Stability change in first quarter second of flight

[Azamiryou]

I used OpenRocket to throw together this design by my six year old "just to see" if it could possibly work. I'm not sure we'll actually ever build it.

Anyway, with all the bits inside (the coupler and bulkheads in the middle represent a baffle, just to get the mass in the ballpark) and a motor, it's clearly not stable: 0.05 calibers with a C11, negative stability with a D12.

Just for fun I ran the simulations anyway, and... it seems to work. By the time it leaves the launch rod, it's got around two calibers of stability with either of those motors. By the time it leaves the rod, the CP has moved back more than three inches!

Is this correct and reasonable? Is it safe to launch a rocket like this? Or have I misunderstood something along the way or made a mistake in how I put it together in OR? (I'm new to OR.)

 

[lakeroadster]

I used OpenRocket to throw together this design by my six year old "just to see" if it could possibly work. I'm not sure we'll actually ever build it.

Anyway, with all the bits inside (the coupler and bulkheads in the middle represent a baffle, just to get the mass in the ballpark) and a motor, it's clearly not stable: 0.05 calibers with a C11, negative stability with a D12.

Just for fun I ran the simulations anyway, and... it seems to work. By the time it leaves the launch rod, it's got around two calibers of stability with either of those motors. By the time it leaves the rod, the CP has moved back more than three inches!

Is this correct and reasonable? Is it safe to launch a rocket like this? Or have I misunderstood something along the way or made a mistake in how I put it together in OR? (I'm new to OR.)

Are the fins really cardboard? In any event, add an ounce of nose weight, insert the D12-5 and light the fires.​

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[Azamiryou]

Are the fins really cardboard? In any event, add an ounce of nose weight, insert the D12-5 and light the fires.​

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View attachment 616330​

If we ever build it, the fins will be balsa and I'll probably add nose weight.

My question is not really about this specific rocket though. It's whether the big shift in stability is real, and if it is, whether it's safe to fly a rocket that has no stability sitting still but two calibers of stability by the time it leaves the launch pad.

 

[neil_w]

Investigating.

 

[lakeroadster]

If we ever build it, the fins will be balsa and I'll probably add nose weight.

My question is not really about this specific rocket though. It's whether the big shift in stability is real, and if it is, whether it's safe to fly a rocket that has no stability sitting still but two calibers of stability by the time it leaves the launch pad.

I run an OpenRocket sim for every rocket I build, and then plot that data to ensure stability throughout the flight. But I don't concern myself with stability while "On the Rod", the rocket needs to be "flying" for stability to occur.​

 

[neil_w]

Here are some preliminary findings?

1) It is not entirely clear why the stability margin jumps so much on this design at rod clearance. However, it is a characteristic of the calculations that margin increases once the rocket gets moving.
2) Once the margin jumps like that, the rest of the simulated flight is going to be fine, as you have seen.
3) To get a rocket like this to simulate as unstable, you need to fiddle with wind speed, wind direction, launch angle, etc. This rocket is asymmetric and these things matter quite a bit.
4) This is yet another good example of OR giving is inadequate feedback to the user, and the sort of thing I hope we can improve as we move forward.
5) For sure, I would not attempt to fly a rocket with .05 static margin, regardless of what the flight sim says. Add nose weight or whatever until static margin looks reasonable.

 

[Azamiryou]

Here are some preliminary findings?

1) It is not entirely clear why the stability margin jumps so much on this design at rod clearance. However, it is a characteristic of the calculations that margin increases once the rocket gets moving.
2) Once the margin jumps like that, the rest of the simulated flight is going to be fine, as you have seen.
3) To get a rocket like this to simulate as unstable, you need to fiddle with wind speed, wind direction, launch angle, etc. This rocket is asymmetric and these things matter quite a bit.
4) This is yet another good example of OR giving is inadequate feedback to the user, and the sort of thing I hope we can improve as we move forward.
5) For sure, I would not attempt to fly a rocket with .05 static margin, regardless of what the flight sim says. Add nose weight or whatever until static margin looks reasonable.

Thank you!

 

[JoePfeiffer]

You actually aren't getting any stability margin at all until you clear the launch rod, which is deliberate -- as long as you're constrained to be on the rod, aerodynamic stability really doesn't matter.

I think what you're seeing in the difference between the stability quoted when you're not flying and the stability shown in the simulation is coming from the rocket not being symmetric. You've got a very stable rocket in one axis, and a very marginally stable one in another. The stability figure being shown in the design window is worst case, while the stability you're seeing in the flight looks like best case. That should really be reporting worst case as well, not sure why it's not...

 

[Azamiryou]

You actually aren't getting any stability margin at all until you clear the launch rod, which is deliberate -- as long as you're constrained to be on the rod, aerodynamic stability really doesn't matter.

I think what you're seeing in the difference between the stability quoted when you're not flying and the stability shown in the simulation is coming from the rocket not being symmetric. You've got a very stable rocket in one axis, and a very marginally stable one in another. The stability figure being shown in the design window is worst case, while the stability you're seeing in the flight looks like best case. That should really be reporting worst case as well, not sure why it's not...

Maybe it has to do with wind angle? When I get a chance I'll try putting the rod straight up and run simulations with the wind coming from different directions to see what difference it makes.

 

[Azamiryou]

Maybe it has to do with wind angle? When I get a chance I'll try putting the rod straight up and run simulations with the wind coming from different directions to see what difference it makes.

With the wind from 0° instead of 90°, it leaves the rod with about -0.5 calibers of stability on a D12 .