Rocket stabilization

[TYRONEEZEKIEL]

I recently purchased 3/4" rocket tooling, and it will produce 7.5" rockets. My problem is, is I cant find the suggested 4' square dowel. The next best thing is 4' round dowel, as my hardware stores around here only go to 36" in their square dowels. I was thinking, maybe i could use 2 square 36" dowels, or use the table saw and make 1/4" by 4' dowels, although Im not to sure about the reliability of my table saw at that small of an increment.

 

Any suggestions?

[firetech]

Just cut your own with 2x4's. You can make them any size. Doesn't matter too much what the shape is, but flat surfaces are preferred.

[derekroolz]

Just cut your own with 2x4's. You can make them any size. Doesn't matter too much what the shape is, but flat surfaces are preferred.

 

 

Actually the shape of the dowel does make a difference in flight stability. I have found that round sticks are too aroedynamic and do not stabalize the rocket efficiently, but when flown with a square dowel they fly straight and are much more reliable. The reason behind this is drag, just like a kite you see flying with kites. The more drag the more stable, as long as the dowel doesn't severly throw off your projection vertaclly or horizontally.

[dagabu]

Actually the shape of the dowel does make a difference in flight stability. I have found that round sticks are too aroedynamic and do not stabalize the rocket efficiently, but when flown with a square dowel they fly straight and are much more reliable. The reason behind this is drag, just like a kite you see flying with kites. The more drag the more stable, as long as the dowel doesn't severly throw off your projection vertaclly or horizontally.

 

I am so sorry to do this but Passfire (I know, I know) has exhaustively gone over this issue and it all comes down to, If it works, do it. There have been books written by rocket scientists and such belying the uses of surface tension to guide the rocket.

 

I am going to take you to task with your ideas of drag = stability:

 

Using slow black powder rockets, mostly end burners with a heading will need a longer stick to help with guidance. This is not so with high power whistle motors. They have been flown (PGI video is proof) that the balance can be 2/3 the way up the engine when using whistle. A 3# rocket with a small salute can balance with a stick only 18" long!!!!

 

Now, lets try two sticks, one on each side, they are rectangles, not round or square, rough cut, 1/4" x 1/2" only 24" long and tape them with the narrow edge on the motor and you will have a very stable rocket with two very short sticks. Even with whistle, they stay straight and the issue of thrust torque is nullified due to equal surface and weight on both sides.

 

Drag is one of the variables, it's true but windvaneing is more important than drag. Weight is also important for balance. There is a test for sticks you can do at home. Take a good 6' long stick and tape it on your motor, go ahead and throw a header on it and tie a string 1/2 way up the motor. Now spin it over your head, play out some string, it will fly straight. Now lets cut off some of the stick, lets lop off a foot at a time. Likely it will fly true right up to 1' long.

 

Yes, it will corkscrew like crazy, it will come at you at a 90° angle out of a tube but fired from a long mortar, it will fly.

 

Cedar is probably the best 2x4 to use for rocket sticks, it is light and strong, cut them up on your table saw, you will love them.

 

D

[TYRONEEZEKIEL]

Dagabu, I must say you are the king of pyro rockets. Its amazing how much more difficult... er.. challenging pyro rocketry is compared to model or even high power amateur rocketry.

 

I really appreciate your advice.

[dagabu]

Dagabu, I must say you are the king of pyro rockets. Its amazing how much more difficult... er.. challenging pyro rocketry is compared to model or even high power amateur rocketry.

 

I really appreciate your advice.

 

LOL!

 

No way, HP is way more complicated by far!!!!!

 

It is all of the variables in pyro rockets that has me all jazzed up and thanks but I am not a pro at all, I surround myself with experts and ask them questions constantly and then bring their answers back here to share with APC. Ask, ask, ask, that is whatyou need to do and when you have tried and proven your hypothesis, throw in your two cents.

 

D

[JMA375HH]

I just got into rockets after wanting to do something different. I love it because of trial and error aspect and the great feeling you get when you get it rite. I went through the tail thing and I too tried round dowels and got spinning results as well. I have shop equipment and make my own wood tails with 2x4 cut to the length I am looking for then I rip them to 1/4'' x 1.5.'' 1.5'' is of course the thickness of a 2x4.

