PRIME EXAMPLE !!!!!

[Daddyisabar]

What does 3FNC mean?

The thing pictured is an oddroc abomination. 0FNC! There are worse designs out there. At least this one might be simmable. At least it is straight, mostly symmetrical and the motor is on the bottom where it should be.

My inner RSO gives it a mild stink eye and a good ole country tyme giggle. "But Mr. RSO, Grandpa says he seen 'em fly fer real back in the day... in that nader back in '36!"

3-4FNC rules. Safety First.
Kits from a trusted manufacturer. Good construction with correct adhesives. Good flying conditions. Sound rocket science! Happiness and success Mostly Guaranteed.

 

[BABAR]

Especially if there is any doubt. This rocket flew great until they put a bigger motor in it, and evidently did not check CP-CG relationship.

Thus the rationale for the “Away Pad.”

 

[Daddyisabar]

View attachment 642819

Oh my! My inner child RSO is huddled behind the backstop, curled up in a ball of fear! What did the simulation say?

 

[Daddyisabar]

Thus the rationale for the “Away Pad.”

The excited spectator looks up and says "Lord Vader, the far far away pad worked. No grievous bodily injury! Just put a bigger motor in it. So cool!"

Lord Vader "An unchecked and unstable O motored sports scale with holes in the fins. The Level Three TAPS are not as forgiving as I am."

 

[BABAR]

The excited spectator looks up and says "Lord Vader, the far far away pad worked. No grievous bodily injury! Just put a bigger motor in it. So cool!"

Lord Vader "An unchecked and unstable O motored sports scale with holes in the fins. The Level Three TAPS are not as forgiving as I am."

 

[Daddyisabar]

My new 3-4FNC simulation software with the motor loaded is now the most powerful stability tool in the universe. Don't try to frighten us with your sorcerers ways of mindsim or reliance on ancient ways, swing testing or disproven pendulum theory. You sad devotion to such foolery has not given you the clairvoyance to prove your oddroc abomination's center of aerodynamic pressure. Nor will it get you a pad assignment on the far, fa, ffff...aaah...

 

[rsbhunter]

OK!!!!! Hoping I'm there! OR with over ridden weight , 24.3 oz total weight without engine. CP is 30.814. CG without engine is 27.7. In or, the caliber is 1.17 with the F40-7W. With a G64-W4, stability shows 1.06. Am I getting close? Thanks for all the replies, help, advice....it really helped...rsbhunter

 

[WizardOfBoz]

I don't use OR or any other program. The sim programs are beyond me.

I very much doubt that, teepot. If you can open up a spreadsheet with Excel, I bet you could use a simulator.

I'm a big fan, because I write simulations (of human physiology) for a living. But even so, I'd encourage you to try either rocsim (free for a month, I think) or open rocket (available free). There's a couple of huge benefits. First is the stability calc. Second is the simulation which tells you if your motor delay is about right or if you are going to zipper your rocket because deployment is too early or late. Plus, it gives you a picture of the rocket, which is cool.



I suspect you could be simulatin' in about 20 minutes, if you relax and budget some time. Simple steps

1. Download one of the programs (rocsim or openrocket) for your computer (that is, Windows or Linux)
2. Install it, using the instructions. For rocsim you might have to do some registration foo
3. Download a file of a rocket you are interested in (it will be either an.ork or .rkt file - you may have to click a button to tell the program which)
4. Specify a motor. In OR, go to "motors and configuration". Click New Configuration.
5. Here's the really cool part. In the motor dialog box start typing a motor spec. "G80" for example. In the window above, all the commercial G80 versions appear - all the data for those motors are in the program!
6. Select motor and click "add configuration". You've just simulated a flight! Also, note that the Cg and Cp are shown on your diagram.
7. Go to "Flight Simulations" and right click on the listed simulation. Select "Plot and Export". When the dialog box comes up, right click on the listed simulation and when the dialog box opens, click plot. You get your simulated flight trajectory. Predicted altitude tells you if you are exceeding the field's limitations (a 4000 foot flight on a 500' xy 500' field? Think again!. Also, super useful is the prediction of velocity at deploy, so you can adjust your delay accordingly, and avoid zippers.

Seriously, you don't have to do simulations, but if you're building and successfully launching rockets, saying that simulation is "beyond me" is almost certainly not true. I encourage you to try it!

