RockSim gurus?

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Well-Known Member
TRF Supporter
Dec 13, 2011
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Georgetown, TX
Any RockSim gurus on the list? I've just installed 7.04 and wanted to try a quick and dirty sim to get started so I 'designed' a rocket to match my existing DynaCom Scorpion. I used four components:
1) Body tube
2) Nose cone
3) Fins
4) 'Mass object'

The mass object represents the missing weight for things like motor tube, altimeter bay, recovery hardware, etc. I calculated the weight of the mass object by subtracting the weight of the body tube, nose cone and fins from the actual weight of the Scorpion (without motor, of course). I placed the mass object inside the body tube so that the finished CG matches were the actual CG is on the Scorpion (again, without motor).

So far, so good. However, when I try to sim the rocket, I can't see any motors to select. When I select 'Prepare for Launch' under the 'Simulation' tab, there are no motors listed under the 'Engine Selection' tab, and selecting 'Load All' doesn't do anything.

Since this is my first sim with RockSim, I suspect that the problem has something to do with the initial configuration. I have reviewed the manual and help files, but can't find anything that addresses this problem.

Thanks in advance!

Tad Danley
TRA #4501
You need to specify the body tube as a motor Mount, (there is a check box on the right hand side under Finish) and RockSim will let you select motors for it. Also put in a recovery device of some sort so the simulation runs properly.

Bruce S. Levison, NAR #69055
Welcome to the forum :). Rocsim is a great program. Yeah, you need to set your motor mount size in the design window :).
Thank you Bruce and Doug! I missed the check box to set the body tube as the motor tube - Doh! I also added a chute as you suggested Bruce.

Here is what I got, based on the following rocket:
Diameter: 5.15"
Length: 105" (including 15.75" Ogive nose cone)
Fins - 3 'standard" trapezoidal:
root cord length: 12"
tip cord length: 5"
span: 6.75"
Weight: 25' w/o motor
Motor: AMW1850GG

Sim projections:
Max altitude: 10,450'
Max velocity: 1,106'/s
Max acceleration: 454'/s/s

Does this look like I'm in the ball park?

Thanks in advance for all the help!
I ran the simulation, my file is attached. I get about the same max velocity but I see only 3000 ft. AGL even with added nose weight to bring the model up to 25 pounds total weight.

Bruce S. Levison, NAR #69055

I may have given you some bad info, the fins are not 'typical" trapezoidal. Here is my sim, it has a weight of 25 lbs without motor. Could the difference be due to mine being closer to optimal mass?

This is my first attempt at attaching RockSim files - I 'think' I got it right!
You got it right!!! A+ on your first try. Now just add the rest of the details.

I tried loading the AMW engine files into RockSim and the program was apparently not doing the calculations correctly giving me the wrong altitudes (yet another software bug?). After I rebooted the computer I now get 10,000' flights AGL with AMW1850GG.

I looked at the RockSim file you posted, there are several minor issues. the PML G10 (0.125") is not setup properly in the materials data base in this version of the program. This was discussed in another thread just a few days ago; visit:

What is the purpose of the 16.8125 mass object located 75 inches from the tip of the nose cone? Was it only to bring the weight to 25 pounds for optimum height? You can use RockSim to calculate the optimum weight for this flight. I get an optimum cost weight of about 24.5 pounds, which I believe includes the motor. So you wouldn't any extra added weight if you are flying this rocket on an AMW1850GG. Typically these weights are included inside the nose cone to improve stability, since your design has a margin of about 3 (which is not bad on such a long skinny rocket) I would suggest just leaving things the way they are.

Bruce S. Levison, NAR #69055
Thanks for the additional information, Bruce. The purpose of the mass object is to account for the additional 'stuff' in the kit. I wanted to do a quick and dirty sim, with the length, diameter and fins - then use the mass object to account for the motor tube, centering rings, altimeter bay, coupler tube, recovery hardware, etc. When it is built, the all fiberglass rocket will weigh 22 - 25 lbs without motor.

Now that I know the sim results are in the ballpark, I will design the RockSim rocket so that it matches the kit exactly, with all of the internal components in the right places!
Rather than adding the mass object, you can use the "mass override" settings to fix the overall mass and cg of the rocket. This disables the RockSim calcs that sum up all the components.