 

Then it's just a matter of ripping again. But I use a push stick and a side guide for all of this making sure I have good pressure on my material at all times. A table saw will eat up quickly the thin lightweight material and send it flying everywhere if you don't.

 

 

Also look for clear wood when doing this, wood with knots will cause you grief. A scroll saw works well also, I have a 15'' deep scroll saw and it's the way to go for 1/8th'' x 1/8th'' sticks.

 

Have fun JMA

[Arthur]

The old fashioned way used to include pin/stapling a triangular fin onto the stick at the bottom if the stick was otherwise too short. This creates additional aerodynamic stability once the rocket was flying. The added mass could also help with mass balance during the initial acceleration phase of flight. Think of it as adding a triangular tailplane or arrow flights. A bit of drag, and a bit of aerofoil dynamics, and a shorter stick.

[ghost808]

Well stablilization really confuses me atm. You see i have a pretty big rocket and i don't know if i need to stabilize it my stick is way to long already.... Someone please help me? just wondering what happens if its not perfectly stabilized.?

[Gunzway]

Well stablilization really confuses me atm. You see i have a pretty big rocket and i don't know if i need to stabilize it my stick is way to long already.... Someone please help me? just wondering what happens if its not perfectly stabilized.?

 

Well, I'm not an expert with rockets but this is how I determine if my rockets are stabilized or not. The stick needs to be obviously straight and relatively long/skinny aka. not a stump. Basically you put your finger behind the exhaust nozzle to determine if it is 'stablized' or not. When you do this, if the stick is heavier than the rocket and the rocket topples backwards it will make reduce your rocket's apogee and it won't perform as optimized. Thus you need to shorten your stick/reduce weight on that side.

 

If the rocket is heavier than the stick so that the rocket topples forward, it might make your rocket fly unpredictably and that's a scary thought especially if you have a header. You will have to find a longer/heavier stick if you wish to have a more predictable flight. I generally like to have my rockets made so the stick is slightly heavier (but hardly noticable), just to ensure my rocket won't fly unpredictably. This, of course, will affect my rocket's apogee but I can't take the chance of it flying unpredictably.

[dagabu]

Well stablilization really confuses me atm. You see i have a pretty big rocket and i don't know if i need to stabilize it my stick is way to long already.... Someone please help me? just wondering what happens if its not perfectly stabilized.?

 

Uh... Pretty much this LINK

 

I will get a lot of heat for saying this but the length of the stick is not important.

 

Let that set in a while,

 

I had 600g on top of a 3/4" ID Hybrid rocket motor in the video above. The motor had the power to lift that heading to 300' but the stick (single 1/4" x 48") flexed twice so the rocket went sideways. The shell was 2g flash and the rest was kitty litter. I put two sticks (2 x 1/4" x 48") on the same shell a week later and it lifted straight up and broke at around 275'.

 

The difference was NOT the weight of the sticks, it was the inability of the thrust to bend both sticks and thus bend the flight.

 

In contrast, I lift 1.5" cans with a handful of poka stars and break with a 1" x 4" ID BP motor too 300' with a 24" stick. Total weight of the whole thing was a scratch over 100g.

 

The most critical items to determine stick length are: Thrust, burn time (impulse), length of rocket and heading, center of gravity and balance point before firing.

 

Take for instance a 3# whistle rocket. The motor may be 10" long, contain 200 grams of whistle mix and have 45# of raw thrust. With a 10g flash heading, a pair of 1/4" x 20" long sticks may be enough to provide stabilization for the flight while a end burning BP rocket of the same dimensions with a 100g 3" ball shell may need a 60" long 3/8" stick in order to provide the stability to the slow rocket as it gains speed and thus stability.

 

Long answer longer, there is no perfect way to ensure the perfect length except trial.

