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rklapp

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FWIW, I played around with it. I removed all of the overrides except the stage weight. By switching to 4 fins and adding 200g of weight, I got the cal to 1 with the engine. I know that's not helpful to you now. I'm sure neil can be more helpful.
 

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neil_w

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So: as previously stated, but problem is that once you override the entire stage, changing the nose weight *inside* the stage has no effect. For now, there are two hack ways to get around this. I am hopeful that a cleaner solution will be implemented in the program in the future.

EDIT: I forgot to remove the motor during certain of the below steps, although I seem to have gotten to (approximately) the correct answer anyway. Start with following post instead, for quicker answer.

First method:
1) Add a new stage to the rocket. Name it "Nose + weight", and set "Separation" to "Current Stage Ejection Charge ("Never" also works):
1594651523773.png

2) Drag the new stage up to the front:
1594652765300.png

3) Now move the nose cone to the new stage:
1594652826798.png

4) Now, the overrides for the Stage are wrong, because it no longer includes the nose. So I will ask you to re-measure the stage mass and CG without the nose, everything else the same. I can estimate the mass pretty well (5950g), but CG is harder. For now I'll just guesstimate it at 33". Remember that CG for the stage is now measured from front of body airframe, not including nose cone! Here's where we are now:
1594653922999.png

If everything was done correctly, the total rocket mass and CG should match the original measured values (I'm too lazy to go back and check.

5) Now the easy part: Add a mass object to the nose cone and adjust as needed:
1594654108754.png


So I show about 37 oz in the nose to achieve 1 caliber of stability. Yeah that's a lot, and it probably agrees pretty well with your original measurements. I don't think it's crazy for a short 20 lb rocket though. Experienced HPR guys can comment here. Also I approximated the weight position; in real life it'll be distributed around the nose a bit differently. But this is probably a good estimate.

If you enter the rocket this way in the first place (nose in separate stage) then it is easy to do the nose weight approximation in OR before you start pouring shot. Of course you will need to confirm everything with real measurements when you're finished.

I'll do another post describing hack #2 later. For now the important thing is that it validates your initial finding that this thing will need plenty of nose weight to fly with that motor.
 
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neil_w

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ARGH, I did the whole thing above with the motor in! Might have totally screwed up the numbers. Let's double-check it using hack #2, which is definitely easier although it has its own downsides.

1) Make a note of stage measurements: mass 230 oz, CG 50". Now disable the overrides on the stage:
1594656452105.png

First observation: Is 69 oz difference between original model weight and actual measured mass really correct? When you measured for the override did you do it *without* the motor?

2) Let's assume it's all correct, even though it sounds doubtful to me. We have 69 oz difference to make up between model and actual. So we'll add a 69 oz mass object and adjust its position until the CG matches the 50" original measurement. It doesn't matter which component we add the mass object to; I'll add it to the top of the Fore Recovery Airframe Section. I get its final position at 29.5" from top of the parent component.
1594656886304.png


3) Now you can add your noseweight and adjust. Because the total rocket mass has not been overridden, it will factor in the added mass appropriately. This time, I achieve the same 1.02 stability margin with...
1594657091849.png


35 oz! Well whaddya know, I guess I didn't screw up hack #1 in the previous post. 35 oz vs. 37 oz might be accounted for just because I positioned the weight slightly differently.

Bottom line: you're gonna need 2+ pounds of weight in the nose to fly that configuration.
 

Steven88

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Wow that was a very nice amount of information that you gave me, Neil. I appreciate it very much. I will read over this better tonight when I’m off work and try to digest it all. Boy howdy, Loc says this rocket flies on L motors, but do they really expect you to stick two lbs of weight in the nose to accomplish that! That is a ton of weight. I’m thinking about lengthening it instead by about 18”.
 

Steven88

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I’m gonna run a sim on it being 18” longer and see if that helps out quite a bit!
 

rklapp

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Yikes, 2lb is about 900g of BBs. I was also wondering what LOC initially intended.
 

Steven88

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Yes, my rocket is 69 0z heavier than the file I received. Mine had the two altimeters, three batteries and some added plywood to hold batteries switches etc added to it. I also, used a number of quick links and have a home made thrust plate.
 

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Do you guys recommend a cal of at least one on OR before you fly or as long as its close, say .85 or greater you’d trust your rocket to fly?
 

rklapp

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My understanding is that a cal of <1 means the CG is too close to the CP and might somersault because the aft wants to go faster than the fore. Over 2 is overstabilized and not reaching it's full potential (too much NC weight or massive fins). Sorry if I got this backwards. Of course this all depends on which motor you use so the cal without the motor is not telling because motors have different weights.
 

Steven88

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If I enter a J135 into OR for my Big Nuke it gives me a Cal of something like .971. Is that close enough to 1 Cal to fly without adding nose weight?
 

neil_w

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1-2 cal stability is a rule of thumb. There are many possible modifiers. @Nytrunner prefers to go with 10% of the length of the rocket, which is another good rule of thumb and covers modifier #1 below.

1) longer skinnier rockets need more margin, short stubby rockets need less.
2) rockets that will go supersonic will need considerably more
3) In calm conditions, you can do with a bit less margin

Most of the time .97-ish will fly perfectly fine. However, on your Big Nuke, 1 cal is 5.54" which is quite a bit less than 10% of the rocket length. I personally would prefer to have more safety margin than that (I'd probably aim for 1.1-1.2, minimum). But I'm just an LPR guy, don't take my word for it.
 

