Wholesale Nosecones

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polaris

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I am looking for "plastic" nosecones at wholesale prices. Primarily to fit BT60, 70, and 80 tubes.

Any suggestions?
 

powderburner

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I don't know this for a fact, but I suspect that you are not going to find an answer to this one.
I would bet that the plastic NCs are all made in China, or Mexico, or somewhere like that, and that Estes owns the molds (or at least the rights to them). I have never heard of an independent company selling Estes-style plastic NCs.
Although I too would love to locate just such an alternate supply.
I know that there are a few companies like Apogee that also sell plastic NCs, but again you are stuck with buying them only through Apogee.
Let us know if you find anything?
 

polaris

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If I am going to go looking for stuff, can you help me with a shopping list?

Top of my list is an ogive nosecone.

I think Estes is about 3:1 fineness with a 10% bluffness. They are closed at the bottom with an attachment for the shock chord.

I was thinking about something a little finer. What do you think about a 5:1 ogive with 10%bluffness? Leaving it open at the bottom. If you need to attach something to the nosecone you could glue in a centering ring or bulkhead.

Any other thoughts on nosecone requirements?
 

powderburner

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If you like the looks of a 5:1 ogive, or if you need the space inside for some special payload, then go for it.
If you think that a 5:1 is more streamlined than a 3:1 (or a 2:1), then you would be mistaken. The optimal fineness ratio for a subsonic ogive/parabolic/rounded NC shape is somewhere around 2:1. More than that and you are just adding surface area (drag) and NC shell material (weight) and you are penalizing the performance of your rocket.
As to the base configuration, I would vote for the molded-in-place solid base (Estes style) any day. It reinforces the NC cross-section to help keep it round (along with the forward end of the BT). If you don't need the closure disc at the base of the NC, you can get rid of it quite neatly with a razor saw.
 

polaris

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You make some really good points. Things I had not thought about, like the idea of an optimum fineness. My curiosity got the best of me. So I “built” some parts in ROCSIM. I realize that it is not a wind tunnel. But it was interesting to see the results.

Optimize for Cd

First came the nosecone alone. Just like you said there is a dip in Cd. The Cd falls quickly from 1:1 down to about 2.5:1. The minimum looks like it is between 3:1 and 3.125:1. Then it rises smoothly and gently all the way out to 8:1 where I stopped. So there is an optimum fineness for Cd.

Next came a BT60 3FNC rocket with an ogive nosecone. I checked out the Cd as I changed the fineness ratio from 1 to 6 in 0.5 increments and also Mach numbers at 0, .24, .5, .75, and .99. I found no dip.

Finally I tried a BT60 tube alone. Again there was no dip.

I think this means savings in the Cd from an optimal ogive nosecone are swamped by the effect of the rest of airframe.

Optimize for weight

I looked at the increase in weight. Basically it appears to increases linearly. As the fineness increases the Cg does shift forward. So, I am not sure there is an optimum fineness for weight.

Optimize for volume.

I remembered that the best packing shape in terms of minimum surface area and maximum volume is the sphere. If you took a sphere and squished it, without changing the volume, the surface area would have to expand. This means the shape that comes from deforming a sphere are not as efficient as the sphere. Then I thought about a nosecone like a hemisphere that got stretched. As long as you leave it rounded, it should work the same way a sphere does. This suggests there is no optimal fineness ratio.

I am not sure I have thought about this right. What do you think?
 

powderburner

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I cannot lay my hands on the exact reference/source that I'm trying to remember, but I clearly remember seeing a drag-vs-fineness ratio chart with a bucket that minimized at around 1.5 or 2.
If you do some digging in old aerodynamics textbooks you may also come up with some data.

A good (well-shaped and smoothly finished) elliptical NC design is even capable of generating *thrust* due to the accelerated airflow around the front end.

The body tube component should make a constant drag contribution, but should not overpower the rest of the vehicle drag (unless you are talking about extreme BT lengths as in super-roc design).

RocSim is probably an OK overall design/performance simulation, but I don't know exactly what sort of drag model was used for nose cones (or any other part of the rocket). Without further checking, I am not sure that I would accept as 'gospel truth' anything that came out of RocSim . . . or from CBS news.
(mods--that was supposed to be a joke, not a political statement)
 
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