Rocket Stick design with OpenRocket?....

[Tad]

Just wondering if anyone has tried to design a rocket tube and motor on OpenRocket showing stability, apogee, thrust, etc.. on a long stick (like the fireworks rocket-on-a-stick design)

Thanks,
Tad

 

[NateB]

I have, as a single free-form fin with the dimensions of my stick. For a 1" rocket motor, I use Poplar ripped to 1/2" square sticks and 60" long. I don't think Open Rocket sims it accurately. Stick rockets are way overstable and will weather vane unless they have really high thrust to weight ratios. OR doesn't show it as stable at all.

 

[BDB]

I've always wondered why bottle rockets like this are stable. I know that if you make the stick long enough so the CG is just behind the motor nozzle, they fly straight, but it sure seems to me that CP is likely in front of CG in that configuration.

 

[neil_w]

I've always wondered why bottle rockets like this are stable. I know that if you make the stick long enough so the CG is just behind the motor nozzle, they fly straight, but it sure seems to me that CP is likely in front of CG in that configuration.

Have wondered exactly the same thing. Would love to hear a proper explanation that doesn't rely on the pendulum fallacy.

 

[NateB]

According to OR, the CP is way in front of the CG. When I have flown these rockets, I try to put the CG in the bottom 1/3 of the motor and ideally right around the nozzle. Sometimes using 2 sticks is easier than a really long one. If you spice up the fuel with things more powerful than BP, you can muscle through some instability.

I have made many rockets with this basic design and with this shape of heading typically weighing 4-5 lbs. They always fly into the wind like you would expect. My guess has always been the drag from the stick stabilizing it, but I really don't know the right math or how to apply it to figure it out. I learned through trial and error and aiming for a 10:1 thrust-weight ratio.

 

[neil_w]

Although the shape is non-traditional vs. a standard finned rocket, it nonetheless is quite simple, and should be susceptible to analysis. Anyone know for sure?

 

[dhbarr]

Well the bit of the finstick that's at the very end has an extremely long lever. Might have more luck simming it in OR as a single tubefin having the same surface area.

 

[Tad]

As NateB stated, I too, have used a single freeform fin shaped like the stick I use that I know has worked in real life scenario but OR does not show it as stable at all. Just wondering if I can still rely on the other stats like apogee, velocity, etc...

 

[NateB]

I don't trust the other values when OR sims an unstable flight in this scenario. If you look at the side view plot, the shows the flight doing funny things that these rockets don't do in real life and that would affect the predicted altitude.

With Eggtimer releasing the inexpensive Apogee altimeters, I'm going to back to my plan of making some stick rockets with 29mm and 38mm motor mounts and recovery instead of stars. I just need to get my maple formers out and make the transitons and nosecones from kraft, wheat paste and probably a layer of fiberglass so I can reuse them.

 

[Tad]

I don't trust the other values when OR sims an unstable flight in this scenario. If you look at the side view plot, the shows the flight doing funny things that these rockets don't do in real life and that would affect the predicted altitude.

fair enough. Thanks, Nate.

 

[mbeels]

Well the bit of the finstick that's at the very end has an extremely long lever. Might have more luck simming it in OR as a single tubefin having the same surface area.

I'd bet it has got to be this. I think that the Barrowman equations assume that the body tube itself doesn't provide any stabilizing force, which is probably mostly correct for a "normal" rocket. But that long stick has probably a fair bit of drag, and at the end of a long arm, it can provide an effective restoring force. That'd be my answer if this was a test question.

 

[JoePfeiffer]

I'd bet it has got to be this. I think that the Barrowman equations assume that the body tube itself doesn't provide any stabilizing force, which is probably mostly correct for a "normal" rocket. But that long stick has probably a fair bit of drag, and at the end of a long arm, it can provide an effective restoring force. That'd be my answer if this was a test question.

You are correct. Barrowman (and OR) assumes the body tube contributes nothing to the CP, which is accurate for "normal" designs.

OR might do something vaguely reasonable if you modeled the stick as a body tube with a radius of 0 and four fins, each 1/4 inch high. But I sure wouldn't trust the result.

 

[JoePfeiffer]

Spent a little time playing with your bottle rocket this morning --

What I expect is the biggest problem is that Barrowman (and OR) assume thin planar fins. To the best of my recollection, OR's drag calculations don't consider the effects of off-center drag -- and my best guess is the main contribution the stick makes to stability is off-center drag, so OR doesn't really deal with it at all.

Something else worth mentioning is that CP really doesn't behave like CG -- it isn't a single point that can be calculated or measured. If you think of a design like a Bomarc that has wings, the restoring force in pitch is very different from in yaw -- in effect, you've got a different CP depending on which one you're looking at. I'd like to think of a good way to present that in OR, but any idea I come up with seems like it would have more capacity for confusion than clarity. My best idea is to have it move as you rotate the rocket with the slider on the left side of the rocket design window... What OR actually does is calculate the CP at all angles, and present the worst case -- the one that's farthest forward.

