OpenRocket - Low Altitude and error

[Pyropetepete]

HI,

I've redesigned a L1 rocket I once flew to 2818ft. The redesign is using lighter materials, shorter length but the simulation on same motor says 275ft for apogee.

Not sure what I am doing wrong, perhaps have one wrong.

The build will be

PML 4'' nose cone
PML 4'' to 3'' tail cone
Loc 38mm motor mount 14' long
Loc 4'' body tube 34'' long

Hopefully attached is the OR file. Anyone able to help please.

Thank you

 

[neil_w]

Extremely low altitude estimates usually means the rocket is unstable. What does stability margin show for your chosen motor?

 

[kuririn]

I looked at your sim. Your CG is behind the CP, margin of stability is -.568 caliber on the I540 motor.
When I add 10 oz. of nose weight I get caliber of stability close to one, and an apogee of 3665 ft. on same motor.

 

[Pyropetepete]

I looked at your sim. Your CG is behind the CP, margin of stability is -.568 caliber on the I540 motor.
When I add 10 oz. of nose weight I get caliber of stability close to one, and an apogee of 3665 ft. on same motor.

I just tried that and it worked, 3884ft it now says. Still not sure how to know it's on the right caliber for stability.

 

[neil_w]

I just tried that and it worked, 3884ft it now says. Still not sure how to know it's on the right caliber for stability.

As a very general rule of thumb, you want stability margin to be 1-2 calibers (higher for very long skinny rockets.) This is reported in the upper right corner of the rocket display, along with CP and CG. What you care about is stability margin when the motor is selected (stability with no motors doesn't tell you anything meaningful).

Stability margin = (CP - CG) / max diameter of rocket

Therefore CP must be greater than (i.e., behind) CG to maintain positive stability.

 

[Pyropetepete]

As a very general rule of thumb, you want stability margin to be 1-2 calibers (higher for very long skinny rockets.) This is reported in the upper right corner of the rocket display, along with CP and CG. What you care about is stability margin when the motor is selected (stability with no motors doesn't tell you anything meaningful).

Stability margin = (CP - CG) / max diameter of rocket

Therefore CP must be greater than (i.e., behind) CG to maintain positive stability.

Neil thank you, been about 6 years from doing this so racking my brain a little.

Having to rethink as just ran the sim with it being 54mm and a K1440WT. 1244mph, not sure paper will work