Rocksim Altitude Prediction Issues

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Well-Known Member
Jan 17, 2009
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After designing several rockets in version 7 the altitude predictions are sometimes skewed. But it doesn't happen every time. For instance I designed a 21lb rocket (attached) and simulated it on a M1315, total weight of 33lbs, and the predicted altitude is 4500'. But if I go to WRasp it sims to 12,000'. :confused:

I had the same issue on my Minnie-Magg when I added a 20" av bay. In this case WRasp proved to be right on.

Yet my level III cert design comes out within 300' between simulations.

Any thoughts or suggestions? Am I doing something wrong?

Thanks in advance.

View attachment Solar Flare.rkt

Your sim didn't include a saved flight simulation, so it's hard to compare. I had to change the motor mount setting; it was set to load 11mm motors, I changed it to 75mm. I also have my atmospherics set for my average summer launch. In any case, I am using ver, and a M1315W sims to 11970' for me.

I guess we need more info to diagnose the problem.

Hope that helps.

When I load that simulation, it has RockSim attempting to calculate the Cd. I suspect this may be the discrepancy. Try manually setting a Cd of 0.75 in both your simulation programs and see if you start getting more consistent results. Automatically calculating Cd seems to be a difficult thing...
I had to load the RS7 software on my computer, but I figured out why you are getting the discrepancy.

Click on the Simulation Prep tab.
Load the M1315 motor and specify ALL for the delay.
Click on the Flight Events tab.
Here is where the error is.
You have two parachutes defined in your design and both are set to deploy at engine ejection. Change this to Apogee for the drogue chute and altitude for the main. Then run your simulation again. I got 12032' AGL on your design.

BTW, nice upscale.
Great job Mark.

Without a flight sim, I always start with an "All" or very long delay for my first sim.

I guess you can tell I fly mostly LPR/MPR, and no electronics. :blush:
Works perfect. As you can tell I'm somewhat of a novice with Rocksim. But I'm always willing to learn.
I've been using RockSim since 2001 for everything from model to high power rockets. It is a very good tool for modeling the behavior of any rocket that can be modeled in RS. There have been lots of improvements over the years.

One of the nice things about using ALL in the delay parameter is that it allows you to see the optimal delay for the flight, but you must specify Apogee for the main (or drogue if dual deploy) to get the highest altitude. The ALL delay is not just for electronic flights, it is also valuable to know where to cut the delay grain for Pro29,38,& 54, motors.
Thanks Mark. That's good insight to cutting down delays. This is the great aspect of TRF. It allows those with experience and expertise to share their knowledge. I can't tell you how much I've learned over the years reading posts.