RockSim Stability Calculations

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Piled Higher and Deeper
TRF Supporter
Jan 26, 2009
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thank u, next
So... for you RockSim gurus out there...

I've been thinking about designs like the one attached. It would be really cool to not have to chop up my nice LOC nose cone to add nose weight. It balances out using the RockSim numbers, but not using the Barrowman numbers. Then again, I don't want something this complex going unstable off the pad.

What thinkest thou educated ones about the stability prospects of this design when loaded with a full compliment of motors?
I think you are fine. My level 3 cert rocket had worse Barrowman numbers than this and flew fine. (Rocksim file hiding in the level 3 pages of my web site.)

Try plotting the static stability margin in Rocksim. Then you can check it when the rocket somes off the rod/rail which is when stability will be at its worst because of the high relative wind angle. Be sure to put in accurate values of the rod/rail length adjusted for the location of your lugs/buttons. Also set the wind speed to the maximum you expect to fly in. Use the Rocksim CP for this as the Barrowman equations assume low angles of attack.

If the margin drops below 0, you could be in trouble. Or maybe not. If the rocket has a high moment of inertia, then by the time its starts rotating, its speed might have increased enough that it has become stable again.
The design looks good to me! I put the main and drogue chutes as far down as they would go and used some heavier motor combinations (M core and J outboard) and there is still more than one caliber of stability using the RockSim equations. I trust the RockSim equations more than I do the Barrowman. I have flown designs with less than a one caliber stability margin using the RockSim equations that are unstable by Barrowman equations and they flew fine. For added security, you might want to find a way to mount the electronics and timer farther forward if possible.

I hope you like walking, in a 12 mph wind this design likes to land nearly a mile from the pad even with the drogue chute!

Bruce S. Levison, NAR #69055
Thanks for the votes of confidence, you guys! Eric (rocwizard) and I are hoping to get this project together for ROCStock this year. We launch on a dry lakebed, so walking isn't a problem :D
OK, here is my RockSim file for my L3 cert rocket. I would appreciate any comments on stability, etc. I plan to fly it at Wayside, TX in October on an AMW 75/6000 motor.

I made some updates and modifications to your design. First I made the tube coupler hollow instead of solid G10! I put in the electonics and associated hardware in as a mass object. I changed the parachutes so the rocket can be recovered on a reasonable range with dual deployment, I had to change the flight events tab on the flight simulation preparation to deploy the drogue at apogee and the main at 500ft AGL. I put in launch lug/rail buttons. I added a shock cord and one pound of nose weight. The 0.125" thick G10 rings and bulkheads seem kind of thin for this design could you double them up and use 0.25" thick? You could use larger parachutes I just used "off the shelf" B2 Skyangle stuff. The design looks good and should fly good with a static margin of around 3.

Bruce S. Levison, NAR #69055
THANKS Bruce! I was going nuts trying to figure out why the rocket ended up 15 pounds heavier than I estimated, with the CG too far forward! I thought I checked everything, thanks for finding the solid coupler.

I made the centering rings 1/4" and upgraded the chutes as you suggested. I also moved the mass for the AltAcc rearward to match where it will be mounted in the bay. I tried to size the rail buttons more accurately, is there a better way to do this?

RockSim is a great product, and this forum is fantastic - THANK YOU!
I looked at the RockSim file again everything seems resonable. I see a drift of almost 1800 ft downrange in a 5 mph wind, are you sure you want to open the main at 1200 feet AGL? I also guessed at the weight of the electronics and supporting hardware you should update this value if you know it. I noticed you are using mass overrides for the weights of the fins and motor mount tubes, is this because you measured their weights and entered them into the program? For the rail buttons do you know their weight, height and diameter? I can help you simulate the rail buttons if you post these values.

Bruce S. Levison, NAR #69055
Bruce, Thank you so much for all the help - you have made getting starting with RockSim very easy! I'll plug in the actual weight of the avionics and hardware as soon as I have the info, and will post the specs on the rail buttons as soon as I receive them.

I used the mass overrides to account for epoxy, fillets and reinforcements in the adapter and motor tubes and the fin can area. One of the reasons for deploying the main at 1200' (the AltAcc can deploy at 600', 1200 or 2400') is so that it may be easier to spot the rocket when it is on the main. The other reason is that it gives me slightly more time for full deployment in case there is an 'anomoly' with the main chute. The field is quite large and I will also be using a Rocket Hunter transmitter for tracking.