Obvious but "non-existent" nose cone concept

[SolarYellow]

I've done a ton of reading about nose cone aerodynamics. A spherically-blunted conical nose cone is one of the standard designs that's talked about in many, many technical reviews. But the sharp angled transition to the cylinder of the airframe is troubling. That never seems to be addressed, except by going to a parabolic or ogive nose cone with a tangent at the rear. I've sort of been mulling over an alternative construction, and got detailed about it today. The idea is simple.

Instead of just a spherically blunted cone, the cone is two spheres connected by a conical section that is tangent to both spheres. The larger sphere is the diameter of the base of the cone where it meets the airframe. The smaller sphere can be defined however one wants, but a logical parametric model would make it a percentage of the larger sphere's diameter. The other parameter needed would be either the distance between the sphere centers, which would additionally define an overall fineness ratio, or the overall fineness ratio of the cone, which would then define the distance between the two spheres. The cross section is simply two circles with two tangent lines connecting them. Rotate that about the line connecting the centers of the circles to define the nose cone.

This seems like such an obvious way of giving the simple blunted cone shape a tangent union with the airframe, it blows me away that I haven't seen it in any literature or represented in any nose cone design. Is there something I don't know about aerodynamics that would make it a bad idea? I can't see how it would be worse than the simpler blunted conical shape that is widespread, and I don't know that it would be a lot worse than something like a blunted ogive.

 

[teepot]

I have made several rockets with styrofoam balls as nose cones. I have use half of a fillable ornament to make a domed nose cone. They fly well. The balls are 6" and 8" in diameter. Lots of drag. Need big fins too.

 

[cls]

What flight regime is this for? Mach % * time? If it's to optimize CD and drag from turbulence at the nose to airframe joint.

 

[SolarYellow]

I haven't really looked at paying for motors that will take anything I'd want to build past about Mach 1.3, so use ~M1.4 as an upper bound.

There's plenty of literature on blunted conical nose cones, but all of it that I've seen uses a hard angled transition from the cone to the airframe. Not one have I seen that implements a double-tangent ogive (which I guess is what this would be) transition to the airframe.

Also, I realized that if thinking of it as a double-tangent ogive, there's no reason to limit the radius to the radius of the airframe. Transition ogive radius could be anything from zero (no transition, as in all the literature) to whatever ends up being tangent to both the airframe and blunting sphere, eliminating the conical section. That would be a "blunted ogive," which is also discussed in the literature.

 

[BABAR]

Speaking of “blunted”, I am lost without pictures. Can you draw a rough diagram of your concept on a Piece of paper and post a picture, please.?

as of now this sounds like hump back midget whale syndrome (or more politcally correct, hump back little person whale syndrome.). BTW, there is a reason you can’t successful google this syndrome which relates to the title of this post.

 

[boatgeek]

It's a bit hard to speculate as to why you don't see something like this in the literature. I'd guess that they're trying to limit the complications. With the "standard" blunted nose cone, your parameters are basically sphere radius/base radius and cone length (probably divided by base radius). While you don't exactly add another parameter with your proposed approach, the geometry isn't as easy to generate.

I'm also not sure there's really any benefit over elliptical nose cones (for subsonic speeds) or ogive/Von Karmann/conical nose cones (for supersonic speeds). With a limited amount of space in tables in textbooks/wind tunnels for testing, you pare down the list of candidate options to the easy ones or the ones that have an obvious flight regime where they excel.

 

[SolarYellow]

Speaking of “blunted”, I am lost without pictures. Can you draw a rough diagram of your concept on a Piece of paper and post a picture, please.?

as of now this sounds like hump back midget whale syndrome (or more politcally correct, hump back little person whale syndrome.). BTW, there is a reason you can’t successful google this syndrome which relates to the title of this post.

Cross section whipped up in AutoCAD. Two circles joined by tangent lines. Chopped the base circle off to make clear what part attaches to the rocket.

On the left, the transition ogive radius is equal to the base/airframe radius.

On the right, the transition ogive radius is 2x the base/airframe radius.

In a parametrically defined version, you would just specify the blunting and transition ogive radiuses as well as some variable controlling length: could be the cone angle, the overall length, the overall fineness ratio, the distance from the base to the center of the blunting sphere, etc.

I think I know enough trig and algebra to work it out. Will try to build it in OpenSCAD.

 

[Rschub]

The reason its not used probably has something to do with this image:

It looks like a shock wave propagates at the transition, a rounded transition there would probably increase drag. For subsonic rounded there would probably be around the same drag as another type, but then you get into the issue of making a more complex shape.

Now that we have 3D printers, we can experiment with any nose cone shape we want to without worrying about.
This is a quick model I whipped up, and the sketch that generates it.

So it can be done if you want to.
If we increase the radius to 6 in it looks like this:

12 in looks like this:

and 22 looks like this:

Which is probably what you get when you buy a fancy metal tip nosecone, because the tip is easier to machine as a straight taper.

 

[jqavins]

Green is the shape you propose; it's hard to see the little circular arcs at the bottom that give the tangent intersection to the body tube, but trust me, they're there. Pink is a "regular" elliptical. for subsonic flight, the perennial science fair result is that a tall elliptical nose cone gives the lowest drag of the common shapes. That seems to imply that a smaller radius to the round-over is better than a bigger one, just as long as there some round-over. Yours has the larger front radius, so my hunch is that it has higher drag than the ellipse.

I too like the idea of a nose cone profile that 1) has a rounded tip; 2) has parallel sides, tangent to the body tube; and 3 is slimmer along the length than an ellipse. I create those in silico as an ogive part of the way up, meeting a parabola for the top.

 

[Alan15578]

I have been in the room where the missile Aerodynamics guy is summing up his presentation on his optimized nose shape and a propulsion guy from the back of the room interjects: But ya know we could still round two inches off your nose shape, add two more inches of propellant, and still get greater performance.