Wednesday, April 22, 2015

Mid Power: Building the Quest Big Dog (Part 9) - Dressing the Nose Cone

Click here for Part 1

I love the larger Quest nose cones!

Nose cones for the Quest Quadrunner and Magnum Sport Loader
This shape is the most common one in hobby rocketry, and is known as a tangent ogive.

A selection of rockets with ogive nose cones -
six tangent ogives and one secant ogive

An ogive is a shape made of segments of two circles intersecting.

You find ogives not only in rocketry, but also in architecture.

A Gothic arch is an example of an ogive in architecture.

An ogive nose cone has a profile defined by these imaginary circles intersecting. A tangent ogive is one in which the airframe is tangent to the imaginary circle. Another, less common nose cone shape is the secant ogive. This is again defined by the intersection of the segments of two circles, but the base of the nose cone is not tangent to the body tube.

The Honest John rocket is a good example of a rocket with a secant ogive nose cone.

Estes Mini Honest John

Many ogive cones come to a sharp point, but the Quest Big Dog nose cone is actually a spherically blunted tangent ogive, meaning that the tip, rather than being pointy, is a section of a sphere.

On the Big Dog, below the ogive, there is a cylindrical section to the nose cone, so the tangent doesn't come off the airframe itself, it comes off the cylindrical section.

This is a nose cone with a very pleasing shape.

But, like all plastic nose cones, it needs a bit of working before it looks its best.

First of all, it's always a good idea to wash a plastic nose cone to remove any oils from the manufacturing process. You can do this with dish soap. Wash and dry it thoroughly.

Due to the molding process, there is a bit of flash, or extra plastic, where the two halves of the mold came together. This is a visible, raised ridge on the nose cone. There may also be (and in this case, there were), some troughs, or low spots, along the seam. This can all be smoothed out with a little sanding and filling.

Raised flash, to be sanded off
Trough, or low spot, to be filled with putty
First, you sand off the high spots. I used to do this with a very fine grit sandpaper, worried I would make a rough surface that would never be smooth on the finished rocket, but the putty I'll use on the troughs will take care of any low spots, as will the primer. So now I use a 220 grit sandpaper - still "fine" as defined by the package, but definitely scratchy. It won't be smooth when you're done.

Sand all the high areas off, and don't forget the tip of the nose cone. When you're done, you'll still see some low areas.

There are a few ways to fill these, but I like plastic putty. You can find this where you'd find supplies for plastic model cars or military vehicles. A popular brand is Squadron. I use Tamiya, because I can easily find it.

These putties come in various colors. Tamiya comes in "basic," which is gray, and white. I prefer the gray, because I can see it, whether it's on a white or black plastic nose cone, so I know if I've sanded it correctly.

The stuff smells noxious, and if you get it on your fingers... well, your fingers will be silvery gray for a day or so.

I gently squeeze the tube and apply a bead of the putty all the way from the base to the tip of the nose cone on both sides, using a craft stick as a putty knife to scrape off some of the excess. You won't be able to get a lot of it off, but any excess you can remove now will save you some sanding later.

The putty will begin to cure immediately, and you could sand it in as little as 15-20 minutes. But I usually set this part aside for a day or so.

Once the plastic putty has fully dried, you'll want to sand off the excess. The best way I've found to do this is to wet sand. This means that you'll either dampen the sandpaper with some water, or better yet, sand the item under a thin stream of running water. This helps keep the sandpaper from loading up with dust, and helps you get a nice, smooth surface.

To do this, you'll use wet/dry sandpaper, which is black, and waterproof. Like ordinary sandpaper, it comes in various grits. I start with 220 grit to knock off the mass of the putty, then move up to 400 grit. Sand in little circles under a trickle of running water from the faucet. I prefer warm water - I don't know if it helps (it feels like it does, but that might be my imagination), but it's certainly more comfortable.

Stop sanding now and then and dry off the nose cone to check your work. If you see any spots that look like there's a ridge, focus on that spot. You might want to check with the back of your fingernail. If you can feel a raised spot, you'll see it when you paint. As before, don't forget the tip of the nose cone. The tip is the trickiest part to sand, and it's where you're most likely to accidentally leave a little extra putty.

If you go too far, and sand some putty out of a low spot, you'll have to re-fill the spot, but that doesn't happen very often. Just use the pads of your fingers to cushion the sandpaper around the contours of the nose cone, and you'll probably take off just enough putty.

This works very well. I do this with all my nose cones. Sometimes my fin filling doesn't come out great, but my nose cones are beautiful. Here's what you'll end up with.

Out of focus, but you get the idea. You should've seen Rx, by the way - great show.
The last step I take in prepping the nose cone is to wrap the shoulder in masking tape in preparation for priming and painting. This will lead me to a little painting trick I like to use.

Next time, it's finally time to prime and paint the rocket - we're nearly done!

Note: For months, I thought it was pronounced "OH-give," as in "Oh, give me a home..." My paper dictionary says that "ogive" is pronounced "OH-jive." Since the first appearance of the word is in France in the 1200's, I think "OH-zheev" sounds OK. In any case, "OH-jive" doesn't sound like a word to me.

The Big Dog has exactly the same nose cone as the Quadrunner - they're identical parts. Many Quest rockets have similar spherically-blunted ogive nose cones, including the Super Bird, the Minotaur, the High-Q and the Li'l Grunt. These are attractive rockets! 

Click here for Part 10

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