OpenRocket Decorative Engine Nozzles

I am designing a scale Kerbal-X in OpenRocket as a potential next project. I will give it a 3D-printed decorative rocket engine on the back of the central tube. However, attempting to simulate the aerodynamics of this engine, by using a pair of transitions, reduces the stability by more than a half caliber. Is this accurate? If not, what could be causing OpenRocket to do this? I have included photos of my simulation with and without the engine bell for reference.

1) You are using an unreleased and very buggy alpha build of the next OR update. Tread with caution.

2) Tail cones are known to move CP forward. So your first transition would have that effect. The final tail cone in back... not sure, but I wouldn't be surprised if the two transitions together *do* have the effect of moving CP forward. I suggest dropping the two side pods and trying the same experiment in the official 15.03 version of OR and see if you get the same effect from those trailing transitions.

3) When the trailing transitions are in there, it is not clear how the motors can still be mounted where they are. If they (or one of them) gets moved back into the nozzle then that'll move the CG back somewhat, making your stability problem even worse.

neil_w said:

1) You are using an unreleased and very buggy alpha build of the next OR update. Tread with caution.

2) Tail cones are known to move CP forward. So your first transition would have that effect. The final tail cone in back... not sure, but I wouldn't be surprised if the two transitions together *do* have the effect of moving CP forward. I suggest dropping the two side pods and trying the same experiment in the official 15.03 version of OR and see if you get the same effect from those trailing transitions.

3) When the trailing transitions are in there, it is not clear how the motors can still be mounted where they are. If they (or one of them) gets moved back into the nozzle then that'll move the CG back somewhat, making your stability problem even worse.

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1) I'm using the buggy alpha build because it is the only way, besides spending the money for Rocksim, for me to simulate the radial boosters.
2) Thanks for the tip about tail cones, I will test it in the 15.03 build to see what happens.
3) The motors are only there because trying to mount them in the radial boosters was doing weird stuff (buggy alpha version!) so I put them in phantom mounts inside the main tube. In reality they will be inside the radial tubes.
Thanks for the tips, I will try them out.

PedroTheRocketNerd said:

3) The motors are only there because trying to mount them in the radial boosters was doing weird stuff (buggy alpha version!) so I put them in phantom mounts inside the main tube. In reality they will be inside the radial tubes.

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Did you try using pods instead of the boosters? It is my impression that the booster object in the OR is for boosters that drop off; in your case the boosters are presumably fixed, and should work fin in pods.

Also: what "weird stuff"? I'd like to test it out.

Finally: having motors *only* in the pods but not in the center is very iffy from a clustering standpoint, if one or more motors fail to light.

neil_w said:

Did you try using pods instead of the boosters? It is my impression that the booster object in the OR is for boosters that drop off; in your case the boosters are presumably fixed, and should work fin in pods.

Also: what "weird stuff"? I'd like to test it out.

Finally: having motors *only* in the pods but not in the center is very iffy from a clustering standpoint, if one or more motors fail to light.

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Thanks for all your ideas. Just now tried using pods rather than boosters. No change, either for better or for worse.

Frankly, I can't remember what the "weird stuff" I was talking about was, so it most likely was just me being dumb and/or misreading the situation.

And finally, as far as having motors only in the pods, I realize that is iffy, but I want to keep the scale look through the whole flight, so I decided on putting motors in the radial boosters rather than removing the main decorative engine for flight and flying on a single 29mm motor. I figure if I use good igniters and test them thoroughly, as well as use my 12V launch controller (2 6V lantern batteries in series), I should be able to get reliable cluster ignition. I have converted an empty 3D printer filament spool into a 2x24mm cluster rocket, and I plan to fly it a couple times for clustering practice before I build this design.

Also, as far as combating the rocket's instability goes, I know of two options: add roughly 10 oz of nose weight, or add clear fins for stabilization.
Nose weight pros: lower flights for smaller fields, doesn't change appearance of rocket
Nose weight cons: 10 oz is *LOTS* of nose weight, and the amount needed only goes up the more motors are used (possibly use an adjustable nose weight system?)
Clear fins pros: less mass = higher flights; unlike nose weight, no need to adjust for different motor configurations
Clear fins cons: Could potentially detract from the looks of the rocket; from what I have heard the commonly-used clear fin plastic is brittle
At this point I am leaning towards nose weight - probably just lead shot epoxied in the nose cone, as well as a piece of threaded rod so the amount of weight is adjustable. If anyone can see any potential problems with this, feel free to correct me or suggest another solution.