Brain lock

[RocketAl]

I have been out of rocketry for a few years. I want to start up again. I am using Open Rocket. I can't remember if Cp is calculated with the motor accounted for or not.

Al

 

[lakeroadster]

CP is based on the flying configuration of the rocket. So yes, the motor needs to be accounted for.

 

[Hobie1dog]

to the forum

 

[ThirstyBarbarian]

The Cp is the center of aerodynamic pressure. The Cp calculations take into consideration the parts of the rocket that touch the air. Because the motor is inside the rocket, it doesn’t affect the Cp. All OR really needs to calculate the Cp is the nosecone, body tubes, fins, or other parts on the outside of the rocket. You don’t need to specify a motor, motor tube, centering rings, parachute, or anything else inside the rocket and you don’t need to have any of the weights correct. All of that is important for Cg, but not for Cp.

 

[RocketAl]

CP is based on the flying configuration of the rocket. So yes, the motor needs to be accounted for.

Thanks, I thought so but I needed confirmation.

 

[ThirstyBarbarian]

CP is based on the flying configuration of the rocket. So yes, the motor needs to be accounted for.

Thanks, I thought so but I needed confirmation.

I disagree on this. CP in OR doesn’t change depending on the motor. CG does. You can try it out in OR — make a rocket design in OR, check the CP, add or remove motors, and the CP will not change. The CG will change, but the CP won’t.

 

[RocketAl]

The Cp is the center of aerodynamic pressure. The Cp calculations take into consideration the parts of the rocket that touch the air. Because the motor is inside the rocket, it doesn’t affect the Cp. All OR really needs to calculate the Cp is the nosecone, body tubes, fins, or other parts on the outside of the rocket. You don’t need to specify a motor, motor tube, centering rings, parachute, or anything else inside the rocket and you don’t need to have any of the weights correct. All of that is important for Cg, but not for Cp.

OK I received two different replies. Thank you both.
So if I am getting the gist of this, I use OP with a the motor for my Cg and without for my Cp. Or can i leave the motor accounted for and use the caluated Cg since it should only use the exposed rocket parts.

Al

 

[ThirstyBarbarian]

OK I received two different replies. Thank you both.
So if I am getting the gist of this, I use OP with a the motor for my Cg and without for my Cp. Or can i leave the motor accounted for and use the caluated Cg since it should only use the exposed rocket parts.

Al

Basically for CP, it doesn’t matter either way if a motor is set up in the sim or not.

But most likely what you really want to know is if the rocket is stable or not, and for that, you do need to add the motors you want to use.

The way OR accounts for motors is with the flight configurations. After you set up the rocket design, you set up the flight configurations, and for each configuration, you specify a motor. Each motor has a different weight, length, and CG for the motor itself.

If you want to check your stability margins, there’s a drop-down where you can pick which motor/configuration you want to check (I think it’s above the right side of the display of the rocket). In that drop-down, you can pick “no motor”, and the info inside the rocket display area will show the weight, length, CP, CG, stability margins, etc. of the rocket with no motor loaded. Or you can pick one of the motor configurations you have specified, and the info will be updated to show the weight, length, CP, CG, stability margins, etc. of the rocket with that motor loaded.

If you do that, you’ll see that the weight, CG, and stability margin will change with each different motor, but the CP will stay the same.

 

[lakeroadster]

OK I received two different replies. Thank you both.
So if I am getting the gist of this, I use OP with a the motor for my Cg and without for my Cp. Or can i leave the motor accounted for and use the caluated Cg since it should only use the exposed rocket parts.

Al

@ThirstyBarbarian is correct. I stand corrected and apologize for the confusion.

 

[MidOH]

Motor nozzle sure does hang outside of the rocket, on many I see.

 

[ThirstyBarbarian]

Motor nozzle sure does hang outside of the rocket, on many I see.

Do you think OR takes into consideration motor nozzles?

 

[neil_w]

Do you think OR takes into consideration motor nozzles?

It does not.

CP: Motor does not matter
CG: Always include motor to get a real stability estimate

 

[ThirstyBarbarian]

It does not.

CP: Motor does not matter
CG: Always include motor to get a real stability estimate

Yep. That’s what I thought.

 

[mh9162013]

CP: Motor does not matter

Unless part of it sticks out the bottom of the rocket...right?

 

[neil_w]

Unless part of it sticks out the bottom of the rocket...right?

OpenRocket does not factor that in. You’ll notice that the CP doesn’t change when you change motor configuration.

 

[JoePfeiffer]

Do you think OR takes into consideration motor nozzles?

Motor overhang doesn't make a significant difference to CP, so OR doesn't worry about it.

 

[ThirstyBarbarian]

Motor overhang doesn't make a significant difference to CP, so OR doesn't worry about it.

Makes perfect sense to me.

 

[cerving]

The air is so dirty by the time it gets to the bottom of the rocket (even with a tailcone) that a relatively small nozzle sticking out of the back isn't going to make any difference.