Strange Rocksim Flight Profile Flight

[bcook7135]

So I have built a standard sized Estes Interceptor and have launched in actual launches with C6-3’s successfully. But I am only getting a little over 500’. My Interceptor weighs in at 4.36 oz. So it is on the heavy side.
But I wanted to get more altitude and was considering Aerotech/Quest Q-jet 18mm D16’s. But when I load my heavy weight and the C6-3 engine into Rocksim, the flight/launch simulation shows the rocket doing a 360 then straightening out for what it seems a safe flight that only reaches about 400 feet. I have done the simulation several times with the same result (more out of curiosity than anything).
Any clue as to what is happening? Should I even attempt a real flight?

 

[waltr]

A guess is the rocket is Un-stable, CP ahead of CG, during first part of flight until the fuel burns and reduces the motor weight then bring CG ahead of CP.

Check where the CG is with that motor compared to the CP. CG should be at least 1 cal ahead of CP.

If that is ok then check Speed off the launch rod. General rule is 12m/s or better.

 

[KevinT]

Can you send the rocksim file. I can take a look at it tonight.

 

[OverTheTop]

Do a swing test on the rocket with the motor and everything onboard. If it flies stable there then you should be safe to launch it.

As others have said, it might be initially unstable but then goes stable as the CG shifts forward. You don't want this to happen, even in a small rocket. The direction when it goes stable is completely random. I have seen one do this on a C and then come at the spectators and nearly nail somebody's foot to the ground.

 

[bcook7135]

Can you send the rocksim file. I can take a look at it tonight.

 

[BEC]

Odd..... I haven't weighed them, but a Q-Jet D16 should be VERY close in mass to an Estes C6.

I can't see Rocksim accounting for fin flex or something on what would be a MUCH faster boost. Hopefully @KevinT will see something in your file.

 

[waltr]

I ran your .rkt file in Open Rocket.
Weight no motor = 124gram
with C6-5 = 147gram
with D16-5 = 150gram

OR can not sim the 'pods' so just fins. Stability at 2.4Cal with D16 so looks ok.

Now interested in what someone finds with Rocksim...

 

[neil_w]

OR can not sim the 'pods' so just fins.

Use the 22.02 beta. A full sim of the Interceptor should now be possible. Hey @K'Tesh , you working on one?

 

[bcook7135]

Thanks for the feedback so far.

 

[H. Craig Miller]

OR can not sim the 'pods' so just fins.

Use the 22.02 beta. A full sim of the Interceptor should now be possible.

OpenRocket file with the pods for the .rkt interceptor design provided (22.02 beta 4).

 

[KevinT]

The design in Open Rocket looked good and ran fine with both motors.

In Rocksim it looks like it is calculating some type of oscillation/resonance that occurs at the higher velocity which is causing the rocket to go unstable. The only way I could find to correct this from occurring was to add a slight amount of weight to the rocket and move the CG closing to the CP (Margin of 1.12) - this seemed to have dampened out the oscillation. I used the estimated weight from the Estes website. Here are some screen shot of the results:

C6-3


D16-4 (you can see the oscillation kick-in


D16-4 with slight increase in weight and moving the CG towards the CP but still stable.


I would not have guessed this behavior with the slight change in the motor. And I am not sure what Rocksim is depicting is correct. If using the D16 motor will truly cause this behavior then someone with more aerodynamic knowledge will need to step in to help explain it.

Hope this helps.

 

[OverTheTop]

Are you getting a crosswind during ascent? That can make a marginally stable rocket go unstable. Usually stable with near-zero angle of attack but a wind shear can give a big AoA and trip the bird into instability.

 

[KevinT]

Are you getting a crosswind during ascent? That can make a marginally stable rocket go unstable. Usually stable with near-zero angle of attack but a wind shear can give a big AoA and trip the bird into instability.

Good point, I thought that too, so In the 3 sims that are posted, I basically set the wind condition 0. The rocket still wants to do loop-de-loops.

 

[waltr]

KevinT,
In the sims I see that the max acceleration (G's) for the D16 is 71, very high. But once you adjusted weight a little it drops to 15.9 which is reasonable compared to C6 of 12G. This seems to indicate something in Rocksim is going wacky.

OR sims calculate a bit lower max acceleration, 8.5G for C6 and 13.4G for the D16.

Craig,
Thanks for the OR file.When I open the ,rkt file OR 22.02 Beta4 removed the pods.

This design, even with the D16 looks good. I'd fly it.

 

[KevinT]

I totally agree - i think Rocksim is doing something wacky with this particular design.

The rocket looks totally flyable to me.

 

[bcook7135]

The design in Open Rocket looked good and ran fine with both motors.

