OpenRocket CP Outside of Rocket

[CoyoteNumber2]

Obviously the design isn't finished, but shouldn't the CP at least be on the rocket? This makes be nervous about trusting the final CP.

 

[CoyoteNumber2]

The problem seems to be with the boattail. If I modify the transition to have a larger aft diameter than fore, the CP moves back to on the rocket.

 

[rocket_troy]

From my understanding (from chatting to someone who has studied the OP code), there's a few assumptions utilised in OP with various things, so I'd imagine that some non standard designs and geometries might not quite simulate perfectly.

TP

 

[neil_w]

Until you have fins, don’t worry about it.

You will be having fins?

 

[CoyoteNumber2]

Until you have fins, don’t worry about it.

You will be having fins?

Just a few.

I've been reading more on the issue. Apparently this is Aerodynamics 101 - a boattail reduces base drag (duh) which alters the CP (naturally).

I had been thinking in over-simplified terms of CP being midpoint of the surface area...which I couldn't understand how that could be off the rocket.

This is why I dropped out of engineering.

 

[OzHybrid]

Swing test is your best validator for weird designs. The diameter of the swinging effectively reduces your angle of attack as it increases. So use as large a diameter swing as possible. And your rocket should be capable of taking the side load of the tether.
Until the design has fins don't worry.
Short stubby designs may need the base drag hack applied in the sim. But an actual swing test is a real world test of a real rocket. Of course you cannot swing test till it's built.

 

[caveduck]

All of our current sim programs rely on the Barrowman CP estimators, whose validity depends on the assumption that the vehicle is relatively long compared to its diameter. You would need CFD to accurately simulate a shape like this.

If anybody remembers the days of egglofters that had a similar shape, they were notoriously prone to instability and pitch-roll oscillations, so it's not far-fetched that the CP would be well forward of what you would expect.

 

[JoePfeiffer]

You have to consider both the location of the CP and the amount of restoring force being applied. That rocket has so little restoring force it's barely even accurate to say it *has* a CP. I'm a little surprised it actually estimated it forward of the rocket, but not surprised enough to worry about it.

I'd like to think of a good way to set some threshold on restoring force, and if it's under that threshold not display the CP on the figure and show it as N/A in the data summary. I just don't know how to pick a good one.

 

[caveduck]

Hey @JoePfeiffer , you could have a situation in a longer vehicle where the CP location was plausible but there was little or no restoring force just because it was coincident or forward of the CG. I'd suggest a warning if the CP lands in a patently implausible location, i.e. significantly too far forward. I'm not sure where you would draw the line though. Negative station wrt the nose tip is definitely out of bounds, but so are small positive values. The warning might need to incorporate the fineness ratio as well to attach more suspicion to short+wide designs.

Another useful metric might be the maximum AoA before the restoring force hits zero as the CP shifts forward. This value must be nonzero and at least several degrees. With some designs a 1-2 caliber static margin at zero AoA can disappear really quickly as the angle of attack increases.