Over stable

[Skye]

I am working on a new rocket .What does it mean in Rocksim when it says the rocket is over stable? An what will happen if I would go ahead and build it that way?
Im new at this !!!

Thanks Tom

 

[powderburner]

It means your CP is 'excessively' far behind your CG

You could therefore get away with fewer or smaller fins
You could reduce the amount of nose ballast (if any)
You could add small forward fins (for style)

If you launch as is, your rocket will be more prone to weathercock into the wind. On a windy day, your rocket could turn toward horizontal very early in its flight and end up impacting the ground before the ejection charge pops.

 

[Rick James]

Before you make any design changes... try loading a motor and then checking stability... sometimes the weight of the motor will change things enough...

 

[Skye]

Thanks guys for all the input

 

[dragonshiprider]

here's a little something from Apogee's web site on overstability
https://www.apogeerockets.com/education/newsletter05.asp

For all of those who don't know Apogees newsletters are one of the best sources on the net for info:
https://www.apogeerockets.com/education/newsletter_archive.asp

 

[shreadvector]

YES! He is very thorough in his explanations, BUT I've noticed that readers tend to skip parts of the important information.

For instance: someone already advised in this thread that if it's overstable it will weathercock and could turn horizontal and then impact the ground. that is NOT the complete story. It will only weathercock badly IF it is moving at a slow speed relative to the side wind. Only then will it rotate (weathercock) badly. If you are flying with a good thrust to weight ratio and a long launch rod, the rocket will be moving plenty fast when it leaves the launch rod and it will barely notice the side wind.

In other words, a rocket moving 200 miles per hour upward will barely notice a 10 mile per hour side wind.

Your parachute WILL notice the side wind, so take a photo of your rocket before you launch it.

Originally posted by dragonshiprider
here's a little something from Apogee's web site on overstability
https://www.apogeerockets.com/education/newsletter05.asp

For all of those who don't know Apogees newsletters are one of the best sources on the net for info:
https://www.apogeerockets.com/education/newsletter_archive.asp

 

[Missileman]

Another factor too is how much overstable is it?
If its just slightly over 2 calipers then it is barely overstable and not enough to even be concerned with.
If its like 4 calipers I wouldn't fly it on a windy day but otherwise it should be ok.

 

[teflonrocketry1]

Skye,

Could you post your RockSim file so us RockSim experts can take a look at it?

I agree with Shreadvector about the Apogee Peak of Flight Newsletters being great sources of information; I even wrote a few of them!

Bruce S. Levison, NAR #69055

 

[JRThro]

Originally posted by shreadvector
... It will only weathercock badly IF it is moving at a slow speed relative to the side wind. Only then will it rotate (weathercock) badly. If you are flying with a good thrust to weight ratio and a long launch rod, the rocket will be moving plenty fast when it leaves the launch rod and it will barely notice the side wind.

In other words, a rocket moving 200 miles per hour upward will barely notice a 10 mile per hour side wind.

Your parachute WILL notice the side wind, so take a photo of your rocket before you launch it.

So my bashed triple-length (32.75" long versus 12.75" for the stock one; < 2 oz. empty weight versus 1.0 oz. for the stock one) Quest Commander with 10+ calibers of stability should be okay if there's little to no wind, or if I fly it on C6 motors.

(I knew that, I just wanted to say it again.)

First flight should be this coming Saturday.

And let's note that it's "calibers," not "calipers" as someone said.

 

[powderburner]

Do you really really have ten 'calibers' of margin in your design with a motor installed? That's a LOT of margin.
I am not familiar with the designs you are talking about, so I don't know what you have up front that would pull the c.g. so far forward. Maybe you could snap a picture of this thing and post it?

 

[JRThro]

Originally posted by powderburner
Do you really really have ten 'calibers' of margin in your design with a motor installed? That's a LOT of margin.
I am not familiar with the designs you are talking about, so I don't know what you have up front that would pull the c.g. so far forward. Maybe you could snap a picture of this thing and post it?

It's a long skinny rocket, based on the Quest Commander, but using 3 10" body tubes instead of just one.

Looking at the attached picture, the mass items are as follows:
Mass Item 1: BT #1
Mass Item 2: BT #2
Mass Item 3: BT #3
Mass Item 4: C6-5 Motor, Unused
Mass Item 5: Coupler #1-2
Mass Item 6: Coupler # 2-3
Mass Item 7: Motor Tube
Mass Item 8: Motor Hook
Mass Item 9: Motor Block

Basically, it's not that the CG is pulled very far forward, it's that a caliber is only 20 mm (0.787").

 

[powderburner]

Wow, you're not kidding, that thing really IS long.
And if you add a parachute (or streamer) and some wadding up in the front, your c.g. will go even farther forward.
I would say you could safely trim off a bit of fin area---quite a bit. Here is your chance to style something way different (visually) from the basic kit.

 

[JRThro]

Originally posted by powderburner
Wow, you're not kidding, that thing really IS long.
And if you add a parachute (or streamer) and some wadding up in the front, your c.g. will go even farther forward.
I would say you could safely trim off a bit of fin area---quite a bit. Here is your chance to style something way different (visually) from the basic kit.

Not a bad idea, but it's already built.

I think I will play around with it in VCP, though. I've always liked the look of the trapezoidal fins that a lot of bigger/faster/higher-flying rockets have. I could trim the fins if I decided to. Just don't want to touch the leading edge, since the grain runs along that edge, and it's reinforced with CA.

 

[powderburner]

You could minimize the rework/repaint necessary by trimming directly off the tips of the fins.

Might not make aesthetically attractive fin shapes, however?

 

[JRThro]

Originally posted by powderburner
You could minimize the rework/repaint necessary by trimming directly off the tips of the fins.

Might not make aesthetically attractive fin shapes, however?

Trimming both the fin tips and the trailing edges might make nicer shapes.