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How can I stop my rocket from tumbling?

I have been entering the realm of high powered rocketry and have been having trouble with my rocket frame design. My rockets accelerate from the ground at 20 m/s2 with a custom built rocket motor which weighs 80 grams, the problem I have is that the rocket spins head over tail after take off at around 10 revolutions per sec. I suspect that the design is too tail heavy... The specifications for the rocket are; Height 40cm Fins 9 by 5 cm 90o triangles x 3 Width 5 cm Any help will be praised appropriately.

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Denger7 years ago
Read G. Harry Stine's Handbook of Model Rocketry. Many public libraries have it on their shelves. Even an old edition is well worth the read, as it addresses all these issues.

That said, it sounds to me like your fin pattern and surface area may be of insufficient proportion to the airframe length and diameter. The triangular fin pattern is not one of the more aerodynamically efficient designs -- you may get better stability by maintaining the same fin surface area but instead choosing a raked or swept fin pattern, which will move your rocket's center of pressure closer to the tail (the CP should be a minimum of 5cm behind the CG, in the case of your design; the farther back, the better).

Your design sounds small enough to possibly be flown on commercially available low power (D-F) motors. Have you tried that? If so, it may just be a matter of adding suitable additional mass to the nose cone to offset the mass of your custom motor.

Otherwise, I think I'd be tempted to build a smaller exact-scale model testbed which could be flown on low power motors, (the CG must of course be in the same location as for its larger sibling). If the small model is very stable, the bigger one should also be fairly stable, all other things being equal. Higher accelerations will of course put different stresses on the larger rocket, but that is another matter...

Good luck, and happy skies!
Johenix7 years ago
G. Harry Stein's "Handbook of Model Rocketry" and Capt. (USAF) Bertrand R. Brinkley's "Rocket Manual For Amateurs" are good places to start.

I agree it needs larger tail fins.

Simple test: Find the center of gravity by balancing the rocket in a loop of string. Mark the balance point. Tie the end of a string around the rocket at the balance point. Now swing the rocket around in a circle by the string. If it prefers to fly nose forward you have enough finns on your rocket.
The center of pressure has to be behind the center of gravity. Suspend your rocket from a string (so it balances horizontally at the center of gravity) and aim a fan at it so the airflow hits it. If the rocket is stable (center of pressure is right), it will turn into the air stream. ~Bob~
jtobako7 years ago
There's a way to balance the rocket by the difference between the profile and the actual center of mass-I don't remember the details.

try the info at
http://inventors.about.com/library/inventors/blrocketcontrol.htm
and
http://www.wikihow.com/Calculate-Stability-of-a-Model-Rocket
(even if it's not a calculation...)
Prfesser7 years ago
I suspect you're right, tail-heavy is bad. A very rough rule-of-thumb is to find the center of gravity of the loaded rocket. If the CG is at least one body diameter ahead of the front of the fins, you may be good-to-go. But from your description, the fins are probably too small; you may need to add a good portion of nose weight.

Steveastrouk's suggested site is a very good idea. Do as much of the testing and calculations *before* attempting to fly. A busted rocket is disheartening... I know...
I suspect you have issues with the relative positions of your centre of pressure and your centre of gravity. There are freeware programs that help you work out the details, but this is an excellent reference.
http://sizzlerocketry.com/Rocket%20Design%20and%20Balancing.pdf