Bertha/Ranger/etc. & commercial "elliptical" nose cones

[SolarYellow]

Got a discussion going in the What did you do rocket wise today? thread. Figured I'd make it its own thread to make it more easily findable as well as easier to follow. So quoting all the relevant posts here below:

Snagged an original Mini Bertha on Ebay. Listed as missing a couple of parts but they were in the body tube. Started the build tonight. It's being rereleased this year but as a snap together. I'm glad to have an original. Decals look pretty good.

Cool! Take a bunch of photos of the nose cone before you sand it, if you can. I'm interested in exactly what shape those were back in the day.

Isn't it elliptical?

Extremely unlikely that it's exactly elliptical. Most generally "bulbous and rounded" nose cones aren't. The archetype "elliptical" nose cone was the Ranger/Big Bertha, and it was actually a more or less spherically blunted ogive of small fineness ratio, as far as I can tell. Today's plastic replacement that was tooled for the goonies is not really elliptical, as far as I can tell.

I've seen various photos of the Mini Bertha NC and it doesn't look really elliptical. However, they are always of the finished part, after someone has sanded it. With the small size, it's really easy for someone to change the profile just getting it smooth and trying to even everything out by hand.

There are at lease several old kits from the balsa era where the manufacturer's marketing photos show a rounded NC tip, but then you see a vintage bagged kit and the NC that's provided was very much closer to a pointed tip. So someone rounded the tip by hand in finishing the demo/display sample.

FWIW, I'd be pretty astonished if any COTS nose cones prior to the common use of CNC machinery (Carl/Semroc) were actually elliptical within any reasonable tolerance.

Interesting. That would (to a small degree) invalidate the results of hundreds of science fair projects.

Not necessarily. If they tested a bulbous and rounded shape against several other shapes, and their results showed the bulbous and rounded shape to be the best, then those are their results. However, if they used a commercially-manufactured nose cone, it may well be the case that their "elliptical" shape wasn't actually an elliptical profile, just sort of ellips-y.

As a practical matter, the difference between an elliptical and a spherically blunted ogive nose cone of equal overall fineness ratio is utterly splitting hairs at speeds below about Mach 0.75. I haven't seen any science projects looking at that difference. The basis for saying elliptical is best, if you trace it back all the way, is a series of tests done in a water tunnel in about 1947, looking at what would be the best nose shape for torpedoes. They just happened to choose elliptical as one of the shapes to test, and it was better than anything other shape they happened to choose to test. That in no way implies it's the best possible. (By the way, the reports on those tests never calculated a CD. I presume that the chart of CDs in Hoerner, which was then repeated in TIAMR and later in Stine, was generated by someone - likely Hoerner or an assistant - re-analyzing the data published in the water test report.) It may well be the case that alternative shapes could outperform elliptical up to around Mach 0.75-0.8, at which point the Von Karman shape would likely take over by its minimization of pressure drag through the transonic region.

My earlier request for photos of the Mini Bertha NC before it's sanded was based on an interest in accurately reproducing the shape that was actually shipped by Estes, rather than simply assuming it to be an elliptical shape.

Well, that was my point. If the project write-up says they tested several shapes, including elliptical, but they actually tested some other shape, then the project is, to a small degree, invalid. They'll have shown that the bulbous mystery shape is better than the others tested. I am most certainly splitting hairs.

Yes, I understood. I'll be interested to see what you find.

BigMacDaddy linked to this other discussion in 3D Printing that is the best I've seen so far discussing the shape of the PNC-80BB (Bertha family) and PNC-60MS (current Big Bertha & Baby Bertha kits):

In my experience with various CAD programs and reference images, the Bertha nosecones are not elliptical (agreeing with previous comments). Best result I got was ogive + sphere (spherically blunted ogive - again agreeing with what was said) but ultimately just traced the outline to create a Bertha NC shape. True ellipsoid was more pointy than the Bertha NC (looking at both virtual CAD designs as well as physical 3D prints).

https://www.rocketryforum.com/threa...not-a-default-one-anyway.174269/#post-2309575

Among these quotes, Jeff Curtis posted the requested photo of his original, vintage Mini Bertha nose cone, fresh out of the bag, but the forum software won't quote it, perhaps because the only content of the post was the photo.

I did a little playing in AutoCAD last night with that and a couple photos I found in the Estes - (Astron) Ranger (K-6 or #1206) Gallery thread. In each case, I aligned the nose cone so its axis was vertical, then drew an ellipse over it to illustrate the departure from an actual elliptical shape.

