If by "dry" sim you mean a rocket without a motor, your comment is incorrect. Many folk design such that the simulation, with the motor, has a stability of +1.
A simulation, without a motor, tells you nothing about stability of the rocket.
My comment is not incorrect when taken in full context of the post, sorry. I stated that after you did the dry sim for an initial estimate you added nose weight if needed (or you lengthen the design or you make the fins bigger, they all affect stability) and
"then you sim with motors to see how the caliber changes". That was in the second sentence. Duh! If you read the post from beginning to end, it was actually attempting to add a bit of humor to the conversation. Alas, it failed miserably on you.
Everyone
needs to design for a caliber of 1+ with a motor installed, unless, of course, it's fat and stubby (.75 is usually sufficient) or has tube fins, or finless rockets using Gas Stabilized Induction for which you will never get a decent sim (because the CG will always be behind the CP but will fly just fine if you understand the physics and built it right). Sims with motors don't always tell the full story; sims don't cover all the bases (unless you only fly 3FNC - boring!).
My method is what you do during a build to get an early read on CG/CP. If you aren't using standard parts in the DB (and a LOT are
not in the parts DB), you weigh each part and update the model in the software. What attachment points are you using for your recovery harness on the top centering ring? A single welded eyebolt? Two u-bolts for a Y harness? No two builds are the same. Many things can change those CG/CP locations depending on how the individual does the build (and seemingly small adjustments in the design can have big affects on stability), especially if it is a scratch build or a kit bash (and I kit bash virtually everything). For example, I cut off the butt end of any plastic nose cone on models 2.6" or greater and custom build a removeable bulkhead design, so I can add a tracker
and/or weight in the finished model. Essentially, all of my bigger birds are custom.
Some folks try to build light and strong for altitude, some overbuild for survivability. I'll guarantee if you use a Rocksim or OpenRocket file from someone else's build and you make it with the exact same parts, it is not going to weigh the exact same nor have the CG/CP in the same locations. One build could fly fine on a big motor and be unstable with the same motor in a seemingly identical bird. For example, there are a zillion ways to fillet a fin can and how you do it will change the weight (e.g., injection for internal fin can fillets vs. keeping the rear CR off until the very end). Which epoxy you use and how much can make big differences. I tend to fly with a wide range of motors in any given model, starting low and slow to see how it performs and ramping up if it passes muster. So, I know I need a dry sim caliber greater than 2, maybe even 3, from the get-go. That comes from experience. I weigh, update, evaluate as I go. I'm an engineer by training. My build method comes with the territory.
Finally, when I'm done with the build, I weigh the finished (
DRY) model, find the real (
DRY) CG, do a (
DRY) swing test if I need reassurance of the CP and put overrides into Rocksim. THEN, I rerun all of the motored sims.
