OpenRocket: How to ignore stability issues?

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Kelly

Usually remembers to get the pointy end up
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I'm simming a short/stubby rocket with lots of base drag. To get an idea of stability, I add a massless transition cone at the tail as recommended elsewhere, and I get a stable rocket. To sim the altitude, I remove the transition. However now OpenRocket is telling me that the rocket experiences "large angle of attack" and tumbles, so my altitude sim is garbage. (I don't seem to remember having had this problem before?)

Is there some way to do the sim such that OR will assume the rocket is stable throughout the flight, and give me an accurate flight sim?
 
Don’t do that; it will alter the sim.

Easiest approach here is simply to override the CG do you have about 1 caliber stability. You should then get a clean flight sim.
I thought the point was that it -was- altering the sim. Because the design is outside the envelope the simulator is designed for. So you add a hack to manipulate it.

I can image that hacking the Cg vs hacking the Cp could have different resulting dynamic stability for the same static stability - but haven’t tested it myself.
I thought the infinitely long launch rod was a clever hack.
 
I thought the point was that it -was- altering the sim. Because the design is outside the envelope the simulator is designed for. So you add a hack to manipulate it.
As I understand it, it is a hack to correct CP calculation only. It has additional affects on drag that you don’t want when simulating flight.

I can image that hacking the Cg vs hacking the Cp could have different resulting dynamic stability for the same static stability - but haven’t tested it myself.
I thought the infinitely long launch rod was a clever hack.
In my opinion, overriding CG slightly to achieve stability In the sim is a much smaller alteration of flight characteristics than, say, the infinitely long rod (although that is indeed clever).
 
As I understand it, it is a hack to correct CP calculation only. It has additional affects on drag that you don’t want when simulating flight.

And I thought the aft nosecone hack was to 'fix' an incorrect base drag calculation with very low L/D designs.

Caveat: I haven't looked at the code myself.
 
What exactly happened? I used that technique to help a friend simulate his TVC rocket and it's always worked for us.
I just tried it, and it worked for me. Still think it's kind of funny!
 
What exactly happened? I used that technique to help a friend simulate his TVC rocket and it's always worked for us.

I took a stable rocket that has a 565 ft apogee and changed the mass in the nose cone to 0 ounces. Ran the simulation and the rocket became unstable with an apogee of 128 feet. I changed the launch rod length to 7,200 inches long. Then re-ran the simulation and the rocket is still unstable and it's apogee dropped to 127 feet?

See attached .ork file
 

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And I thought the aft nosecone hack was to 'fix' an incorrect base drag calculation with very low L/D designs.

I've always assumed - based on zero knowledge of how the code works - that while the code can calculate the base drag for use in velocity/altitude calculations, it is not able to apply it to Cp determination, as this is a completely different calculation. I think that's plausible, but it's still just a guess.

I just ran a sim where I attached that transition backwards - the pointy end at the back - so now it's acting like a good boattail. The altitude simmed about 33% higher than "no transition". That tells me (I think?) that it is seeing the base drag when doing velocity/altitude calculations.

So my standard practice is to use the transition for Cp, take it off for normal sim. I don't have enough experience with these rockets to know how accurate this is.
 
Then re-ran the simulation and the rocket is still unstable and it's apogee dropped to 127 feet?
Hmm, it worked for me, on your file. I'm using OR 15.03, and changing the launch rod using the "edit simulation" button on the "RUn Simulations" tab.
It didn't work if I edited the launch rod using the 'preferences' tab, for some reason this doesn't change the simulations.
 
I've always assumed - based on zero knowledge of how the code works - that while the code can calculate the base drag for use in velocity/altitude calculations, it is not able to apply it to Cp determination, as this is a completely different calculation. I think that's plausible, but it's still just a guess.
That is my belief as well.
 
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