LOC I-ROC Questions

[-Slick-Ice]

Hi everyone! I recently built a LOC I-ROC and have a few questions. The calculated Center of Pressure is nowhere near where it was on the package. Can someone review my calculations for this? I have read that Open Rocket doesn't handle short/fat rockets but either way is not anywhere near a 1-2 caliber stability. I can't find a single person saying they added nose weight and it isn't mentioned in the instructions.

The other issue I am having is that it looks like they significantly changed the rocket design and shortened the motor tube. Modest calculations are saying around 3g BP charge. I have the 38MM Aerotech standard forward closure and it maybe holds 1.5g at the most. Does the floating forward closure hold more? If not is there a plastic cap that would fit?

I also put the forward rail button somewhere around the CP (where it was in the original rocksim file if mounted to forward centering ring). The The actual kit did not match the rocksim file at all. The fins, motor tube, and body tube were all different.

 

[tsmith1315]

have read that Open Rocket doesn't handle short/fat rockets but either way is not anywhere near a 1-2 caliber stability

Check out this thread:

https://www.rocketryforum.com/threads/the-rocksim-openrocket-base-drag-trick.169558/

And welcome to the forum!

 

[Buckeye]

Post RockSim file please

 

[-Slick-Ice]

Post RockSim file please

 

[Buckeye]

I assume all the dimensions in the file agree with your actual kit? I would disregard any info or simulation files from LOC. Manufacturers are notorious for changing parts and not updating instructions/data/simulations.

I stripped out all the internals, leaving just the nose, body-tube, and fins so that there were no weird translation issues when exporting to RockSim. In RockSim, CP = 39.9 in (Barrowman method) and 43.3 in (RockSim method). This difference is pretty typical.

You have mass overrides on the nose cone and parachute, but nothing else. Since you have the complete rocket built, weigh and measure CG, then override the entire stage. Then check the stability margin. Shoot for 8%-12%.

This rocket L/D is 10:1, the classic rocket shape. No need for base drag tricks. They are not validated anyway.

 

[tsmith1315]

This rocket L/D is 10:1, the classic rocket shape.

My bad, I thought the I-ROC was shorter than that.

 

[-Slick-Ice]

These were the actual measurements from the completed rocket.
32.75 in CG no motor
72.2 oz no motor

13.6 motor (I180W-8)
CG with motor 35.0

39.9 in (Barrowman) (35-39)/55 = .0727
43.3 in (Rocksim) (35-43.3)/55 = .1509
So... "should" be fine?

Yeah, the IROC is exactly 55 in and 5.5 in

 

[wsume99]

These were the actual measurements from the completed rocket.
32.75 in CG no motor
72.2 oz no motor

13.6 motor (I180W-8)
CG with motor 35.0

39.9 in (Barrowman) (35-39)/55 = .0727
43.3 in (Rocksim) (35-43.3)/55 = .1509
So... "should" be fine?

Yeah, the IROC is exactly 55 in and 5.5 in

Why are you dividing by 55 and not 5.5? Stability is normally expressed in calibers of the rocket's diameter.

 

[-Slick-Ice]

Per above - Then check the stability margin. Shoot for 8%-12%

In this case being 5.5" diameter would be the same as the caliber.

 

[Buckeye]

These were the actual measurements from the completed rocket.
32.75 in CG no motor
72.2 oz no motor

13.6 motor (I180W-8)
CG with motor 35.0

39.9 in (Barrowman) (35-39)/55 = .0727
43.3 in (Rocksim) (35-43.3)/55 = .1509
So... "should" be fine?

Yeah, the IROC is exactly 55 in and 5.5 in

Yes, I would say it is fine. The Barrowman CP (both OR and RS) is more like 40 in, not 39, so about 9%

 

[Buckeye]

Why are you dividing by 55 and not 5.5? Stability is normally expressed in calibers of the rocket's diameter.

% of length is an alternative form of stability margin. OR allows you to choose your preference. The target usually is expressed as a range of 8% -12% (or 15%, can't remember). This approach is more reasonable than "must be greater than 1 caliber" rule of thumb that freaks everybody out when the margin is less than 1 caliber.

 

[wsume99]

% of length is an alternative form of stability margin. OR allows you to choose your preference. The target usually is expressed as a range of 8% -12% (or 15%, can't remember). This approach is more reasonable than "must be greater than 1 caliber" rule of thumb that freaks everybody out when the margin is less than 1 caliber.

Interesting. I've never seen or looked for that option in OR. One possible reason people focus on the greater than one caliber rule of thumb is because it's on the NAR L2 exam.