Rocksim versus real world flight predictions.

[Tominator 2]

I have been fiddling with rocksim lately and the flights are going well beyond Mach. I am wondering if there is some rule of thumb as to the altitude and speed that rocksim predicts versus real time data. I am talking about supersonic rockets. In the Mach 1-2.5 region. I do have Rasaero and I hear that is more accurate. But the simulations I have been doing are multi staged and Rasaero can only do single stagers.

Thanks,
Tom

 

[UPscaler]

rocksim doesn't do too well with supersonic flights, I highly recommend RASaero, it does very well in predicting speed and altitude for high performance flights.




Braden

 

[Tominator 2]

I already have Rasaero (that was said in the original post). And I was doing two stage designs and the last time I checked Rasaero can't do multistages.

 

[n5wd]

What seems to work well for me is to go ahead and do the sim in Rocksim, putting in actual weights and weather conditions. Then, after the flight, adjust the derived CD so that the sim matches the actual flight.

After that, most flights are very close to the sims as you adjust weight up and down, and adjusts for changing weather conditions.

We use this method for TARC to get the kids used to inputting real flight data into a simulator to be able to predict, accurately, the subsequent flights.

 

[hardinlw]

The biggest problem with Rocksim's predicted CD is that it assumes a polished finish. I have found that changing the surface finish to rough or unfinished comes pretty close to the CD derived from flight tests. Also note that the average CD varies with wind speed. When the rocket comes off the rod in a high wind condition, it is flying at a non-zero angle of attack and the CD is higher than if it were at zero angle of attack. Rocksim attempts to predict this behavior if you are letting it compute CD on the fly, but if you have CD override turned on, it just uses the value you input. That average value will be higher in windy conditions than in calm winds. If you are going to use a flight-calibrated CD, you need to tabulate that value as a function of wind speed. Using actual launch conditions, including wind speed and launch rod angle, is really important in getting accurate results.

 

[Tominator 2]

Thanks for the tips guys. I'll give it a try. I also heard that the conic cone is the same perfomance wise as a real Von karman at >mach 1