Here's a sim of the OP's DX3, with a 2 ounce weight on the nose bulkhead, using an I200W (apogee of 4100 feet):

 

[David_Stack]

OK!!!!! Hoping I'm there! OR with over ridden weight , 24.3 oz total weight without engine. CP is 30.814. CG without engine is 27.7. In or, the caliber is 1.17 with the F40-7W. With a G64-W4, stability shows 1.06. Am I getting close? Thanks for all the replies, help, advice....it really helped...rsbhunter

Sounds positive. Is your 'empty' CG the baseline one derived by the sim, or one where you actually balanced the completed rocket, measured, and entered that value as the CG Override?

 

[WizardOfBoz]

OK!!!!! Hoping I'm there! OR with over ridden weight , 24.3 oz total weight without engine. CP is 30.814. CG without engine is 27.7. In or, the caliber is 1.17 with the F40-7W. With a G64-W4, stability shows 1.06. Am I getting close? Thanks for all the replies, help, advice....it really helped...rsbhunter

Sounds like you are on the right track. OR estimates your weight and weight distribution based upon components. I get the same Cp. But with the engine, I get a Cg at 26.96" and a caliber metric of 1.49. Can you help me understand why and how weight is being overridden?

 

[BABAR]

I’m not a sim guy, but I theeeeenk I understand the confusion.

For CP of basic rockets (think NOT @lakeroadster , @Daddyisabar , or @neil_w ), sims are pretty good.

For CG, sims give a ball park estimate but builder variability can sway that considerably.

For a completely built rocket you would like to get an actual CG of the rocket ready to fly EXCEPT for the motor. Why? You are NEVER going to use this sim for planning a flight! BUT, you can then enter this REAL value into the sim as an over-ride, THEN for actual FLIGHTS you can put in ANY motor you want and it should give you decent sims.

So for ACTUAL flight planning the sim will use
1. The sim CP
2. The ACTUAL rocket CG without motor AND
3. You enter the selected motor and the sim adjust the CG for the motor, then spits out estimated stability, speed off rail or rod, apogee, etc.

Don’t know the details on why the full scale Phoenix on the O motor encountered fecal-turbine interaction, but possible it previously flew quite well on a smaller motor, but was not re-simmed with the larger motor and the CG change reversed the CG to CP positions.

Or maybe the BlueBird of Happiness traded shifts with the BlackBird of Crappyness that day!

Meconium happens, but in this hobby it’s our responsibility to do our homework to decrease the probability.

 

[rsbhunter]

I did install a piece of all thread (1/4") into the nose , to the tip from nose cone to the " nipple" off bottom of cone. It actually threads in nicely. I know the cg with the complete motor installed is 1.17 caliber from cp....is this not the goal? New to this, so I'm hoping this is a flyable combination? Thanks..rsbhunter

 

[OzHybrid]

OK!!!!! Hoping I'm there! OR with over ridden weight , 24.3 oz total weight without engine. CP is 30.814. CG without engine is 27.7. In or, the caliber is 1.17 with the F40-7W. With a G64-W4, stability shows 1.06. Am I getting close? Thanks for all the replies, help, advice....it really helped...rsbhunter

I don't think you've ticked the subcomponents box. The results I get are less than 1 caliber with an F40 loaded. And did you then add the threaded rod and increase the weight and move the CG forward?

 

[Zman1961]

Always check the CG with the motor in place and all the "internals" pushed down to the top of upper centering ring, which is where all the loose stuff will end up as the rocket accelerates. Manaufacturers list motor weights in their specs pages. Balance the rocket for the heaviest motor you plan to use. You don't need to have the motor, I use a ziplock bag filled with BB's.

 

[rcktnut]

The only real way to determine CG is to actually balance it when completed, flight ready. The sim crap with components calc. is just that crap. I've never determined CG or mass by that. Also I've always used the RS calculations for CP not the Barrowman. Never ever had a problem. RS -CP calcs. put CP at 33.4 instead of 30.8 .Here is the DX3 CP with RS calcs., and a long read about CP calcs.
https://apogeerockets.com/education/downloads/Newsletter238.pdf

 

[rcktnut]

BTW, I have a buddy that loves scratch building rockets and he has some beauties. All used the RS- CP calcs, to determine CP and never had a problem. Just a few here:

 

[rsbhunter]

Always check the CG with the motor in place and all the "internals" pushed down to the top of upper centering ring, which is where all the loose stuff will end up as the rocket accelerates. Manaufacturers list motor weights in their specs pages. Balance the rocket for the heaviest motor you plan to use. You don't need to have the motor, I use a ziplock bag filled with BB's.

