PRIME EXAMPLE !!!!!

[rsbhunter]

Newbie here, and now TOTALLY confused. I posted a question about finding CG on a now completed build. MADCOW DX3 2.6. I have been told to remove engine case and load, in other words, nothing in rocket body engine wise, AND I have been told to put complete loaded engine in the rocket .With the engine in, i need to add nose cone weight, without, i don't. I have all the internals in the rocket, including Jolly Logic chute release.I would really like to get a definite answer to which way is correct? It makes a big difference between the 2 scenarios!!!! Thanks for any and all advice...rsbhunter

 

[Hobie1dog]

you find it by having the rocket just exactly like it's going to be when you get ready to put it on the launch rod/rail

 

[rsbhunter]

I have read that, that's why I was totally confused when I've been told by 2 other gentleman to remove the engine???

 

[Raythain]

What are you trying to find the CG for?

If you already know the CoP, and you want to verify stability, find the loaded CG. Put everything in like you were going to fly - motor (or dummy mass), Jolly Logic, recovery gear, wadding, the whole kit and kaboodle.

If you want to use a sim program like OpenRocket to check stability with multiple motors, get a “dry” CG by loading everything but the motor. Override the mass and CG in your sim program with your values, and allow it to make adjustments based on the chosen motor.

 

[rsbhunter]

The finished rocket, without anything in it, weighed more (+-3 ounces) and I wanted to make sure I hadn't affected the "caliber" distance between cp or cg....first mpr build, have $100.00 plus in it, would like it to go up for the majority of time...rsbhunter

 

[Raythain]

Found your other thread, I think I understand more of what you are asking.

The thing to consider when locating your rocket’s CG is that the CG location will change based on the selected motor. Your CG on an Estes E12 will be significantly further forward than your CG on an Ameritech G80. So if you measure the CG with a motor installed, that measurement is only valid for that particular motor.

If you measure CG without the motor installed (usually referred to as a dry CG), this will be same location no matter what motor you choose to fly with. And once you have this location, finding the “wet” CG (Center of Gravity when ready to fly) is just calculating the average location of two point masses: your rocket mass at the dry CG location, and your chosen motor mass at the center of its mass. This is something you could calculate by hand, but is trivial for a sim program like OpenRocket or RockSim.

 

[rsbhunter]

I guess it is just confusing to those of us getting back into rocketry, especially mpr and hpr. There are so many variables that can be induced on purpose, or inadvertently, that can effect the outcome for success or failure. It was alot simpler when ABC rocket company told you how to do everything, it was just a glue proposition....rsbhunter

 

[OzHybrid]

CP= centre of pressure( found from Open Rocket simulation)
CG=center of gravity( theoretical initially in OR and the real one once completed)

You build a theoretical rocket in Open Rocket. You use this to get the location of the CP.
You refine the design in OR and make the component parts as realistic as possible, rough paint finish, thickness of parts and correct material.
This gives you a theoretical simulation that is going to be close.
You now add a motor in the simulation and see if it will fly and how high. play with the motors till you get the rocket to be flying to an altitude you're happy with.

If you then decide to build the rocket you have designed theoretically, then as the REAL bits get built you can(if you want to) change each individual component mass and cg to the REAL mass and measured cg as you build.

Once you have COMPLETED your REAL build, mark on YOUR BUILT ROCKET where the CP is.
You now have a built rocket with the CP marked on it and from the previous simulations you know what motor you intend to use.

NEXT.

Weigh the finished rocket without the motor or motor casing.
Balance it horizontally and locate the CG
Go back to Open Rocket and select the Stage tab.
Make sure there is No motor loaded into the simulation.
Override mass for Stage to the correct mass as built. Make sure all subcomponents box is ticked.
Same for CG.



You now have the design and real world mass and cg into your SIMULATION in Open Rocket, but no motor loaded.
Load the motor you intend to use into the simulation. Make sure the position of the motor is correct. Check where OR NOW shows the CG. It should be 1 calibre in front of the CP. Rerun the simulation and make sure you are happy with the flight profile. Use it to check what the delay from motor burnout to apogee is to use as a figure to set your delay to.


If you have your REAL motor. You can fit it into the rocket and check that your cg is REALLY 1 calibre in front of CG.

If you need to add nose weight, figure out where you will be able to add it to the rocket and tape a bag with the weight temporarily to the ( nosecone where you will be adding it hopefully)

Remove the motor/ casing.
Reweigh and find the new balance point. Put those numbers back into Open Rocket in the stage override without the motor added in the sim.
Once changed to your latest REAL WORLD weight/CG position use OR rerun the sim with the motor added again.

Your OR Sim will now give you the best expected altitude information. Check the delay is again correct.
Good luck with the flight.

