Actual weight +CG vs Rocksim CG, please help

I'm working on a rocket, and am running into some major discrepancies, and I'm looking for advice.

I find that the actual weight of the rocket with NO ENGINE is 4.55oz, while Rocksim says it should be 4.26. Okay, the difference is likely to be the glue weight. However, the measured CG is 13.375" from the rocket nose, whereas Rocksim says it should be 11.55". I manually added a mass object to the rear of the rocket because the way the tube sizes are, likely most of the glue weight is there. This brought the CGs to 13.375 actual vs 12.52" in Rocksim. If I bump the weight up to .9oz from .45 then the numbers are close enough (13.375 actual vs 13.33 in Rocksim). But that is way heavier than the rocket actually weighs.

I used actual part weights and manual override instead of rocksim's database weights.

I'm inclined to go with actual weight and measured CG vs Rocksim, but I have to wonder why the big difference?

If I add the engine in and re-measure the actual CG, then it moves backwards, but Rocksim is still .75" off of actual measured CG. What gives?

I'm going to spin test the rocket today, but I suspect that I am going to have to increase the size of the rear fins to move the CP back to ensure the rocket is stable enough to fly.

What is the actual distance (in diameters of body tube) of the CG to the CP? As long as the actual CG far enough in front of the CP for stability, the Rocksim value for CG is not that important. You can override the CG in Rocksim to match your actual CD as well.


Tony

From the manual:

1. RockSim will, by default attempt to calculate the mass and center of gravity for the rocket. If you wish to override these calculations, you can do so by selecting the "Use the values shown below for all simulations" option. This can be useful when you want to run a simulation against a partial rocket design, or to help find optimal engine configurations.
   

You are worrying too much about the RS estimates, but are doing the right thing. Entering the actual measured weight and CG is the way to go. Adding weight to the nose is a way to adjust your CG to CP relationship also, rather then adding bigger fins. Do you have an RS file to share?

If you are doing manual override for each component, you must input both mass of the component and CG of the component. That is easy to do for body tubes, but measuring the CG of a nose cone or a complex fin set can get tricky.

Here is the Rocksim file... its got different body tube diameters.

I just did a swing test and it proved unstable, so I definitely need to change something. I'm going to cut some bigger fins and tape them on and see if it helps. I don't want to have to add nose weight unless I really have to, as that would impact altitute more than I would like, probably.

I'm using the Rocksim stability margin calculations, not the Barrowman.

Bill,

I'd suggest to not worry so much about the sim unless you're REALLY OCD (note that I'm really OCD). The program will get your CP pretty much dead-on. As for weight, get the actual weight correct via adding some weight here and there within the simulation program. I say "here and there" because you can add mass to individual components or do an "override" of mass for a program group (as shown in the Design tab), but you need to move that weight fore or aft in the program to match your actual CG as measured by you putting the rocket on an edge and measuring from the nosecone to the pivot point. That point, where the rocket balances, is your CG.

I've used this method on large rockets of 4" and 5" diameter for which I had no simulation file, but I had the actual rocket. I can input all the parameters I know: the weight of the total rocket, the length, the shape and location of the fins, shape of the nosecone, the width of the airframe, etc. Once all those are input, the program will nail your CP (center of pressure, where the aerodynamic forces moving perpendicular to the length of the rocket are equal from nose to tail). If you were to tie a string to the rocket at the CP, suspend it in the air, then blow air at it from the side, the rocket will basically stay perpendicular to the flow of air at the CP (air flow has to be consistent/equal along the length of the rocket). CG can be measured by laying the rocket on a dowel and finding where it balances (that's the CG). Move some weight fore and aft in the program, being sure you don't change the total actual weight of the rocket until it matches. You're now ready to run motor options.

In short, get your actual values to match your simulation values (and you're not far off on this rocket), and then run your flight simulations. You'll be fine...

Buckeye said:

If you are doing manual override for each component, you must input both mass of the component and CG of the component. That is easy to do for body tubes, but measuring the CG of a nose cone or a complex fin set can get tricky.

Click to expand...

Is Rocksim different than OR? I've never had to input a different value for CG when changing the total mass, and I've NEVER had an unstable rocket out of dozens of scratch builds...

I tried a couple of things to get it stable; I taped on larger fins (Lexan), which should have worked, no dice, still flying sideways.

I then added .60oz noseweight (3 quarters) to the front of the rocket and spin tested again. This moved the CG forward about 2/3rds of an inch, no real gain as it still flew sideways.

I switched over to using the more conservative Barrowman equations, and it says 1.42 stability margin. Yet when I swing test it, it flies sideways... Kinda driving me nuts. At this point, I trust the spin test more than RS.

Trust RS, lots of stable rockets fail the swing test, especially light weight ones. I did some sims with your design( pretty neat) seemed to fly just fine. I would add nose weight to get to 1 caliber stability with motor in and fly it. It looks good flying in the sims on .50 stability margin.

I thought of one possible issue; I didn't have any "string" around the house, so I used a long length of 1/8" round elastic. It occurred to me that maybe it is causing some twisting or somehow interfering with the rocket's movement? I am going to get some plain old cotton string and try again, trying to get the initial spin faster (maybe I was going too slowly?).

***edit, after some more searching, and reading Apogee's Newsletter #53 on the subject, I'm going to trust the software and not worry about the swing test, as I think I wasn't doing it correctly anyways (dangling the rocket on the string and trying to swing it hard and never did point correctly to begin with). I'll just bring a copy of my Rocksim file to the first launch and provide it when asked; they'll give everyone the heads up that its an untested design and keep your eyes on it and be ready to duck).

I try to get the weight close in sims but mostly concentrate on getting dimensions right so the CP should be real close. Than just load the rocket ready to fly; chute, shock cord, motor, everything. Than check the actual CG. If it is real close to 1 caliber from the marked CP it should be good to go. It would be real hard to get a swing test to match the actual air speed over the fins as a rocket leaves the rod.

likely your string was too short(making your angle of attack to high), common problem when the rockets get over 15" long.
Rex