I did balance it " launch ready" and I did have the chute, etc at the upper center ring. When I put the info into OR , I get 1.17 stability with the motors I have. Using the G motor sim, I still show 1.07. But I don't know OR at all...I'm sure if I was 30 or 40 years younger, I would understand it alot better. Maybe I honestly need to stay with lpr kits....rsbhunter

 

[rcktnut]

I did balance it " launch ready" and I did have the chute, etc at the upper center ring. When I put the info into OR , I get 1.17 stability with the motors I have. Using the G motor sim, I still show 1.07. But I don't know RO at all...I'm sure if I was 30 or 40 years younger, I would understand it alot better. Maybe I honestly need to stay with lpr kits....rsbhunter

Don't worry, be happy, and launch it!! It will fly just fine on any MPR motor of your choice.

 

[rsbhunter]

Don't worry, be happy, and launch it!! It will fly just fine on any MPR motor of your choice.

That's getting to be my mindset....I didn't mess around with changing anything, didn't get "creative" with fin placement, bt length, etc. If it doesn't fly, I'll be the first one to know!!! rsbhunter

 

[OzHybrid]

The only real way to determine CG is to actually balance it when completed, flight ready. The sim crap with components calc. is just that crap. I've never done that. Also I've always used the RS calculations for CP not the Barrowman. Never ever had a problem. RS -CP calcs. put CP at 33.4 instead of 30.8 .Here is the DX3 CP with RS calcs., and a long read about CP calcs.
https://apogeerockets.com/education/downloads/Newsletter238.pdf

View attachment 642880

That's a significant difference from OR. OR is close to the CP quoted by the kit manufacturer.

Let's see what RasAero says.
30.89 inches from nose to CP.
I'd trust RAS Aero every time. It's CP is in agreement with OR within 1/8". Perhaps someone from RockSim can explain the significant difference between it and the other 2 main simulation programs. Why does it put the CP more than a full caliber further back.

 

[rsbhunter]

That's a significant difference from OR. OR is close to the CP quoted by the kit manufacturer.
View attachment 642904
Let's see what RasAero says.
30.89 inches from nose to CP.
I'd trust RAS Aero every time. It's CP is in agreement with OR within 1/8". Perhaps someone from RockSim can explain the significant difference between it and the other 2 main simulation programs. Why does it put the CP more than a full caliber further back.

I followed as many people's opinions and advice as I could. The one piece of info that seemed concrete was to find cg with a "launch ready" rocket. So I did, and I came up with a 3.435" difference between the cp and cg, on a 2.6" rocket. Seeing as I've read that anything over 1.0 caliber is stable, I figure I'm good to go. Good thing is, I can remove the all thread from the nose cone, add shot and Gorilla glue, screw the all thread back in to secure . rsbhunter

 

[tsmith1315]

The one piece of info that seemed concrete was to find cg with a "launch ready" rocket. So I did, and I came up with a 3.435" difference between the cp and cg, on a 2.6" rocket.

Exactly.

At a club launch, you'd walk up to the RSO with your fully loaded rocket, he'd ask you where the CP is and then balance it to see if it looks stable. No further info needed.

Maybe I honestly need to stay with lpr kits

Nope. Put this one under your belt and you'll fell better.

 

[rcktnut]

That's a significant difference from OR. OR is close to the CP quoted by the kit manufacturer.
View attachment 642904
Let's see what RasAero says.
30.89 inches from nose to CP.
I'd trust RAS Aero every time. It's CP is in agreement with OR within 1/8". Perhaps someone from RockSim can explain the significant difference between it and the other 2 main simulation programs. Why does it put the CP more than a full caliber further back.

I got the RS file from the Madcow site: https://www.madcowrocketry.com/2-6-dx3/
Edit: In RS you can use Barrowman calcs or RS calcs. Using Barrowman calcs. the CP is at 30.8 in. Again why RS is more "lenient" on CP calcs: https://apogeerockets.com/education/downloads/Newsletter238.pdf

 

[rsbhunter]

Exactly.

At a club launch, you'd walk up to the RSO with your fully loaded rocket, he'd ask you where the CP is and then balance it to see if it looks stable. No further info needed.



Nope. Put this one under your belt and you'll fell better.

Thanks...I'm not looking for handholding, but it's nice to hear someone say, fly it! Thanks...rsbhunter

 

[rcktnut]

Now that your confident that your rocket will fly nice and stable let's discuss how you are rigging your JCLR. Do you really need it? How big is your field? Lots of trees in area?

