I had always used SpaceCAD myself, and it came close to my L3 altitude. SpaceCAD predicted 8581, and RockSIM 8 predicted 8900 and change. My altitude, IMO, wasn't properly recorded by the blacksky electronics, which is why for the next N launch, our member Jim's new altimeter (MWC) will be used... anyway, blacksky said 8700 something, almost in the middle of both. You shouldn't have that much variance between them. Check the design in both software packages again (units, exact fin shape, length, etc.) to make sure they are as close as possible. You may have made a mistake along the way. Trust me it happens. Get the RockSIM package--well worth the price. There are many in this club who can use it within 1% accuracy. Let it rest for a night or two, and then go back. After that talk to your certification guy. He'll lead you in the right direction. You are in for one of the best experiences of your life. 😉

They definitely shouldn't be that far off.

I have used rocksim with great success many times, and trust it for reasonably accurate simulations. However, it is only as good as what you enter into it. Check and make sure everything is entered correctly, including altitude of the launch site and such. I have had a few weird sims where I discovered that the altitude of the launch site was way off (forgot a decimal or a zero usually). Also make sure that it is calculating the Cd at simulation time rather than using a pre entered value.

8) I might be way off, but a 30.lb rocket on an M1297 to 12,000+ feet does not seem right. A very nice motor, but only a baby M. Just a thought. 8)

It depends a lot on the rocket's diameter and drag...

19.5lbs isn't that much for an M (30lbs with motor and all), but as you say, it is only a baby M. On a typical rocket, I would probably expect 8-10k, but so much depends on the rocket's diameter and design. I have one 17 pounder that sims to 13k on an L and 15-16K on a smallish M, but it is also a relatively small diameter, low drag rocket with a high ballistic coefficient.

That M is or was an L and recerted as an M. I can tell you that you should be fairly close to about 8K with that weight of rocket and that particular motor.

yes Diameter can make a difference SOME... But not nearly as much as the weight will for Optimal mass. Even a rocket with a sleek line can be a hog and not get really good altitude. if its a heavy pig. But again it would be based at that point on Optimal mass for that specific motor. A heavy slim and aerodynamically designed rocket would do better with a much harder hitting motor with a moderate burn time to lower burn time then it would with longer burn. BUT I would then again question a 3 inch or 4 inch diameter rocket that would weigh 38lbs. Full throttle is 15ft tall and 6" diameter and all its mass weighs in at only 46 lbs fully loaded minus the motor. So even with size weight does not have to be a huge factor. for its size it is fairly light. I have seen rockets go much higher then expected with larger diameters. Dave Tarks Berford for instance flew at the June research launch on a N2400 red and it flew to 15K + change. Its bigger diameter then mine even but had less weight. the weight was and is normally more of a factor then drag. Gravity is a son of a gun. not saying aerodynamics arent a imposing factor Just that Optimal mass is even more so important and gravity will take more wind out of a flight then areodrag will normally.

It would be my belief that you have some data set wrong somewhere. Do you have a file you can share and let me look at it of the rocket and run sims on it to verify what the settings are? its probably something very simple.

Email me the file from rocksim of your rocket and lets look at it and see if its a setting..

my email constevens@cowisp.net

Conway

Well, considering that the frontal area goes up by the diameter squared, a 7.5" rocket will have more than triple the drag of a 4" rocket at the same speed, assuming similar coefficients of drag. You're right that this can be partially mitigated via optimum mass, but at that weight, the 1297 should be easily capable of pushing it to 12k and change if it's a 4". There are definitely a lot of factors though, and many that you wouldn't expect to make much of a difference really do.