First, Jeff's original Mini Bertha. It's straighter near the base and fuller than the ellipse in the forward section, before tapering to sort of follow the ellipse near and over the tip.



@nosecone posted his cool Mini Brute-liveried Baby Bertha. The fuller shape of this continues all the way to the top, giving it a more blunted shape than an ellipse. Remember that this plastic NC was created for the "Goonie" series of rockets in the '70s, which were meant to be kind of low & slow, silly and goofy. Not really intended for any aerodynamic result. See the thread linked by BigMacDaddy in the quote above for some excellent discussion of how to match the actual shape of this and its BT-80 cousin.



Finally, @bjphoenix posted his original year-old Ranger from 1967. It differs beyond just the additional length discussed in this thread. It's pretty close to the ellipse, but if you look closely, you can see that in the shoulder region, where the Mini Bertha nose cone is at its fullest relative to the ellipse, this balsa BNC-60L Ranger cone is slightly flattened out, just a hair slimmer than the ellipse. Also, my earlier comments about hand finishing appear to be on-point relative to this example. The right side of the cone in the image matches the ellipse more closely, while the left side may be infinitesimally fuller down near the base (or it could be photo effects), while the straightening out and slimming is noticeably greater near the tip. Neither of these differences is great enough to notice without the ellipse drawn over it, which just underlines the difficulty of getting things perfectly even by hand. It's a very cool rocket, apparently built by a young man who took commendable care of his things.



I hope others find this interesting. Just wanted to land the plane on a) why I was interested in having Jeff post a photo of his Mini Bertha nose cone and b) the non-elliptical-ness of nose cones that we, collectively as a community, almost always refer to as elliptical. Additionally, Estes has similar "elliptical" nose cones in the NC-5 pack, in the NC-20 pack (both shorter and longer), and the BT-60 part number 072825 (Astron Sprint XL). They all seem to have somewhat different profiles, beyond just variation in fineness ratio.

And, yeah, I need to get the 3D printer making smelly.

 

[RCBrust]

A little more info on the Bertha/Fat Boy nose cones here.

Randy

 

[SolarYellow]

That's the one BigMacDaddy linked in the quoted post above.

 

[PeterAlway]

I have collected a few vintage Estes balsa cones over the years

Back in the 1990's a southeastern Michigan store had learned how to turn a profit buying old stock from hobby distributors and selling it at collectors' prices. But the collectors at the time wanted kits, and didn't care about parts. So the place was selling old stock nosecones and payload sections dirt cheap. I got my hands on a few.

When I looked at them after having drawn lots of scale subjects, it became obvious to me that the ellipses weren't ellipses and the ogives weren't ogives. The shapes were pretty willy-nilly. Some, like the original Alpha nosecone, were more cylinder-ogive, but not exactly. The Bertha cone was closer to cylinder-ogive-hemisphere. The best theory I've heard is that the cones were laid out with one of those old "French Curve" templates that went the way of the slide rule decades ago.

It doesn't help that the tooling wore out over time, so some of these shapes were variable. Often, the original balsa cones had an excessive diameter step between the shoulder and the base of the exposed part of the cone, and required a ton of sanding to fit well.

 

[SolarYellow]

The best theory I've heard is that the cones were laid out with one of those old "French Curve" templates that went the way of the slide rule decades ago.

It doesn't help that the tooling wore out over time, so some of these shapes were variable. Often, the original balsa cones had an excessive diameter step between the shoulder and the base of the exposed part of the cone, and required a ton of sanding to fit well.

I've reverse-engineered several of the blow-molded cones, and it's common for Estes to use three radii between the base and the spherical blunting at the tip. So while the other thread here linking to analysis of the PNC-80BB indicates a French curve is a match, that's not the case for the PNC-55AO, PNC-56, or PNC-60AH.

Not only did the balsa NC tooling wear during use, it was originally contoured by hand. Once it wore to the point a manager decided it needed refurbishing, it was again recontoured by hand. So you had both the gradual change due to wear and step changes from time to time when the tooling was reconditioned. Just looking at Estes balsa Goblin noses over time, they are all over the map. It's fallacious and silly to pretend there is one "correct" contour for them. The PNC-55AO is still a different shape. At least, however, the durability of the blow mold tooling has kept it consistent over time, subject to the variation and irregularities inherent in the blow molding process.