Stability is getting drop at before Apogee in Openrocket

[ogimogi98]

Hello,

I am new at the Forum. Because of that ı don't any idea for Forum rules. Anyway.

I am designing High Powered Rocket at Openrocket. Rocket flights 3.000 Meter (10.000 feet). But i have a problem with Static Margin at Apogee, when i simulated.As you can see below Graphic, my Rocket's static margin suddenly getting drop. I could't figure out what the reason is. anyone have an idea?

 

[HHaase]

As it nears zero velocity, you've got rapidly decreasing airflow over the airframe and over the fins. Need airflow to have aerodynamics.

This is actually normal to see and not a concern. If a rocket goes straight up, and comes straight back down, there is a period of instability at apogee. If you didn't deploy a chute you'd see the rocket tumble briefly at apogee. Then when velocity increases again during the descent, the aerodynamic forces build back up again, and it will become stable once more...... just pointing down.

In practice it's an uncommon situation unless you've got a perfect perfect flight. Typically a rocket will have some lateral velocity at apogee and arc over more gracefully. But if the wind is calm, and you're dead vertical, you can sometimes see a rocket just hang for a second and tumble for a moment. Gets really crazy if conditions are perfect and it back-slides all the way down.

-Hans

 

[Zeus-cat]

As HHaase said, once you reach apogee who cares. The rocket is done "flying" at this point. Recovery is the issue you have to deal with once you get to apogee. Get the chute out or at least split the rocket into two pieces if you are doing dual deploy.

 

[OverTheTop]

Welcome to the forum ogimogi98. The two previous posts cover the answer nicely.

 

[HHaase]

On a side note, please tell us more about that rocket. The three parachutes are intriguing

 

[ogimogi98]

Thank you all for your explanations. This rocket was preparing for a competition in Turkey. Maybe you've heard the name Teknofest.

According to the competition rules, there must be a minimum of three parachutes. There is a payload and its parachute on the shoulder of the nose. Just below the drag parachute and above of the engine block there is the main parachute.

The rules say that Rockets' static margin must be stable throughout the flight. So the instability in the apogee part worried me. I think it will not be a problem if I explain why this is happening.

And i couldn't see a title on this topic before. I will check it again.

 

[OverTheTop]

Thanks for the information. Good luck in the competition .

 

[Voyager1]

I think you’ll find that the rules require the rocket to be stable throughout its ascent up to apogee. As stated above, once the initial deployment has occurred at, or after apogee, then stability is not really relevant.

 

[ogimogi98]

Yeah I think I got it. Like Hans said that, If conditions are really close to perfect, there is a possibility that the rocket hangs in the air for a while and then spins like a propeller.

Since the drag parachute will open at apogee, the rocket will be prevent from during descent by tumble.

Thank you for your explanations and nice wishes. Have a nice day

However, if you have any other opinions or suggestions, it will be pleasure to read for me.

 

[Funkworks]

Gets really crazy if conditions are perfect and it back-slides all the way down.

... Like Hans said that, If conditions are really close to perfect, there is a possibility that the rocket hangs in the air for a while and then spins like a propeller.

Does anyone have videos of actual rockets at apogee in perfect or near perfect conditions? I'd like to see that. If videos are hard to find, simulations would also would fun to watch.

 

[HHaase]

Since it's so large, this one really shows a pretty good video of a peaceful apogee and deployment. Otherwise I'm having trouble finding videos with a clear view at apogee. That's just such a difficult part of a flight to get a clear zoomed video of.



-Hans