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Old 06-08-2018, 09:21 AM
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blackshire blackshire is offline
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Join Date: Jan 2009
Location: Fairbanks, Alaska
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Quote:
Originally Posted by teflonrocketry1
I have plans to offer the 3D printed nosecones to eRockts to use in their remake of the Centuri Hummingbird boost glider kit. I looked at the Centuri Moonraker build on Chris Michielssen's blog: http://modelrocketbuilding.blogspot...ild-part-1.html , and now I am not sure I got the shape exactly right. I printed the shape as elliptical and made it match to the profile tracing from: https://sites.google.com/site/centurihummingbird/

Can some one provide exact measurements and/or a good close-up profile of this nosecone next to a standard measuring scale (ruler) of some sort?

The best thing about 3D printer is that I can easily change the print color so a red, white or blue #7 nosecones are possible. I adjusted my 3D print file so the nose cone fits in the ST-7 tubing that I got from eRockets. I made the nosecone shoulder slightly loose to accommodate the sticker or pressure sensitive tab, as supplied in the original kit, which was used for gluing the nosecone into the paper body tube.

I can easily make decent 3D prints in PLA of the current design. I already have printed a half dozen or so in white and blue PLA. I am worried that the PLA plastic will not be durable enough for this purpose. I have read stories about PLA prints melting in a hot car parked in the sun or shattering when dropped on a hard surface on a cold day when the temperature is below freezing. These nose cones weigh in at 3.1 +/- 0.05 grams I am wondering if that is too heavy for the boost glider?

My ABS prints are much smoother than the PLA prints and should be much more durable and they weigh less at 2.8 +/- 0.05 grams. I currently have about a half dozen white ones since I only have this color of ABS filament.
I have--packed away somewhere [*argh*]--a cast polyurethane resin duplicate of the Hummingbird and Moonraker #7 plastic nose cone. This is the PNC-70, whose total length (not counting the tenon [shoulder] that fits inside the body tube) is 1.5 " (see: http://www.ninfinger.org/rockets/no...a/80cen036.html ). Also:

The PNC-70 isn't an elliptical nose cone (the one on the Hummingbird clone in your second link above [*not* Chris Michielssen's blog]) is a hand-carved elliptical nose, made from a balsa block (as listed in its parts list). The PNC-70, shown in the Moonraker kit photograph on Chris Michielssen's blog, is a tangent ogive nose cone with a rounded tip (knowing its length, 1.5", maybe the picture on Chris Michielssen's blog could be of some help?). I can "bracket," to an extent, the PNC-70's shape for you:

The PNC-70 nose cone's base diameter--which is the same as the #7 body tube's outside diameter--is 0.759" (the #7 body tube's ^inside^ diameter is 0.715", see: http://www.ninfinger.org/rockets/no...a/72cen058.html ). A 2-to-1 tangent ogive nose cone for the #7 body tube, if it had a pointed (un-rounded) tip, would be 1.518" long from base to tip. Since the PNC-70 is 1.5" long, and has a rounded tip (if memory serves, the radius of its tip is ~3/16" to ~1/8" or so), its length--if its tip wasn't rounded, but was "allowed" to come to a sharp tip--would be somewhat more than 1.518". In other words, the PNC-70 isn't a 2-to-1 ogive, but is a longer ogive--perhaps 2.25-to-1 or 2.5-to-1, or some other ratio "in that neighborhood." Here (see: http://www.google.com/search?source...1.0.XVcuZ0ETB5c ) are several websites that show how tangent ogive nose cones (and secant ogive nose cones--the Bristol Aerojet Skua and Petrel [see: https://hu.wikipedia.org/wiki/Petrel and http://www.astronautix.com/p/petrel.html ] and the Honest John had secant ogive nose cones [the Honest John's was a "super-caliber" one]) are constructed geometrically; the Wikipedia article shows how both ogive nose cone types are constructed geometrically.

I hope this information will be helpful (and hopefully someone has a PNC-70 handy to measure, or photograph next to a ruler).
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