Leviathan CP stable issue

I just painted and finished my Leviathan and wanted to mark CP on the body - which several places says 31 1/8" from tip of nose cone. Also most places show CG 22" ish dry 2.88 overs table.
So I through a g76 in it and pack the shoot just to check ... However my CG is only 2" forward of CP not over stable. Could be the location of my recover equipment.

Has anyone added nose weight?
Without using Rocksim or OR,
What is a good method to try different nose weight while balancing the rocket? If I put weight in cone, while rocket balancing, the weight settles to the middle of nose cone affecting CG location. Afterward, I thought to make a slurry to weight the cone then 2part foam it into place.

Can the rocket brains here teach me nose cone weight techniques?

Thanks for your help, I have a couple shorter rockets that I think will need the same treatment
LOC Aura
FG Alpha MPR

I should have done all this prior to painting lol

Pick the biggest motor you plan on using in rocket. Adjust CG so you get at least 1.0 margin of stability. Try stuffing rags in NC to keep weight stable while measuring CG. Once you find the sweet spot, you can mix your weight with epoxy and pour into NC. Or even use foam like you mention. Just make sure you scuff up inside of NC so epoxy will adhere.

John Coker has a video about how to add nose weight. You want it not only scuffed up but drilled and pinned for absolute retention.
I'll go find you the video.

Here you go:

[video=youtube;WDjgq0hfsgw]https://www.youtube.com/watch?v=WDjgq0hfsgw[/video]

Rocket looks great by the way!!!

Nice looking rocket!! To adjust your weight, put it in a plastic baggie, wrap the baggie around the tip of the nose cone, tape in place. This way no tape on paint job. Adjust amount of weight until satisfied. Recently I added 6oz to my 3" Bullpup. Rocket was also built, painted. The nosecone was stock, only had the 3/8" hole in the shoulder to work with. I poured 6 oz. of lead shot in, added a few drips of water and squirted in some Gorilla glue to top of shot. Keep tip down until glue has dried. No sanding inside the nose cone needed. The weight hasn't and never will come loose.

I am wondering if you have the CP right? What did you do when you built the fin can? I built my Leviathan stock except for the 1/8" tubular Kevlar shock cord from the MMT to about 4 ft out the BT where the original elastic is attached. I've flown it with AT H250G motors which add 256g to the back end. I've never added nose weight and never had a problem.

I believe you are over thinking this. I don't know of any motor you could put in the Leviathan that would require additional weight in the nose cone unless you over built the fin can and added a lot of weight there. If you put a 38mm or 54mm MMT in it, that would be a different story.

thanks Everyone for the help - good techniques and your explanations should be easy to follow - the Coker video is great and saw that but I don't think he addressed how he knew how much to add (maybe I need to rewatch) - I like the cross pin idea and just hate to repaint - and didn't want to cut the shoulder...seams like many of my projects are "half done" right now.

I appreciate the rocket compliment, I prepped and prepped to finally get a decent paint job and it worked out. That's why I was like "really? cross pin, cut shoulder off and re paint - dang it I was done with it!" , whats the matter with my forethought - its a freaking Leviathan."
I believe I either have a CP disparity or yes over-thinking this, I never hear of people adding nose weight on a Leviathan but it is time I learn how to do nose cone weight and how to figure out the right amount.

I built the fin can stock except for the motor tube is a little further back .25" so maybe I should not worry about it and fly it.
I looked a few different places for CP location which shows right above the fins and roughly 31" and CG at 22" from the nose cone tip, but doubtful Estes would have a stock kit this recent not stable and no directions to add weight.

So I just thought - hey I want to learn more and know where my locations are on all my rockets. So I started by loading up the Levi and checking CG -... wow - that is farther back than I thought - the 22" advertised must have been dry weight. Maybe I should do the cardboard cut out thing and check CP - again I more concern about learning - why.. I have some scratch builds coming soon and want to know how to do this.

Handeman - I don't really intend anything more than a baby H with motor deploy 29mm - so yes, proly stuff the motor in and light it.

Nick@JET said:

thanks Everyone for the help - good techniques and your explanations should be easy to follow - the Coker video is great and saw that but I don't think he addressed how he knew how much to add (maybe I need to rewatch) - I like the cross pin idea and just hate to repaint - and didn't want to cut the shoulder...seams like many of my projects are "half done" right now.

I appreciate the rocket compliment, I prepped and prepped to finally get a decent paint job and it worked out. That's why I was like "really? cross pin, cut shoulder off and re paint - dang it I was done with it!" , whats the matter with my forethought - its a freaking Leviathan."
I believe I either have a CP disparity or yes over-thinking this, I never hear of people adding nose weight on a Leviathan but it is time I learn how to do nose cone weight and how to figure out the right amount.

I built the fin can stock except for the motor tube is a little further back .25" so maybe I should not worry about it and fly it.
I looked a few different places for CP location which shows right above the fins and roughly 31" and CG at 22" from the nose cone tip, but doubtful Estes would have a stock kit this recent not stable and no directions to add weight.

So I just thought - hey I want to learn more and know where my locations are on all my rockets. So I started by loading up the Levi and checking CG -... wow - that is farther back than I thought - the 22" advertised must have been dry weight. Maybe I should do the cardboard cut out thing and check CP - again I more concern about learning - why.. I have some scratch builds coming soon and want to know how to do this.

Handeman - I don't really intend anything more than a baby H with motor deploy 29mm - so yes, proly stuff the motor in and light it.

Click to expand...

The CP Center of Pressure if you built your kit stock, no modification of fins, length of rocket, etc. is going to be at 31 inches from tip of nosecone period. The only thing you can change is the CG Center of Gravity. Being a 3 inch diameter kit you want that CG at least 28 inches from tip of nosecone, at least one body diameter forward of the CP. If your CG is only 2 inches forward of CP now, you will have to add some weight. Having the CG at anywhere between 26-27 inches would be best.


If you plan on scratch building Open Rocket is a free sim program, entering your design will establish correct CP for you and approximate CG. Correct CG can only be established after the rocket is completed and manually measured.

I say punch it. 1 cal of stability is not a law of anything. It's a guideline. You're not breaking mach, and you're not launching in a hurricane.

Less than 1 cal is not unstable.

DizWolf said:

I say punch it. 1 cal of stability is not a law of anything. It's a guideline. You're not breaking mach, and you're not launching in a hurricane.

Less than 1 cal is not unstable.

Click to expand...

+1 to what DizWolf said, a stability margin of 1 is a safety factor, I routinely build and fly rockets with Stability Margins of +.3 to +.5 and they fly great.

Sounds good, if it goes unstable and lawn darts, well guess I'll do have to do a nose cone mod .

There are guidelines for stability margins, thrust to weight ratios, speed off the pad, etc. How far you wish to sway off the guidelines is up to you. My answer was based on my thought that you wanted to achieve a 1+ stability margin. Marginal stability is marginal stability, not marginal unstability. It may fly just fine at +.6. How much you adhere to the other guidelines, wind conditions, and what the design itself can withstand all contribute to whether you have a good flight or not. I for one don't wish to add any weight to a rocket unless absolutely necessary, but do follow the +1 stability minimum guideline.

The one thing I don't understand is how your rocket with a normally (less engine) over stable design ended up with the CG where it is. Lots and lots of glue/epoxy??

rcktnut said:

There are guidelines for stability margins, thrust to weight ratios, speed off the pad, etc. How far you wish to sway off the guidelines is up to you. My answer was based on my thought that you wanted to achieve a 1+ stability margin. Marginal stability is marginal stability, not marginal unstability. It may fly just fine at +.6. How much you adhere to the other guidelines, wind conditions, and what the design itself can withstand all contribute to whether you have a good flight or not. I for one don't wish to add any weight to a rocket unless absolutely necessary, but do follow the +1 stability minimum guideline.

The one thing I don't understand is how your rocket with a normally (less engine) over stable design ended up with the CG where it is. Lots and lots of glue/epoxy??

Click to expand...

I'm really just wanted to learn and be safe, for the record, I'm going to add some weight to get to the 1 caliber with the big motor just to get the experience since I have some scratch builds in the works.

Also I have started messing with OR for those future scratches

I think CG is there due to maybe a few reasons, Fillets were a little generous , I used conform rail guides, I didn't use an eyelet, but did use 1/4" Kevlar that I tied around motor tube and epoxy liberally. Really didn't consider how much a little overbuilding will affect CG but it is a pretty light rocket so it makes since.

Thanks for all the responses.

Most of the mods you described should not have altered the CG as much as your measurements are indicating. I'd double-check the balance point again just to be absolutely sure you measured it correctly. I'd check it once without the motor and once again with the motor and record both of those CG numbers. I'd also weigh the rocket without a motor in it --- if the rocket's CG has shifted as much as you think, then the rocket is probably heavier than stock by a significant amount. You might also weigh it with a motor loaded.

If you have an OpenRocket file for the Leviathan, you can enter a mass and CG override for the entire rocket, using the weight you measured and the CG you measured without the motor. Then you can select the motor you are planning to use and see if the CG moves to the position you are measuring for the loaded rocket, and see if the mass matches up. That would just be a good way to be sure everything is matching up nicely.

If you already know the mass of the loaded rocket, CG when loaded, and the CP, then there are easy ways to calculate how much nose weight is needed to move the CG to where you want it. Here is a PDF I found Googling the topic: https://tccrockets.com/documents/report.pdf

If you do decide you want to add some nose weight, one easy way is with Gorilla Glue. It seems to stick really well to nose cone plastic. I don't think you need to rough up the inside or pin the nose weight --- it couldn't hurt, but the glue is pretty strong. I just squirt a little bit of Gorilla Glue in thorough the hole in the NC so that a pool of glue forms in the very tip of the NC. When you ad the glue, it's a good idea to try to drip it in straight so that it lands right in the tip, not all over the inside surface. Then dump in a weighed amount of steel BB's into the hole so they settle in the glue. Maybe squirt some more Gorilla Glue in on top for good measure. Gorilla Glue can cure without added moisture, but if you want, you can drip a drop or two of water into the NC to help it along. Leave it to cure in a cup or a can standing up so that the BBs are all in the very tip.

That's how I added nose weight to my Leviathan. It flew just fine stock. But after a few hard landings, the fins were so banged up I cut them off square with the bottom of the rocket, moving the CP forward. And then it core sampled at some point, and I cut about 3 or 4 inches of crushed cardboard off the body tube, moving the CG aft. Just to be sure it was still stable, I added 2 oz BBs to the nose. It flies great. I also think the added weight in the NC, helps the NC pull the chute out of the BT on deployment. The Leviathan has a reputation for failing to deploy fully, so a good tight-fitting seal on the NC helps the NC pop off with speed like a champagne cork, and a little nose weight adds momentum to the NC to help pull out the chute.

Good luck!

Alright - I posted this originally to learn from you Rocket gurus - so I know , I may add a little weight and fly the bird .

Extra weight probably from the quick link and swivel.

But check this out - I decided to check CP the old school way , trace outside of rocket and cutout a price of cardboard. Balance this and should be CP. see picture - 23" from Tip of nose - not even close to advertised CP of 31" so now I must be doing something wrong.


Now load the chute and motor G76. I loaded the chute back towards motor with the cord and nomex protector.. Why by the motor ? Because launch it will all wind up back there anyway.
Balanced the rocket and this will be CG - 28.75" from tip of cone ... BEHIND my measured CP but close to the advertised CP - still will add a little weight. See pic



So which CP is correct? The reason I am not letting this go is I am also instructing 4-H and what to be able to pass on correct info not just my hunch.

Can you see that the cardboard cutout is essentially your rocket with only two fins? Little experiment: make another cutout, but with the fins twice as wide (thus simulating the effect of four fins) and see where it balances.

The cardboard cutout method is a very conservative guestimate and doesn't take into account a lot of factors. The quoted CP is probably calculated from the Barrowman equations (which is the basis of what is used in both RockSim and OpenRocket) which is much more accurate than the cutout method, but still makes assumptions that don't hold up in extreme cases. The simulators both have tweaked versions of the Barrowman equations (and other more expensive simulators have equations that go far beyond Barrowman) in order to deal with those possible extreme cases.

So, the short answer is, if the design is fairly simple, and the CP calculation came from a model rocket simulator, you can trust it.

the cut out method gives you the cp for the absolute worst case scenario, a 90 degree angle of attack( flying sideways). normally our rockets rarely experience an angle of attack greater than 5 degrees. rock sim and open rocket calculate the cp location based on an AoA of less than 1 degree(the cp is a dynamic location based on airspeed and angle of attack), so both cp locations are likely correct. bit late now but, I have been know to install a shelf for the recovery gear to keep it from sliding aft under boost.
Rex

And while I'm at it, let me throw another spanner in the works. The one caliber stability margin is based somewhat on the fact that the CP will move forward at higher "angles of attack" (AOA). That means the direction the rocket is pointed relative to its motion through the air. One of the assumptions of the Barrowman equations is that the rocket is flying at very near zero AOA. However, a gust of wind or high winds, some thrust imbalance, launch rod whip, or fin misalignment, can increase the AOA beyond those assumptions, and the rocket may find itself suddenly with the CP very close to the CG or even briefly ahead of it.

So, the one-caliber rule of thumb is usually a good one.

royal - I don't think you can go on 4 fins, the wind side wind would only hit surface area of side view, but yes, OR or Rocksim and good to know the also make assumptions. Also good to know that I'm not taken into consideration the AoA.

Rex, - great point ... Cross wind and angle of attack, the cardboard CP would be 90 degree ,

In summary, IMHO Cardboard CP not a good estimate, I've also heard a better estimate is where the fin angles would meet.

Looking at the picture, it looks like the CG is at 28.375, just a little bit forward of 28.75. As I recall, the CP, as calculated by OR's modified Barrowman calcs is a bit aft of 31, not exactly 31. So maybe you actually have very close to 3" separation between the two.

If you'd like to add some nose weight, I don't see a problem with an ounce or two, especially if you are going to fly it on high thrust G motors. There would likely be more issues with low thrust motors like Econojet F's. Trust in Thrust!

I would avoid pushing the chute down into the rocket. It's true it might slide back under thrust, but I would not shove it down there. As I understand it, the dynamics of the deployment are not that the ejection blows the chute out of the BT like a wad in a shotgun. Instead, the pressure inside the BT from the ejection charge pops the NC from the BT like a champagne cork, and the momentum of the NC drags the chute out after it. A lot of people think you want a kind of tight parachute and protector wad and a sort of loose NC, but it is actually the opposite. The chute rolled inside the protector should slide easily within the tube. I would still use a bit of dog barf. Put that in first. Z-fold the shock cord and slide that in next. Slide in the chute and protector burrito. Then fold the shock cord leader that goes to the NC, and put that in last. Fit the NC in place. If the NC is loose enough that it will slide out if you turn the rocket upside down, then put a strip of tape around the shoulder before putting the NC in. You want a tight seal and a little friction, enough that you would have to shake the rocket a bit to get the NC to fall out. When the ejection blows it will pop the NC forcefully, and the NC will pull out the leader behind it and then yank out the chute. If you do add an ounce or two of weight to the NC, it will help with the deployment process.

Here's my stock Leviathan this past weekend at Bong. On an 'Estes' F26-6. The local airport was reporting 13-16 mph winds for this time frame.

[video=youtube_share;p_Nd3dsfous]https://youtu.be/p_Nd3dsfous[/video]

Apologies for the low res. Can't upload HD at the moment.

Angles into the wind and takes off. I haven't checked the CG on mine.

Nice explanation of dragging the chute out.

I like the slow mo video - really can see what's happening

I made a epoxy slurry with 1.5 oz of weight and then 2 part foamed it into the nose cone - man did it shift the CG forward - roughly 3.5"+ forward - very touchy. I'm not holding the tape very well but now CG 24.5" and CP the estimated 31.125" for a over stability of 2.21.
Before


After



Thanks for the help all of you - great learning experience, and yes I know I could have flown it. However I want to know what why how do I can troubleshoot myself.

OH! OH! Now watch out on the windy days!! Just kidding!! Wishing you lots and lots of great flights. I just love the looks of that thing!!

Where is the ideal cg and cp of any rocket


IJEEPIN
Paul Sandyck
Membership Officer
/|_|,[______],
I--I--L--OIIIIIIIO-
( )_) ( )_)--o--)_)

Nick@JET said:

royal - I don't think you can go on 4 fins, the wind side wind would only hit surface area of side view, but yes, OR or Rocksim and good to know the also make assumptions. Also good to know that I'm not taken into consideration the AoA.

Rex, - great point ... Cross wind and angle of attack, the cardboard CP would be 90 degree ,

In summary, IMHO Cardboard CP not a good estimate, I've also heard a better estimate is where the fin angles would meet.

Click to expand...

I agree that the cardboard is not a good estimate, but the idea that only the cross wind would affect stability is faulty. Once the rocket is moving, most of the affects of the fins on stability is from the air moving across the fins from the forward movement not any sideways winds. This also keeps the AoA very low.

djpaulys said:

Where is the ideal cg and cp of any rocket


IJEEPIN
Paul Sandyck
Membership Officer
/|_|,[______],
I--I--L--OIIIIIIIO-
( )_) ( )_)--o--)_)

Click to expand...

CG should be closer to the nose cone than the CP by at least 1 body tube diameter. The is the stability, but like some have said in this thread that is a good rule. This way they would in theory go straight up and won't tumble, invert, etc.

During flight CG moves forward due to motor weight reducing as it burns which gains stability. There is way more To this than I am posting but this will give you the basic. I'm sure someone will take that and expand on it.

My Leviathan has 3oz of nose weight, 26 oz total, and has beautiful flights on F44 economaxes. I'm not sure I'd put an F26 in it.

flyskippy1 said:

My Leviathan has 3oz of nose weight, 26 oz total, and has beautiful flights on F44 economaxes. I'm not sure I'd put an F26 in it.

Click to expand...

Why did you add the nose weight? and why not fly the F26?

My Leviathan was built stock without any nose weight and works great with Hobbyline G motors, AT H128W and AT H250G motors. There are no stability issues at all.

Was able to fly the Levi today on a G74-6W , nice flight, there was about 7mph wind, you can see the pics, a little wind rotation, but pretty good flight considering the wind.
Up and almost out of sight, recover about 100 yds from pad. A fun rocket