Open Rocket: Problem with altitude calculations.

So I have run into this with OR many times. I build a rocket, put the motor in, have good stability, but the predicted altitude is WAY lower than I expect. But if I add just a little weight, the predicted altitude jumps up to there I would expect. Please see the two screen shots below. Everything is the same in both except in the first, the nose weight is 19 grams and in the second, the nose weight is 20 grams. Obviously, 1 gram won't make a difference of 5,314' (over 1 mile). Have others noticed this before? I am using version 15.03.

I assume you are looking at the apogee in the lower left? This is a quick value and may not be refreshed. Go to Flight Simulations tab and properly re-run the simulations there.

Please attach ork file.

Buckeye said:

I assume you are looking at the apogee in the lower left? This is a quick value and may not be refreshed. Go to Flight Simulations tab and properly re-run the simulations there.

Please attach ork file.

Click to expand...

I did that. And I still get a large difference with just 1 gram weight difference. Here is the file. But keep in mind, this is just one example. I have noticed this on many other files of rockets that are very different.

View attachment OR Anomoly example.ork

CZ Brat said:

I did that. And I still get a large difference with just 1 gram weight difference. Here is the file. But keep in mind, this is just one example. I have noticed this on many other files of rockets that are very different.

View attachment 278737

Click to expand...

Do a flight simulation, and plot stability vs. time. I suspect at higher mach your CP is shifting forward causing the rocket to go unstable right around burnout, giving you the low altitude in the first picture. Adding one gram of nose-weight seems to be just enough for your model to keep it just barely stable at mach, allowing it to the coast to its full potential apogee.

Yep, something is going on with stability with the G motor. Looks like you are right on the edge.

I turned off wind and repeatedly ran the simulation over and over again. There is still something causing variation in stability and the result. Not sure how that is possible. Must be some random number is still applied somewhere in the sim. 9 out of 10 sims the flight is unstable and ~1700 feet. Attached is the 1 of 10 sims where it is barely stable and ~6000 ft altitude.

I turned your mass object to 20g. Now 9 of 10 sims are more stable and ~5000 feet. However, 1 of every 10 sims is still unstable.

So, why is the simulation showing variation with no (known to me) external influences? If there is no change in launch conditions, then the result should identical every time, right?

View attachment OR Anomoly example.1.ork

The very low altitude is when it's going unstable during the flight.

Edit: I thought the differences from run to run were due to some randomness applied to the wind strength in the model but you said you turned wind off so not sure what else is going on.