 

[rsbhunter]

Size is a minor concern...almost half mile square. My want for the Jolly Logic is low altitude , big chute deployment. The ground here is hard pack. It will tear up a rocket. Plus, it is the old airport, and the couple asphalt runways aren't much harder than the ground...I had erronusly said I was going to deploy at 200 yards, but I'm now thinking 2-300 feet. Though I'm going to pick the flying days, wind here is a given...last couple days, we've had gusts up to 63 mph!!! S.E. New Mexico...rsbhunter

 

[bjphoenix]

I don't think you've ticked the subcomponents box. The results I get are less than 1 caliber with an F40 loaded. And did you then add the threaded rod and increase the weight and move the CG forward?
View attachment 642873

I don't understand the need for the second checkbox(s). It seems that if you are entering an override for the mass then by definition this automatically overrides all masses. If you don't check the second box then what are you overriding? And also when it says "all subcomponents" that isn't technically correct because it doesn't override the mass of the motor.

 

[Azamiryou]

I don't understand the need for the second checkbox(s). It seems that if you are entering an override for the mass then by definition this automatically overrides all masses. If you don't check the second box then what are you overriding?

No override: OR computes mass and CG of component according to its material and dimensions.

Override without subcomponents: OR uses the given mass and CG for this component. (E.g., "this is the mass of the body tube.")

Override with subcomponents: OR uses the given mass as the sum of this component and all subcomponents. (E.g., "this is the mass of the body tube, fins, couplers, and motor mount.)

And also when it says "all subcomponents" that isn't technically correct because it doesn't override the mass of the motor.

True, but I think "component" here means the things that show up in the rocket design component list/tree.

It's also a practical consideration. The same rocket design is often flown with different motors, which change the launch mass and CG. If overriding the mass and CG included the motor, you'd have to remember to change the override values every time you want to sim with a different motor.

 

[bjphoenix]

No override: OR computes mass and CG of component according to its material and dimensions.

Override without subcomponents: OR uses the given mass and CG for this component. (E.g., "this is the mass of the body tube.")

Override with subcomponents: OR uses the given mass as the sum of this component and all subcomponents. (E.g., "this is the mass of the body tube, fins, couplers, and motor mount.)

But in the example given, and the way that I've done it, the override is applied to the entire stage so doesn't that automatically apply to all of the subcomponents because the stage is the assembly? I understand how if you just override for a single component it would apply to that component, or if you do with subcomponents then it applies to the parts that are subordinate to that component, but everything is subordinate to the stage.

 

[teepot]

I very much doubt that, teepot. If you can open up a spreadsheet with Excel, I bet you could use a simulator.

I'm a big fan, because I write simulations (of human physiology) for a living. But even so, I'd encourage you to try either rocsim (free for a month, I think) or open rocket (available free). There's a couple of huge benefits. First is the stability calc. Second is the simulation which tells you if your motor delay is about right or if you are going to zipper your rocket because deployment is too early or late. Plus, it gives you a picture of the rocket, which is cool.

View attachment 642832

I suspect you could be simulatin' in about 20 minutes, if you relax and budget some time. Simple steps

1. Download one of the programs (rocsim or openrocket) for your computer (that is, Windows or Linux)
2. Install it, using the instructions. For rocsim you might have to do some registration foo
3. Download a file of a rocket you are interested in (it will be either an.ork or .rkt file - you may have to click a button to tell the program which)
4. Specify a motor. In OR, go to "motors and configuration". Click New Configuration.
5. Here's the really cool part. In the motor dialog box start typing a motor spec. "G80" for example. In the window above, all the commercial G80 versions appear - all the data for those motors are in the program!
6. Select motor and click "add configuration". You've just simulated a flight! Also, note that the Cg and Cp are shown on your diagram.
7. Go to "Flight Simulations" and right click on the listed simulation. Select "Plot and Export". When the dialog box comes up, right click on the listed simulation and when the dialog box opens, click plot. You get your simulated flight trajectory. Predicted altitude tells you if you are exceeding the field's limitations (a 4000 foot flight on a 500' xy 500' field? Think again!. Also, super useful is the prediction of velocity at deploy, so you can adjust your delay accordingly, and avoid zippers.

Seriously, you don't have to do simulations, but if you're building and successfully launching rockets, saying that simulation is "beyond me" is almost certainly not true. I encourage you to try it!

Here's a sim of the OP's DX3, with a 2 ounce weight on the nose bulkhead, using an I200W (apogee of 4100 feet):
View attachment 642830

I have tried OR several times and have gotten nowhere so I gave up and do my own thing. The last time I made a spread sheet was 2002.