Elliptical nose cones in OpenRocket

[SolarYellow]

Many sources I've read, including the technical documentation of OpenRocket, indicate that at speeds below 0.8 Mach, there is essentially no pressure drag for any nose cone with a tangent merge to the body tube. The specific formula used by OR says this: drag is proportional to the square of the angle of intersection, so zero angle means zero drag. The least drag should be something like a hemisphere because it has the least area, especially on a rocket that's staying below 500 mph.

However, when I select any elliptical nose cone, the OR calculated apogee tanks compared to a similar fineness-ratio parabolic series nose cone.

Anybody know what's going on with that?

Am I going to have to do a series of launches with a good altimeter and different 3D printed cones?

 

[waltr]

Have you gone to "Tools/Component analysis" and examined the "Drag characteristics" of the various nose cone shapes?

 

[SolarYellow]

Had not done that before. Interestingly, when I switch from parabolic to ellipsoid, the friction Cd increases (logical, as the elliptical cone has more surface area for a given fineness ratio. However, there is also an equal pressure Cd, which the technical documentation suggests should be zero or very close to it.

 

[SiboVG]

I do not see this effect in the latest OpenRocket beta. Did you test this behavior in OpenRocket 15.03 @SolarYellow?

 

[SolarYellow]

I'm running 15.03.

 

[SiboVG]

I'm running 15.03.

That explains It should be fixed in the beta, but just to be sure you could do some extra testing in the OR beta. If you still find an anomaly there, we'll take a deeper look at it.

 

[SolarYellow]

Tried the Beta. Overall apogee and velocity calcs changed a bit (increasing). The difference was most notable with a C6-7 as compared to the B6-6 and A8-5, so maybe it's a tweak to the engine model parameters?

On the nose cone issue, I'm using the model for a BNC-20B, but shortened to 2.54 cm (1 inch) long. It comes up in the parabolic series automatically. Drag characteristics show pressure Cd of 0.00 and friction Cd of 0.02.

When I select the series to "Ellipsoid," it shows an ellipsoid nose cone on the model. Drag characteristics show pressure Cd of 0.01 and friction Cd of 0.02.

Sim shows max velocity as 174 m/s.

It's not as big a difference as it was in 15.03, but it's still a difference. It contradicts the findings here https://www.apogeerockets.com/education/downloads/Newsletter346.pdf and in a few other places, though it's hard to know how many of those places are just echoes of the same information being repeated.

I have a friend with a shiny new 3D printer who's looking for excuses to click "Print," so I'm going to have him make me a shortened BNC-20B from JackHydrazine's Thingiverse files.

 

[SiboVG]

@JoePfeiffer may have a better opinion on this (although he's quite busy atm).

 

[JoePfeiffer]

At what velocity are you calculating the pressure drag? Cd is not constant, and the pressure drag gets more significant at higher velocities.
Using the component analysis tool on an ellipsoidal nose cone all by itself, I see a pressure drag of 0 up until Mach 0.39, at which point the value shown is still 0, but 1% of the total drag. At Mach 1.2, pressure Cd is .11
The calculation is described in the tech doc in Appendix B.3.
Tracking down references, the tech doc cites NASA tech report R-100; the original source of the data is a NACA research memorandum, RM A52H28.