Negative stability at launch guide clearance in OR

I am playing with simulations on a Wildman Blackhawk38 with CTI J394. I am using Open Rocket, as I have read on this forum that it is the best for stability calcs.

I am not well-versed in 6 DOF rocket flight dynamics, so bear with me. I am wondering if this is a problem or not:

1. Static stability margin = 2.75 calibers. Check. All is good.

2. Flight simulation with 15 mph wind. At launch rod clearance, AOA = 14 deg and stability margin = -1.0 calibers as shown in this picture.



Stability margin recovers very quickly and there are no other anomalies in the flight data.

Does this brief, negative stability off the launch rod have any any real meaning to worry about, or is it just a numerical thing? I think Barrowman's stuff was only good to 4 deg AOA. I also forced the simulation time step to 0.001 s if resolution is an issue, and I got the same result.

Thanks

You probably do not have enough rod length to launch your rocket at sufficient velocity to be stable in a 15 mph wind.

To be stable in a cross wind, the rod velocity needs to be 20 mph or 4 x the cross wind velocity, whichever is greater. In a 15 mph crosswind, the minimum rod velocity should be 60 mph. Reference. https://www.nar.org/wp-content/uploads/2014/08/launchsafe.pdf

Try increasing the launch rod length and see what happens.

Bob

In case you didn't know:

Default setting is 36in on launch rod/rail.
Go to preferences to change it along with site temperature , winds & humidity to get a more accurate sim.

blackjack2564 said:

In case you didn't know:

Default setting is 36in on launch rod/rail.
Go to preferences to change it along with site temperature , winds & humidity to get a more accurate sim.

Click to expand...

Yep, did all that. My launch rod is 60 in.

bobkrech said:

You probably do not have enough rod length to launch your rocket at sufficient velocity to be stable in a 15 mph wind.

To be stable in a cross wind, the rod velocity needs to be 20 mph or 4 x the cross wind velocity, whichever is greater. In a 15 mph crosswind, the minimum rod velocity should be 60 mph. Reference. https://www.nar.org/wp-content/uploads/2014/08/launchsafe.pdf

Try increasing the launch rod length and see what happens.

Bob

Click to expand...

Rod velocity is 59 mph. Close.

It takes about 200" of launch rod to cross over to positive stability margin in this simulation. I highly doubt that.

So, what I am hearing is that I need an adequate stability margin throughout the entire flight. As I mentioned, there are no other "bad things" happening in this simulation that I can tell. So, should I believe the predicted negative stability in the first fractions of a second?

For those playing along at home, here is the ork file. Thanks for any advice.

BTW, the more I edit the simulation and run/plot, the slower and slower the execution time. Some kind of latency issue in OR on my system.

View attachment blackhawk38.rkt.ork

This rocket has a high L/D which means that the CP can move much more than you would think in response to non-zero angle of attack. Combine that with using a large heavy motor that moves the CG much too close to the static CP and instability is not a surprise.

It might be the case that the rotational moment of inertia is high enough that the rocket passes through this instability without rotating too much but the smart thing to do is to add nose weight or use a smaller motor.

Longer rail and a little nose weight.


Mark Koelsch
Sent from my iPhone using Rocketry Forum

1 Ounce of mass in the NC makes it stable.

Bob

Barrowman's goal was to make it unnecessary to have conservative stability, usually resulting in overstability and weathercocking. The plot you show is typical or actually better than average. It's not a "numerical thing" its a plot of what actually happens. A momentary instability is only problematic if it doesn't recover as the rocket gains speed. The ideal would be critical stability going into slight positive stability, until it gets a chance to move sideways with the wind as a unit, allowing low angles of attack as speed increases.

You could try other plot views, and try making it worse and better in the sim, see if you're close to the edge and if the oscillation in the angle can even be made better or seems inherent. Or just fly.

https://www.rocketryforum.com/showthread.php?119906-Multi-Stage-Rocket&p=1377037#post1377037

Check out the video in the above link which illustrates why you want to make sure you have a high enough stability margin and T/W ratio in a fresh breeze to insure that your rocket will not do this........

Bob

bill_s said:

Barrowman's goal was to make it unnecessary to have conservative stability, usually resulting in overstability and weathercocking. The plot you show is typical or actually better than average. It's not a "numerical thing" its a plot of what actually happens. A momentary instability is only problematic if it doesn't recover as the rocket gains speed. The ideal would be critical stability going into slight positive stability, until it gets a chance to move sideways with the wind as a unit, allowing low angles of attack as speed increases.

You could try other plot views, and try making it worse and better in the sim, see if you're close to the edge and if the oscillation in the angle can even be made better or seems inherent. Or just fly.

Click to expand...

bill_s: This is exactly the response I was looking for! Yes, I figured my plot was telling me the way it really is (I am a simulation guy), but more importantly, is it a problem in my design? I also thought the momentary instability was typical, given that everything else in my sim looks good and is as expected - speed, altitude, lateral motion, etc. Nothing else in the sim says this rocket will veer wildly off the rod.

Yes, I plotted other views and variables, adjusted parameters to make better and worse. Not much change. The angle oscillation looks inherent. What other variables should I look at in the simulation that may indicate this is a risky flight?