# Model Rocketry/Model Rocket

Question
Mr. Bell,
Good evening. I am hoping you could help m figure out the best weight distribution on my son's 3rd grade bungee cord model rocket.
Thank you

Dear LM -

I will admit I am not familiar with the concept of bungee cord rockets, but I did look them up through a variety of web sources.  In essence, I think the main thing you are looking for is basic rocket stability.  A rocket launched from a bungee cord should be as stable as a rocket launched by any other means.

Rocket stability is primarily concerned with two factors: Center of Gravity (CG) and center of pressure (CP).  The CG is where the rocket literally balances when laid flat.  It is the point where the weight toward the nose balances the wait toward the tail.  You can do this very easily with a pencil ... put it under the body of the rocket and locate that tipping point where it balances.

The Center of PRESSURE is the point where the rocket has equal DRAG between the nose of the rocket and the fins.  A rocket is stable when the CG is FORWARD (toward the nose) of the CP.  Think of your rocket as a flat piece of cardboard suspended on a rod like a weathervane.  If the attached pivot point is at the nose, the rocket would always weathervane INTO the wind, which is what you want.  If you attach the pivot point at the fins, the rocket will weathervane AWAY from the wind, which makes the rocket unstable.

Here is an easy way for a third grader to calculate CP ... have him make an exact cutout of the rocket on a flat piece of cardboard, stiff enough not to bend.  Just like you did with the actual rocket to calculate CG, put the cardboard cutout on a round pencil or dowel to locate its balance point between nose and tail.  Once found transfer that measurement to the rocket.  Now compare the CG to the CP.  The CG needs to be toward the nose of the rocket compared to the CP.  Think again of the weathervane example.  The CG is where the rocket balances.  If the CP is BEHIND the CG (toward the tail), the rocket will weathervane into the wind and be stable.  If the CP is forward of the CG you need to add weight to the nose to bring it forward.  The idea CG to CP relationship is one body tube diameter apart.  This means if your rocket tube is one inch, you want the CG to be one inch FORWARD of the CP.

Hopefully I explained that adequately.  If not, please don't hesitate to write back.

Model Rocketry

Volunteer

#### James Bell

##### Expertise

I am a member of Tripoli Rocketry Association, and am certified to fly hi power rockets. I can answer questions anyone may have about design, materials selection, stability calculations, motor selection, altitude and performance predictions, and parachute and recovery options.

##### Experience

I make my rockets from existing kits or parts readily made through commercial channels. I do NOT make my own motor propellants (they are challenging enough even for the companies that do this for a living), and I do not advocate making rockets using "typical household item" such as paper towel rolls.