OR Simulation Techniques

[davdue]

I have never done anything fancy with OR but I have an upscale Estes Swift (13X) that I want to build. I want to make the fins out of either bass wood with composite honeycomb in the middle with stringers to make them lightweight. Is there a way to simulate this or do I just override the mass after I have built one?

 

[Bat-mite]

The simplest way to get accurate sims is to override the CG and total mass of the entire rocket after it is built. But for checking things as you go along, then yes, just override the mass of the fins.

 

[Bat-mite]

I should add that the densities of materials are generalizations, and almost never accurately model any rocket I am building. Plus, it doesn't account for epoxy/glue mass, static ports, etc.

 

[davdue]

Thanks that is what I was thinking.

 

[Rex R]

if you use a mass override for the fins, keep in mind that the mass you enter will need to be multiplied by the number of fins(the override covers all the fins per group of fins).
Rex

 

[lcorinth]

I usually weigh all the parts before building and override all the components when I create an initial, "dry" sim (without glue).

Then, as I build, I weigh again after each step and override the mass of the rocket and the CG. So I can see how each step - glue, primer, sanding primer off, paint - affect the possible flight of the rocket. It also helps me monitor my building, and I'm sometimes surprised how much or how little weight each step adds. The idea is that this might help me keep from overbuilding too much. Also, I just find the information interesting.

Once I am finished, I have a sim that reflects the real mass and CG of the model, and I know how each step changed it.

 

[Nytrunner]

If I'm modeling a built rocket, I usually just have nosecone, BT, Fins, and motor tube with no internals and override the CG to reality.

If I build and take the time to weigh all the parts ahead of time, I'll keep that up throughout the whole build and at the very end I'll add a free-floating mass object "paint and glue" that I adjust to account for final weight and balance.

(and after you've flown and honed your sim to within 50', you'll get an extra peppy motor on the far end of the manufacturer impulse tolerance and be left scratching your head wondering where the extra 700' came from.....)

 

[Bat-mite]

(and after you've flown and honed your sim to within 50', you'll get an extra peppy motor on the far end of the manufacturer impulse tolerance and be left scratching your head wondering where the extra 700' came from.....)

Or the reverse. For most of my rockets , if I adjust things to get an accurate sim with a particular bunch of motors, it will be wrong for another bunch.

That is why altimeter deployment is optimal. Get that rocket apart at apogee every time without having to rely on delay timing. Sims are good to give you a feel for whether or not your rocket will stay under the waiver, but they can be way off for timing of delay charges in motor deployment. Helpful? Of course. Close enough? Usually. But all it takes is one time ....

 

[davdue]

Or the reverse. For most of my rockets , if I adjust things to get an accurate sim with a particular bunch of motors, it will be wrong for another bunch.

That is why altimeter deployment is optimal. Get that rocket apart at apogee every time without having to rely on delay timing. Sims are good to give you a feel for whether or not your rocket will stay under the waiver, but they can be way off for timing of delay charges in motor deployment. Helpful? Of course. Close enough? Usually. But all it takes is one time ....

Most likely this rocket and most of my high power rockets are altimeter deploy. As far as waiver that is not a problem. Kloudbusters have a 25K waiver with call ins to 50K so I don't plan on busting either one of those altitudes anytime soon. I am a see the rocket for most if not all of the flight type of guy. My highest flying rocket right now the one in my avatar. It is my L3 cert rocket and it goes about 8100' on an Loki M1650 motor. Someday I would like to break 10K but right now I am having fun with the rockets I have.

Thanks everyone for your suggestions on overriding CG. I really have never used that part of OR. As long as I have a good stability caliber number I am good then I will find the final actual CG and compare it to the OR CP and fly it and see how high it goes compared to OR. For the most part my rockets haven't gone as high as OR says but that is probably due to epoxy, paint and other things that aren't accounted for in the simulation.

 

[neil_w]

Thanks everyone for your suggestions on overriding CG. I really have never used that part of OR. As long as I have a good stability caliber number I am good then I will find the final actual CG and compare it to the OR CP and fly it and see how high it goes compared to OR. For the most part my rockets haven't gone as high as OR says but that is probably due to epoxy, paint and other things that aren't accounted for in the simulation.

You might find that the mass override for the whole rocket accounts for most of the difference. Try adjusting the OR models of one of your rockets and seeing how much closer the new altitude estimates match your actuals. I'd be really interested to hear the results.