Rocket Failure?

[TYRONEEZEKIEL]

Alright, today, I made a 7/8" ID endburner rocket that had a 13/64" nozzle, which was not off center nor canted to any appreciable value.

 

The tube was 2" tall handrolled paper that massed at 5.3g

 

I used 5g of rammed kitty litter for nozzle. It was solid and didnt fail.

I used 15g of BP for fuel

I used 4.7g kitty litter for bulkhead.

 

I used 14" of 1/4" dowel rod for stability which weighed 12.3g

 

I taped it tightly on, and the center of gravity was just below the nozzle.

 

My black powder would have roughly a 2.5:1 lift ratio with this design

 

So far everything looked correct, and felt right.

 

Upon launch, I slightly angled the rocket away from me. The rocket took off correct, but at about 10ft up, it turned completely the opposite direction, and flew almost completely horizontal very fast toward the street.

 

Luckily, no one got hurt, nothing was damage, burned, nor harmed in any manner, save for my confidence.

 

I learned that I have outgrown my current site, and must find elsewhere for pyro, but what was the problem with the rocket? Ive succesfully launched many very similar to this.

 

Thanks,

Tyroneezekiel

Edited October 5, 2009 by TYRONEEZEKIEL

[NightHawkInLight]

A lull in the burn can cause that. Perhaps a bit of clay fell in the tube while ramming, or a pocket of charcoal that failed to be incorporated into the fuel properly.

[TYRONEEZEKIEL]

I dont believe either of those are plausible, since it was 12hr milled BP. And I cleaned both the tube and the rammer between each ramming.

 

Could it be too much thrust for the sttick, causing it to overcome the stability of the stick?

[FrankRizzo]

Yes. The stick was improperly sized for that type of rocket. Round dowels are too aerodynamic, so they don't present enough drag to keep the rocket from canting unless you use a very long one. It's a matter of putting your center of gravity higher than your center of pressure. Square sticks will give a much better result if you're limited in the amount of length you can transport.

 

Round dowels will work, but you need to use 1/2" diameter, or place two thinner sticks 180-degrees from each other.

[TYRONEEZEKIEL]

how do I calculate Cp on bottle rockets? I dont have fins like a model rocket, so I would imagine that it is in the motor somewhere. Also, wouldnt having a longer stick lower the Cg, pulling it below Cp?

[firetech]

Well a very elementary way to do it is to outline the entire rocket on paper, stick and all, and cut that out. Find where that balances and that is your COP. I balance ALL my rockets at the nozzle on the stick at 30 degress off vertical. They all fly perfectly straight. Just make sure your COP is above your COG. Which is fairly easy and simple. If it balances at 30 deg. and the stick is at least 7x as long as the rocket motor you are pretty much set. The stick on the rocket is like a tail on a kite. The longer the tail the more drag and the straighter it will go.

Good luck.

[FrankRizzo]

how do I calculate Cp on bottle rockets? I dont have fins like a model rocket, so I would imagine that it is in the motor somewhere. Also, wouldnt having a longer stick lower the Cg, pulling it below Cp?

 

Yes, you're correct. The length of the stick has to be balanced with the additional mass gain and it's center-of-gravity influence. The easiest way of measuring the CP of a rocket is to stick it like you normally would, then put a bright light on one side, and look at the shadow the whole assembly casts. Or make a simple tracing and cut it out like Firetech suggests. If you're using a round dowel, the tracing is not entirely accurate since the dowel isn't effective over its full diameter. Anyway, that will give you a rough approximation of the surface area the air/wind sees when trying to cant your rocket one way or the other. Again, that is why square sticks are better than round dowels, because they present a large surface area without needing to be excessively long.

 

I think Firetech meant the opposite of what he said regarding CP vs. CG. If the CP is ahead of the CG, the rocket will be unstable.

[TYRONEEZEKIEL]

Yeah, Im Cg always comes before Cp.

 

Thanks guys, I picked up some square dowels, and I will use about 8x rocket length, and see where that gets me for now.

 

Also, What is the correct nozzle size for a 7/16" ID rocket? It should be 1/3" ID IIRC?

[Mumbles]

The general rule of thumb is 1/3 ID for core burners, and 1/4 ID for end burners. You'd be looking somewhere in the vicinity of 1/8" if not a bit smaller.

[Eric70]

Yeah, Im Cg always comes before Cp.

 

Thanks guys, I picked up some square dowels, and I will use about 8x rocket length, and see where that gets me for now.

 

Also, What is the correct nozzle size for a 7/16" ID rocket? It should be 1/3" ID IIRC?

 

Try your 8x rule for length and adjust from there. I am sure there are many ways to go about it but this is what I do for achieving balance:

 

Get some of those tiny rubber bands, use 3 or 4 to attach the stick to rocket. I put the stick about 2/3 of the way up the rocket. With the stick attached using rubber bands, I try to balance the rocket on my finger at the base of the rocket engine - not at the very end edge but within the last 1/2" of the engine. Take care to not drop it. When balancing it on your finger, if it wants to tilt towards the stick end you will need to cut an inch off and repeat the process. If it tilts towards the rocket engine side, move the stick up a hair and see if that helps. Once I achieve balance I glue it down.

