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LOC V2 4" CP problem

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SRBell

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Just finishing up a 3D printed adjustable ballast system for my LOC V2. LOC instructions say center or pressure is 21.9, but Rocksim has it at 23.9. Two inches seem to be a lot of difference. Which one should I trust?
 

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Banzai88

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Double check your sim and make sure that you have the form factor as accurate as you can based on actual dimensional measurements of YOUR rocket....and trust that.

Same with the 'actual, all up ready to fly weight' minus motor casing.

Same with 'actual measured GC'.

All of these should be things that you check and override in the program, if necessary, for YOUR individual rocket.

What you measure and input will almost always be more accurate for YOUR physical object than anything else.

The only exception I can think of would be short/stubbys less than 10:1 Length: Diameter, but there's a simple fix to figuring a more accurate CP for that, too.
 

Nytrunner

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On the other hand, I'm inclined to go with Loc's CP recommendation here. The V2 is a challenging form factor to accurately model with our limited simulation programs (small fins, boattail, fins ON boattail, etc...). Unless you have a CFD tool or other method of accurately determining the CP, I'd trust the guys that made it.

When it comes to CG, I 100% agree with banzai above. your design choices could significantly impact where the CG ends up for your as-built rocket.
 

Buckeye

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On the other hand, I'm inclined to go with Loc's CP recommendation here. The V2 is a challenging form factor to accurately model with our limited simulation programs (small fins, boattail, fins ON boattail, etc...). Unless you have a CFD tool or other method of accurately determining the CP, I'd trust the guys that made it.

When it comes to CG, I 100% agree with banzai above. your design choices could significantly impact where the CG ends up for your as-built rocket.
Do you think LOC is running quality wind tunnel tests or CFD? I don't. Maybe they iteratively moved the CG in flight tests until the rocket went unstable? I doubt that, too. LOC is probably using the same consumer grade CP tools like the rest of us.
 

Nytrunner

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Do you think LOC is running quality wind tunnel tests or CFD? I don't. Maybe they iteratively moved the CG in flight tests until the rocket went unstable? I doubt that, too. LOC is probably using the same consumer grade CP tools like the rest of us.
I don't think they had a real analytical solution, but they do have the experience of producing, testing, and selling that kit.

When in doubt, ask them how they arrived at the number?
 

SRBell

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Thanks everyone. I sent LOC a message and see what they say. Pretty easy for me to add weight with the modified nose : )
 

Buckeye

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Do you have RockSim set to Barrowman or RockSim method? That may be the answer right there.
 

RainierWolfcastle

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The V2 is a funny one, for the LOC shaped ones CP appears to be very close to the very front edge of the fins. It’s a short fat rocket but the boat tail negates any base drag, so don’t cut it too fine with stability margin, one caliber ahead of the fin front edge has always worked well for me. Flying the Estes 2.something” and a LOC 5.5” V2 I’ve found the faster you get off the rail the better the flight, high speed at the start reduces the coning often seen in V2 flights.
 

Dipstick

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Do you have RockSim set to Barrowman or RockSim method? That may be the answer right there.
Yes, go with the Rocksim method for a V2. I have some experience with this, originally building a removeable noseweight system based on Barrowman, then finding a huge difference with Rocksim and not requiring that weight after all...apparently Rocksim does a better job calculating the fin area on the boat-tail...
 

shawn_rocket

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Double check your sim and make sure that you have the form factor as accurate as you can based on actual dimensional measurements of YOUR rocket....and trust that.

Same with the 'actual, all up ready to fly weight' minus motor casing.

Same with 'actual measured GC'.

All of these should be things that you check and override in the program, if necessary, for YOUR individual rocket.

What you measure and input will almost always be more accurate for YOUR physical object than anything else.

The only exception I can think of would be short/stubbys less than 10:1 Length: Diameter, but there's a simple fix to figuring a more accurate CP for that, too.
What is that fix? Working on a Nano Magg and having some doubts/issues with openrockets stability calculation. I have not been able to find any info about this.
 

Banzai88

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What is that fix? Working on a Nano Magg and having some doubts/issues with openrockets stability calculation. I have not been able to find any info about this.
Not to clutter up this thread, but you add a ZERO MASS transition cone pi*Diameter in length and with the point of it set at 0 (zero) and the back of it set to (D) to the back of the rocket to get the CP. Remove it to get altitude and flight predictions for setting delay grains.

Below are the links to the relevant POF newsletters and my personal cowabunga file so you can see how it's done. (Note, I discovered this method AFTER putting too much nose weight in the rocket. Adjust the nose weight and see what it does to the CG as a learning tool on how effective the aft cone is!)

POF 154 (pt1)
POF 157 (pt2)
POF 162 (pt3)
 

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Gramparocket

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This shouldn't be that big a deal. I trust rocksim and agree with most all the posts here..but... I have a buddy that flies hpr's never had a copy of rocksim.
He builds the rockets according to manufacture instruction. He goes by the manufactures specs. Physically balanced the rocket on your finger, hand, screwdriver whatever you have and make sure that it is at least one caliber ahead of cp and your good.
If your using rocksim, same as any other program... Garbage in=garbage out. Check everything then check out again. I have a 5.5 Loc V2 also and agree with Ranier Wolfcastle about getting it off the pad faster.
 

David Schwantz

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LOC kit also. Used LOC's CP location. Flies great. J570W. She moves off the pad :)
 

burkefj

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I'll confirm that in many cases Rocksim with rocksim simulation is less conservative than openrocket or using borrowman method and I have not had an issue with my models using rocksim as the CP calculation.
 

Zielijo1

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I have this kit, added some nose weight, 5 washers w an eye bolt. 1400 ft on a G80. My issue w the kit are the fins at the motor mount after the boat tail. Very weak.
 
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