LOC Big Nuke 3E stability?

Hello,

I'm planning to launch a Big Nuke 3E this Saturday on a K600 motor. However, on Open Rocket my sim says that stability with this motor is just 0.526. Even adding a kilogram in the nose cone does not get stability over 1.0.

I started with the stock file downloaded from https://www.rocketreviews.com/unknown-locprecision-pk-82-3e-big.html and modified the values to match my actual build, the values were fairly close. I am thinking that the issue is with the CP value in the file. The Rocket Reviews page says:
"CG: 49.6559 inches from front
CP: 63.8747 inches from front
Margin: 2.59 Overstable"
The actual file, however, says the CP is 149 cm (58.66") from the front, with "no motor" stability at 1.66.

My actual "no motor" CG is 49.7, about the same as what the web page says it should be. So it seems the question is whether the CP is 63.87", as the web page states, or 58.66", as the file states. Or am I missing something? If CP is 63.87", how to override the CP value in the file?

Another flier posted a similar question about the original Big Nuke (not the 3E) at https://www.rocketryforum.com/threads/big-nuke-stability.55495/, there was also an issue about the file.

Any thoughts on which CP value is correct? And has anyone encountered any stability issues flying a K600 or similar motor on a Big Nuke 3E?

Thanks,
Bill

Openrocket uses the more conservative pure Barrowman form of the CP calculation, while Rocksim has an option to use a less-conservative form that supposedly incorporates body-fin effects.

I've put about 8 oz of weight in the nose of my Big Nuke 3E and flown it on motors as large as the Aerotech L1000 with no stability problems.

Watch out for small things in OR causing issues with stability. I had a small set of "fins" which were actually motor retainers for a MD rocket that were totally throwing the stability off, something like .3 caliber on a Loc Weasel, once those were removed the stability jumped up to a more realistic number.
Can you post your OR file for us? And did you remove all the overrides from the original file downloaded?

I'll post it when home, time permitting I'm going to try to make a new file from scratch, without using the one on Rocket Reviews as a model. Interestingly, I'm getting a green check with all motors, even with wind, and even with an L1000 motor, with no weight added and stability less than 0.6. I did not see any way to override the CP on Open Rocket, is that so?

billdz said:

Interestingly, I'm getting a green check with all motors, even with wind, and even with an L1000 motor, with no weight added and stability less than 0.6. I did not see any way to override the CP on Open Rocket, is that so?

Click to expand...

There's no way to override CP.

1 caliber of stability is just a guideline. I could easily imagine the design would be fine with 0.6 cals of stability, as it is both fat and comparatively short. I chose to add some nose weight just so I wouldn't have to worry about it. Being stable in OR doesn't mean anything, it's whether it's stable in reality that's important!

mine's flown quite nicely on 54/4 motors of all type... and a baby L

mikec said:

There's no way to override CP.

1 caliber of stability is just a guideline. I could easily imagine the design would be fine with 0.6 cals of stability, as it is both fat and comparatively short. I chose to add some nose weight just so I wouldn't have to worry about it. Being stable in OR doesn't mean anything, it's whether it's stable in reality that's important!

Click to expand...

THIS ^^^^

Use sim programs to calculate the CP. Use the actual loaded rocket to determine actual CG. Is the true balance point of the rocket 1 caliber in front of the CP derived from the sim program (in this case, 5.54")...if so Go fly!

Yes, some rockets can get away with less than 1 caliber.

When I use rocksim, CP is 63.87 and CG with a K600 is at 55.9. I have no nose weight added to mine.

1.44 cal of stability.

I think a K600 would be a fantastic motor for that rocket.