 

This works for me and I am able to establish a stick length that will work for the same design. I think you should have the stick attached to the rocket body at a minimum of 1/2 way up. If I make the rocket with a bigger or smaller header, with or without the header, I repeat the rubber-band-balance-on-finger routine.

 

I suppose twist ties or wire would work in place of rubber bands - something light in weight.

[dagabu]

Likely not the problem here, but remember the rocket will tend to fly into the wind, if there is any.

 

Sometimes, wind will be occurring above housetop level- wind that you'll be unaware of until your rocket veers.

 

Good point. At PGI in Mason City, there was one night with a steady breeze that was pushing rockets over to Target. I pulled my stick off and cut it in half and re-taped it on the motor, one piece on each side. That took care of the wind vane and it shot pretty straight up.

 

Also, round sticks suck!! You would think that it's all about the weight, it's not. The area of the stick is very important and rough cut sticks are much better at catching the air then dowels.

 

I take a cedar 2x4 and cut it down to 4', than I cut it into widths of 1/4" by 1/2". Thats pretty close to 5/16" in surface area, half the weight of any dowel I have found and a nice flat surface for the air to push against.

 

Don't take my word for it, go ahead and make one. Go try them side by side, the rectangle of rough wood will fly straighter every time.

 

D

[derekroolz]

On the topic of rocket fails, my rockets fly up perfectly straight but close to the end of the flight. They begin to cruve back toward the ground, the rocket has enough fuel and thrust to go up, But it appears that the rocket becomes off balanced and curves. Any solutions.

[TYRONEEZEKIEL]

Your rockets are fine, or are working. They can be tweaked.

 

That is gravity working. As the fuel burns, the amount of thrust diminishes, therefore it no longer accelerates upward as hard to fight gravity as well, so they will begin to fall while still burning.

 

What fuel are you using?

 

There are several ways to fix this,

 

If you are endburning your fuel, try coring. This changes everything!

 

Depending on your fuel, you can adjust the nozzle. For endburners it should be 1/4 ID and Core burners they should be 1/3 ID IIRC.

 

Adjusting the fuel will also make a huge difference.

 

OH. BTW. I did try the square sticks, and my rockets fly straight like arrows now, even under more aggressive thrust. Thanks everyone

[derekroolz]

Your rockets are fine, or are working. They can be tweaked.

 

That is gravity working. As the fuel burns, the amount of thrust diminishes, therefore it no longer accelerates upward as hard to fight gravity as well, so they will begin to fall while still burning.

 

What fuel are you using?

 

There are several ways to fix this,

 

If you are endburning your fuel, try coring. This changes everything!

 

Depending on your fuel, you can adjust the nozzle. For endburners it should be 1/4 ID and Core burners they should be 1/3 ID IIRC.

 

Adjusting the fuel will also make a huge difference.

 

OH. BTW. I did try the square sticks, and my rockets fly straight like arrows now, even under more aggressive thrust. Thanks everyone

 

 

I am using Kno3/And sugar 60/40 ratio and I use endcore burning. I use replicated a-26 estes size tubes but obviously filled to the top with fuel. So I guess that makes them C class motors. And I feel like a noob saying this, but could you explain a proper way to core a rocket. Would you need full dimensions?

Edited October 18, 2009 by derekroolz

[firetech]

Endcore burning? I have no idea what that means. If you are using KNO3 and sugar you should definitely make them coreburning. Try 3/4'' ID PVC with a 1/4'' nozzle. You're going to have to experiment with the length of the core and the length of the motor, it's been a while since I've made those. (Might be a 3'' section of PVC with a 2'' core).

[derekroolz]

Endcore burning? I have no idea what that means. If you are using KNO3 and sugar you should definitely make them coreburning. Try 3/4'' ID PVC with a 1/4'' nozzle. You're going to have to experiment with the length of the core and the length of the motor, it's been a while since I've made those. (Might be a 3'' section of PVC with a 2'' core).

 

 

O sorry I meant end burning not endcore. And my brain just now started working, I woke up about 15minutes ago but if I remember correctly the core of your rocket should be equal on all sides Ex. 1"of fuel around the core and on the top of the core.

Edited October 18, 2009 by derekroolz

[TYRONEEZEKIEL]

O sorry I meant end burning not endcore. And my brain just now started working, I woke up about 15minutes ago but if I remember correctly the core of your rocket should be equal on all sides Ex. 1"of fuel around the core and on the top of the core.

 

That is correct.

 

Also, are you recrystallizing your KNSU? That greatly improves performance, although. KNSU rockets are not the greatest.

 

 

The way to core them is to get a drill bit the right size and use a drill press, a drill, or your hand, and drill out the correct core.

 

Also. Try priming your core if it still is not where you need it.

[derekroolz]

That is correct.

 

Also, are you recrystallizing your KNSU? That greatly improves performance, although. KNSU rockets are not the greatest.

 

 

The way to core them is to get a drill bit the right size and use a drill press, a drill, or your hand, and drill out the correct core.

 

Also. Try priming your core if it still is not where you need it.

 

 

Alright will do and thanks for all the information, but I will have to wait to put up the trialing of it till next weekend. Stupid work coming in at just the right time