do you get them straight enough for good flights.

The obvious thing when making cemented joints is to roll them on a table top while the cement is wet. You can do this with coupling joints, too, and I suppose you can use the sleeve covering the joint to correct slight bends.

What can you do to test your finished rocket for straightness before risking your work and possibly expensive electronics on board?

This seems like a simple test. Attach a little screw eye or similar hook overhead in your workspace. Get a carpenter's plumb bob, (a little pointed weight on a string used to find true vertical), and suspend it with the point barely off the floor. Put some wide tape on the floor below the point. When the weight stops moving, mark on the tape the exact point below the plumb bob. Take down the plumb bob.

Now drill a tiny hole in the exact center of a screw-on bottle cap that fits your bottles. Put a string through the cap, knot it inside, and hang it from the screw eye. Screw the cap on your rocket's nozzle end and suspend it with its nosecone just off the floor. When the rocket stops swinging, shouldn't its tip be exactly over the mark on the tape?

If not, the rocket body may not be straight or there is significant weight off-center of the rocket's vertical axis. Now you have a chance to make adjustments without risking a crash.

Dennis

[SaskAlex]

Hi. That's an interesting idea, but I'm not sure if it's necessary. I've never made robinson couplings, so I can't speak for those, but I make symmetric splices with long sleeves, and my rockets always come out perfectly straight. It just happens naturally, as long as your sleeve is long enough and a good fit.

Also, your method would accurately tell you if the COG was nicely on the vertical axis, but not necessarily if the rocket is straight. For example, if the rocket was a symmetric "S" shape (doesn't have to be dramatic, just two opposite "kinks") then it would still have it's COG perfectly in line with the nozzle and nose cone, but it certainly wouldn't be straight.

So it sounds like a good method for balancing your rockets, spliced or not, but not the greatest for determining "straightness".

[RebelRockets]

Point taken about checking for straightness, and I agree. That made me realize that I had left something important out. When the rocket is hanging upside down and no longer swinging, the string can be given some twisting and then let it unwind, and the rocket will revolve like on a slow moving lathe. Then any bend in the rocket should be visible and obvious. If you happen to have one, I believe there are motorized hangers for revolving ornaments that would be handy for this.

Do you make long sleeves from bottle sections or have you ever tried plastic mailing tubes? There are some 4 ft. long clear mailing tubes made from a PETG material made at a factory near my area. Anybody know if PETG is compatible with PET & PETE cements, and would it take similar pressure so the rocket body could be made from it? It would save making a lot of splices.