RockSim or Barrowman: Which CP Do You Trust?

[Kruegon]

I just completed a RockSim file for a semi-scale 1/6 Patriot. The RockSim CP says 1.64 calibers and the Barrowman says 0.36 calibers. That's a pretty drastic difference. Which would you trust? Just for reference the cardboard cutout says 0.82 calibers. These are without the conduit covers for now.

 

[RadManCF]

Have you rechecked your Barrowman calcs? The 1.64 you quoted as coming from RockSim sounds much more reasonable, whereas your hand calcs sound like you screwed something up.

 

[REK]

I have trusted more the Barrowman. The reason for this is, because both the barrowman and the rocksim CP have different point of view's when it comes to the CP location. Rocksim predicts more while the Barrowman predicts less. I would use Barrowman's prediction, because when you make the rocket stable. Rocksim's CP predication is a guarantee for your rocket to be stable.

 

[Kruegon]

It's the internal RockSim calcs. It shows all 3 of them. All those numbers come from RockSim. It's just the calcs shown for each algorithm.

And the patriot is as close to scale as it can be gotten given nose cone selection. This means the fins are scale also. Not semi scale.

 

[rharshberger]

Scale Patriots need the nose weight OR larger fins, otherwise they aren't stable, I would probably go with the lowest number as the worst case and weight according to what brings the model to stable.

 

[Kruegon]

Ok. So back to the question. Which CP should I use? Because that answer is required for me to know which to adjust to.

 

[Kruegon]

So no one really knows which of the CP formulas is more accurate?

 

[aerostadt]

Like others I think that I would trust Barrowman, but I don't know of a definitive answer. Barrowman developed analytical equations to find the CP. You could add Apogee's opinion to your collection. They are good are responding to questions, although, sometimes very briefly.

 

[EXPjawa]

I'm under the impression that the original Barrowman formulas made a number of conservative assumptions, meaning that they might make something look worse than it is. I expect that the calculations that Rocksim does are a little more refined, and might be overall more accurate (generally speaking). I've also been led to believe that the Barrowman equations are best suited to fairly traditional shapes, and the more a design diverges from a standard configuration, the less accurate they'll be - or, perhaps, the more conservative they'll be. The Patriot is a pretty conventional shape, though, so it strikes me as odd that the calculations are that far divergent. I haven't come across that phenomena myself, most of the time the difference is minor. That said, comparing calibers of stability might be leading us astray here - what are the actual differences in predicted CP location, delta and as a % of the overall length?

 

[Charles_McG]

You've read this?
https://www.apogeerockets.com/education/downloads/Newsletter238.pdf

 

[Bat-mite]

Gary T. always uses a value somewhere between Barrowman and RockSim, for what it's worth.

 

[bobkrech]

1. The Patriot missile does not have a recovery system and it has a rather heavy warhead so the full scale fins are most likely too small of a "scale" model rocket which would not be expected to be very nose heavy, and additionally it is actively guided and designed to make high G maneuvers and hence has a low moment of inertia aka low stability margin.

2. Why is everyone so hung up on have a small "stability margin". It is absolutely ludicrous. The stability margin is a measure of how quickly your rocket reacts to an environmental change: like a cross-wind, turbulence or wind shear, or active control. The Patriot is a high G actively guided missile and doesn't need a high stability margin. The bottom line is that unless your rocket has active guidance, you want a large stability margin to keep the rocket trajectory from being effected by turbulence, wind shear or a cross-wind, or it will be thrown off course or become unstable. It is all about apparent wind and angle of attack. If you have enough initial thrust to get your rocket to a velocity > 4+ times the wind speed before it leave the rod and you have a stability margin of 2 or more, the angle of attack will be less than 15 degrees so your fins will not stall and you rocket will not go unstable.

Bob

 

[markkoelsch]

Bob, I am with you on the static margin. More, to a point, is better.

As to scale models- many are based on actively guided rockets/missiles. In most cases these will need nose weight. Just the way it is.

As to Barrowman. Barrowman made assumptions that we frequently break, especially in high power. First is velocity- as I recall a big assumption was subsonic velocity.

 

[Kruegon]

I'm already in bed, but I'llL post the .rkt file tomorrow. Maybe we can figure it out. The point for me is simoly to be as scale as possible. It's a challenge I enjoy. It takes time and calculations to get it right.