OpenRocket simulation altitude did not match up to actual launch on a J570 twice

[andrewlathrop87]

I launched a Blue Tube ARR Blue Phenix 3inch on a Aerotech J570 on two different occasions. The first launch was to an altitude to 4,200 feet and the second launch went to 3,800 feet. The simulated altitude using Open Rocket said it would go 5,970 ft. The rocket weighs in at 84.6 oz. I have now idea why the altitudes did not match up. Both launches were during similar weather conditions. Is my Perfectflight altimeter not working properly or did I just listen to the beeps wrong?

[sylvie369]

In my experience sims typically run high. There are a whole bunch of factors affecting altitude that are probably not accounted for in the sim, notably variable winds aloft, and a non-vertical trajectory (which is a BIG factor).

I'd guess that your altimeter is working fine, you heard the beeps right, and your sim was just wildly optimistic.

[stickershock23]

I agree with the above. your sim is run in a perfect world. and we all know the world is not perfect.

Sims are best for proving facts,, like stability CG etc. and thats only if you input the info exactly correct.

Basically sims are a good guideline, use them as that and be happy with it.

[cjl]

I'm surprised it was that far off - I've very rarely used OpenRocket, but I've found rocksim to be surprisingly close most of the time (the exception being flights that spend a significant time supersonic).

That having been said, I'd trust an altimeter over sims every time.

[mikec]

The simulated altitude using Open Rocket said it would go 5,970 ft.

Did you override the weight in OpenRocket to match what your rocket weighed? What surface finish did you specify?

The Blue Phenix has an odd fin shape. Maybe OpenRocket was confused by that. I've flown a 3" G3 (quite similar in weight and diameter) on various J and K motors and OR has always been within 20% or so of the actual.

[berlinetta]

I have not tried the OpenRocket Sim yet. However, I use wRASP and RockSim a lot, and wRASP is usually about 300' off.