Fiddling Around

My schedule is all over the place. Sometimes I work late at night; sometimes early, early in the morning. All this back and forth has left me with less time than I'd like for rocket building or blogging. I get maybe two good building days a week, and I work slowly.

Fortunately, I have OpenRocket - free rocket design and simulation software.

As any gamer or couch potato can tell you, even if you're really tired, you can still spend hours fiddling around on your computer.

So, when I don't have the time or energy to build rockets or write about them, I play around with different designs. It's a way of staying an active rocketeer when I'm too busy to be actually building.

Once you know the basic principles of stability and rocket construction, it isn't that hard to design a basic rocket - or at least to get one started. You might need to make revisions, weigh components and change a few things, but you can come up with some cool basic designs in short order. I've even built and flown rockets that took me 20 minutes to conceive, and I only had to make minimal revisions.

Here are some designs I've been working on.

The Circe Series

I started this the other night. I wanted something simple to replace my lost Estes Hi Flier - a small, lightweight, high-flying rocket. It performs so well because it's a minimum diameter rocket - meaning it is only as big around as it needs to be to hold the motor. It has no motor mount - the body tube is the motor mount!

Circe A - a small rocket which will hold a standard 18mm diameter (A-C) motor, and go quite high

Though I am trying to build bigger and bigger rockets (and currently building two of my Estes Pro Series II rockets), I just had the urge to design this little thing.

OpenRocket has a scale function, so you can take a rocket of a particular size and scale everything up or down by the same amount. So while Circe A (pictured above) will only take a C sized motor, Circe B looks exactly the same, but is larger and will take a D or C motor. When you scale things up in OpenRocket, sometimes you have to adjust components, because, for example, the mass of the nose cone might actually end up being much greater than the part you actually have on hand. Other than that, it's pretty simple.

I went up to a BT60 sized rocket with the Circe series before going to bed. BT60 is a 1.637 inch diameter tube, and is the same size as the Big Bertha. The cool thing about the BT60 is that it's just the right size to fit a cluster of 3 standard motors.

Circe C2 - looks much the same, but has a cluster of 3 motors. You'll notice the nose cone is slightly shorter - this is due
to the size of nose cones I have on hand, which are a of slightly different ratio than the BT20 nose cones I have.

If you're an advanced rocketeer and are good at making stuff from scratch, you can make your own parts. All of them - even the body tubes. And, of course, nose cones.

I'm not to that level yet. I don't have access to a wood lathe, and I haven't picked up the skill of designing and making a custom-shaped nose cone. Fortunately, there are a lot of good parts on the market.

So what I do, for now, is look at the parts that are available - either in my parts box, or online, and design around that.

Here's a lovely balsa wood nose cone I picked up from JonRocket months ago.

This shape is a spherically blunted tangent ogive, and the nose cone fits a BT70 tube, which is about 2.2 inches in diameter. This is getting toward the higher end of body tube size you'll find for low power model rockets, and into the mid power range. If you're new to model rockets, this thing will look huge to you - there are not many BT70 kits out there!

I have realized I need to get more of these, or get some other BT70 nose cones, because over the months, I've designed a number of rockets around this very nose cone.

One of the first was called Horus.

The Horus Series

People give rockets all kinds of zany names. A quick look at an Estes catalog gives you some idea - Sizzler, Prospector, HiJinks, etc. A rocket can have any kind of name. But I guess I'm more of a classical kind of guy. I like to name my rockets after Greek, Roman, or sometimes Egyptian gods or mythological characters, or after celestial bodies. Horus is the falcon-headed Egyptian god of the sun.

Seemed like an appropriate name for a rocket.

I wanted to design my own rocket which looked vaguely like the Sirius Rocketry Eradicator - a beautiful rocket which has been on my wish list since I first saw it - but would be simpler to design and construct for a n00b.

The Eradicator, from Sirius Rocketry, available here

(Eradicator - there's an interesting one. I love this rocket, but the name reminds me of this Kids In the Hall sketch)

No offense, Sirius Rocketry - you guys are awesome!

Anyway, I love the two differentiated diameters of the airframe on the Eradicator. I designed this one:

Not too bad, I thought. But in order to get a minimum of 1 caliber stability, I needed to have 5 fins, if they were to be swept forward. I was able to decrease the number of fins to four by adding fin vanes.

This increases the surface area of the fins, moving the center of pressure aftward, without making the fin span inordinately wide.

The Eradicator has a lot of great detail - it looks like a real launch vehicle. That level of design was a bit advanced for me. That's not to say you can't add additional detail to the build, but it's tricky to add it to the OpenRocket design.

