Tips and Guidelines for Aerodynamic Stability?

[Interstellar]

I've been designing a simple one stage rocket with OpenRocket, which allows the user to design and simulate their designs. I was always told that a general rule of thumb for a rockets stability is that the center of gravity should be 1-2 calibers above the center of pressure. With this rule applied to my model, in the simulation the rocket violently pitches in an oscillating motion, but when the stability margin is less than one but greater than zero the rocket is semi-stable (still has stability issues but they are reduced). Is this the result of a flaw in the simulation or... ?

model0.png - Contains design of rocket with stability margin of 0.193 cal
graph0.png - Graph containing simulated flight results with stability margin of 0.193 cal

model1.png - Contains design of rocket with stability margin of 1.55 cal
graph1.png - Graph containing simulated flight results with stability margin of 1.55 cal

 

[dhbarr]

Attach your sim and a screenshot of your launch settings?

 

[neil_w]

Yeah, post your ORK file and we can have a look.

Two quick comments of your side view: the launch lug is way too far forward (should be around the CG) and the motor is apparently inset into the BT, which you Do Not Want. The motor should stick a bit out the back (I typically do 1/4"). Neither of these issues should have anything to do with your weird simulation results, though.

 

[dhbarr]

Wait you don't have a -0 delay, do you?

 

[Interstellar]

Wait you don't have a -0 delay, do you?

Oops. I went ahead and fixed that but still, no effects on the "tossing and turning". I'll leave the rest of the file as-is while I attempt to find a solution.

 

[jlabrasca]

What is the pitch rate supposed to look like? I opened a couple of my designs to check, and I am getting the same kind of jagged catastrophe on all of them.

This is a plot of the rate at which the pitch is changing -- not a plot of the pitch, and it looks like it oscillates around 0 for both of the OP's rockets.

I've never before looked at these plots for one of my designs, but I'd guess that the lift and drag of the fins exert a smaller torque on the less stable rocket, which means a smaller angular acceleration, and a smaller angular speed over any interval in the simulation -- hence the smaller amplitude in the pitch rate plot.

(also, something is up with the "file upload" on the forum).

 

[Interstellar]

Yeah, post your ORK file and we can have a look.

Thanks for the advice, I'll keep it in mind. Say, why do you want the motor to stick out the back a small bit?

 

[Interstellar]

What is the pitch rate supposed to look like? I opened a couple of my designs to check, and I am getting the same kind of jagged catastrophe on all of them.

Good question. Based on the example models in OpenRocket, I'm seeing fluctuations that are less than five degrees per second. I'm aware that its degrees per second and not the actual pitch, but if you reverse engineer the data, the pitch is still out of whack.

 

[neil_w]

Thanks for the advice, I'll keep it in mind. Say, why do you want the motor to stick out the back a small bit?

Practical considerations, mostly:
1) To make it possible to grab it and remove it
2) To avoid cooking the inside of the body tube

Recessing the motor like you did there could have the beneficial effect of moving the CG forward a bit, but usually there are better ways to address that (e.g. nose weight).

 

[neil_w]

What is the pitch rate supposed to look like? I opened a couple of my designs to check, and I am getting the same kind of jagged catastrophe on all of them.

This is a plot of the rate at which the pitch is changing -- not a plot of the pitch, and it looks like it oscillates around 0 for both of the OP's rockets.

I've never before looked at these plots for one of my designs, but I'd guess that the lift and drag of the fins exert a smaller torque on the less stable rocket, which means a smaller angular acceleration, and a smaller angular speed over any interval in the simulation -- hence the smaller amplitude in the pitch rate plot.

Same here. I just tried out a few of my old files and came up with pitch rate plots similar to the OP. So I'm gonna say that it's normal.

I am also inclined to say that for "typical" rockets, that's not a useful parameter to plot, unless you really know how to interpret it properly (I sure don't).

 

[jlabrasca]

Same here. I just tried out a few of my old files and came up with pitch rate plots similar to the OP. So I'm gonna say that it's normal.

I am also inclined to say that for "typical" rockets, that's not a useful parameter to plot, unless you really know how to interpret it properly (I sure don't).

Yeah. I am seeing the same rapid saw-toothed oscillation in the pitch rate during thrust for every sim I've opened -- from an Estes Viking to the 3 inch rocket I flew for L1.

I'm aware that its degrees per second and not the actual pitch, but if you reverse engineer the data, the pitch is still out of whack.

Since the profile of the oscillation is very nearly triangular, it is pretty easy to eye-ball an estimate of the pitch. Looking at the .ork you posted, the maximum pitch is about ±4.9° in the first -.25 seconds -- just after the rocket leaves the rod. The oscillations decay through the thrust and coast phases. I agree that it is safe to ignore this plot for a conventional model rocket.

However, your design may not be conventional. It is a odd in several respects. The "body joint" and "engine anchor" are unfamiliar components, and the thick-walled 20mm O.D. body tube is unusual. I am not certain that I understand what you are trying to do.

What is certain is that the unstable version of the rocket would actually be unstable. It would go sideways.

 

[Steve Shannon]

Remove the off-axis mass (launch lugs, etc.) and see if the pitching is reduced. Any off-center mass will throw the Cg slightly off center. The thrust, being centered on the axis, will cause a slight rotation around that off center Cg.
Then, as a second experiment, increase the mass of the launch lug to see if the pitching increases.

 

[Bat-mite]

Thanks for the advice, I'll keep it in mind. Say, why do you want the motor to stick out the back a small bit?

You don't want to burn your MMT, nor leave residue that builds up and makes it difficult to remove the motor. Speaking of which, it is far easier to remove a motor that is sticking out than one that is jammed up in there.

 

[jlabrasca]

Looked back at the OP's previous posts on the forum. I am unwatching this thread.