Your bottle rocket design, having only one fin, only has a restoring force in yaw, nothing in pitch. So OR is showing you the CP of the nose cone and the transition. If I remove your fin completely, the CP doesn't move. I can add a second fin identical to the first but at 90 degrees around the body tube, and I see the CP gets a little less unbelievable -- it's still in front of the nose, but closer to it.

 

[neil_w]

My best idea is to have it move as you rotate the rocket with the slider on the left side of the rocket design window... What OR actually does is calculate the CP at all angles, and present the worst case -- the one that's farthest forward.

That is exactly what I would do as a starting point... possibly, though, there could a bit more nuance to the UI to make it clearer.

 

[shockie]

I found this online:

Does the stick on a bottle rocket firework provide any stability or is that the common pendulum rocket fallacy?

It doesn’t keep the bottom end down towards the ground. That’s the Pendulum rocket fallacy - Wikipedia you mentioned. But it does provide other kinds of stability. It slows down how fast the rocket will curve off course. That is mostly due to the aerodynamic forces acting on the stick. If the rocket gets tilted with respect to the direction the rocket is moving, those forces apply a counteracting force that corrects the pointing of the nose back towards the direction of flight. The long stick also increases the moment of inertia. That slows down the rate at which the rocket will rotate off course if the thrust vector is not quite through the center of mass of the rocket. And, at the start of the flight. the stick is used to point the rocket in the desired direction of flight (usually upwards).

About 10-12 years ago a pyrotechnics person did a research study on the weights/lengths of the sticks versus the power and weight of the motors attached....I've lost to in the fog of time but I'll search and see if I can find it again.

we've been here before:

https://www.rocketryforum.com/threads/cg-cp-relationship-in-stick-rockets.25791/
Based upon what I have read in the various pyrotechnic forums and sites, they usually balancethe CG at or right behind the engine .

Instead of using square cross-section wood stick, consider using a fiberglass or carbon stick

from John Demar:

the CG is far forward. The corrective moment (force at a distance) is larger for the stick because its CP is far away from the CG. The upper (unstable) moment due to the body tube in front of the CG is smaller because it is not very far from the CG. Subtracting the two moments, the stick wins.

add some small fins at the rear aka the Chinese fire arrow for added stability

 

[BDB]

Are we saying that the Barrowman equations simply weren’t made for this scenario? If that’s the case, would it be helpful to measure CP via the old-fashioned cardboard cut-out method?

 

[JoePfeiffer]

As NateB stated, I too, have used a single freeform fin shaped like the stick I use that I know has worked in real life scenario but OR does not show it as stable at all. Just wondering if I can still rely on the other stats like apogee, velocity, etc...

No -- it sims it as behaving like an unstable rocket. Run a sim and plot the angle of attack. It shows it as looping (like an unstable rocket would do), which kills all the results.

 

[JoePfeiffer]

Are we saying that the Barrowman equations simply weren’t made for this scenario? If that’s the case, would it be helpful to measure CP via the old-fashioned cardboard cut-out method?

Hmm.... this is actually a case where the cardboard cutout method might be reasonably close.... the only case I think I've ever seen.

 

[NateB]

Stick rockets scale up to 100kg+ as well. The Thai's seem to keep them stable.

 

[mooffle]

Spent a little time playing with your bottle rocket this morning --

What I expect is the biggest problem is that Barrowman (and OR) assume thin planar fins. To the best of my recollection, OR's drag calculations don't consider the effects of off-center drag -- and my best guess is the main contribution the stick makes to stability is off-center drag, so OR doesn't really deal with it at all.

Something else worth mentioning is that CP really doesn't behave like CG -- it isn't a single point that can be calculated or measured. If you think of a design like a Bomarc that has wings, the restoring force in pitch is very different from in yaw -- in effect, you've got a different CP depending on which one you're looking at. I'd like to think of a good way to present that in OR, but any idea I come up with seems like it would have more capacity for confusion than clarity. My best idea is to have it move as you rotate the rocket with the slider on the left side of the rocket design window... What OR actually does is calculate the CP at all angles, and present the worst case -- the one that's farthest forward.

Your bottle rocket design, having only one fin, only has a restoring force in yaw, nothing in pitch. So OR is showing you the CP of the nose cone and the transition. If I remove your fin completely, the CP doesn't move. I can add a second fin identical to the first but at 90 degrees around the body tube, and I see the CP gets a little less unbelievable -- it's still in front of the nose, but closer to it.

This would make sense, I've tried to make sci-fi plane rockets before and adding 2 fins (the wings) does nothing to change the CP, if I add a 3rd fin then it calculates it out.