How to calculate the stick lenght for a rocket?

[Pirotecnia]

For example, i have a rocket (propulsor & head) that weights 50 gram.

How can i calculate the lenght of the stick that it needs to be stable in the air?

 

Thanks!

[val77]

you need to your finger on the front of nozzle, if the length is perfect, and the rocket will stick horizontal

if the stick is too long the rocket will err on the side of the stick

if it is too short, the rocket will lean forward

[Pirotecnia]

you need to your finger on the front of nozzle, if the length is perfect, and the rocket will stick horizontal

if the stick is too long the rocket will err on the side of the stick

if it is too short, the rocket will lean forward

 

Thank you!

[AirCowPeacock]

Figure out what puts Cg just behind the nozzle. You'll have to know the density of the motor and stick (awell as header if you have one.) S(0, l)[M'(x)*x dx]/M(l) where M(x) is the amount of mass behind point x. l is length of rocket, so M(l) is total mass.

[AirCowPeacock]

Of course this will only give you an estimate assuming with your rocket the Cp is well behind the nozzle.

[dagabu]

I am going to disagree with the "normal" line of thought here in dealing with rockets sticks and "proper length". Take a look at the Rocket threads and find the last drawn out conversation about rocket sticks, you will see some tests I did in regards to stick length and Cg locations.

 

The truth about rocket sticks is that the length, weight and Cg can vary enormously from rocket to rocket. A case in point is the 4th place "Best Small Rocket Motor" at PGI this past summer. This was multi-tone whistle rocket, 5/8" ID and had two 1/8" sticks placed 180 degrees apart that were 14" long each. The balance point was nearly 1/3 the way up the motor and flew straight as an arrow.

 

Another example is the "Drinking Straw" rockets I flew in 2011 as a test for APC. I used 4oz. motors using the UT design with a nozzle using mild BP for fuel. I used a single drinking straw on each of them and every one flew straight for the first 100' or so until the plastic straw was melted off. The Cg was virtually 1/2 the way up the motor.

 

I am not trying to refute anybodies experiences with this nor am I trying to discredit anybody but simply asking that you test fly a few rockets with guidance sticks out of the norm to see for yourself what stick/s are actually needed to guide your rockets before taking Landcaster's or other hundreds of years old instructions as the rule of thumb.

 

I'll try to take some pictures of my missiles for you as well, they have no more than 6 sq inches of surface and are balanced at the point of exhaust exit but fly arrow straight once off the rod which is less than 12" long. A 36" long 1/4" square stick has 36" sq inches of surface area and my 36" long sticks weigh in at 20g while the three fins weigh in at 7g.

 

Obviously there is much to learn about guidance and we have barely scratched the surface with sticks and fins, take a look at the gyroscopic "gerbs" that are being worked on in another thread... amazing.

 

-dag

Edited November 2, 2012 by dagabu

[Col]

I use 3ft sticks for 5g salutes or 3" ball headers, the extra weight doesnt seem to make much of a difference to the flight. I had to use 4ft sticks on slower rockets even without headers so i guess speed plays a part too.

[dagabu]

Speed plays a great role in the guidance needed.

 

This past weekend, I was able to see some great pyro, about 100 rockets went up as well and there were some 1/2" X 48" (96 square inches of surface area!) sticks attached to #3 rockets that corkscrewed like they were opening a bottle of wine and then some that went straight as Tim Teebow!

 

In any case, it's better to get rockets up in the sky and learn from the experience then it is to be so rigid in your methods that you cannot enjoy the flight if it it wiggles a bit.

 

-dag

Edited November 2, 2012 by dagabu

[moondogman]

"Speed plays a great role in the guidance needed' I have to agree with that!! I have to think after going to the pgi this year and seeing alot of people with 2x4's attached to their rockets most people are overdoing their sticks!!! I use small/short sticks and they fly fine. Personally i like corkscrewing for a fireworks rocket. I use 3/8 to 1/2 sticks less than 4ft long even with a 6 inch header on up to a 3# rocket. on heavier headers I use 2 sticks. (smaller shorter sticks are easier to transport too). My wife even says she likes my smaller shorter stick.

 

Steve

[dagabu]

EWWWWWW!!!!

