Cg and CP for rockets with wooden stick

[Sulphurstan]

Hi all. I've been through the forum, but couldn't find the answer. I'm beginning to build some rocket, with C6 motor, and then a 2 inch shell glued on top. Stability is done with a wooden stick. My concern is how to compute all that stuff, in order to have center of mass ahead of center of pressure... the rules, formulas found on Internet are based on rockets with fins and seem not to apply to this stick design... Grandmaster T Rebenklau just show that center of gravity is just down the nozzle ecit, but nothing about center of pressure... Does I worry too much about theorics? But I would like to avoid my first rocket to fly bad, just because design is not done properly... If someone has experience with this, and would like to share, I woulb be thankful.

FYI the shell (2inch) is 35 grams, the motor ( diam 18 mm, length 70mm) is 19,5 grams, the stick (600mm) is 12 grams.

[pyrokid]

I would say that the findings of TR are consistent with my experience. The quick test for stick rocket stability is to balance the finished device on your finger, with your finger at the base of the engine. Many exceptions to this rule exist in my experience. Using powerful fuels allows you to use shorter sticks or heavier payloads; higher velocity allows you to use a shorter stick since interactions with the air and the stick are what keeps the rocket flying straight, and these interactions scale with rocket velocity.

 

I am almost inclined to say that experience and testing are more valuable than theory as far as firework rockets are concerned.

[dagabu]

With a known motor such as your C6, you should use TR's instructions for balance. PK is correct about all the fuel types and various energies each fuel can create, the sticks will vary WIDELY when going from low & slow to fast & furious!

[Sulphurstan]

Ok, thank you for your replies. I'll fire this one tomorrow and if the flight is OK, I'll keep on the experimental way, and no more headache with maths and physics.

[Mumbles]

You may want to consider firing a few rockets with out headers or with dummy headers first. This well help to ensure everything is going well before adding live explosive charges to the top.

[stix]

Yes, there is indeed a difference between the way model rockets fly or simply attaching a model rocket motor to a stick. There is also a difference between rocket motors that have been developed for low powered model rockets (Estes etc.) and those made for launching a pyrotechnic shell.

 

I've never launched a rocket motor with a ball shell attached to the end, but hope to some day, nevertheless I think what I have to say is relevant.

 

What I've done in the past is pretty much as PK suggests, ie. once the stick is attached, then using a finger (as the pivot like a balance scale) place it on the stick behind the nozzle exit area. My standard rule was to place it approx. one finger thickness, 3/4 inch (20mm) away from the nozzle. More often than not the motor end would be too heavy. Having already attached the stick, the easiest option is to add a bit of weight to the end of the stick. I use thin solder wire (electronics) and wrap it around the end of the stick until it balances, then tape it there.

 

Possibly "over-engineered" but I've never had an instability issue providing the stick was rigid and the motor provided enough thrust. A few times I didn't have the right stick, so I taped a few thin sticks together - the "flexible pendulum effect". not good.

 

It's important to also mention what Dags said about power and energy. He is correct. In general you could say that a higher amount of power will improve stability, but there is a point that it won't. Therefore nozzleless motors that have huge amounts of initial thrust are used.

 

Finally, Mumbles suggestion of a few dummy headers should be the starting point. If you simply MUST test, then please make sure to do it in an open area and no-one close.

 

Remember, "Impatience is the enemy of the pyrotechnist". I say this, not from something I've read, but by experience and pure random luck, that no-one got hurt.

 

[EDIT] Sulphurstan, check out this link http://www.nar.org/SandT/pdf/Estes/C6.pdf the basic performance of an Estes C6. Do a bit of math and work out how many g's your rocket will take off at. It's important to know for stability.

Edited December 25, 2016 by stix

[Sulphurstan]

Thanks to all the advices from you here, that rocket took of just straight and stable and gravity center was adjusted with STIX's method,BUT....Blow blind 😢😢😢

So I must better coat my veline blue stars (I putted some veline priming, then BP...) and ricehulls in thé center. Well, keep practicing and testing....

[stix]

That's good that it took off straight and stable (and so it should), but there is a limit to my method of adding weight to the end of the stick - that's why I posted the link to the thrust performance chart of the Estes C6.

 

All because we have stability, does not mean or ensure that the rocket will leave the launch device.

 

Imagine a heavier rocket and shell. In order to stabilize it using my method we need to use lots of weight on the end of the stick. This makes the "whole" rocket & shell heavier. We could easily end up in a situation where the overall rocket is too heavy. It launches upward with wonderful stability, but slowly - and the shell breaks 10ft off the ground!!!! Not good and very dangerous!!!

 

No harm done if it's a simple rocket - but with a shell????

 

Therefore PLEASE look at this link. It has built in Estes motors performance and will give you a good idea of the altitude expected at apogee.

http://www.unm.edu/~tbeach/flashstuff/RocketAltitudeFixedSize.html

 

If you don't understand then please ask.

 

The reason my method works is because I've previously worked it out and confident enough where apogee will occur - or close to anyway.

[ExplosiveCoek]

Interesting topic fella's. I have a question regarding the balancing.

 

Normally I balance my rockets behind the header, not behind the rocket engine. Is this a big risk?

It was fine for my 3 and 4'' shell's so far.

 

However, I noticed with my whistle rockets (lifting 5 and 6'' shell's), they tend to leave the launch pad, and arc/bow out of trajectory. Then, once core fully ignited, they fly stable.

They are also balanced behind the 1kg headers. I used one stick, and even doubled the sticks (taped them together), but this didn't change much. I'm now going to ignite it with QM in the top, so I have immediately full power. You can see the arc in this video, right after lift-off:

https://www.youtube.com/watch?v=Ta9jUy9JBVc

 

A small indication about the stick's for 5'' I use roughly 110 mm x 10 mm x 20 mm (rectangular) and for 6'' I double those, so 110 x 20 x 20.

 

Could you also elaborate about your 'flexible pendulum effect' theory? I'm using two sticks for my 6'' shell's now. The previous one in the vid only had one stick, and wasn't balanced that well.

I'm curious what you guys think.

Edited December 27, 2016 by ExplosiveCoek

[OldMarine]

On larger rockets I use 2 sticks but on opposite sides of the rocket and have had good success. I've not tried whistle fuel yet so YRMV.

[NeighborJ]

Explosivecoek, I've had all kinds of guidance problems when using large sticks like yours. I've taken advice from others to shorten my sticks and make them thinner. My 5" shells when launched with whistle motors use a single stick only 6mm X 8mm and 75-80mm long. They now fly straight every time and never break any more. I suggest you try it once to see the difference, your shells will go higher without the lost energy during the direction change. I know that size stick seems inadequate but it's not.

[Sulphurstan]

That's good that it took off straight and stable (and so it should), but there is a limit to my method of adding weight to the end of the stick - that's why I posted the link to the thrust performance chart of the Estes C6.

 

All because we have stability, does not mean or ensure that the rocket will leave the launch device.

 

Imagine a heavier rocket and shell. In order to stabilize it using my method we need to use lots of weight on the end of the stick. This makes the "whole" rocket & shell heavier. We could easily end up in a situation where the overall rocket is too heavy. It launches upward with wonderful stability, but slowly - and the shell breaks 10ft off the ground!!!! Not good and very dangerous!!!

 

No harm done if it's a simple rocket - but with a shell????

 

Therefore PLEASE look at this link. It has built in Estes motors performance and will give you a good idea of the altitude expected at apogee.

http://www.unm.edu/~tbeach/flashstuff/RocketAltitudeFixedSize.html

 

If you don't understand then please ask.

 

The reason my method works is because I've previously worked it out and confident enough where apogee will occur - or close to anyway.

 

 

 

YEP, That's why I'm now always building DUMMY payloads (same shape, same weight), mount them on the rocket, fire it. Only then, I'll put the real payload (shell) on it.

Regarding the HTML page you sent here, it is just GREAT!! (I made my own excel sheet before, but this one is really better, example the optimum mass calculation is a must have!!) THANX a lot STIX, you made my day with this page :-)

Edited December 28, 2016 by Sulphurstan

[Sulphurstan]

Interesting topic fella's. I have a question regarding the balancing.

 

Normally I balance my rockets behind the header, not behind the rocket engine. Is this a big risk?

It was fine for my 3 and 4'' shell's so far.

 

However, I noticed with my whistle rockets (lifting 5 and 6'' shell's), they tend to leave the launch pad, and arc/bow out of trajectory. Then, once core fully ignited, they fly stable.

They are also balanced behind the 1kg headers. I used one stick, and even doubled the sticks (taped them together), but this didn't change much. I'm now going to ignite it with QM in the top, so I have immediately full power. You can see the arc in this video, right after lift-off:

https://www.youtube.com/watch?v=Ta9jUy9JBVc

 

A small indication about the stick's for 5'' I use roughly 110 mm x 10 mm x 20 mm (rectangular) and for 6'' I double those, so 110 x 20 x 20.

 

Could you also elaborate about your 'flexible pendulum effect' theory? I'm using two sticks for my 6'' shell's now. The previous one in the vid only had one stick, and wasn't balanced that well.

I'm curious what you guys think.

Just one thought: I balanced the rocket with TR's isntruction, and it works OK!. But, what I've seen on commercial rocket (sammler, maybe maximum 60 grms), is that the Center of Gravity lies somewhere in the middle of the motor stage, and not in the rear of the nozzle. Does anybody knows why?

[ExplosiveCoek]

Explosivecoek, I've had all kinds of guidance problems when using large sticks like yours. I've taken advice from others to shorten my sticks and make them thinner. My 5" shells when launched with whistle motors use a single stick only 6mm X 8mm and 75-80mm long. They now fly straight every time and never break any more. I suggest you try it once to see the difference, your shells will go higher without the lost energy during the direction change. I know that size stick seems inadequate but it's not.

 

Thanks for the information, but I found that the 10x10x100 mm already barely holdup the weight of the shell. I think if I'll go thinner, it might break. I also can't really understand what the benefit would be. It has even less stability like this, right?

 

Can you give some information on what type of whistle motor (dimensions) you're using?

Mine are 65 mm core, the pin's base is 11 mm until roughly 6 mm at the end. Whistle engine's dimensions are 22mm diameter.

For 6'' I use the same engine's, but 90 mm core instead of 65.

[NeighborJ]

coek, the key to useing the thinner sticks is to not allow the stick to suport te weight of the shell. I rest the bottom of the motor on the top of the launch tube to protect the stick from added stress, its only function is stablization during flight.

when i used the thicker sticks, the motor thrust would push against the large surface area of the stick causing broken sticks, misdirected launches, and loss of thrust. I now face the thinnest side of the stick towards the motor to provide the ninimal crosssection for the exhaust to push against.

You asked about the motors used, a 5" shell gets a 1lb motor capable of lifting 600g, the spindle dimensions are 1/2" (12.7mm) x 2-1/2" (63.5mm). The fuel is cuoxycl catylized salisylate 76-23-1+2.5. I often use several motors on larger shells to dial in the lift easier and to add to the effect but no changes are made to stick sizing.

[ExplosiveCoek]

That makes sense. I'll try to rest the motor on the launching pad too.

 

What I still don't get is how the stick could break from the thrust of the rocket? And why my rockets go straight from the launching pad, bend, and go then straight again in that direction.

This phenomena occurs only with these larger rockets. I would test fire some more rockets if given the possibility, but they make a lot of noise, and >1kg test header coming down (if I don't use a bit of flash to pop it open), is quite dangerous too. This NYE I'll shoot some of them, I'll keep you posted about the results.

 

Your dimensions and fuel are really a lot similar to mine, I use kbenzo, instead of salisylate though. But even with a slightly smaller engine you can lift the 5'', interesting.

 

Do you use quickmatch to ignite the top of the core, or just visco/bm touching the end of the grain?

[stix]

@ExplosiveCoek

 

Could you also elaborate about your 'flexible pendulum effect' theory?

 

Sorry, not a theory, but more of an observation. What I was getting at is this. One time I didn't have a decent stick and was desperate to launch. I had one of those "matchstick" blinds so I decided to tape 3 of the sticks (approx. 3mm diam.) together, ie. in parallel. The stick was quite flexible but I went ahead and balanced it at usual. When it took off I noticed that it started to sway back and forth like a pendulum, ever increasing - due to the flexibility of the stick. If the burn time was longer it probably would have went out of control, although sometimes good if you like that sort of thing . Otherwise you simply MUST have strong sturdy rigid stick(s). Thin or thick, but strong and rigid nonetheless.

 

@All

 

I like the idea of the 2 sticks method (one each side). Never tried it and a bit more work. If I had a shell on the end that's what I would do - but still following my own balancing method. Overall, two shorter sticks will be more rigid than just a single longer one, whilst also adding more weight to the aft end. A benefit.

 

In my view this type of rocket is more stable if it's balanced BEHIND the nozzle, that's where the force is coming from. Taken to the limit, If you move the cg too much forward it's going to spin at some point. Think about it. I call this the carousel of no return effect.

 

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If you're using a powerful rocket motor (like an internal burner nozzle-less) I can see how the stick might snap off at launch. Minor misalignment's could be exaggerated. Regardless, the OP Sulphurstan stated he is using an Estes C6, that particular burn profile (link posted) has a bit of a boost at the beginning to get it going, but for the most part is sustained and even thrust. For stability, slower take off rockets require the cg to be further back, whereas faster take-offs allow more leeway.

 

My method initially sacrifices altitude for stability. But if we are consistent with our methods, then we can always fine-tune to an optimum performance of that design.

 

---

 

Thinner sticks indeed!!... mumbo jumbo. Yeah thinner sticks could be beneficial, but more important is rigidity.

Oh yeah, this assumes of course that the stick(s) are attached rigidly to the body.

 

I mean, bloody hell guys, come on, It's not exactly rocket science!!

 

[EDIT] Sorry about all the edits.

Edited December 29, 2016 by stix

[ExplosiveCoek]

So it does appear to be rocket science .

 

https://youtu.be/wINHvkDOg_0

 

There are some whistle rocket's in there, 5'' & 6'' header. Seems like they didn't have enough power, and slightly too much delay.

Maybe I've to add another whistle rocket.. I'm not sure. The 5'' had a 65 mm core, and the 6'' 90 mm core. Both 22mm ID whistle rockets.

[NeighborJ]

We're those whistles made with the thinner ,shorter sticks? I had a few rockets which flew with that corkscrew flight but I think it was because I used a less powerful fuel mix and they flew slower. I also had three catos so I had to pull the rest of the fuses back to the aperture to resolve the issue. I don't know why but all of my whistles without a Ti delay flew fast and true. You can experiment as far as length but the skinny sticks are the way to go.

[DavidF]

Nice rockets, too bad about the fog! I also use skinnyish sticks. I use 2 sticks, opposite each other. I have had some erratic flights using 3lb nozzleless motors with hot BP and 6" shells on top. I was launching them with one stick in the launch tube, and one hanging free. After one session, there was one broken stick on the ground for each rocket. Then I used a 'ring stand' apparatus, with both sticks inside. That worked better. I got flamed by the 'weak rocket people' for mentioning it Still not satisfied, I started taping the sticks together at the bottom. Presto, perfect flights! My rockets balanced about halfway up the 12" long 3lb motor. The taping of the sticks increases drag, but also gives stable flight. I always do it now. I have only had one erratic flight since, out of probably 200 rockets. For the 1lbers I do the same, with shorter and skinnier sticks, like 5/16"X40" or so in length. The double-sticked rockets with the sticks taped together have not corkscrewed on me ever.

 

For the 1lbers I made up a jig that holds the motor and lines up the sticks, so they can both be taped on at once, perfectly straight. I simply mount the jig in a vise and git 'er dun.

[Mumbles]

Dave, could you describe the ring stand apparatus a little more? It is what it sounds like, just rings attached to a post or pole to keep the rocket oriented upwards? This is a chemistry lab one, but is the idea about the same?

 

[Arthur]

I've heard that square sticks are better than round ones, but have no data to prove or disprove that. The Cp and Cg calculations are for a stickless rocket, if the rocket leaves the launch tube fast enough then the square stick acts as an aerodynamic guide, 2 sticks,2 guides.

[DavidF]

Yes, that's what I mean Mumbles. I mostly do round shells, so I can actually hang the rocket by the ball and aim it somewhere. The ring could always be a short tube if preferred. Or the ring could be a bit lower, and the rocket sits on the sticks instead of hanging. I really think taping the sticks together evens out and mellows the forces- for want of better words. The sticks can't whip around and they give each other strength. I'm gonna keep doing it

[NeighborJ]

Dave I use the taped ends on my BP motors also but I've found the whistle motors are too powerful and the exhaust gasses pry out on the crook of the sticks. I stopped taping those motors after turning a slew of test motors into toothpick makers. They do fly nice, always perfectly strait.

[ExplosiveCoek]

Thanks for support guys.

 

So regarding the questions:

I was too afraid to use only one stick on the 6'' , they weren't balanced at all that way. Now I used two, and taped them on both side of the engine, like you suggested. They flew straight, but with quite an arc. Maybe I need a more powerfull whistle rocket, to get them up higher.. But the 5'' should have flew fine, as it was a 22 mm rocket too, so I don't get it. As you can see from the breaks of the shell's close to the ground, they weren't high enough too.

 

The 5'' had one stick only, was too afraid to use half the size of the current stick. Mostly due to the balance being completely off.

 

I shot a 16 mm ID whistle rocket too, with roughly 7x7 length of.. 500 mm, dunnow exactly.. stick. That one was nice and straight.

 

After balancing, all rockets had their balance slightly beneath the headers (top of the rocket engines).

After balancing, all rockets had their balance slightly beneath the headers (top of the rocket engines).