Every year, I join the teams at Pegasus during their practices, and every year someone always complains that their rocket is not reaching the altitude predicted by the simulations, falling short by as much as a couple hundred feet. Such a huge mismatch must be due to some cause, and according to the team, it is usually a bad batch of motors or lots of drag on the rail, which everyone knows is not accounted for in the simulations. And nearly 100% of the time, the team is wrong - it ain't the motor, and it ain't the rail.
It's the sims - they put too much faith in the altitude predictions.
Open Rocket and Rocksim are fabulous tools, and they are absolutely the things to use in designing hobby rockets in this day and age. However, both programs make some simplifications and assumptions, with the net result that they almost always overestimate the altitude the rocket is capable of achieving on a specific motor. I can prove this with a specific example - my Geezer TARC entry for this year, the Eggsploder. Here is the altimeter profile of its flight from September of last year:
|Eggsploder PNUT altimeter data (Click to enlarge).|
|Open Rocket simulations for Eggsploder (Click to enlarge).|
So Open Rocket over predicts... What about the other guy on the block, Rocksim?
|Rocksim simulations for Eggsploder (Click to enlarge).|
Bottom line - If I choose a motor combination for Eggsploder that gives simulated altitude of 800 feet, I would be very unhappy with the performance, as the rocket would struggle to make it to 700 feet, well short of the goal. This is why, year after year, I always tell the teams to sim high - you can always add weight to bring the altitude lower, but it is very hard to raise the altitude without changing to a more powerful motor, which can mess with stability (bigger motor = more weight in back = less stable).
So I write this post, hoping that next year's teams will read it and pay attention. Maybe next year will be the year.
But I'm not going to get my hopes up...