OpenRocket: Please review file to see if I did things correctly

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Sandy H.

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I made an ork file of a LOC Lil 'Diter that we've flown a bunch of times. After the shakedown on a G or small H, it has gone up on an I435 at least 5 times and the only damage has been on landing.

My friend was wanting to see what it would do on a J, so I made a simulation to see if we were getting into danger territory as far as speed goes. The sim says that it goes Mach on the I435 and we didn't do anything special when building it (i.e. no fiberglass, just 5-min epoxy and decent size fin fillets).

I not that the file is complaining about 'discontinuity in body tube diameter' so I'm wondering if I need to model it differently to get a more accurate result.

Any help is appreciated.

Sandy.
 

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H. Craig Miller

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The "discontinuity in body tube diameter" is fixed; the problem was that the nosecone base diameter was set at 2.3", not 2.26". You may also want to check your delay charges to ensure that you are modeling the actual delay charge being used; if you are drilling the delay charge you should input the new shorter delay so that the ejection at high speed warning is not triggered.
 

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Sandy H.

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Thanks for the fast solution!

I believe we were drilling the delay down about a second, so that follows the simulation. Again, I surprised that the rocket is simming at mach+.

Is your gut feeling that it really is going mach or is the simulation likely optimistic and it is slower. The one time we flew an altimeter on it, something went wrong and we didn't get data.

Thanks again for the help!

Sandy.
 

H. Craig Miller

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The Mach numbers for your design and motors are substantially the same in both OpenRocket version 15.03 and the new release in development. Acceleration can be significantly affected by building methods, especially paint finishing techniques. The best way to verify the altitude projections is to use actual flight data to adjust the drag coefficient to match actual altitudes for various motors.
 

Sandy H.

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Understood and logical as well. Good combination!

I think the altimeter issue was that they were Jolly Logic units that had not been chanrged for years so the battery life was junk. I think the next time we fly the rocket it will be with a conventional 9V powered altimeter. We could re-fly the I435 (which we enjoy and the crowd seems to like at times as it screams off the pad) and back out the flight data like I used to do when I flew the Raven.

I think we have a plan moving forward. Not sure if the end result will be beefing up the fin-can a little for mach+ flights or to build a new rocket for the J motors. Either way is a good time.

Thanks again for the help!

Sandy.
 

boatgeek

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I don’t really know the kit, but I have a little experience in this realm. A good friend flew a rocket based on a 54mm LOC heavy wall tube on a J well past Mach. It had 1/8” plywood fins and sturdy though not beautiful fillets. Everything recovered just fine. I’ve flown a MD 38mm that summed past Mach with similar fins. It’s definitely doable though a little more reinforcement may make you feel better.
 

GlueckAuf

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Your launch lug is embedded inside a fin. Use the Radial Position function to move it between fins as shown below.

1623397989054.png


I suggest halfway as shown, but with a rod you can affix it right up against a fin...just not inside it.

Good skies,

GlueckAuf
 

Sandy H.

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I don’t really know the kit, but I have a little experience in this realm. A good friend flew a rocket based on a 54mm LOC heavy wall tube on a J well past Mach. It had 1/8” plywood fins and sturdy though not beautiful fillets. Everything recovered just fine. I’ve flown a MD 38mm that summed past Mach with similar fins. It’s definitely doable though a little more reinforcement may make you feel better.
Good information. Hopefully next time we fly it, we'll get altimeter data to try to verify the real events vs the simulation and iterate to improve. But, having real world examples like yours help to remove uncertainty for sure.

Your launch lug is embedded inside a fin. Use the Radial Position function to move it between fins as shown below.

[snip]
Thanks for pointing that out. I did update the model and the simulation results did not change. In reality, the rocket has rail buttons, not a launch lug, but I agree that making the model accurate vs a dumb error like putting the launch lug in the fin makes sense. I'll make sure to pay attention to details like that in the future.

I apprecaite the continued help. Hopefully I'll be able to update the thread in a month with measured values. This month's launch was cancelled.

Sandy.
 
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