Steven88

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ARGH, I did the whole thing above with the motor in! Might have totally screwed up the numbers. Let's double-check it using hack #2, which is definitely easier although it has its own downsides.

1) Make a note of stage measurements: mass 230 oz, CG 50". Now disable the overrides on the stage:
View attachment 424368
First observation: Is 69 oz difference between original model weight and actual measured mass really correct? When you measured for the override did you do it *without* the motor?

2) Let's assume it's all correct, even though it sounds doubtful to me. We have 69 oz difference to make up between model and actual. So we'll add a 69 oz mass object and adjust its position until the CG matches the 50" original measurement. It doesn't matter which component we add the mass object to; I'll add it to the top of the Fore Recovery Airframe Section. I get its final position at 29.5" from top of the parent component.
View attachment 424369

3) Now you can add your noseweight and adjust. Because the total rocket mass has not been overridden, it will factor in the added mass appropriately. This time, I achieve the same 1.02 stability margin with...
View attachment 424370

35 oz! Well whaddya know, I guess I didn't screw up hack #1 in the previous post. 35 oz vs. 37 oz might be accounted for just because I positioned the weight slightly differently.

Bottom line: you're gonna need 2+ pounds of weight in the nose to fly that configuration.
Well, what do you know! When I download this file into Rocksim it shows this rocket is much more stable than in OR. The stock file shows the cal at .99 in RS and .012 on OR. How can this be? Dave Barber with Loc says that there must be something wrong with the file in OR as the Rocket should NOT need 2lbs of nose weight to fly it on an L. Now I’m having to try to learn RS on my thirty day trial after being used to OR. So far I haven’t figured out how to make the rocket match my exact build on RS but I’ll keep playing with it. Any ideas on what could be going on with the discrepancies between OR and RS?
 

neil_w

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I dunno, the CP seems like it's in a reasonable location in OR (about 59"). Where does Rocksim put it?
 

stantonjtroy

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I wonder why the difference?
Been a while since I've used RS but I do recall you had a choice of 3 different methods of calculating the CP. Barrowman, RocSim and a third I can't recall. There was also an option as to which one (or combination of them) were displayed. I remember on a couple of designs the difference in location was, I felt, significant. Not sure if this is still an option but it's a good place to start.
 

Buckeye

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OR uses straight-up Barrowman equations which are the most conservative for CP. Rocksim offers the same Barrowman as well as their own method, which is the most liberal. RASAeroII offers their modified Barrowman which fall in the middle. Pick your poison.

In my experience, OR is far too pessimistic. I wasted a lot of time stressing over OR's CP predictions and added nose weight where it wasn't needed on a few of my builds. I stopped using it and now stick with RS and RAII.
 

neil_w

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It is surprising to me that there is that much difference in the results even for a simple 3FNC. I could imagine they'd diverge when things got more complex, but figured things were pretty solid and consistent for the simple stuff.
 

QFactor

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I wonder why the difference?
I use RS all the time. Would you be up for posting your RS file so that I could take a look at it?
Also, are you using Z-clips or an Aero Pack retainer to hold the motor in?
 
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Steven88

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Certainly, do you want it in ORK file format?
 

Steven88

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OR uses straight-up Barrowman equations which are the most conservative for CP. Rocksim offers the same Barrowman as well as their own method, which is the most liberal. RASAeroII offers their modified Barrowman which fall in the middle. Pick your poison.

In my experience, OR is far too pessimistic. I wasted a lot of time stressing over OR's CP predictions and added nose weight where it wasn't needed on a few of my builds. I stopped using it and now stick with RS and RAII.
On RS it is set to Barrowman equation as well so we should be talking apples to apples as far as I know.
 

QFactor

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Here is it. Let me know if it doesn't work for you for some reason.
Thanks for the file! It opened fine. I did notice that when you entered mass on some of your components you left the CG location as "0". You need to give a proper CG location for the component or RS will set the component's full mass at the Forward End ("0" End) of the component. So you rocket's overall CG will be incorrect.
 

Steven88

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Ok. Thanks for that tip. This file I sent you is from Jason with Loc and that is the stock file. I haven’t done anything to it. So I should weigh the avbay, main section and booster separately and then with a string balance and find the CG of those and input each one into RS? Is there anything else that I should weigh and correct? Is it not good enough to just balance the final built Rocket all put together and input the CG of that isn’t RS? I have not done that into RS yet as I am still learning this program. Thanks for the help.
 

QFactor

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Ok. Thanks for that tip. This file I sent you is from Jason with Loc and that is the stock file. I haven’t done anything to it. So I should weigh the avbay, main section and booster separately and then with a string balance and find the CG of those and input each one into RS? Is there anything else that I should weigh and correct? Is it not good enough to just balance the final built Rocket all put together and input the CG of that isn’t RS? I have not done that into RS yet as I am still learning this program. Thanks for the help.
I would start off "simple" so that you can get the feel for RS. Weigh the body tubes, 75mm motor tube, coupler & stiffy tube, centering rings, etc. All the items that have simple shapes. Then just set their CG location at their midpoint. For example, your 75mm motor tube is 23" long - so set its mass at 13.5" from the front of that tube. Your coupler is 11" long - so set it's CG at 5.5". And the CG for the 1/4" thick rings is 1/8" (0.125"). You can also do this for the fins, but finding the CG of a fin is a little tricky but doable.
 
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