Building the Aerotech Barracuda for HMC E80

Pictorial Outline of building the Aerotech Barracuda rocket for Harvey Mudd College's E80 class, equally valid for the Aerotech Arreaux kits used as well.  It replaces the kit instructions, which are generally fairly poor in my opinion, but it is probably also worth reading them anyway to get a sense of how the rocket goes together. 

(Yes, the picture is of an Arreaux and not a Barracuda.) 

Also, this tutorial was prepared not while building a normal for-E80 Barracuda, but one being custom-build around a very expensive set of accelerometers. Therefore, some steps are not pictured as they were altered. All necessary steps are described here, however. 

It is highly recommended to read the entire tutorial before beginning construction. 

Finally, a note cyanoacrylate: the pink cyanoacrylate glue we are providing for these kits is kind of the worst of both worlds-when you don't want it to set up instantly, it always does, but when you need something to stick in place, it never cures fast enough. I have tried to avoid these problems in the build layout, and have also tried to point out places where either of these tendencies  will cause problems, but in general, think ahead and be careful. 

The main body tubes has slots in it for fins and for the launch lugs; these must be cut out with a sharp knife (carefully, of course). I used my pocket knife for this rocket, but a better tool would be a boxcutting knife, an x-acto,  or other straight razor, sharper and thinner.

Slip your chosen blade into the slot, and then slip it back out the other side of the slot, as shown. Now you can (carefully!) slide the knife down the length of the slot to the end, and slice through the cardboard holding the strip of extra material in place.  

Remember to get all 5 slots:

There are two tubes requiring cutting. 

Payload bay: this is the 1.9" diameter tubing, the larger stuff. Your kit came with two pieces-the one with slots cut out of it, and the one without holes; a payload bay. The E80 rockets require a payload bay that will be 14.75" long; this is longer than the Arreaux payload bay, and shorter than the Barracuda payload bay. Therefore, Barracuda kit users need only to cut down their existing payload bay, whereas Arreaux users will need to acquire a longer piece of tubing-ask a proctor or the section Prof. 

Motor Mount this is the smaller, tougher ~1/25" tubing. Both kits require a motor mount, or MMT, that is ~15" long. Users of both kits will need to ask a proctor or section prof for a long piece of MMT tubing and then cut it down to 15". 

WHen you are done, I highly recommend going through all of the parts of the kit, and discarding the ones other than what is shown in the kit above; they will not be needed, and you should make sure you have all of what is shown above. 

Shock cord
Parachute
motor mount hardpoint/baffle, next to the parachute
'cooling mesh', steel wool next to the parachute
fiberboard centering rings
fins
launch lugs, small black plastic next to the fins
two eyehooks
fiberboard bulkhead, with a small hole in the center
Fin-Lock rings, below the parachute-verify that both have spots for three fins
the coupler, shown standing upright
nosecone
motor mount tube, cut to 15" in length
Payload bay tube, cut to 14.75" in length
Body tube, with excess paper slits removed
Plastic motor retainer, consisting of two parts, shown next to the eyehooks

All other parts can be safely discarded. 

The motor mount front end assembly is fast and convenient, so we will start with that now. Take the 'cooling mesh' of steel wool and drag it out into an elongated mass about 4-6" long; then, stuff it into one end of the motor mount tube as shown. 

Take the hardpoint/baffle, and check that it fits inside the motor mount tube; if it does not, sand one or the other. Then, apply a thin bead of cyanoacrylate as shown, and swiftly slide the baffle/hardpoint thing into the end of the motor mount tube where the cooling mesh is. After a second or so, it should be firmly in place. It doesn't hurt to twist the baffle/hardpoint thing as you slide it in. If you do this insertion/twist too slowly, the cyanoacrylate might seize partially in place; the joint can be reinforced with epoxy and fiberglass if this happens. Ask a proctor for help if it does, but if you are swift it shouldn't be a problem. 

FInally, tie the shock cord to the eyebolt as described in the Aerotech instructions, and thread the eyebolt into the small hole in the baffle/hardpoint. 

Take the opposite end of the MMT, and measure out and draw lines around it 1" and 3.5" from the end of the tube, as shown below. These measurements are identical for both the Barracuda and the Arreaux, and for the plastic motor retainer or the metal AeroPac one if those are being used in the future. 

Then, slide the two fin-lock rings and the two centering rings onto the tube, as shown. This is just a test-fit; do not cement them in place now. The aft edged of the fin-lock rings should line up with the drawn lines on the MMT. If they do not slide easily, sand out the inside of the rings until they do. 

[Not pictured, but it is helpful to actually snap (but do NOT glue) one of the fins into the fin-lock rings at this point to ensure proper rotation-alignment between the two cylinders.]

Move the centering rings aside, and put healthy fillets of cyanoacrylate around the edges of the fin-lock rings. Then, while the cyanoacrylate is still liquid, slide the centering rings back up against the outside edges of the rings. 

