Hi All
There is going to be many factors that need to be considered in this question and most errors are not the sim but the numbers put in;
First rocksim has the ability to choose finish for each of the parts, most people seem to choose the polished or maybe gloss finish. I believe that is used to help determine Cd. Play with those parameters for the exterior surfaces and use the calculate Cd option to see if that gets you closer. Use the Cd analysis function (rocket tab/Cd Analysis ), to show you some of the complexity that RS considers.
Second, under the prepare for launch function and launch conditions. Make sure you have these values correct Altitude, humidity temp and pressure. Setting I use for the Rock Lake site in Alberta are 3500 feet, 20% humidity, 25.999" in Hg for pressure. Using the actual numbers for flight conditions will be best. These numbers are used to determine air density which is in the Cd formula, one of the reasons aircraft on long haul fly high, less air density therefore less drag. Note that pressure is station pressure not the adjusted sea level pressure used by weather folks. As you gain altitude air pressure is less but to keep things consistent for consumers of weather info they adjust the pressure numbers for comparison to other areas. STP (standard temp and pressure) formula also is interesting in that high humidity conditions, air density is actually less than desert conditions. Temp also plays a big role. So much more to explore here.
Third, is the motor actually performing as stated in the RS file? Remember that the motor burn characteristic is based on a few test motors on a test stand, maybe several years ago. has the manufacturing process changed? formula slightly adjusted? does the age of the motor change the thrust profile? With the Cd you calculated you can do some calculations with previous flight acell data from the flight computer to see if the motor file in RS is accurate or not.
Forth, wind arcing will reduce altitude. Arcing will have a significant loss of altitude. again if you have acell data that will tell you the probable straight up flight but it will be a barometric data that is correct and generally, ok always lower, than the acell altitude.
Fifth and one not really considered is rail drag, I can't tell you how much effect this has but it is there, flight observations is probably best to determine if this is big or not.
Sixth is the altimeter and its accuracy. just the barometric types can have issues due to vent hole placement and even accuracy when temperature changes. as mentioned before accell types can have their own issue too, low sample rates being the biggest error maker, and as mentioned arcing will show higher altitudes than reality
Hope this helps
David