CG/CP relationship in stick rockets

[r1dermon]

i assume bringing the CG as far back as possible would be the optimal design for a rocket, such as a bottlerocket. something finless, relying on the weight of a stick for stability...what role does the CP figure play in a finless rocket of this design? is it simply overcome by the rearward weight of the CG? are these rockets never truly "stable" in all aspects, resulting in less than predictable flights? if i were to build a large HPR bottle rocket, as seen on the discovery channel coverage of LDRS, what method would i use to ensure the vehicle is going to fly as reliably straight as rockets with fins at their bases? obviously, a standard 3FNC rocket is succeptable to any number of anomolies due to weather or equipment, like rod whip, or a stiff breeze leading to a weathercock...but beyond those variables, are there any more variables which would contribute to a "bottle rocket" design being any less stable overall?

 

[llickteig1]

i assume bringing the CG as far back as possible would be the optimal design for a rocket, such as a bottlerocket. something finless, relying on the weight of a stick for stability...what role does the CP figure play in a finless rocket of this design? is it simply overcome by the rearward weight of the CG? are these rockets never truly "stable" in all aspects, resulting in less than predictable flights? if i were to build a large HPR bottle rocket, as seen on the discovery channel coverage of LDRS, what method would i use to ensure the vehicle is going to fly as reliably straight as rockets with fins at their bases? obviously, a standard 3FNC rocket is succeptable to any number of anomolies due to weather or equipment, like rod whip, or a stiff breeze leading to a weathercock...but beyond those variables, are there any more variables which would contribute to a "bottle rocket" design being any less stable overall?

I think your idea of CG back is correct. I don't know the theory, but the fact that the thrust is pulling the bottle rocket instead of pushing it like the rockets we usually use, probably changes the CP rules.

At the start of the flight, you have all the weight forward due to the fuel. That changes over time, although you still have a bunch of weight from even the spent motor. The stick has to be long/heavy enough to keep the bottle rocket from spinning. I would suspect that the stick needs to weigh about the same or a little more than the stuff at the top in order for the bottle rocket to stay somewhat stable.

Some smart person will probably provide the theory.

--Lance.

 

[ClayD]

I think your idea of CG back is correct. I don't know the theory, but the fact that the thrust is pulling the bottle rocket instead of pushing it like the rockets we usually use, probably changes the CP rules.

At the start of the flight, you have all the weight forward due to the fuel. That changes over time, although you still have a bunch of weight from even the spent motor. The stick has to be long/heavy enough to keep the bottle rocket from spinning. I would suspect that the stick needs to weigh about the same or a little more than the stuff at the top in order for the bottle rocket to stay somewhat stable.

Some smart person will probably provide the theory.

--Lance.

I dont think (pull or push) thrust, does much as far as aerodynamic stability.
It does have a lot to do in situations with tourqe tho...

By Bro-Inlaw and i play the game "artillery" with bottle rockets, each has a circle the first to bail looses...

CG/CP relation ship is true in rockets, cars, trains, and planes.... anytime, they are too close, or reversed, you will have a reversal when the CP takes a higher value than ceter of gravity. or an attempt at it atleaset
(CP also moves as lift coeficients change)

Velocity and pressure dictate here.

My Guess... is that its not instability (skywriting) your fighting, its a low polor moment of inerta...(ir high.. im not a rocket scientist) the stick doesnt offer much stabilizing lift, or corrective value. However, it has surface area, (and thanks to exaust velocity) much higher lift coefficient than does the motor.

Also spin, and yaw will help to stablize a pop bottle rocket... Since it also increases lift on the stick.

 

[r1dermon]

one thing i find interesting is that, often times, the stick is not heavier than the payload. upon burnout of the motor, the rocket normally continues on in flight...i presume with enough weight to overcome the header's weight, upon burnout, the rearward weight would continue "pushing" due to newtons first law. the momentum of the greater (aft) mass will be greater than that of the momentum of the forward mass, leading to instability in the vehicle...

am i off-base with this assumption?

 

[ClayD]

one thing i find interesting is that, often times, the stick is not heavier than the payload. upon burnout of the motor, the rocket normally continues on in flight...i presume with enough weight to overcome the header's weight, upon burnout, the rearward weight would continue "pushing" due to newtons first law. the momentum of the greater (aft) mass will be greater than that of the momentum of the forward mass, leading to instability in the vehicle...

am i off-base with this assumption?

Launch enough pop bottle rockets, and you will see they tend to go unstable at burnout......

your not looking for the stick to add weight, infact lighter the better. you want your weight forward as far as possible.

If i built an HPR bottle rocket i would use a tube, not a stick.