Setting the motor mount aside, let's build the coupler and install it in the payload bay tube. Take the fiberboard bulkhead and the remaining eyehook, and thread them together. A drop of cyanoacrylate in the last half-turn isn't necessary but it might help prevent problems. A vise or at least pliers will probably be necessary, unlike the plastic baffle, which tends to tap easily.

Now, you will want to take a short length (~6-8") of thick white rope, or the flat kevlar strap shown here, and tie it to the eyehook. The other end of it should either have a loop tied in it, or be tied off to a quick-link as shown here; you will have to consult a proctor about the availability of quick-links, however. The point of this cutoff is that the eyehook will be nigh inaccessible later on and you might want to easily separate the payload bay from the booster. 

Apply cyanoacrylate to the rim of the coupler tube as shown; make a nice, thick bead all the way around, when you are done, carefully lower the cutoff through the coupler, cementing the bulkhead across the end of the coupler. It will help to do this last bit against a surface (like the table with paper towels on it or something) to get it properly centered. WARNING-this is an easy step to glue your fingers to the rocket. Use gloves and be extra careful. 

After it is dried, test fit it into the payload bay. If it does not fit (probably because the bulkhead isn't centered on the coupler), sand it until it does. Measure out the 2.5" shown in the last image, apply cyanoacrylate in a manner similar to that shown in step 3 onto the upper portion of the coupler, and slide-while-twisting the coupler into the payload bay tube up to the line you drew. 
 

Take the three fins, and carefully with pliers, snap off the extra set of fin roots from the bottoms of the fins. With sandpaper or a sharp knife, remove the remaining flashing as shown in the second image. Finally, I recommend sanding down the snap-lock ridge on the fin root as shown in the third image; just a little bit to make it easier to snap in and out of the mounts on the motor tube. (the left fin has been sanded a little). 

[not pictured]
Returning to the motor mount, feed the shock cord through the booster airframe, starting at the fin slot end. You can then insert the motor mount tube (without glue!) into the booster tube. 
[/not pictured]

I highly reccomend dry-fitting all three fins in place at this time, using the snap-lock system to snap the fins all the way in place. This will hold the motor mount tube in place while you glue it in. Do not glue in the fins at this time, however. 

You want to dribble superglue into the back of the motor mount tube, trying to build up a fillet of sorts around where the aft centering ring meets the booster airframe.

The instructions suggest that you should do this for the centering ring that is part of the forward baffle/hardpoint too, however I have no idea how one would go about doing so. I have not found it necessary. 

Anyway, you can mix up some JB-weld at this point and use it to epoxy on the male-threaded portion of the motor retainer, slipping it over the exposed end of the motor mount tube. You should wipe up any excess JB-weld from the interior of the motor mount tube now. The last image shows the retainer already bonded on, and the built-up cyanoacrylate fillet inside the rocket (as best as I could photograph). 

Remove the fins from the previous step, after ~15 minutes to ensure that the majority of the cyanoacrylate is solid. 

With ~120 grit sandpaper, rough up the area around the fin and launch lug slots, as demonstrated in the third image. You don't have to (and shouldn't) sand very deep, just take the shine off of the paper. 

One fin at a time, you should apply a thick bead of glue along the motor mount as shown (and up inside the fin-lock rings), and a thin bead around the fin (the part of the fin highlighted in purple should be covered in cyanoacrylate). 

Then, all you have to do is carefully (but swiftly, ~15 seconds at most) push the fin down into the slot where you put the glue bead, and then snap the fin back down into the fin-lock rings. Make very sure that both the front and back snap into place; part of the purpose in the previous step's dry fit was to make sure both snaps work. you probably don't even need to hold it, since the snaps provide excellent mechanical support; you can go straight on to the next fin. 

After doing all three fins, you can apply cyanoacrylate as shown to the launch lugs, and push them into their slots. They won't sit flat-because they are flat-bottomed and you are applying them to a round airframe-but just hold them there for a few seconds, until they don't fall back off. A piece of blue tape can also be used to hold them in place. The slots in the airframe actually doa great job of making sure that they align properly, so you don't really have to worry about that. 


That's all I cover in this tutorial. You will still need to tie the free shock cord end to the payload bay via the cutoff you installed in step 5, tie the parachute to the shock cord (about 12" from the payload bay end works well), and drill various vent holes and rivet holes for the nosecone-but these are covered well in the existing instructions, and the vent hole placement and sizing, as well as the nosecone attachment, should be something you calculate based on payload requirements, anyway. That's it!

Painting the plastic nosecone and fins isn't impossible, but requires care. Clean them thoroughly with a damp cloth and soap, and rinse all the soap off with a damp cloth. Then use a plastic primer. FInally, after waiting the appropriate time for a full cure (48 hours for Rustoleum plastic primer), apply a general purpose primer to the entire rocket body, including the fins. Follow the primer instructions with regard to the topcoat paint.