First Rocket Issue

[Albanach]

When I used OpenRocket on my first HPR it showed the CG and it worked out fine. When I balanced it with a loaded motor it was off. To balance it I would need to add weight to the nose. My first thought it to figure out if I have too much epoxy in the aft end.

 

[neil_w]

Stability *only* matters with the motor loaded. If you have insufficient stability with the motor in, then indeed nose weight would be a solution. Too much adhesive in the rear can definitely pull CG too far back, but some rockets are just destined to need nose weight with certain motors. In other words: it depends.

If you give us some info about the particular rocket in question, and post the OR file, we can give additional help.

 

[Steve Shannon]

When I used OpenRocket on my first HPR it showed the CG and it worked out fine. When I balanced it with a loaded motor it was off. To balance it I would need to add weight to the nose. My first thought it to figure out if I have too much epoxy in the aft end.

Are you trying to make sure that the Cg is where the sims say?

 

[Albanach]

Are you trying to make sure that the Cg is where the sims say?

Sort of. I understand it won't be precise but I should see the Stability Margin in the locations.

 

[waltr]

I only use the sin (OR) to determine CP and get an idea of where the CG and thus the stability in Cals is.

CG is very easy to determine on a rocket by finding where it balances (loaded and flight ready). If it is a couple cals ahead of CP then good to go. CP is the hard location to determine so I trust the OR sim to do this and then mark this on the rocket (in line with launch lug/button where it isn't seen.

I then go back to the OR Sim and add/move Mass to obtain the CG location actually measured. Same with total weight (usually without motor and add/subtract weight to get sim the same as rocket as built).

 

[Albanach]

Here is the OR file. And RS.

 

[neil_w]

It looks fine. Why do you think there is a problem, or that you need nose weight?

 

[Albanach]

I think I need nose weight. I used a cord to determine the balance point. Without the motor, it is where OR said it was but with the motor it is very close to the CP.

 

[lakeroadster]

Without the motor, it is where OR said it was but with the motor it is very close to the CP.

Weigh your motor. Does it match the weight of the H125W-6 motor in Open Rocket = 7.9 ounces?

Also, your are selecting the motor in the dialogue box, correct?

 

[neil_w]

I think I need nose weight. I used a cord to determine the balance point. Without the motor, it is where OR said it was but with the motor it is very close to the CP.

How do the measured mass and CG of your rocket compare to the OR model?

In general, once you have built the rocket, you should override mass and CG in OpenRocket with the measured values, and then after that you will get accurate numbers when add motors.

It is common for a built rocket to have CG further back due to epoxy, which is often unaccounted for in the model.

 

[OzHybrid]

When checking the actual Cg make sure that the parachute is where it will be when the motor KICKS it to at launch. Especially if it’s marginally stable. Also with a swing test if it’s stable it will definitely be stable at launch. In a swing test because it’s moving in an arc which moves the Cp forward.