"Rounded Ogive" (Semroc SEM-BNC-60L)?

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neil_w

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Trying to put a parts order together.

An upcoming build requires a 3.1", BT60 ogive nose (1.9:1). Well, the actual profile looks like it might not be quite ogive, but that should be a good enough approximation. It's what I used in my OR model:
upload_2019-11-22_11-49-36.png


So... the closest I can find is that erockets has a 3.1" "Rounded Ogive". Here's the website picture, but I'm not convinced it's very accurate.


I've emailed Randy to ask if he has a photo or accurate drawing of that part, but I don't know when or if he'll have the time to respond. Does anyone know what that nose cone actually looks like? I don't believe that "rounded ogive" is a precise technical term, so it leaves quite a bit of wiggle room for the actual shape. I don't even know what to put into OR to approximate it so I can see what it'd look like.

I'm having a hard time deciding whether to compromise on the shape, or get one custom turned by Roachwerks, or 3D-print one (I worry about finishing with 3D-printed parts.
 
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mbeels

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I get a permission error trying to view that attachment.
 

neil_w

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Fixed, I hope. Some weird business when pasting images into the editor sometimes.
 

kuririn

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You can do what some of the TLP kits do and put a cardstock tip on the rounded nose cone. Then blend in with cwf or putty and harden with CA.
 

kuririn

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After re-reading your first post let me retract and say that since the length is correct, all you need is to re-shape the cone to a pointed ogive. Draw up a template and sand, sand, sand.
 

neil_w

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I feel like I really need to have an idea about the initial shape before formulating a proper plan.
 

BEC

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That picture is pretty accurate.The 60L is the Big Bertha (and many others) nose cone. But they have LOTS of nose cones. Also look at BC-16s (which fit Centuri T-16, which is VERY close to BT-60). For example, the Goliath/Vega nose cone:C59E8D72-1E45-45BE-93E9-13383D4CC8FB.jpeg

As an aside, I don’t know of any good way in OR to represent ogive nose cones with rounded tips (Alpha III, Big Bertha and family, etc.).
 

kuririn

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I believe the "rounded ogive" is a spherically blunted tangent ogive. Anyway I wasn't a wiz at trig in high school so here's a section of a wiki article that might help you get the right shape:
Tangent ogive[edit]


Tangent ogive nose cone render and profile with parameters and ogive circle shown.
Next to a simple cone, the tangent ogive shape is the most familiar in hobby rocketry. The profile of this shape is formed by a segment of a circle such that the rocket body is tangent to the curve of the nose cone at its base, and the base is on the radius of the circle. The popularity of this shape is largely due to the ease of constructing its profile, as it is simply a circular section.

The radius of the circle that forms the ogive is called the ogive radius, ρ, and it is related to the length and base radius of the nose cone as expressed by the formula:

ρ = R 2 + L 2 2 R {\displaystyle \rho ={R^{2}+L^{2} \over 2R}}

The radius y at any point x, as x varies from 0 to L is:

y = ρ 2 − ( L − x ) 2 + R − ρ {\displaystyle y={\sqrt {\rho ^{2}-(L-x)^{2}}}+R-\rho }

The nose cone length, L, must be less than or equal to ρ. If they are equal, then the shape is a hemisphere.

Spherically blunted tangent ogive[edit]


Spherically blunted tangent ogive nose cone render and profile with parameters shown.
A tangent ogive nose is often blunted by capping it with a segment of a sphere. The tangency point where the sphere meets the tangent ogive can be found from:

x o = L − ( ρ − r n ) 2 − ( ρ − R ) 2 y t = r n ( ρ − R ) ρ − r n x t = x o − r n 2 − y t 2 {\displaystyle {\begin{aligned}x_{o}&=L-{\sqrt {\left(\rho -r_{n}\right)^{2}-(\rho -R)^{2}}}\\y_{t}&={\frac {r_{n}(\rho -R)}{\rho -r_{n}}}\\x_{t}&=x_{o}-{\sqrt {r_{n}^{2}-y_{t}^{2}}}\end{aligned}}}

where rn is the radius and xo is the center of the spherical nose cap.

Finally, the apex point can be found from:

x a = x o − r n {\displaystyle x_{a}=x_{o}-r_{n}}


From the first picture,
If I read this right, you want L to be 1.9(2R). (Since 2R is the diameter of the tube). So if you get a compass and paper and draw a circle so that L = 1.9(1.637") = 3.11", then you will have the proper contour.
Then you can shape whatever nose cone you get to the proper shape. The nose cone you are looking at only needs the tip to be sanded down and a paper tip added on, I believe. That will replace the amount removed by the "spherically blunted ogive" configuration.
DISCLAIMER: I'm not an engineer, anyone who has technical training please chime in and correct any errors.
 

kuririn

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BEC's BC-1631 would be a better match. ID for a BT-60 is virtually the same as an ST-16. ST-16 is a little thicker so the cone may have to be SLIGHTLY sanded at the lip of the shoulder to blend in with the tube.
 
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neil_w

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That picture is pretty accurate.The 60L is the Big Bertha (and many others) nose cone. But they have LOTS of nose cones. Also look at BC-16s (which fit Centuri T-16, which is VERY close to BT-60). For example, the Goliath/Vega nose cone
Hey, that one looks very good. Looks like only a tiny bit oversized compared to BT60, should be able to sand it down easily enough.

That's probably the winner.

As an aside, I don’t know of any good way in OR to represent ogive nose cones with rounded tips (Alpha III, Big Bertha and family, etc.).
There are ways, but they are not "good". ;)
 

neil_w

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BTW Randy emailed me back and said that those website images are generated by the CAD software so they should be pretty accurate.
 

cwbullet

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Send me the OR file and I will print this for you. Easy Peasy
 

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