CG with loaded engine

[rsbhunter]

Ok, I know the cg needs to be found with the rocket launch ready. I have researched and some say to not load the Aerotech29/40-120 until ready to launch, others say as long as you don't tighten the forward or aft caps tightly, it doesn't hurt it? I have used open rocket, but I will be using a JL chute release with chute, and streamer release at chute release.SO, just to be sure I don't need nose weight added, I'd like to get an accurate cg. Any tips or advice would be great....rocket is Madcow DX3 2.6" engine load is a F40-7W...rsbhunter

 

[rharshberger]

Make a weight to fit inside the motor that brings it up to the specified loaded weight for the motor which for the F40 in the 29/40-120 case is 126g (according to ATs catalog).

 

[OzHybrid]

Once you have built the rocket, mark on your rocket where Open Rocket says the center of pressure is. CP
Weigh your completed rocket WITHOUT the motor but everything else. Balance it and measure the position of the CG from the nose cone.
Open your simulation in OR and right-click on the Stage. Select override mass, tick box for all subsequent components,and put the measured mass in. Override cg and tick box for all components. Put in measured cg.
Then select the motor you want (you'll then see total mass and cg position change as you change selected motors. OR includes the weight of the motor case for you) and OR will sim with real weights, and real cg position.
As long as your center of gravity is 1 caliber ( rocket body diameter) in front of CP you are good to go.

 

[Art Upton]

Ok, I know the cg needs to be found with the rocket launch ready. I have researched and some say to not load the Aerotech29/40-120 until ready to launch, others say as long as you don't tighten the forward or aft caps tightly, it doesn't hurt it? I have used open rocket, but I will be using a JL chute release with chute, and streamer release at chute release.SO, just to be sure I don't need nose weight added, I'd like to get an accurate cg. Any tips or advice would be great....rocket is Madcow DX3 2.6" engine load is a F40-7W...rsbhunter

Leave the O-Rings out, load the motor grains and tubes into motor case; test CG. Then take parts out of the motor case and put them all back into a Zip lock bag. Do inventory to make sure you did not loose anything before launch day.

Edit: Use sharpie to write on Ziplock bag the motor and delay incase launch is rained out and weeks later you forget the motor type in the bag. [I've learned the hard way on this]

Or just use pen on masking tape and put tape on bag.

 

[Art Upton]

Ok, I know the cg needs to be found with the rocket launch ready. I have researched and some say to not load the Aerotech29/40-120 until ready to launch, others say as long as you don't tighten the forward or aft caps tightly, it doesn't hurt it? I have used open rocket, but I will be using a JL chute release with chute, and streamer release at chute release.SO, just to be sure I don't need nose weight added, I'd like to get an accurate cg. Any tips or advice would be great....rocket is Madcow DX3 2.6" engine load is a F40-7W...rsbhunter

What is your final weight of the DX3 without the motor ?

 

[3stoogesrocketry]

Or just take the motor case and install it into the rocket, and take a rubber band or if your fins are big enough, place the reload in the valley between the fins. Leave the reload right in the bag.

 

[rsbhunter]

Or just take the motor case and install it into the rocket, and take a rubber band or if your fins are big enough, place the reload in the valley between the fins. Leave the reload right in the bag.

 

[rsbhunter]

This is probably the easiest way yet....I am amazed at the genius idea's that the members here show....you All have given me multiple ways to do what I asked, and they are all great!! Honestly, you guy's make this hobby great because of your willingness to help anyone who asks! rsb

 

[Buckeye]

Or just take the motor case and install it into the rocket, and take a rubber band or if your fins are big enough, place the reload in the valley between the fins. Leave the reload right in the bag.

I do the same.

Also:

It is perfectly fine to fully assemble the motor ahead of time and let it sit indefinitely. No need to loosen the closures - that is TRF folklore.

If all the input masses of your design are good, then the final mass and CG will be good when OR adds the motor to the configuration.

The DX3, if built stock-ish, will be plenty stable on a wide range of motors. You should have no problems with an F40.

 

[bobbyg23]

Go ahead and assemble the motor. It will be just fine. Or lay it in the valley of 1 of the fins. Either way will give you excellent results.

 

[rsbhunter]

I do the same.

Also:

It is perfectly fine to fully assemble the motor ahead of time and let it sit indefinitely. No need to loosen the closures - that is TRF folklore.

If all the input masses of your design are good, then the final mass and CG will be good when OR adds them to the configuration.

The DX3, if built stock-ish, will be plenty stable on a wide range of motors. You should have no problems with an F40.

Thank you, words of encouragement are always welcome! I just want to do this right. I picked the reload based off of Madcow's suggestion. I built it (or overbuilt) by the book. I did put epoxy fillets on the inside fin/ mm junction as well as fin/bt junction...therefore, my weight came out 3 oz heavier without engine/reload...thats why I'm nervous bought getting the cp/cg to a 1 caliber or more (1.5). Again, I know this stuff is ho-hum to you guys, but it is GREATLY appreciated by us old newbies. rsbhunter

 

[rsbhunter]

I do the same.

Also:

It is perfectly fine to fully assemble the motor ahead of time and let it sit indefinitely. No need to loosen the closures - that is TRF folklore.

If all the input masses of your design are good, then the final mass and CG will be good when OR adds them to the configuration.

The DX3, if built stock-ish, will be plenty stable on a wide range of motors. You should have no problems with an F40.

Thank you!!! I am grateful for yours, as well as everybody else's help.. rsbhunter

 

[jkovac]

Good advice above about physically checking the CG. Remember to push the recovery gear down before balancing, because that’s what happens when the rocket takes off.

 

[Ron Soto]

as norman states in his message you should be calculating CG without your motor.

ron-

 

[rsbhunter]

Ron, this is a perfect example of what I stated in my previous post...I AM NOT DOUBTING YOU. But I have other members who say to have the complete motor either installed, or at the place of ×here it would be at launch ??? This is a really good time to get a definitive answer. Most say to load the motor you will use in open rock or rocsim, to obtain the cg??? I don't know enough to say one way or the other.... I am going to post this question to the forum....Thank you for your help, rsbhunter.

 

[jkovac]

as norman states in his message you should be calculating CG without your motor.

Maybe I missing something here, Ron? The physical CG before launch is where the rocket physically balances at. With a motor and everything else in it (except the igniter), ready to fly. A simulation will also show you where that fully loaded balance point (CG) should be, but the simulation could be wrong if one or more of the inputs are wrong. If the simulation doesn’t match where the rocket actually balances at, the simulation is wrong. The simulation doesn’t fly, the rocket does, in physical reality, so where the rocket balances in reality is what matters. Let me know if I’ve misunderstood you, or if you think I’m wrong.

 

[Ron Soto]

Maybe I missing something here, Ron? The physical CG before launch is where the rocket physically balances at. With a motor and everything else in it (except the igniter), ready to fly. A simulation will also show you where that fully loaded balance point (CG) should be, but the simulation could be wrong if one or more of the inputs are wrong. If the simulation doesn’t match where the rocket actually balances at, the simulation is wrong. The simulation doesn’t fly, the rocket does, in physical reality, so where the rocket balances in reality is what matters. Let me know if I’ve misunderstood you, or if you think I’m wrong.

yes.. i read it wrong. now i realize that he's loading motors. sorry for the confusion..

r

 

[OzHybrid]

Maybe I missing something here, Ron? The physical CG before launch is where the rocket physically balances at. With a motor and everything else in it (except the igniter), ready to fly. A simulation will also show you where that fully loaded balance point (CG) should be, but the simulation could be wrong if one or more of the inputs are wrong. If the simulation doesn’t match where the rocket actually balances at, the simulation is wrong. The simulation doesn’t fly, the rocket does, in physical reality, so where the rocket balances in reality is what matters. Let me know if I’ve misunderstood you, or if you think I’m wrong.

Until you ACTUALLY launch, the simulation is your only source of information for the altitude you will achieve and therefore the flight to apogee time and therefore the DELAY REQUIRED FOR PARACHUTE EJECTION AT APOGEE. The OR sim is the best way to obtain that.
You can easily verify if the sim weight of the motor weight is correct by checking in the sim what additional weight is with the motor loaded in the sim and validate that against your actual motor case and reload weight together.
If the motor sim weight matches the real motor weight, and I've not seen any issues with that to date, then you can use the sim motor weight in OR with confidence.
You then add any additional weight to get the CG 1 caliber in front of CP and go back around the loop one last time.
I hope that makes it clear why you need OR or RockSim. To get your delay time. Expected altitude. Estimated apogee.
All are required for the information on a flight card for the RSO to review before launch. Even with no RSO at a private non-Tripoli or NAR launch, you should still know that information.
With my rocket , Blunt Instrument, and real weight information without motor fitted, OR said the weight would be 1470g with a G64 fitted and it was 1470g as weighed at the RSO table. Maybe I was lucky.

https://www.rocketryforum.com/threads/what-did-you-do-rocket-wise-today.48649/post-2577572

Norm

 

[jkovac]

Until you ACTUALLY launch, the simulation is your only source of information for the altitude you will achieve and therefore the flight to apogee time and therefore the DELAY REQUIRED FOR PARACHUTE EJECTION AT APOGEE. The OR sim is the best way to obtain that.
You can easily verify if the sim weight of the motor weight is correct by checking in the sim what additional weight is with the motor loaded in the sim and validate that against your actual motor case and reload weight together.
If the motor sim weight matches the real motor weight, and I've not seen any issues with that to date, then you can use the sim motor weight in OR with confidence.
You then add any additional weight to get the CG 1 caliber in front of CP and go back around the loop one last time.
I hope that makes it clear why you need OR or RockSim. To get your delay time. Expected altitude. Estimated apogee.
All are required for the information on a flight card for the RSO to review before launch. Even with no RSO at a private non-Tripoli or NAR launch, you should still know that information.
With my rocket , Blunt Instrument, and real weight information without motor fitted, OR said the weight would be 1470g with a G64 fitted and it was 1470g as weighed at the RSO table. Maybe I was lucky.

https://www.rocketryforum.com/threads/what-did-you-do-rocket-wise-today.48649/post-2577572

I don’t disagree with any of that. I’m definitely not making an argument against simulations - I use both RASAero and Open Rocket for every flight. The OPs question was about CG, and I’m just saying one can easily check that physically before launch.

 

[neil_w]

Because I sense (maybe) a lingering bit of uncertainty, I'm going to restate what everyone else has already stated correctly. I think I'm also going to make a tutorial about this. I'm consciously ignoring stuff about CP shift at Mach, angle of attack, and such. Sticking with the basics here.

1) Stability of the rocket is determined by the stability margin (CP - CG) of the rocket in its full flight configuration. Period.

This is ironclad, and dictates that the CG measurement includes the motor. If you want you can stop reading right here: put the motor in, adjust your nose weight to achieve desired stability margin, and fly the rocket. QED.

However, there are at least two other approaches. All are designed with the same end goal: achieving desired stability according to equation (1). Even if you use one of the methods below, you can still measure CG of your fully-loaded rocket just before launch, just to be confirm previous calculations and/or measurements.

2) Use a sim program.

For this approach you weigh and measure CG of the fully-built rocket *without* the motor, and then enter those values into the sim program (as overrides for the entire rocket). When you tell the sim program what motor you want to use, it can then figure out the CG (and therefore stability) of the full flight configuration, because it knows the weight of each motor, and the location of the motor in the rocket. This way you can check the stability with different motors, including ones that you don't even own but are considering. You can also, of course, do a ton of other useful stuff (e.g. calculate altitude and delays), but just focusing on stability here.

This is where the confusion comes from with regard to with or without motor. In this approach *you* are measuring the CG without the motor, but the sim is then finishing the calculation and determining CG (and stability margin) *with* the motor, which is ultimately what matters.

Note that the sim program will still report stability margin even when you have not configured a motor. Generally this should be ignored, it's not relevant to anything.

3) Measure rocket with heaviest planned motor.

Let's say you're not using a sim program, but you want to configure your rocket to fly safely with a variety of motors. What you can do is load up the heaviest motor you plan to fly, and measure CG. Adjust nose weight as needed to achieve desired stability margin. Fly happily on all different motors.

This works because the heaviest motor is going to be the worst case, with CG furthest back, and therefore the worst stability. Lighter motors will yield higher stability margin, which generally is fine (unless it gets way out of hand, but that's not usually a result of swapping motors).

Note that this method requires that (a) you *know* the heaviest motor you'll want to fly, and (b) you have one to measure.

 

[jkovac]

Because I sense (maybe) a lingering bit of uncertainty, I'm going to restate what everyone else has already stated correctly. I think I'm also going to make a tutorial about this. I'm consciously ignoring stuff about CP shift at Mach, angle of attack, and such. Sticking with the basics here.

1) Stability of the rocket is determined by the stability margin (CP - CG) of the rocket in its full flight configuration. Period.

This is ironclad, and dictates that the CG measurement includes the motor. If you want you can stop reading right here: put the motor in, adjust your nose weight to achieve desired stability margin, and fly the rocket. QED.

However, there are at least two other approaches. All are designed with the same end goal: achieving desired stability according to equation (1). Even if you use one of the methods below, you can still measure CG of your fully-loaded rocket just before launch, just to be sure.

2) Use a sim program.

For this approach you weigh and measure CG of the fully-built rocket *without* the motor, and then enter those values into the sim program (as overrides for the entire rocket). When you tell the sim program what motor you want to use, it can then figure out the CG (and therefore stability) of the full flight configuration, because it knows the weight of each motor, and the location of the motor in the rocket. This way you can check the stability with different motors, including ones that you don't even own but are considering. You can also, of course, do a ton of other useful stuff (e.g. calculate altitude and delays), but just focusing on stability here.

This is where the confusion comes from with regard to with or without motor. In this approach *you* are measuring the CG without the motor, but the sim is then finishing the calculation and determining CG (and stability margin) *with* the motor, which is ultimately what matters.

Note that the sim program will still report stability margin even when you have not configured a motor. Generally this should be ignored, it's not relevant to anything.

3) Measure rocket with heaviest planned motor.

Let's say you're not using a sim program, but you want to configure your rocket to fly safely with a variety of motors. What you can do is load up the heaviest motor you plan to fly, and measure CG. Adjust nose weight as needed to achieve desired stability margin. Fly happily on all different motors.

This works because the heaviest motor is going to be the worst case, with CG furthest back, and therefore the worst stability. Lighter motors will yield higher stability margin, which generally is fine (unless it gets way out of hand, but that's not usually a results of swapping motors).

Also, this method requires that (a) you *know* the heaviest motor you'll want to fly, and (b) you have one to measure.

Great summary.

 

[Ron Soto]

Thank you, words of encouragement are always welcome! I just want to do this right. I picked the reload based off of Madcow's suggestion. I built it (or overbuilt) by the book. I did put epoxy fillets on the inside fin/ mm junction as well as fin/bt junction...therefore, my weight came out 3 oz heavier without engine/reload...thats why I'm nervous bought getting the cp/cg to a 1 caliber or more (1.5). Again, I know this stuff is ho-hum to you guys, but it is GREATLY appreciated by us old newbies. rsbhunter

When building, i input my 'overrides' and let the computer and RockSim software do it's work. Then.. all i did was sim different motors as RockSim, and OR both have the correct variables for the different motors. SO, in my opinion, you can build up your motor, place it in the MMT and get your CG (but that would be for every motor).. But if you override the CG for the sustainer, without the motor installed, the software will place the CG on target when a new motor is sim'ed by its weight.

side note: i was in the ER for 2 days for extremely high BP. this said i'm ok now, but when i originally responded to an earlier thread yesterday, i was not focused on what i was trying to say, when agreeing with norm on his post.

r-

 

[rsbhunter]

Just fyi....my calculation was a physical balance point. With the weight taped to the end of the nosecone(where it will be) the cg was 3.5" ahead of the cp ( as per Madcow. ) Caliber is 2.6". Thanks for all the info....rsbhunter

Great summary.

That was my understanding, and as said, th rso checks the cg physically....not off a computer screen...reminds me of the old saying "junk in, junk out" rsbhunter

 

[jkovac]

When building, i input my 'overrides' and let the computer and RockSim software do it's work. Then.. all i did was sim different motors as RockSim, and OR both have the correct variables for the different motors. SO, in my opinion, you can build up your motor, place it in the MMT and get your CG (but that would be for every motor).. But if you override the CG for the sustainer, without the motor installed, the software will place the CG on target when a new motor is sim'ed by its weight.

side note: i was in the ER for 2 days for extremely high BP. this said i'm ok now, but when i originally responded to an earlier thread yesterday, i was not focused on what i was trying to say, when agreeing with norm on his post.

r-

Glad you are ok. The conversation was confusing, not your fault. Neil W’s post straightened it all out.

 

[jkovac]

Just fyi....my calculation was a physical balance point. With the weight taped to the end of the nosecone(where it will be) the cg was 3.5" ahead of the cp ( as per Madcow. ) Caliber is 2.6". Thanks for all the info....rsbhunter

That was my understanding, and as said, th rso checks the cg physically....not off a computer screen...reminds me of the old saying "junk in, junk out" rsbhunter

Sounds like you’re good to go. When is your flight?