Yes. Your assumptions are correct. The trick is to put the cone in for CP calculations and then remove it for altitude/delay prediction.But now a question - has anyone had success matching both stability and predicted altitude / coast time with the phantom cone? Interested in some empirical results. My suspicion is that the phantom cone is appropriate for getting the CP right, but might end up double-counting the base drag and making the sim results less accurate. Certainly important for selecting delay lengths.
.X2Search "base drag for stubby rockets"......been conversations about the stability of short fat rockets
It's a fine line. I've been playing with OR on a sim for my MadCow Cowabunga, amongst others. The Cowabunga is the only HP stubby that I've done, though (along with a metric butt ton of the usual LP suspects). I've tweaked the Cowabunga sim close enough after 3 flights that I'm at the point now where the last 4 flights have all achieved altitude within 25 feet and timing on ejection to within 1 second of optimal as indicated by my JL ALT2.Banzai88, thanks for the reply - have you had some success matching the final altitudes for stubbies without the phantom cone? The real trick would be if the sim was unstable without the phantom cone - then you wouldn't be able to sim it at all without it. This doesn't seem to be the case for most short stubby kits though, they at least have some notional stability without the phantom cone.
I was not responding to your post but the OP's instead.mpitfield, I'm not sure what you're implying. I've read both the previous thread and the Apogee newsletter. My question regarding simming for altitude was not, at a first read, included in either location. And in fact my whole point was that the Apogee newsletter is 12 years and several RockSim / OR releases old at this point, so I wanted to know if it was still relevant. This was the most recently updated thread on the subject and therefore seemed like the most reasonable place to ask.
Just to stir the pot in a different direction, I'll throw out my usual vote for "Don't measure Calibers, measure percentages!"@Binder - that's a good suggestion too. It's important to remember that "one caliber stability" is really just shorthand for "sufficient moment arm for the net aerodynamic restoring force to act on, plus margin for uncertainties in the calculation", and that depends more on the actual distance from CG to CP and the size of the fins more so than the body diameter itself.