FMLV

This is a simple recent design, but one I like and am building now. It's another BT70 design with a 24mm motor mount. Like the Circe rocket, it's got swept back fins, but as you'll notice, the tips are not parallel to the rocket.

I've seen a few designs like this recently, and I like the look of it. Actually, a lot of classic kits have fins like this, but I've only just recently thought of changing the shape of the fins in OpenRocket.

You can select a few basic fin shapes in OpenRocket. I almost always opt for "trapezoidal," and then simply change the dimensions of the root chord, tip chord (chord is the distance from the leading edge to the trailing edge) and height (the distance from the airframe to the tip of the fin).

That means that most of my designs look like this:

Imperius, formerly known as the "Donor's Rocket"

See how the tips of the fins are parallel to the rocket body tube? Nothing wrong with that, of course.

But part of what gives a rocket its character is the shape of the fins. I went with a few designs that had the fins swept forward, like the Horus series above, and the Copperhead (formerly known as "Keith's Rocket").

And I like these designs. But you learn by playing with the tools you have, so I decided to try a different shape. Still simple enough to cut with a ruler and hobby knife, but a little different from what I'd made before.

FMLV stands for First Massachusetts Launch Vehicle, because it's the first scratch design rocket I'm building here in Boston. I'm using a 24mm Estes "quick release" screw-on motor retainer so that I can use my newly-acquired AeroTech 24mm reloadable casing, or an Estes black powder D or E motor.

That gives me a lot of motor options.

It will also be the first rocket I build from scratch which will have through the wall or TTW fin construction. This means that the fins have tabs at the base which will go through slots in the airframe and attach directly to the motor tube. You fins this as pretty standard in mid power and high power kits, and even some Estes low power kits, like the Cosmic Explorer.

Attaching fins to the Cosmic Explorer. In this kit, the slots are already cut for you.

Of course, in a kit, the slots are cut for you. For the FMLV, I'll have to cut my own fin slots for the first time.

BT80 Rockets

Also included in my parts box is a BT80 sized parabolic nose cone.

You may have noticed that this nose cone has a similar profile to the Big Bertha nose cone, but while the Bertha uses a BT60 body tube - 1.637 inches in diameter, the BT80 is 2.6 inches in diameter. For a beginning model rocketeer, that's really big!

Regular readers of the blog may also recognize this nose cone as part of the ICU2 camera payload bay from Make: Rockets: Down-to-Earth Rocket Science by Mike Westerfield, which I used in the design of the Janus II two-stage rocket.

Since I had another one, I played around a little with a few designs.

A really large rocket, called Titus, after my girlfriend's nephew.

Meh... This kid needs to have a cooler looking rocket named after him.

Also played around with a large two-stage rocket.

Could be fun, especially with the two large 24mm diameter motors. But still a little inelegant. It was probably late at night when I did this one.

I might make a larger, not-quite-proportional version of the Copperhead rocket pictured above.

A larger Copperhead design with the BT80 nose cone, and a 29mm motor mount, for E, F or G motors

Of course, the Janus II with its payload bay was lost on first flight, so I may just rebuild that...

Finally, a design I nearly forgot about. This must have been months ago.

Arrowhead

I had this idea when building both Sounder I, a tiny minimum diameter rocket I lost on its first flight, and the Ceres B booster, again from Mike Westerfield's book.

I stacked one on top of the other with a balsa transition I had in the parts box.

 

It reminded me of some of these:

This looks like a two-stage sounding rocket, but it's really just one stage. Not that you couldn't do a two-stager here, but it would require some electronics I haven't worked with just yet.

I haven't discussed staging on the blog yet, but in brief, most model rockets with multiple stages rely on the first motor igniting the second. The first motor has no delay grain or ejection charge, or even clay cap.

From Apogee Components

The propellant is exposed at the forward end of the motor, so when it burns up, hot particles shoot forward into the nozzle of the second motor, igniting the second stage propellant.

An illustration of model rocket motor staging
from the Estes Model Rocketry Technical Manual

Many multi-stage rockets have the motors touching each other. This is known as contact staging. There can be a gap between them, in what is known as gap staging, but the gap can only be so wide - 12 inches is probably the maximum.

This rocket is longer than that, and the two parts are separated by a solid balsa transition, so there would be no way for the hot propellant particles to reach the top portion. You can make hollow paper transitions, but I'm not very good at that yet, and balsa transitions of all sizes are available online.

Again, for a beginning designer, working around available components is the easiest way to get started.

High power rocketeers do multi-stage rockets using altimeters, or sometimes timers, to electronically ignite upper stages in the air. This is known as an air start.

Still, even as a one-stager, I like it.

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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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