The N00b T00b - A Quick, Easy, Scratch Built Tube Fin Rocket - Part 1: The Basic Build

I have a bunch of random model rocket parts lying around the Rocket Room. Some are things I intentionally stocked up on, and some are leftovers from kits I modified, or from other projects.

I have a bunch of short, BT-20 sized tubes (0.736 inch in diameter). They're most certainly supposed to be motor tubes, though they're a little short, so I'm not sure where they came from. Most Estes engine tubes are about the same length as an 18mm (A, B, or C) motor, while these are shorter. If you used them for a motor mount tube, you'd have to have the motor hang out the back by at least a half inch, whereas most Estes kits have the engine stick out about 1/4 to 3/8 inch.

I think they're leftover stuff from Rocket Camp which ended up in my pile of parts, and I think they might be from Pitsco, which a previous Rocket Camp teacher seems to have used.

I also had one 8.75 inch long BT-20 body tube which I know is a leftover from Rocket Camp. When you teach model rocketry to kids, there will always be parts left over. Not from the kits the kids build, but from the extra kits you have on hand.

Kids at camp will lose parts, glue parts in the wrong spot, shove parts up their noses or get them stuck firmly on their fingers and need them cut off by the nurse (yes, this all happened). Consequently, some of them won't have a rocket to complete if you don't have spare parts, and a kid at Rocket Camp with no rocket to build is likely to get bored and become disruptive. This is really a digression, but if you ever teach model rocket camp, make sure you have extra kits, and be prepared to end up with a bunch of random parts left over.

So, having given away launch lugs, engine blocks, fins, etc., I had one body tube left over from an Estes Viking.

I hadn't started building a rocket in a long time. The N00b family had recently moved, and for a while I was trying to just finish building and painting all the stuff I'd started in the previous two years. My build pile of kits is so big, I didn't know where to start.

Then I thought: maybe I should build a quick tube fin rocket, just to get back into it.

When it comes to model rockets, my tastes are pretty traditional. I like rockety-looking rockets - a nose cone, long body tube, and three or four fins. I'm not as interested in odd-rocs, saucers, boost gliders, or tube fin rockets. That's not a criticism - I enjoy seeing them fly at launches. It's just for my own fleet, I like mainly sport models and the occasional scale model.

But tube fin rockets can be fun. They have a lot of devotees. They can be simple to construct, and unlike rockets with flat fins, they apparently do not weathercock.

N00b Note: "Weathercocking" is when a rocket arcs into the wind. Almost all model rockets do this to some extent. It's a kind of side effect of fin stabilization. As a rocket's fins correct its trajectory in flight, lift on the fins rotates the rocket body around its Center of Gravity. As it oscillates, the rocket will tend to correct more in the direction the wind is coming from. Rockets with larger fins or rockets which lift off slowly tend to weathercock more severely than faster models, which may only weathercock slightly closer to apogee. See The Handbook of Model Rocketry by G. Harry Stine and Bill Stine for more on this phenomenon.

I had lots of parts, so building a tube fin model would be easy. Tube fin rockets can come in different designs, and I'm no expert on them, but it seems that the easiest to build quickly are rockets which use the same diameter tube for fins as it does for the main body tube. You can fit six tube fins perfectly around the center tube, so getting them glued on straight is easy.

As far as "designing" the tube fin rocket would go, there wouldn't be much design. I had parts, so I'd glue them together. I wasn't going to have to cut anything or shape anything. I'd just see how it turned out.

The rocket used a BT-20 body tube, the diameter used for 18mm A, B, and C motor mounts. So this would be what's called "minimum diameter." No centering rings, no engine hook, no motor mount - the body tube is the motor mount. Minimum diameter rockets tend to fly very high. Some examples beginners might know are the Estes Viking, Wizard, Hi-Flier, etc. The motor would be kept in the rocket though friction fit - a wrap of masking tape around the engine until it's tight enough that it won't fall out of the rocket, or be ejected out by the ejection charge.

Construction was simple. First, I glued in a spare engine block I found in my spare parts. I pushed it n place with an old motor casing so that the motor would hang out the back by about 1/4 inch.

Next, I glued on the fins. This is the part that makes a tube fin like this a snap. To get the fins on straight, you start with one fin. Run a thin bead of glue down one side of the tube fin and attach it to the main body tube. Then, lie both tubes on the work table, side by side.

A straightedge helped me make sure the ends of the tubes were even with one another, and the fact that both tubes were lying on a flat surface as the glue dried ensured that they were parallel - that the tube fin was straight.

As the first fin dried, I ran a bead of glue down a second tube fin and attached that to the body tube by laying it down next to the first tube fin.

A third tube fin was glued to the body tube laid on the table opposite the first fin.

I needed to figure out where to put the launch lug, since I'd never built one of these before. I tried to see if I could hide it in a gap on the body tube between two tube fins, but it turns out that BT-20 tube fins are a little too small to hold a 1/8 inch launch lug between them. So I glued a launch lug inside the fourth fin and glued that in place in the remaining gap on the body tube.

I made sure all the tube ends were even using my sanding block, and then I let the glue dry a little bit. The first four fins went on in about 10-15 minutes.

I flipped the rocket over when I felt the glue would hold without things moving, and quickly glued the remaining two fins in place in about 1 minute's time.

That was it. I popped on the nose cone, and had a look. With the exception of putting in a shock cord, the rocket was built.

Coming up: Stability? Payload? Paint? How about a name for this rocket??

Follow me on Twitter.

Like my Facebook page for blog updates and extra stuff.

Have a question you'd like to see addressed on this blog? Email me at iamtherocketn00b@gmail.com.