Building Ogive Nose Cones
nose cone ('nOz-"cOn) : a protective cone constituting the forward end of an aerospace vehicle
ogive ('O-"jIv): a diagonal arch or rib; a pointed arch
tangent ('tan-j&nt): meeting a curve or surface in a single point
secant ('sE-"kant): a straight line cutting a curve at two or more points
Click for larger illustration
This article will focus on one of the more common nose cone shapes -- the ogive -- and an effective way to shape a large version for an extreme project.
The shape of a tangent ogive is formed by a segment of a circle or arc. The rocket body is tangent to the curve of the nosecone at its base, and the base is on the center line of the arc. The ratio normally assigned to a tangent ogive nose cone indicates the cone's length compared to the diameter of the base.
The tangent ogive in the illustration is roughly two-and-a-half times the longer than it is wide, or 2.5:1.
A secant ogive is also formed by a segment of a circle, but the base of the shape is not on the center line of the arc, and the rocket body is not tangent to the curve of the nose at its base. A secant ogive nose cone can actually have two drastically different profiles: the stubby little number labeled Secant Ogive A, or the bulbous beauty labeled Secant Ogive B. In both cases the rocket body bisects the arc at two points.
There are some nifty wiz-bang equations that define these different profiles, but printing them here would put most readers to sleep and make me look like a pompous windbag. So here's my advice: since you are already online, just type "ogive nose cone calculator" into your favorite Internet search engine. You'll find several useful tools -- like this one -- that will give you the output you want without any lessons in calculus.
And while I'm dispensing advice, here's some on the value of belonging to your local rocket club: I learned all of the following by talking to Bob Heninger, a friend and member of both SSS and AHPRA in my hometown of Phoenix, AZ. I've learned more from Bob while sharing a couple of burritos than I could dream up myself with months of ciphering. It's always good to rub elbows with guys who know stuff.
Bob's Level 3 project, the Mini Gila Monster, required a custom 8.5-inch diameter tangent ogive nose cone, and Bob decided to make it himself. After grinding through the necessary calculations, Bob used those measurements to construct a stack of Styrofoam in the rough shape of the cone he needed for his rocket. To the top of the stack he added a wooden tip -- this is both for strength and to avoid trying to do create a fine point out of foam. The stack was mounted on a central tube, and the pieces glued together near the center of the rings (away from the area to be trimmed off).
Bob created a simple beam compass that would scribe the correct arc onto a cutting template (see above, top center). The length of the beam used in the compass equals the radius of the ogive arc, or p as shown in the illustrations. (Bob captured my jig philosophy perfectly with this contraption; it doesn't have to be beautiful, it just has to work.)
The arc is then cut from the template and mounted on a platform adjacent to a horizontal axle, creating a jig that is a lot like a milling machine (see above, bottom left). Bob made a cradle to hold his laminate router firmly in a horizontal position, which is able to slide along the arc of his cutting template. With a rabbet bit mounted in the router, this tool cuts easily through foam, wood -- and flesh if you aren't careful -- so use caution and always wear safety glasses. The nose cone stock is mounted on a horizontal axle made from 5/16-inch diameter threaded rod. Bob simply slides the router back and forth along the template guide and rotates the stock by hand to remove all unwanted material (see above, bottom center).
Some of you may be pondering the advantage of using a milling machine versus a lathe. Lathes rotate the work piece at a high rate of speed while a stationary blade is applied against it to remove unwanted material. This spinning mass can be quite substantial with a large diameter nose cone and any imbalance in the work piece can be quite dangerous and difficult to shape. With a milling machine the blade rotates at a high rate of speed and the work remains fairly stationary. Balance of the work piece is less critical. A lathe would generally be more precise than the method described here, a criterion that is probably more essential on a small cone than it would be on a large diameter one.
Once the milling is complete, the voids are filled with spackle and sanded smooth. Then the whole thing is covered with fiberglass and epoxy finishing resin, and sanded smooth again, primed, and painted. Bob has built a number of incredible nose cones using this method, including the nose for the full-size Gila Monster, a 16.5-inch diameter blazing orange behemoth. Bob will try to tell you that the results aren't perfect -- but that is an argument he typically loses.