 

[rsbhunter]

Ozhybrid, mostly the procedure I followed. The cp is supplied by Madcow. The finished rocket has the cp marked. Seeing as where I launch is asphalt, and hard dry earth, I'm upgrading to a 36" chute and a Jolly Logic chute release, so that in its self changed my cg. I am slightly under one caliber as it sits ready to launch...with weight added to nosecone, I'm slightly over one caliber....I'm thinking I'm going with the loaded engine/rocket to figure cg. rsbhunter

 

[OzHybrid]

Ozhybrid, mostly the procedure I followed. The cp is supplied by Madcow. The finished rocket has the cp marked. Seeing as where I launch is asphalt, and hard dry earth, I'm upgrading to a 36" chute and a Jolly Logic chute release, so that in its self changed my cg. I am slightly under one caliber as it sits ready to launch...with weight added to nosecone, I'm slightly over one caliber....I'm thinking I'm going with the loaded engine/rocket to figure cg. rsbhunter

In OR, you should adjust the override values without the motor in the rocket or the simulation. The Weight that OR will add for any motor selected is pretty accurate. But it does not know how much extra epoxy anyone has added or where.... This is why you need to do the override at the end to real world values.
That process also allows you to change theoretically to any motor you might want to use in the future.
Once you've done it a few times, it's just click, type click., done... ish. Certainly a lot quicker than me typing the wordy explanation. But in OR you definitely override without motor....
When you check it in the real world, before flying, you check with the motor fitted.
Good luck with your flight.

 

[WizardOfBoz]

I think you have the concept correctly, but for stability the Cg should be well in front of the Cp at all points in time. If you get a Cg with the heaviest motor you are likely to use, and your Cg is still about 3 inches (a bit more than the rocket diameter, which is a heuristic that seems to work), you'll be fine because as the rocket burns the fuel the weight in back is reduced an the Cg shifts forward. The way to find the Cg is to find the spot that the rocket balances on a ruler, fully loaded and prepped with engine, parachute, nose weights, electronics... whatever.

The way to find Cp is to use openrocket (openrocket can input rocsim files, get that for your dx3 here), or to go to Richard Nakka's website and get the barrowman spreadsheet in input your rockets dimensions.

I used the madcow rocsim file and in openrocket, and I'm having a hard time figuring out how you are getting this instabiligy. If I load "normal" motors, fitting in motor cases of RMS-29/180 or less, I get more than 1 caliber of stability. If I load an I200W (!!!!) which is RMS-29/360, stability becomes dodgy, and I need 2.5 ounces of weight mounted to the nose bulkhead to get 1.09 caliber of stability.

The rocsim file indicates that the Cg is 71.2 cm from the rocket tip (with the huge I200W and the weight added) and is 68.7cm from the rocket tip with a G80 and no weight. The Cp doesn't change: 78.5 cm from the rocket tip.

What distance does the Mad Cow data indicate is the distance between the tip and the Cp?

 

[teepot]

I don't use OR or any other program. I build a rocket. Make an educated guess of where the CP is then put the biggest motor that the rocket would ever fly on and every thing else to put the rocket onto flight configuration. I then balance it on a knifes edge. If the CG and CP are too close together I add nose weight until I like what I see. I've built over 200 rockets and except for a couple of my first builds when I didn't know anything they have all been stable and fly just fine. I know I'm in the minority by how I do it. The sim programs are beyond me.

 

[Art Upton]

I have read that, that's why I was totally confused when I've been told by 2 other gentleman to remove the engine???

They are mistaken between checking the rocket before flight and setting up a SIM to use multiple motors.

A safety check before flying your rocket with the chosen motor means checking it in reality with the motor you are going to use.

 

[OzHybrid]

They are mistaken between checking the rocket before flight and setting up a SIM to use multiple motors.

A safety check before flying your rocket with the chosen motor means checking it in reality with the motor you are going to use.

This should always be done. Prior to an actual flight, the only way to check things is through a simulation program. Either OR or RockSim. Working your way from theory to reality and then to any reality changes to cg and expected altitude and delay timing.

 

[OzHybrid]

They are mistaken between checking the rocket before flight and setting up a SIM to use multiple motors.

A safety check before flying your rocket with the chosen motor means checking it in reality with the motor you are going to use.

Read my decription of using OR. Please let me know what specific issues you have. As far as i can see it's described accurately.

 

[Art Upton]

Read my decription of using OR. Please let me know what specific issues you have. As far as i can see it's described accurately.

No Issues.

The confusion is setting up a SIM for CG without motor to be able to SIM multiple motors in the future using motor weight, and that of checking your CG in reality with the motor loaded; or at least the motor case in the rocket and the reload package floating on a fin valley.

 

[David_Stack]

No Issues.

The confusion is setting up a SIM for CG without motor to be able to SIM multiple motors in the future using motor weight, and that of checking your CG in reality with the motor loaded; or at least the motor case in the rocket and the reload package floating on a fin valley.

When using current motor files, doesn’t the OR sim add the correct motor casing/propellant weight? If so any sim file inclusive of motor would be as accurate as your real world validation, no?