Edited July 1, 2010 by dagabu

[dagabu]

Well, I'm not an expert with rockets but this is how I determine if my rockets are stabilized or not. The stick needs to be obviously straight and relatively long/skinny aka. not a stump. Basically you put your finger behind the exhaust nozzle to determine if it is 'stablized' or not. When you do this, if the stick is heavier than the rocket and the rocket topples backwards it will make reduce your rocket's apogee and it won't perform as optimized. Thus you need to shorten your stick/reduce weight on that side.

 

If the rocket is heavier than the stick so that the rocket topples forward, it might make your rocket fly unpredictably and that's a scary thought especially if you have a header. You will have to find a longer/heavier stick if you wish to have a more predictable flight. I generally like to have my rockets made so the stick is slightly heavier (but hardly noticable), just to ensure my rocket won't fly unpredictably. This, of course, will affect my rocket's apogee but I can't take the chance of it flying unpredictably.

 

This is about the safest way to do a stick determination however, my 600g heading would have taken a pair of 1/4" x 8' sticks to provide the balance you have mentioned.

 

Gunz- I'm not sure what you mean about the "topples" comment in your first paragraph.

[Gunzway]

This is about the safest way to do a stick determination however, my 600g heading would have taken a pair of 1/4" x 8' sticks to provide the balance you have mentioned.

 

Gunz- I'm not sure what you mean about the "topples" comment in your first paragraph.

 

Dagabu, was hoping you would post here. Someone with expertize with rockets and knows what they're talking about . I simply meant that if the rocket topples (tumble, fall down etc.) so that one side is leaning closer to the ground than the other, it would have influence on the rocket's flight.

 

'Stabilizing your rocket is relatively a simple process as long as you follow these rules. Your stick needs to be straight and it needs to be long and heavy enough to pass the balance test. This test is simple, attach a stick to the side of your finished rocket and try and balance your rocket on your finger. Do this by placing your finger just behind the exhaust nozzle. If the rocket topples forward, then your stick is not long or heavy enough and may cause it to fly in unpredictable directions. If it topples backwards then your stick is too long or heavy and will make your rocket sluggish and reduce it's apogee. If it's too long simply trim a small amount off and do the test again. If it's not long or heavy enough replace the stick with a larger one. Also, if you have for example 2 sticks, which both pass through the balance test, but one is longer and lighter and the other one is shorter and heavier, it's the best to pick the longer lighter stick, so that you rocket will fly better.' --- Pyroguide.

 

Basically what I based what I was writing on (what I've been doing for my rockets since I started).

[dagabu]

Again, as a starting point, I agree but the weight of the stick is probably the third most important part of the equation. The first being the available thrust, second being the surface and third being the weight.

 

The length is not important, the weight and surface take care of that.

 

Balancing the rocket is is a fools game. None of the Chinese rockets balance and as soon as you light the motor, the balance changes dramatically and the stick outweighs the motor.

 

I still say to use your direction, start long and trim back until it starts to spiral then go back to your last length (paraphrased). I am not convinced that straight sticks are needed especially if you are shooting fast rockets. Any bow should be made to go into the exhaust stream or if using two equal sticks, either into or away from the exhaust.

 

I use bowed sticks all of the time and use them to purposefully spin the rockets when placed on either side of the motor with the bow facing opposite directions. Just make sure that the sticks are free of flaws or you will get a wild flight!! This is especially beautiful when using slow BP fuel as the sparks make a lovely cork screw.

Edited July 1, 2010 by dagabu

[Peret]

The rocket has a center of mass (gravity) and a center of thrust, which is somewhere inside the motor. If the center of mass is ahead of the center of thrust, and the rocket isn't perfectly vertical, you have thrust pushing up and gravity pulling down, which sets up a turning couple that will turn the rocket over and make it dive into the ground. The effect of the stick is multiple. First, it adjusts the static center of mass back a bit, though as many people have pointed out it's not necessary to bring it all the way back. The second effect takes care of that - once the rocket gets up to speed, aerodynamic drag acts in the same direction as gravity to bring the dynamic center of mass further back. It needs to be enough to bring it back behind the center of thrust so that the thrust is effectively towing the mass, not pushing it. The third effect is to give the whole assembly enough moment of inertia that it doesn't have time to tip over before it picks up speed, otherwise (as seen in Dagabu's video) it will tip at first and then straighten up and fly in the direction it was pointing once the thrust kicks in.

