Rockets

Finally, a post inline with the name of this blog! I've been working on a rocket for Southern Thunder 2010 for the past week or so. I plan on certifying Level 1 and 2 with it.

Note: This build log is written for people with some previous knowledge of high-power model rocketry.

This rocket is a Blackhawk R&D 3" Stinger kit. It came as a kit with pre-cut fins and tubes, which is very convenient. I cut my own fins/tubes for many of my rockets, and it can be a real pain.
The first steps were to rough up the 3" body tubes and glass them (I'm going to put a J engine in it afterall).

Fiberglassing setup. The tubes are slid on here and laid up. Note the wax paper on the floor.

Glassed tubes. 

Leftover resin formed to the shape of a cup it was in.

I used 4 oz. glass and West Systems 105/205 resin. You can see the rings I trimmed off the ends of the tubes in one of the above pics. The discoloration is where the resin soaked into the tubes differently. I then had to cut out the fin slits.

 

Then I soaked the ends of the tubes in superglue and sanded them to keep them from burring.

The dark ring is superglue.

The next step was to make the motor mount (the thing that holds the rocket engine). 

  Here's the forward centering ring epoxied on to the motor tube. 

Kevlar is used for the first part of the shock cord because it's fire-resistant (there's a black-powder ejection charge that fires to deploy the drogue or main chute depending on setup) and won't wear out as fast as a regular shock cord. This will be attached to a shock cord, which will be attached to the bulkhead.

While that was drying, I started working on the payload section:

Here's the bulk head epoxied in. I used some Kevlar pulp in the epoxy to strengthen it. Another common filler is short carbon fiber threads; I couldn't find our bag of them, so I just went with the Kevlar. Silica/micro-balloons are other common fillers.

Filleting the other side of the bulk head with epoxy.

Then the motor mount was shoved into the aft end of the body tube and glued with some 5-minute and filleted:

The shiny stuff inside the tube is a fillet of about 30 grams of West Systems mixed with Kevlar pulp poured in. That centering ring is a good 18 inches down the front end of the body tube, so the only way I could get the epoxy in there is to aim, pour, and hope I didn't make too big of a mess.
While that was setting, I worked on the back centering ring. I didn't glue the two aft centering rings in (so only the one forward ring got glued in) when I glued the motor mount; you'll see why in a moment. The back centering ring needs motor retention screw mounts:

Motor retention screw mounts epoxied in.

While all that was drying, I worked on the fins. They are laser cut out of 3/16" plywood- very nice by my standards. The edges needed to be tapered/sanded, so I went to the belt sander:

A rounded rear fin.

I also drilled 1/16" holes in the fins near the body tube interface.

 

Forward fins. The fin tabs were cut a little short, so I glued a then strip of balsa to the bottoms.

These holes will act like rivets when filled with epoxy when filleting. The next step was to glue in the forward fins.

First fin glued in.

All forward fins glued in. Note the aft centering ring: it's not glued in, just holding the motor tube centered.

After gluing the fins in, they needed to be filleted. I used West Systems/Kevlar pulp again. Some people do tip to tip fin glassing at this point. I didn't feel that it was necessary with this rocket. My view is that it is only necessary for rockets flying L motors and over, or anything going Mach.

I normally fillet the inside of the fins while doing the outside fillets, but I couldn't on the forward fins because of their displacement from the rear end of the rocket (you'll see in a moment how I did it on the aft fins). So I needed some other type of inside strengthener. Expanding foam to the rescue! 

The setup. Rocket, expanding foam, and MEK for clean up (lost the acetone).

I enlisted dad's hands for this part because foam is messy stuff. We attached some straw extensions to the foam nozzle in order to reach all the way down to the forward centering ring. Then I sprayed a bunch of the stuff in there. The stuff expanded a bit more than we thought, so I grabbed the middle centering ring and shoved it in there to stop the expansion. You can still see some of the mess after the first few rounds of clean up in the following pics:

The straw extension.

Overall, this method worked very well. The next step was gluing in the back fins and filleting them, outside and inside.

Note the inner fillets on the motor tube and body tube.

Instead of inner fillets, many people put sections of fiberglass on the inside. Again, I didn't think it was necessary on this rocket. 

Then the aft centering ring was epoxied in and filleted. 

 And that's it for the structure! Next, primering and painting. 

I sprayed two coats of heavy filler primer, then wet sanded. 

Much smoother!

 However, this primer is also really good at filling in the spots between grains in sandpaper. So I went through a lot of sandpaper:

 

Overall, I did three primer coats. Next came painting. I forgot to paint the tube white before laying down a coat of yellow on the gray primer, so I ended up having to do three coats of yellow...oh well.

Painting the fins black.

Painting black stripes.

Finished paint job! Not my best work, but ok. Time for decal work.

Post-clear coat. Done!

I now have a rocket that is basically ready for an H and J motor at Southern Thunder. It's missing an altimeter bay for dual deployment (necessary for the 7000ish feet high that it will go on the J), but that's a short/easy project.
It's also long enough for a smallish, e.g. I, hybrid engine, so that might happen if I get my cert flights in in time.

The pile of hybrid engine parts.

I'll post a few pics of the launch next week.