If the builder has already updated their file with a CG for an ‘empty’ rocket (ready to fly, lacking only motor) based on actual CG, then all subsequent simulations should be accurate as well.

 

[Art Upton]

When using current motor files, doesn’t the OR sim add the correct motor casing/propellant weight? If so any sim file inclusive of motor would be as accurate as your real world validation, no?

If the builder has already updated their file with a CG for an ‘empty’ rocket (ready to fly, lacking only motor) based on actual CG, then all subsequent simulations should be accurate as well.

You should not trust that always do a reality check.

I've learned the hard way on an N1100 motor that the Thrust curve in [SIM name here] was not what the motor manufacture had.

 

[neil_w]

Having multiple threads about the same question often does not add clarity. See my post in other thread. https://www.rocketryforum.com/threads/cg-with-loaded-engine.185968/post-2577921

 

[Raythain]

When using current motor files, doesn’t the OR sim add the correct motor casing/propellant weight? If so any sim file inclusive of motor would be as accurate as your real world validation, no?

In theory, there’s no difference between theory and practice.

 

[Art Upton]

In theory, there’s no difference between theory and practice.

But in practice there is

 

[rsbhunter]

Having multiple threads about the same question often does not add clarity. See my post in other thread. https://www.rocketryforum.com/threads/cg-with-loaded-engine.185968/post-2577921

You are 100% correct...I don't like typing the same question over and over. That is why it is frustrating when you get 2 different answers to the same question!!! And which answer do you go with? I am going with the rocket fully loaded (NO IGNITER ) and go with a 1.25 caliber goal...3.25" from CP to CG, (2.6" bt). Thanks for all the information and help..rsbhunter

 

[Art Upton]

You are 100% correct...I don't like typing the same question over and over. That is why it is frustrating when you get 2 different answers to the same question!!! And which answer do you go with? I am going with the rocket fully loaded (NO IGNITER ) and go with a 1.25 caliber goal...3.25" from CP to CG, (2.6" bt). Thanks for all the information and help..rsbhunter

Good that your not into Ham Radio, the Ham forums you will get more then 2 different answers to questions, perhaps 5 or more

 

[lakeroadster]

I thought this was another paint thread.... .... Prime Example

 

[jkovac]

You are 100% correct...I don't like typing the same question over and over. That is why it is frustrating when you get 2 different answers to the same question!!! And which answer do you go with? I am going with the rocket fully loaded (NO IGNITER ) and go with a 1.25 caliber goal...3.25" from CP to CG, (2.6" bt). Thanks for all the information and help..rsbhunter

Neil W’s post in the other thread summarizes this well. You are actually not getting conflicting answers, but I can see why it would be confusing. If you are still confused after reading Neil’s post in the other thread, no problem, keep asking questions until you are clear.

 

[BABAR]

I don't use OR or any other program. I build a rocket. Make an educated guess of where the CP is then put the biggest motor that the rocket would ever fly on and every thing else to put the rocket onto flight configuration. I then balance it on a knifes edge. If the CG and CP are too close together I add nose weight until I like what I see. I've built over 200 rockets and except for a couple of my first builds when I didn't know anything they have all been stable and fly just fine. I know I'm in the minority by how I do it. The sim programs are beyond me.

I have the same experience. I suspect however that most of my rockets are likely overstable, or better put, more overstable than average.


If I understand right, as long as CG is forward of CP AND you have dead air (no wind) and no other variable, you don’t need CG to be forward of CP very much. But Barrowman and I suspect all the simulator programs assume a near zero angle of attack. Throw in a crosswind or some rod whip and what was previously (although perhaps minimally) stable becomes UNSTABLE. The more greater distance CG is forward of CP (usually reflected in calibers, although that may be less than ideal for squatty rockets) the better your rocket will correct for actual events like a wind gust.

So I suspect the vast majority of kits are designed to be a bit more overstable than required as well. This allows for variances by builders, such as how many coats of paint, wood glue or epoxy, etc. Estes had a few birds that were a bit marginal, particularly for 18mm motor mounts and the available C6 motors. Venus Probe, MIRV, and 1/200 Saturn V come to mind.

For smooth bodied typical 3FNC or 4FNC (for new guy, FNC means FinsNoseCone) seems like if CG is a bit forward of the fins you are probably good to go. Now, when you start doing crazy stuff like @neil_w , @lakeroadster , and @Daddyisabar , you may need to be a bit more thoughtful.

I am guessing for competition flying you want to push the envelope and get a rocket that has the lowest stability margin (cuz you can use smaller less draggy fins) and hope the winds don’t kick up.

 

[David Schwantz]

What does 3FNC mean?

 

[David Schwantz]

I'm sorry, I could not help myself

 

[rcktnut]

You should not trust that always do a reality check.

I've learned the hard way on an N1100 motor that the Thrust curve in [SIM name here] was not what the motor manufacture had.

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.

 

[BABAR]

What does 3FNC mean?

Not to be confused with 3NCF