Open Rocket Sim error

[TomSmith58]

Folks
I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
I am using this model
View attachment estes_interceptor_e (2).rkt
and I get these results for the G135W-14 example

It shows good results for motors G80 and below.

What gives?

Thanks
Tom

 

[OverTheTop]

Looks like it is going unstable. The G135 motor is quoted as having a length of 20.6cm in thrustcurve, and the G79 a length of 15cm. The longer motor will shift the CG forward a little, perhaps pushing it into unstable territory? I suspect the design is marginally stable according to the sim and only starts skywriting when the AoA gets too far out of whack (any oscillations building up in the sims?), then the fins lose lift and everything goes wild.

Usually things starting stable and going unstable during burn are hybrids. Solids usually start off unstable and then snap into stability at a random direction

Anybody else have any good ideas on this one? Marginal stability is the only thing that comes to mind for me currently.

 

[dhbarr]

I think plotting stability-vs-time will reveal an increasing oscillation off the rod tip. On phone, can't peek.

 

[Cabernut]

Folks
I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
I am using this model
View attachment 331728
and I get these results for the G135W-14 example
View attachment 331730
It shows good results for motors G80 and below.

What gives?

Thanks
Tom

Double check the weight and CG override for the entire model, that they match yours. I'm getting negative stability on everything.

Could also be that the overly complex fin shapes are throwing off the CP calculation for those fins(wings), giving a CP result that is too far forward, resulting in negative stability for the sim.

As a workaround, since the G79 is a known stable motor, add an extra generic fin set to the back. Large enough to pull the CP back enough so that stability is comfortably positive. Then load up the G135/H135 and see if that works for the sim. Should give you a ballpark figure.

Anyone who has it, what does RockSim give for the CP?

 

[K'Tesh]

I'm no help on this one Tom... I can create the appearances of rockets, but when it comes to something as complicated as the Interceptor, the OR version I use doesn't give me anything on flight performance. Sorry

 

[Bat-mite]

How do you have the wind speed/direction set for the sim? Launch rod/rail length?

 

[rstaff3]

Folks
I have this dandy Interceptor-E modified to 29mm. I have flown it once on an Aerotech G79R. I was wanting to sim it with motors bigger than a G80. I tried an H115 and an H135 and got screwy results.
I am using this model
View attachment 331728
and I get these results for the G135W-14 example
View attachment 331730
It shows good results for motors G80 and below.

What gives?

Thanks
Tom

I loaded your file into RockSim and I do not get those crazy results. Are you using RockSim? The graph format isn't the same as mine. Anyway, the margin is only at .22 so maybe it is statically unstable on your system?

 

[Cabernut]

I loaded your file into RockSim and I do not get those crazy results. Are you using RockSim? The graph format isn't the same as mine. Anyway, the margin is only at .22 so maybe it is statically unstable on your system?

So if the margin in RS is .22 with the CG overridden to the same location, then that would only leave a difference in calculated CP between OR and RS.

This is one area that RockSim shines. Simming complicated models.

 

[TomSmith58]

Ok, who has an Interceptor E Model for Rocksim? Last I looked I didn’t find one.