stick theory?

[shagaKahn]

Been searching the Rocketry forum for a discussion of stick theory.

 

In a recent build, I installed my sticks based on the balance point being one third up from the end of the nozzle--but it seemed like a couple of my rockets were really "stick heavy."

 

Is there a hard fast rule of do you all just wing it for stick length/weight?

 

Found an old rocket a friend had built and it had way less stick than anything I've built so far and it flew like a perfect monster straight up.

[dan999ification]

hi, my ideal balance point is right at the nozzle, the stick can be either half way up the motor or all the way to the top but i always balance at the nozzle just my preference, ive recently found out they can fly without sticks

and that some use two sticks that dont really balance at all.

someone did post a rough guide of stick ratios that i cant find. either way i always balance at the nozzle seems to work fine

if theyr'e stick heavy trim some off if theyr'e motor/payload heavy move the stick down the motor or get a longer one.

dan

[ballmill]

I've been firing my 1 pound rockets (with very light headings) with a 5/16" square stick 42" long, glued half way up the motor. They've flown straight as an arrow... however with larger headings that may have been different..

[dagabu]

I do like the name of this thread, "Stick Theory?"

 

Why? Because it is just that, theory and nothing more. I do not have access here to my YouTube account but I will post a video yet again of a 3# Universal Hybrid Fueled rocket with 2" ball shell that uses only three small fins for guidance and a guide rod only 8" long.

 

Also, hot whistle like the newly developed Copper Oxychloride catalyzed fuel, makes motors so fast that sticks (X2) only as long as the motor can be used to guide them well.

 

Endburners as well display rather peculiar results with different sticks, thin and long sticks seem to work spectacularly with the low thrust rockets.

 

As a "rule of thumb", you should start off using a stick that is 1/2 the ID of the rocket motor and X6 as long as the motor casing. This would mean my 3# UT rockets would use a 48" long stick 1/2" square. They do fly well with those sticks but they fly even better with two 24" sticks mounted on opposite sides of the motor.

 

I just picked up 6-6' long cedar fence planks, they are 5/8" thick and 6' long. I will rip these to 5/8" X 1/2" and will use them to launch my nozzleless rockets carrying 600g headings to 400' at the spring shoot. At $1.87 each plank, I will have 60 strong and perfectly straight lightweight sticks for $.20 each. I normally use The Home Depot's scrap white pine tongue and groove paneling for sticks but they are only 4' long and I have to cut them to 3/8" square. They work OK for 90% of my rockets but when I want to have an arrow straight launch, I use the long cedar sticks.

 

You choose, try what you have, think outside the box, see how they all behave and have fun!!

[dan999ification]

http://youtu.be/-iYvYukzXuY not the flight but prefire/build, this was posted in answer to one of my suggestions that square sticks are better, and they are for my endburners. yes dag the two sticks ive seen are mainly on catylized whistle motors/strobes youve taught me an awfull lot past weeks and i agree that its theory about sticks having/needing a set standard of dimensions since individual applications need different or no sticks. im just glad evrything worked out for me first go[ this time].

dan

 

 

[Blackthumb]

My 2 cents....balance mine at the nozzle....usually use 3/16 or 1/4 square for my little guys....about 6x the tube length.

They do fly well with short set of fins...cardboard but the stix and a glue gun is much quicker!

I'm giving the cedar back to my woods...! Recycle!

 

Still refining my sawdust fuel mix...

[donperry]

In theory and reality its all about Center of gravity and center of pressure (from the wind, or air pressure of the atmosphere)

 

For a rocket to be stable the CG should be towards the head and the CP towards the tail. A longer sticks makes sure these two are far apart. Shorter sticks brings them closer and the rocket becomes unstable.

A short stick can work only if you add fins to the base. Also, long tiny dense (thus heavy) stick is not as good as a short light thick stick.

Why? Because CP is affected by surface area

[dagabu]

A short stick can work only if you add fins to the base. Also, long tiny dense (thus heavy) stick is not as good as a short light thick stick.

 

I would like to argue the point that you are making is supposition based on publications that have largely been disproved. Take for instance the whistle battery. While there are certainly a lot of whistles that will spiral and shoot off in odd directions, most follow a pretty flay trajectory considering the shape and complete lack of tail. They also do not spin like a bullet allowing gyroscopic stability.

 

A long thin stick (steel rod) can be just as stable as a fin, which is EXACTLY what "a short light thick stick" becomes when it acts like a rudder.

 

To make a simple statement that one stick design is "better" than any other is disingenuous at best since every fuel, nozzle size, thrust rating, impulse and weight will change the formula in such a drastic way as to make any statement absurd.

 

I have fired rocket motors out of tubes with NO sticks, fins, or guidance at all and they flew completely straight and have flown motors with a straight, firm stick X10 the motor length that have corkscrewed crazily.

 

Why?

 

Because we largely ignore the fluid nature of the motors, thrust curves, shock waves and human variables. 1/2" rockets fly straight using a standard drinking straw (proven in video last month) and single stick Salicate fueled whistle rockets spiral, do 90° turns and corkscrew (search you-tube) even with strong straight sticks.

 

Just this last week, B.O. sent a rocket up with a ball shell that used three sticks attached to the ball shell at a severe angle from each other leaving only a short distance from the rocket nozzle to the ground and it flew like an arrow!

 

Its best not to typify what the "rules" are about sticks and accept that it is purely theory. Test, experiment, try something strange and weird, you may discover the equivalent of the De Laval nozzle for rocket stabilization.

 

-dag

[shagaKahn]

dagabu: "1/2" rockets fly straight using a standard drinking straw (proven in video last month)."

 

Is that 1/2" the I.D. of the rocket motor? 'Cause that's just the size I'm experimenting with currently (5" long tube with BP) and I'd LOVE to think a soda straw would do the trick for a stick!

 

Do you slip the straw down over a launch wire like with estes rockets?

 

Thanx a meg dag,

s

[dagabu]

dagabu: "1/2" rockets fly straight using a standard drinking straw (proven in video last month)."

 

Is that 1/2" the I.D. of the rocket motor? 'Cause that's just the size I'm experimenting with currently (5" long tube with BP) and I'd LOVE to think a soda straw would do the trick for a stick!

 

Yes, the ID is 1/2", any time we speak about the rocket size we will refer to it as either pound (i.e. 1#) or ID (i.e. 3/4"). My end burners were 3" long with about 2" of fuel. I dont think that a soda straw will work but what the heck, go try one . The real problem with the straw was that it completely burned through at apogee, your rocket is 2" longer so I dont think it will work, sorry.

 

Do you slip the straw down over a launch wire like with estes rockets?

 

Nope, I shot them just like a regular rocket from a tube.

 

Thanx a meg dag,

s

[donperry]

I agree with you Dag, and i think I figured out why stick rockets behave differently.

 

The common understanding as to why rockets are stable is to have a CG higher than a CP.

 

The total cg of the flying object [the rocket], is affected by the weight of the stick. If you have a long skinny [but] heavy stick the CG will be shifted towards the bottom and the CP will mostly be towards the top.

 

.

 

^^ that was theory, below is my opinion.

using that theory my immediate assumption was

A light short fat stick maintains most of the CG towards the head of the rocket where the fuel, motor and everything that adds weight resides. In this case the rocket will not rotate around the CG as the head IS the CG

 

Now, in reality the fact that they [stick rockets] are employing a pulling force as opposed to a pushing force makes this theory less applicable. Pull, because the nozzle is at the head of the object. Pushing would be when the nozzle is at the base as with real fin based rockets. I think that is where i went wrong with my post, I forget about their pulling nature.

 

 

 

If you have any links to article based on theories please post them, i'm only here to learn.

 

ps. as for the "Salicate fueled whistle rockets spiral, do 90° turns and corkscrew" I'd love to see them use a long light stick with something like a fin at the end. Even a think piece of cardboard would do. This is just for experiment, of course.

Edited April 28, 2011 by donperry

[dagabu]

ps. as for the "Salicate fueled whistle rockets spiral, do 90° turns and corkscrew" I'd love to see them use a long light stick with something like a fin at the end. Even a think piece of cardboard would do. This is just for experiment, of course.

 

The small fin on the end of a a stick was reported to have been done as early as the 1600's.

 

Don't get me wrong Don, I understand the science and dont dismiss it at all, after all, the law is the law. My concern is that we seem to get caught up in rules and see the physical laws as prohibition for experimenting with drastic out of the box thinking. The fin on the stick is a good beginning but there are other experiments that have yet to be done.

 

How about putting the stick in front of the motor? "Cant be done", they said, then it was done... over 200 years ago. Canards, when they were first thought of were dismissed as a "Foolish notion." by the wright brothers critics and were told that birds dont fly backwards, tails belong in the back.

 

That airplane flew and now all of the highest performance aircraft use canards.

 

The point is that as you state "reality" being the physical laws with leaving out all forward facing stabilization and that closes the door for a lot of those looking for new and different ways to stabilize rockets.

 

One final point and I will leave you to be. Last year I took a hot 1# (3/4") motor made on Universal Tooling and drilled two holes in the side, tangentially to the core, inserted visco into the holes, placed it in a 1" pin and lit it. No stick, no heading, no coins, just a motor.

 

It flew 300' straight as an arrow. There were a couple dozen of us watching it go. Apparently I was the first one to do this with this kind of motor and that was only because nobody told me that it wouldn't work.

 

-dag

[WonderBoy]

Don't worry Dag, we're still experimenting out here. Here's an example similar to yours, but when I made it I had no idea you'd done it previously. It was a 1lb motor made on a BP spindle, with whistle pressed then a clay nozzle, then BP. Creating a "whistle assisted stinger". It had one tangential hole drilled into the BP section, which is where I inserted the visco.

Whistle Assisted Stinger

[dagabu]

Fantastic! I love the two rings at apogee. Nice

 

-dag

[donperry]

Nice dag. I like these kinds of discussions.

nice rocket wonderboy

[alexthegreat00]

I dont want to make this over theoretical, but it is possible for the center of gravity and the center of pressure to be too far apart. The distance is called the stability margin I think. I remember that around 1.5-2 is ideal, but I forgot exactly how to calculate it. Anyway, if you rocket is over stable it would theoretically arc into the wind a fair bit. In a perfect world at least. I agree that a couple tests are much more meaningful than complicated theories. Human error makes it impossible to eliminate all of the variables.

 

EDIT: A program called RockSim does it, but it is not free and is geared mostly towards model rockets.

Edited April 29, 2011 by alexthegreat00

[shagaKahn]

Bamboo grove bordering our estate is the source of my stix. I cut the thinnest longest sticks from last year's dried out canes.

 

Of course this means no two are exactly alike. (Had read someone else's recommendation to buy bamboo nursery stakes or I might not have thought to try this).

 

Balance point AT the nozzle tip seems to be yielding the best results--but then I had one last week fly haywire; hardly fifty feet up out of the launch tube it went horizontal and spiraled.

 

A professional gave me some commercial Chi- rockets the other day (Ball Bullets) and he grasped the end of the bundle and broke off the last four inches of sticks; said they'd fly better.

 

Stick "theory" was indeed an appropriate title for this thread.

[dagabu]

I dont want to make this over theoretical, but it is possible for the center of gravity and the center of pressure to be too far apart. The distance is called the stability margin I think. I remember that around 1.5-2 is ideal, but I forgot exactly how to calculate it. Anyway, if you rocket is over stable it would theoretically arc into the wind a fair bit. In a perfect world at least. I agree that a couple tests are much more meaningful than complicated theories. Human error makes it impossible to eliminate all of the variables.

 

EDIT: A program called RockSim does it, but it is not free and is geared mostly towards model rockets.

 

Absolutely, I forgot the name of the phenomenon but when the center of gravity and the center of pressure are too far apart, a correction is made by the GP over the CG and a "wobble" is created. You will see lots of high powered rockets coming apart a few hundred feet up, in slow motion, you can see the effect on the body tube right before it parts.

 

This phenomenon is best demonstrated in the common arrow when it is fired from the bow, it bends in the middle as the ends seek to center themselves, with a rocket, the CP has more ability to correct and the nose is usually the fulcrum.

 

On a pyro rocket, the stick is able to "wobble" allowing the motor to oscillate without being destroyed like the body of a model rocket would be.

 

This is very different then the phenomenon you are talking about, we call that wind-vaneing in pyro. The stick is pushed freely by the wind and the rocket points into the wind. This is a desired trait and should not be fought against as we are not trying to move the rocket horizontally but vertically to display an effect within a known fallout zone.

 

-dag

[dagabu]

Bamboo grove bordering our estate is the source of my stix. I cut the thinnest longest sticks from last year's dried out canes.

 

Of course this means no two are exactly alike. (Had read someone else's recommendation to buy bamboo nursery stakes or I might not have thought to try this).

 

Balance point AT the nozzle tip seems to be yielding the best results--but then I had one last week fly haywire; hardly fifty feet up out of the launch tube it went horizontal and spiraled.

 

A professional gave me some commercial Chi- rockets the other day (Ball Bullets) and he grasped the end of the bundle and broke off the last four inches of sticks; said they'd fly better.

 

Stick "theory" was indeed an appropriate title for this thread.

 

Interesting, isn't it? You will find that round and smooth sticks will not steer as well as square or rectangular sticks.

 

-dag

 

At a HP rocket meet some years ago, I found a guy that was a steam punk kind of guy and was using a ring mounted on two sticks to launch the motor and chute assembly. He had better heights for the same size motor then all the other guys. Odd guy and definitely outside the box.

Edited May 6, 2011 by dagabu

[donperry]

I want to learn more about that wobble for HP rockets. one of my rockets took off, flew slightly leaned but when fuel is almost done it began to spiral. could this mean the CG and CP too far?

[dagabu]

Only a guess here but it may well be that they were. Alignment has a lot to do with as well. When making my first rockets, I didn't pay a lot of attention to how straight the spindle was in the tube before pressing the nozzle and I always seemed to get wandering rockets, now I make sure the support and tubes are aligned properly so that the spindle is dead center. The rockets do much better now.

 

-dag

[donperry]

Hey dag, about the rocket having CG and CP too far apart you were indeed right.

Overstability, as it is called, allows adverse behaviors such as turning into the wind very easily

[moondogman]

if you dont get the stick on straight you can get a pretty good spiral... but thats not all bad.

 

Steve

[dagabu]

if you dont get the stick on straight you can get a pretty good spiral... but thats not all bad.

 

Steve

 

I know what you mean, all but the few I posted spiraled because I had to tape the sticks on in the field. In the shop, I use a simple jig to align the motor and stick.

 

-dag

[Blackthumb]

I balance mine at the nozzle...never had a bad flier.....

Use hot glue to stick 'em.....have a few blisters to show for it...

Moondogman is correct about alignment.....sometimes spirals are neat!!

While glue is hot, I roll 'em over til the stick hits the work surface...aligns them OK