[Algenco]

"Speed plays a great role in the guidance needed' I have to agree with that!! I have to think after going to the pgi this year and seeing alot of people with 2x4's attached to their rockets most people are overdoing their sticks!!! I use small/short sticks and they fly fine. Personally i like corkscrewing for a fireworks rocket. I use 3/8 to 1/2 sticks less than 4ft long even with a 6 inch header on up to a 3# rocket. on heavier headers I use 2 sticks. (smaller shorter sticks are easier to transport too). My wife even says she likes my smaller shorter stick.

 

Steve

 

not what She told me

[AirCowPeacock]

If I try to use an 1/8" stick it burns and bends inward, the rocket loses its stability and corkskrews. I put Al tape on the suseptable area.

[Col]

1/8" would be pretty whippy unless its a really short stick. Safe bet is 1/4" square for 1/2" motors and 7/16" square for 3/4"

[nater]

I found out at PGI, that I use relatively short sticks compared to some guys as well. 5/16" x 36" for 1lb BP and whistle rockets and I have some sticks I ended up cutting at 1/2" x 48" for 3lb motors, but I haven't tried them yet. One problem I haven't had, is rockets corkscrewing on the way up. I've left the stick in pieces on the launch pad with a CATO, I've made gerbs on a stick, and I've had slow and fast flying rockets, but no corkscrews.

[dagabu]

1/8" would be pretty whippy unless its a really short stick. Safe bet is 1/4" square for 1/2" motors and 7/16" square for 3/4"

 

Just another example of how many different views there are. I use 1/4" square sticks for all my #1 motors if I single stick them and double 5/16" square sticks on my #3 motors. To each his own but I really hate 1" square lumber lawn-darting 2 feet in the ground like what happened at PGI on the rocket line. I head for the truck when they launch the #6 rockets. I think stick busters should be mandatory on sticks bigger then 1/2".

 

Last Saturday, a stick with motor buried itself 4" into hard pasture about 3 feet from a group of guys. I headed for the barn....

 

-dag

[nater]

Like this? That was a 1/2" ID rocket, arched over the crowd and lawn darted near a road. I decided I'm putting stick buster salutes on any rockets larger than 1lb that I make.

 

[dagabu]

Same thing but the motor was almost completely underground and would not come up with the stick. I found that all of my rockets with a report on top break off the stick as i do not tape them but just use a ribbon of hot glue.

 

"STICK!!" "Hey! Point at the damn thing too so we can all see it!"

 

Davey, PGI 2012

[Col]

8mm x 8mm is 5/16", thats what comes with me trying to convert from metric to imperial late at night. 11mm x 11mm would be overkill

I guess it wouldnt need to be a very big salute to detach the stick, shredding the tube would take a bit more

Edited November 3, 2012 by Col

[dagabu]

8mm x 8mm is 5/16", thats what comes with me trying to convert from metric to imperial late at night. 11mm x 11mm would be overkill

I guess it wouldnt need to be a very big salute to detach the stick, shredding the tube would take a bit more

 

There is really no need to "shred" the tube, the tube with a clay nozzle will not kill somebody when it comes down sideways but a tube with a stick travels very straight and can impale somebody. A good sized fire cracker is enough to be a 'stick buster'.

 

-dag

[usapyro]

My general rule with end burners has been to put the balance point at the nozzle. With really slow launching ones... Maybe balance a few inches behind the nozzle. With a fast launch core burner put the balance point half way up the rocket and it's fine!

 

It all depends upon the launch speed of the rocket! If your rockets are acting erratic during flight or hard to launch consistently lengthen the stick or shift the weight back more(heavier stick).

 

Generally the best performance can be gotten with a long light stick with a wood like pine or balsa. Just make sure the stick isn't "whippy"!

Edited November 3, 2012 by usapyro

[Pirotecnia]

Thanks for all your opinions!

 

Today, i've made my first rocket and it worked perfect. It was an endburner configuration, 20mm ID, 10cm long, fueled with a 70/30 mix of fast milled BP and Chrysanthemum 6 composition. I used the "balance" method (finger behind the nozzle) to estimate the lenght of the stick. The clay and fuel were rammed with my homemade wood tool that i've posted on another topic.

 

Thanks!