 

They fly straighter if they get a really quick getaway, which you can help by running black match all the way up the core to the end so that the whole surface lights instantly.

 

I wonder, has anyone ever tried making a rocket where the garnish is at the rear in a ring round the nozzle, lit by quickmatch from the head? If the heat didn't set it off prematurely, such a rocket should always be in balance and fly straight up without a stick.

[dagabu]

They fly straighter if they get a really quick getaway, which you can help by running black match all the way up the core to the end so that the whole surface lights instantly.

 

Peret,

 

The fusing is solely dependent on the fuel type. Pressure spikes shatter the fuel grain and cause CATOs. This is the case in lighting whistle rockets where you only light the very end of the grain and even retard the burn so that as small of a pressure spike can be realized.

 

BP rockets that use a mild fuel can be core lit while a conventional 75:15:10 mix will need a big (if any) nozzle and end grain lighting. That being said, black match burns at a rate of 1-3 seconds per inch and almost 200' per second if perfectly contained so a section of quickmatch is better for this use.

 

You are correct about the rockets that would use a ring of effects at the base not needing sticks as demonstrated by the grandfather of modern rocketry, Robert Goddard. This system has been used several times and I have built them from time to time as well. They work OK but take an incredible amount of reinforcement to secure and are heavy.

 

You do shed much valuable light on one point that I had not addressed, the guidance of the rocket in its initial thrust phase is essential in a good flight so that slow fuels may need longer sticks for no other reason then to guide the upward movement of the rocket out of the guide but not in flight. Two stage rockets make good examples of this phenomenon.

[Peret]

The fusing is solely dependent on the fuel type.

I was only thinking about standard cored black powder types. I know little about end burners and other fuels, except what I've learned from dissecting commercial rockets.

 

I've found that the core itself is enough containment to make the black match burn with a crack. Piped quickmatch wouldn't light the core except at the far end, which I can't see offers much advantage over lighting it at the nozzle, though the initial sweep of flame down the core instead of up might light it quicker. I didn't think of that idea myself, btw - I heard it from the individual known as "The Master".

[dagabu]

Ahhh, The Master, Good man!

 

Yes he uses 1# motors that have a throat diameter of 1/4" which is as I stated above, just about "perfectly contained". The Master only does one rocket size, one fuel type, he is the first to admit that he knows very little about motors and made his own spindle for his fuel which he never changes.

 

Most of us making rockets vary our motors depending on the wanted effect therefore have to vary the ignition technique as well. Certainly not an argument, you are blessed with time with The Master as I have only had phone conversations with the man.

 

I am teaching a beginners course that we put together at PGI this year and I hope to have all of my small 1# motors pressed and ready for the class so the students can make and fly their headings on Thursday.

 

Keep your stick pointing skyward!

Edited July 5, 2010 by dagabu

[Eric70]

This thread is great discussion top to bottom!

 

Nothing to argue but this is what I use:

 

1# BP coreburner: 3/8" X 41" pine; using lathe roofing strips from Menards ripped on the table saw. These strips give me sticks 3/8" or a hair under depending on the strip.

 

3# BP coreburner 1/2" x 52" pine; ripped on table saw from whatever pine I can find.

 

These are square sticks and the length is shortened accordingly depending on header.

 

Perhaps balancing is a fools game. I gave up thinking about it and just do what works for me. I always balance approximately and inch below nozzle and they fly straight.

 

Last year I started balancing approximately and inch up from the bottom of the engine, the 1# rockets veered-off halfway up, the 3# ones had the corkscrew exhaust. I went back to balancing below the motor and all is good. It seems like it should be the other way around but that's what works for me.

 

I always paper pipe my visco fuse to ignite my coreburners at the top of the core.

 

Many different ways to go about things when it comes to rockets. Different variables such as power and weight that affects the outcome.

 

Eric