Satbility

Can any one tell me a simple way to check stability before a first launch?
Thanks

You will first need to detirmine the center of pressure. This can be done with almost any rocket simulation software. Find the center of pressure on your tube and mark it with a sticker or a pencil or whatever elts you want. Next, load your rocket with an engine, wadding, parachute, payload etc. Then find the center of gravity. Stick your finger under the rocket and find the point where it balances. A stable rocket will have the center of gravity closer to the nose cone and the center of pressure closer to the back of the rocket. It is also good to keep the center of gravity 1 body tube width infront of the Center of pressure. If it is not, add some nose weight until it is.


This is not the simplest but it is one of the most accurate. But hey, better to spend 5 more minutes on the stability then to watch it fail in front everyone.

Or, the classic method is taking your rocket and hanging it from the 'balance point' (center of mass) with a strong string, fully loaded for flight (engine, wadding, etc). Once you do that, tape the string in place, go somewhere with lots of room, and swing the rocket over your head. This simulates the rocket in flight.

If the rocket 'flies' in the right direction no matter which way it starts, it's pretty stable.

If the rocket 'flies' in the right direction after you 'start' it that way, it's marginally stable...make sure it has enough velocity at the end of the launch rod and/or fly it on calm days.

If the rocket doesn't ever stabilize in the proper direction, it is unstable...it needs fixing either by design, or, if possible, adding weight to the nose.

Good luck!

FC

What they said.

At https://www2.estesrockets.com/cgi-bin/wedu001P.pgm?p=publicat you'll find the "Model Rocketry Technical Model [sic]" which describes the "swing test" with pictures.

The "Classic Collection" also has a more detailed discussion of stability. It also describes the classic "cardboard cutout" method of determining a conservative Cp that was extensively used before computerized stability software came along.

FastCargo said:

Or, the classic method is taking your rocket and hanging it from the 'balance point' (center of mass) with a strong string, fully loaded for flight (engine, wadding, etc). Once you do that, tape the string in place, go somewhere with lots of room, and swing the rocket over your head.

Click to expand...

One little tip about the swing test: The longer the string (bigger circle) and the more smoothly you can swing the rocket, the better your test results will tend to be. If you can only hold your end of the string up in the air over your head and swing 5-6 feet of line, the tight circle may distort the airflow a bit and cause some problems like causing tumbling for a rocket that is actually stable.

I often take something out in the yard with me to stand on, to get my end of the string a little higher in the air for a bigger circle. Yeah, I use a step ladder. Yeah, my wife gives me a hard time about it every time. ("Can't you do that after dark when nobody can see you!?!?")

powderburner said:

One little tip about the swing test: The longer the string (bigger circle) and the more smoothly you can swing the rocket, the better your test results will tend to be. If you can only hold your end of the string up in the air over your head and swing 5-6 feet of line, the tight circle may distort the airflow a bit and cause some problems like causing tumbling for a rocket that is actually stable.

I often take something out in the yard with me to stand on, to get my end of the string a little higher in the air for a bigger circle. Yeah, I use a step ladder. Yeah, my wife gives me a hard time about it every time. ("Can't you do that after dark when nobody can see you!?!?")

Click to expand...

ok did the test flew the rockets and all was well.thanks for the help all:wave: