CG's and CP's lesson learned

[AfterBurners]

Last weekend I attended the ROC launch at Lucerne, I had a great time and had a chance to meet some people from the forum and talk rockets. All of the rockets we launched boosted straight and had good chutes! All except one. I remember walking to the pad to load my MADCOW Bat Ray and said something to my friend Matt how well we are doing with getting great flights and sure enough the Bat Ray ended that conversation. When it left the pad it went crazy and did a few corkscrews and caused enough damage to where I need to replace the kit, but I was able to salvage the retainer and nose cone.

I also had the MADCOW Scooter kit I wanted to launch, but after launching the Bat Ray I was unsure if I wanted to try it. I was thinking I had the CG and CP calculated right, but I didn't chance it waited until I got home. When I pulled up the ROCKSIM file on my Bat Ray I realized that with the G64-7 loaded it came in at .76 which is considered "marginal" I remember checking the CG and CP per the instructions and they were right on and I figure I was good and went against my gut and decided to fly it as without nose weight. Well now I know not too.

So when I pulled the ROCKSIM file for the MADCOW Scooter kit it came in at .73 Marginal (with a G64-7 loaded) about the same as the Bat Ray. Go figure right?

CG - 16.63
CP - 18.26

Per the instructions it calls for the CG to be 16" from the nose tip, which it is, but I know now this won't fly and requires some nose weight. See file View attachment MADCOW Scooter.rkt

My question to you guys is how much weight would be enough?? I know by rule of thumb that the CG should be ahead of the CP by least 1 diameter length of the body tube (1.5 is better), so in this case it should be 2.2" ahead. Taking the difference above I get 1.63 so not quite 2.2

So I added 1/2 oz and came up with the following:

CG - 14.50
CP - 18.25

For a difference of 3.75 and a "margin" (Not marginal) of 1.67 so a much more stable rocket.

So how far should you go when adding weight? Do you think staying in the 1.50 range a solid range for stability or more or less? I'm know this has a lot of variables, but I thought this would be a good discussion to have on the forum. I would image kits like the Jayhawk or BOMARC for instance aren't your everyday favor 3FNC rocket. Is there anything that I should be aware of when building these kits? The reason I ask is because I have both of them in my build pile and I would hate to see them do what my Bat Ray did.

As it is now I plan on buying a new balsa nose cone for the Scooter and weighing properly and painting before flying this rocket. The other one I'll save for another kit. I'm sure I'll find some use for it. The reason I want to buy another nose cone is because the one that's on the rocket now already has been filled with epoxy and a wood dowel and I don't want to drill it out. Just as easy to spend $10 and get another one.

Thanks for all your help and suggestions!

 

[powderburner]

Try the archives:

https://www.rocketryforumarchive.com/showthread.php?threadid=3379

But I'll warn ya, there is a guy on there that just won't shut up, he goes on and on and on

 

[Rex R]

you Will have to add nose weight to the Jayhawk. Jayhawks are notorious for sky writing IIRC I needed something like 4oz., but yours may differ and you'll have to trust Madcow's numbers since OR doesn't do Jayhawks. F52s are probably the largest motor I'd use.
Rex

 

[CRC]

Great info before I fly my Scooter, I left the NC hollow so I have lots of room to add weight, even though I dont plan on flying it on a G for a few flights.

 

[terryg]

Were you measuring the cg (where it balances) with a fully loaded and flight ready rocket? The engine and recovery system shifts the cg. That being said I like to use a margin of 1.5 on most of my birds.

 

[AfterBurners]

Were you measuring the cg (where it balances) with a fully loaded and flight ready rocket? The engine and recovery system shifts the cg. That being said I like to use a margin of 1.5 on most of my birds.

I wasn't the rocket was empty. I didn't have a motor handy to load at the time, but agree I think 1.5 is a good margin. Have you flown anything peculiar aside from a 3FNC that required some other adjustments

 

[GregGleason]

Try the archives:

https://www.rocketryforumarchive.com/showthread.php?threadid=3379

But I'll warn ya, there is a guy on there that just won't shut up, he goes on and on and on

Wow. Good stuff there. In your writeup you confirmed something that I suspected as I was thinking through these things a while back.

Here is another place to get fancy again; you can optimize
(reduce) the amount of ballast required by locating it all the way
forward at the tip of the nose cone.

So if you had a slug of tungsten in the nose, it could do the same amount of "work" of many washers at the base of the nose cone. Weight placement becomes more efficient the further it is from the CG.

Thanks for taking a "moment" to explain things.

Greg

 

[terryg]

My hardest rocket for this so far has been my 7.5 inch pershing 1. A hard one to accurately sim and tiny fins. It balanced out at 1.25 calibers with a K550W. I can add adjust the nose weight by adding or removing fender washers in the nose. I am planing a 4 inch upcale of a solar flare for a night launch rocket which will be harder to sim also (has a ring fin).

https://fotos.nahcpj.com/sb_2013/h5d43aae8#h5d43aae8

 

[Daddyisabar]

I absolutely hate nose weight. It reduces the performance of your rocket and adds nothing to its looks. I have been told covering up bad design features with nose weight and power is just a crutch for those trying to fly silly objects (non 3-4FNC) like rockets. The sillier they get, the more nose weight is needed, more than the average guy would think. There are scale designs that are, for some scientific reason I am not completely sure of, just nasty, like the Jayhawk, BOMARC. Phoenix and others. They will surprise you and are very fickle given wind conditions. You "never" can have too much nose weight on these babies! So for the kits use all the nose weight supplied. If you add anything, especially to the hind end, you have to balance it with more stinkin', performance robbin' nose weight. Bigger motor, nice motor retention, nice thick epoxy fillets, recovery gear shifting, all need more weight up front.

When flying my silly stuff the scientific test I use to see if there is enough nose weight is real easy. I just take the nose cone in one hand and the loaded rocket body in the other and hand it to any certified Level 2 or 3 member standing around. If I do not get an immediate belly laugh then I need more nose weight.



Except for the Lancaster, because it has no nose cone. That one uses the "Level 3 Laugh at First Sight" doctrine. With enough power and nose weight you can fly anything. Generally, fear of adding more nose weight is the biggest factor in unstable flights. Its simple; Guess or calc a conservative CP, then get the CG as high as you reasonably can with out making the whole thing too heavy for the kick given by the motor. They may crash for other reasons but instability has never been an issue. Like Tiny Tim, all I need is my crutch.

 

[AfterBurners]

I absolutely hate nose weight. It reduces the performance of your rocket and adds nothing to its looks. I have been told covering up bad design features with nose weight and power is just a crutch for those trying to fly silly objects (non 3-4FNC) like rockets. The sillier they get, the more nose weight is needed, more than the average guy would think. There are scale designs that are, for some scientific reason I am not completely sure of, just nasty, like the Jayhawk, BOMARC. Phoenix and others. They will surprise you and are very fickle given wind conditions. You "never" can have too much nose weight on these babies! So for the kits use all the nose weight supplied. If you add anything, especially to the hind end, you have to balance it with more stinkin', performance robbin' nose weight. Bigger motor, nice motor retention, nice thick epoxy fillets, recovery gear shifting, all need more weight up front.

When flying my silly stuff the scientific test I use to see if there is enough nose weight is real easy. I just take the nose cone in one hand and the loaded rocket body in the other and hand it to any certified Level 2 or 3 member standing around. If I do not get an immediate belly laugh then I need more nose weight.

View attachment 174907View attachment 174909View attachment 174910View attachment 174911

Except for the Lancaster, because it has no nose cone. That one uses the "Level 3 Laugh at First Sight" doctrine. With enough power and nose weight you can fly anything. Generally, fear of adding more nose weight is the biggest factor in unstable flights. Its simple; Guess or calc a conservative CP, then get the CG as high as you reasonably can with out making the whole thing too heavy for the kick given by the motor. They may crash for other reasons but instability has never been an issue. Like Tiny Tim, all I need is my crutch.

Swell...now I have something to look forward to building those two kits in my build pile.

 

[Daddyisabar]

The Madcow Jayhawk is fun. I love to rip it on a G64. The BOMARC is still in the package and it is still scaring me. Several good TRF build threads on each.

 

[Handeman]

If you had .75 cal of stability on the Bat Ray, it should have been stable. I can think of only one thing that might have messed that up, that is if the CP is calculated with solid fins and the holes in the BatRay make enough difference to reduce the stability.