In edit forgive my dimensia when reading I get easily confused....
Its not the mass distribution that causes the instability, its the loss of lift from the exaust...
if the bottle rocket is going "fast" enough the forward shift in CP at burnout can cause it to go unstable, escpecialy if it is flying horizontal. where gravity can help pull the CG out of line with the cp even further.
Mostly they are draggy and not going that fast, so the loss of lift on the stick, and lack of thrust, they just continue to coast....
I dont think inertia from the aft and forward masses being different will cause enough for instability, since your motor case is much of the mass to begin with.

 

[cjl]

The stick actually moves the CP back. You want the weight as far forward as possible, just like with any rocket. The fact that the thrust is at the front doesn't change the required balance (see the Pendulum Rocket Fallacy for details). What's nice about this kind of design is that the motor itself is basically acting as noseweight, so the CG is inherently very far forwards. The stick only has to pull the CP back a little bit to make the rocket stable. Ideally, the stick should be quite a bit lighter than the motor to keep the CG as far forward as possible. After burnout, they can sometimes go unstable because as the propellant burns, the CG shifts aft. If it shifts far enough, it can drop behind the CP, which leads to instability. Just like with any other unguided rocket, CP/CG relationship is the only thing that determines stability in flight.

 

[r1dermon]

VERY interesting cjl...

so from that, could we conclude that a square stick will be a better option than a round stick? due to the increased surface area?

what you're saying is basically, the stick acts as a sort of very long fin-can?

i know some very experienced rocket builders, who build these types of rockets for fireworks applications...from what i've been told by them, it is possible to use a smaller stick on a heavier payload, by adding weight to the rear of the stick...so what you're saying is almost the exact opposite of that...

presumably then, a longer stick, with a lesser mass, will always be the better option?

a few weeks ago, a fellow launched a rocket at one of our club events. the rocket went up with plenty of thrust, however, pitched over at about a 45 degree angle, and the tail was bouncing in an up and down motion...i presume (based on your post), that the stick had too little mass, not enough surface area, or was too short (kind of the same concept as the too little surface area)...this is directly opposite of my initial response, which was "that thing needs a heavier stick!"

wow...you learn something new every day.

 

[ClayD]

pitched over at about a 45 degree angle, and the tail was bouncing in an up and down motion...i presume (based on your post), that the stick had too little mass,

Did it pitch at or after burnout... if it did like most pop bottle rockets will its from the loss of lift from the exaust plume.

Most (Firework) pop bottle rockets have a verry small propellant mass fraction so motor burn CG shift is much smaller than the CP shift that occurs at burnout. And they still experience this after burnout yaw. Once the "report" takes care of the rest of the CG shift, they start to tumble....

 

[cjl]

VERY interesting cjl...

so from that, could we conclude that a square stick will be a better option than a round stick? due to the increased surface area?

Well, possibly. I'm not sure of the aerodynamics of a square stick vs a round stick of the same dimensions. I would guess that a square stick would provide slightly more stability, but I don't have anything to really back that up.

what you're saying is basically, the stick acts as a sort of very long fin-can?

i know some very experienced rocket builders, who build these types of rockets for fireworks applications...from what i've been told by them, it is possible to use a smaller stick on a heavier payload, by adding weight to the rear of the stick...so what you're saying is almost the exact opposite of that...

Basically, yes. A larger stick will always be more stable, and I wouldn't advocate adding mass to the back of the stick.

presumably then, a longer stick, with a lesser mass, will always be the better option?

I'll give this one a qualified yes. Diameter matters as well for the CP, but given equal dimensions (length and diameter), a lighter stick will result in a more stable rocket.

a few weeks ago, a fellow launched a rocket at one of our club events. the rocket went up with plenty of thrust, however, pitched over at about a 45 degree angle, and the tail was bouncing in an up and down motion...i presume (based on your post), that the stick had too little mass, not enough surface area, or was too short (kind of the same concept as the too little surface area)...this is directly opposite of my initial response, which was "that thing needs a heavier stick!"

wow...you learn something new every day.

The stick was probably too small - if the tail was bouncing, that means that the rocket had very little pitch damping. It might have been stable, but with too small of a stick, it would have inadequate damping which would result in oscillations around the desired flight path.

 

[r1dermon]

Did it pitch at or after burnout... if it did like most pop bottle rockets will its from the loss of lift from the exaust plume.

Most (Firework) pop bottle rockets have a verry small propellant mass fraction so motor burn CG shift is much smaller than the CP shift that occurs at burnout. And they still experience this after burnout yaw. Once the "report" takes care of the rest of the CG shift, they start to tumble....

under thrust.

but most rockets i see shot on the line fly very straight even after thrust is lost.

thanks cjl. big help!

 

[MarkII]

The stick helps to stabilize the rocket by increasing its rotational inertia. The problem is that as the propellant in the rocket is consumed, the forward end of a stick-stabilized rocket becomes lighter in relation to the pole at the aft end. The center of mass moves rearward rather rapidly during the rocket's flight, causing it to eventually lose stability.