Rock Sim accuracy

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Well-Known Member
May 25, 2004
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I was wondering how accurate Rock Sim is when it comes to speed and altitude.
It says my 3" IRIS with a H125-10 will go 3273 Ft high with a max velocity of 578mph. How close is this to a real launch with the same weather conditions.

For RockSim accuracy it all depends on how accurate your measurements of the rocket and ambient conditions are! You can easily get to within 5%, or possibly better. I am reffering to altitude measurements I made with an Adept A1 altimeter; speed measurements should be just as accurate.
I have bracketed the RockSim predicted altitude of a low power model rocket with three or more flights in succession. The motors were selected to have the same weight as the engine file that RockSim uses to within +/- 0.05 grams. I used temperature, humidity and pressure measurements I made at the launch site; lattitude and elevation were obtained from the Terra server at:

Wind speed and variability I got from the Weather Channel
The rocket itself was measured three times in all dimmensions with a vernier calipers, and the average value of all three measurements was programmed into RockSim. The rockets weight was determined immediately before each launch and was adjusted to that used for the simulation to within +/- 0.05 gram (just added or removed a little wadding), and the CG was also kept constant to within 1mm.
I also have an angle finder built into my launch pad, and measure the distance from the bottom of the launch rod to the bottom of the lowest launch lug as the launch guide length.
I can get the landings to better than about 10%. I have actually given public demonstrations where I pushed the button and had the rocket either land back on the pad or in my waiting hands!
Of course everyone wants to know how I did that and I explain to them I spend a few hours programming all this flight data into RockSim to calculate the exact launch angle that would put the rocket right where I wanted it.

Bruce S. Levison, NAR #69055
Bruce is the one guy here at TRF (there may be other regs whom know this program as well as he does ), that I would trust completly on his knowledge and usage of Rocksim.

If I can ever afford to buy it (they'll probaly be up to version 12 by the time that happens), he'll be the first gent I contact with questions dealing with external balsa rods.

Love the detailed information Bruce, do you happen to have any of the data available for online side-by-side viewing?

What the two guys above me said, but where did you get the file for the 3" Iris from? It sounds a bit optimistic.
Well I got 18.879 ounces without motor from that file. That sounds very optimistic for a 3" rocket. With the H125-10 it says 26.836 ounces and (of course) the exact same altitude and speed you got. I don't believe it (becouse of a low weight in the file) and would expect the Yank 3" Iris to go about 1200' - 1500' on an H125, but thats just an edjucated guess.
Well the kit weighs 40oz and im sure after Im done with it it will weigh more with the fin can being filleted inside and foamed. Ill redo the sim with a better weight and see what comes out.