ogive secant nose cone?

[Rex R]

how does one tell open rocket that you are using an ogive secant nose cone? just which shape parameters do you use?
thanks
rex

[JPVegh]

OOOH! This sound like a math problem. I hate math!

I just reduce the shape parameter to less than 1 until it looks about right. For example about .66 for a 3:1 ogive nose cone or .75 for a 4:1 cone .8 for a 5:1 cone. I"m probably way off on these numbers but it looks about right to me. I'm hoping that it's close enough for gov work.

I'm sure one of the resident math wizzes has a formulae for this.

[plano-doug]

how does one tell open rocket that you are using an ogive secant nose cone? just which shape parameters do you use?
thanks
rex

I think the best way to address this is to consider the tangent ogive. The curvature of the cone is tangent where it meets the body tube. That is, the tangent of the curve lines up with the edge of the tube to form a smooth, minimal drag joint.

Conversely, a secant ogive is not tangent. The curvature of the cone meets the tube more like a conical cone does, with a distinct angle at the joint. This should disturb the airflow more and make it a little more draggy.

Here's a secant ogive. There is clearly an abrupt change in angle where the cone connects to the tube. (BTW, this pic has an error. The ray's starting point should be farther aft/left. But the shape of the cone is still a secant ogive.)




For the tangent ogive below, the nosecone transitions smoothly to the tube.



HTH.

Doug

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[troj]

Awesome explanation, Doug!

It even makes sense to me, and Geometry makes my head go numb.

-Kevin

[JPVegh]

I think the best way to address this is to consider the tangent ogive. The curvature of the cone is tangent where it meets the body tube. That is, the tangent of the curve lines up with the edge of the tube to form a smooth, minimal drag joint.

Conversely, a secant ogive is not tangent. The curvature of the cone meets the tube more like a conical cone does, with a distinct angle at the joint. This should disturb the airflow more and make it a little more draggy.

Here's a secant ogive. There is clearly an abrupt change in angle where the cone connects to the tube. (BTW, this pic has an error. The ray's starting point should be farther aft/left. But the shape of the cone is still a secant ogive.)




For the tangent ogive below, the nosecone transitions smoothly to the tube.



HTH.

Doug

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I think I get it, so any value less than 1 and greater than 0 for shape parameter will result in a secant because the rocket body is not tangent to the curve of the nose cone? So if open rocket allowed a value greater than one then that would also result in a secant? So the hojo has a secant ogive nose cone?

Did I mention that I hate math?

[chadrog]

You think too much Rex... See ya' tomorrow.

[plano-doug]

FWIW, here's a drawing I cooked up which hopefully is clearer than the ones I grabbed off the web. HTH. Doug




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[Rex R]

as I understand it, the HoJo nose is in the Haack family, and a shape parm of 2/3(0.667) or more results in the nose being larger than the bt(was looking up a Sears- Haack nose). think I just use .75 shape parm for the ogive nose and call it 'good enough' .
rex

[luke strawwalker]

FWIW, here's a drawing I cooked up which hopefully is clearer than the ones I grabbed off the web. HTH. Doug




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Looking at the picture... you're supposed to slowly reduce the amount of acetylene until the feather in the flame just disappears...

Now you're ready to cut steel...

Later! OL JR

[Sampo]

Hi,

how does one tell open rocket that you are using an ogive secant nose cone? just which shape parameters do you use?

There's been quite good explanations on the geometry, though it's a different (but equivalent) description from what I've seen. I've used this as a source, which contains descriptions of a lot more shapes as well: http://projetosulfos.if.sc.usp.br/ar...eCone_EQN2.PDF

In OpenRocket (as it explains next to the selection field) the shape parameter 1 produces a tangent ogive, and any smaller value produces a secant ogive. The limit at 0 is a conical nose cone.

If you want the gory details, then the following is the exact definition of the shape parameter:

In a tangent ogive the radius of the circle or revolution (rho1) is uniquely defined by the length (L) and radius (R) of the nose cone:

rho1 = (R^2 + L^2)/(2R)

In a secant ogive the radius of the circle (rho) is correspondingly

rho = rho1 / alpha

where alpha is the shape parameter. Thus alpha=1 produces the tangent ogive radius, alpha=0 produces infinite radius (i.e. straight line), and values in between produce the secant ogives.

OpenRocket does not allow values greater than one because that would mess up the aerodynamic calculations (nose cones must be increasing in radius). If you want to create a bulging nose cone, you need to create it as a nose cone and reducing transition.

In practice it doesn't matter that much what the exact parameter is, so eye-balling the shape parameter is quite sufficient.


Cheers,
Sampo N.