In Rocksim it looks like it is calculating some type of oscillation/resonance that occurs at the higher velocity which is causing the rocket to go unstable. The only way I could find to correct this from occurring was to add a slight amount of weight to the rocket and move the CG closing to the CP (Margin of 1.12) - this seemed to have dampened out the oscillation. I used the estimated weight from the Estes website. Here are some screen shot of the results:

C6-3
View attachment 526474

D16-4 (you can see the oscillation kick-in
View attachment 526468

D16-4 with slight increase in weight and moving the CG towards the CP but still stable.
View attachment 526469

I would not have guessed this behavior with the slight change in the motor. And I am not sure what Rocksim is depicting is correct. If using the D16 motor will truly cause this behavior then someone with more aerodynamic knowledge will need to step in to help explain it.

Hope this helps.

Very informative. So the actual rocket is 4.36 oz. which is what I have in my file I hope. So it is heavier than the Estes spec.

 

[H. Craig Miller]

Thanks for the OR file.When I open the ,rkt file OR 22.02 Beta4 removed the pods.

You're quite welcome; it's interesting to see how best to handle conversions with pods since OpenRocket and Rocksim have fundamental differences in how pods are used.

This may be helpful for others as well. Rocksim allows the attachment of pods to all exterior components, except launch guides, while OpenRocket only allows the attachment of pods to body tubes. And, currently, neither OpenRocket nor Rocksim convert pods from the other format.

So, there are two options when converting designs containing pods from Rocksim to OpenRocket. You can probably guess the first. Open the Rocksim design in OpenRocket and recreate the pod elements. That is what was done here:


And, you can tell because the Rocksim created components are blue, and the OpenRocket re-created components are not.

The second option is to import the pods separately, meaning that, in complex designs, you create a Rocksim file that only contains the pod components (not in a pod), open that element in OpenRocket, and copy the components to an OpenRocket pod. Using this design as an example, here is how it's done.


Then, duplicate the OR pod and adjust the rotation.

Not the best, but until the Rocksim pod conversion is upgraded, it's two options that are available. The user will have to decide which method is easier on a design-by-design basis.

 

[msjohnso]

The design in Open Rocket looked good and ran fine with both motors.

In Rocksim it looks like it is calculating some type of oscillation/resonance that occurs at the higher velocity which is causing the rocket to go unstable. The only way I could find to correct this from occurring was to add a slight amount of weight to the rocket and move the CG closing to the CP (Margin of 1.12) - this seemed to have dampened out the oscillation. I used the estimated weight from the Estes website. Here are some screen shot of the results:

C6-3
View attachment 526474

D16-4 (you can see the oscillation kick-in
View attachment 526468

D16-4 with slight increase in weight and moving the CG towards the CP but still stable.
View attachment 526469

I would not have guessed this behavior with the slight change in the motor. And I am not sure what Rocksim is depicting is correct. If using the D16 motor will truly cause this behavior then someone with more aerodynamic knowledge will need to step in to help explain it.

Hope this helps.

This is probably an anomaly in the numeric calculations...all iterated/approximated solutions to complex equations have points where the calculations become inaccurate. Most of the time, it happens because there is a divide operation where the denominator becomes very close to zero for a single iteration, causing the quotient to 'blow up' at that point, and become chaotic. There isn't much you can do about it, although making tiny changes to some parameter in the inputs could very well cause the anomaly to change/move or even disappear.

Floating point division with very small values in the denominator is essentially chaotic. If OpenRocket or some other tool says it's OK, I wouldn't be too concerned. This is not really a software bug, it's an inherent limit in the way computers do math...if the programmer puts in a fudge-factor to clean up this specific calculating anomaly, it will likely just show up somewhere else, in somebody else's sim run!

[All this from the guy who got the 'C' in Numerical Analysis 40-some years ago...]

 

[KevinT]

Very informative. So the actual rocket is 4.36 oz. which is what I have in my file I hope. So it is heavier than the Estes spec.

I suggest weighing the completed rocket with everything less the motor and measure the CG. You can input these values in the Mass Overide Tab in rocksim. I do this for all my rockets to ensure things sim properly and very how close my estimated weight compare to actual weight.

 

[bcook7135]

Yes. I have now done that with CG. CG is at 16". Still flips.

 

[bcook7135]

Last Friday I sent my file to Tim Can Milligan and he did a Rocksim live demo. And he doesn’t know what causes the problem when the D16 is loaded in Rocksim. But he said, fly it. He thinks it will be ok.

 

[OverTheTop]

If it swing-tests ok then fly it!

 

[KevinT]

If it swing-tests ok then fly it!

Agree