Introduction: Assembly of an Aerotech Rocket Motor
This tutorial covers an alternative method for assembling Aerotech-brand rocket motor kits. This method can, with some slight modifications, be applied to any Aerotech motors s54mm and smaller. When there are changes that I know of for different motors than the one described here, i will mention them, though i may not find all changes.
Disclaimer: don't buy rocket motors you aren't allowed to own or fly, don't use my guide alone (use the assembly diagrams available on the aerotech website!!!!), your mileage may vary, etc.. However, I have directly supervised probably almost 100 motors using this technique with zero failures, which is a pretty good success rate.
DO NOT, UNDER ANY CIRCUMSTANCES, TOUCH ANY EXPOSED PROPELLANT FACE. GRIP THE DELAY ELEMENT AND PROPELLANT GRAINS BY THEIR SIDES (COVERED IN PLASTIC ON THE DELAY ELEMENT AND PAPER ON THE PROPELLANT GRAINS), NOT THE ENDS. Most of the time nothing bad will happen, but the Mojave green propellant is toxic, and the delay element can fail to burn if you get oils from your hand on it, leading to a lakestake.
Step 1: Delay Stack Assembly
The Delay Stack is what I am calling the delay grain (gray plastic-lined propellant cylinder), the delay spacer (colored paper tube, very short), and the delay insulator (thick white paper tube). These parts are shown in the first image; the length of the delay element and the delay spacer (and the delay spacer color) may vary from reload to reload.
DO NOT TOUCH THE ENDS OF THE DELAY GRAIN.
The Delay insulator needs to be de-burred, the step referred to in the instructions as chamfering. This is often unnecessary, but is easy to do with a bit of sandpaper-the idea is to remove the rough edge on the ID of the paper tube.
If you are using a delay adjustment tool to change the delay time, now is the time to do it.
After deburring and delay adjustment, assemble the three elements as shown in the image. If you drilled into the delay to shorten it, the hole should face the delay spacer, not the protruding side.
Step 2: Delay O-Ring
Take the smallest Oring, the one for the delay, and grease it using the synthetic teflon super-lube. just a light film is fine, enough so that it is slightly slippery. CAREFULLY AVOIDING GETTING GREASE ON THE EXPOSED DELAY GRAIN FACE, stretch the delay grain over the protruding end of the delay grain as shown.
Step 3: Assembly of Forward Closure
This is a long step, my apologies. First, gather all of the materials as shown in the first image: the previous assembly, the forward closure itself, and the forward delay spacer, a flat black neoprene washer.
Grease the inside of the forward closure. Use a finger to smear grease over the entire interior surface. You will inevitably plug the touch-hole (if this is an open closure) with grease, as shown in the second image; use a small screwdriver or the like to clean it out, so it looks like the third picture.
Take the neoprene washer and lay it flat in the bottom of the delay well, as shown in the 4th image:
Now, take the assembly from the previous step and force it into the forward closure as shown in image 5. The aft delay spacer, hollow space, should face backward, toward the propellant, again as shown. It will take some force to get the oring to compress properly for the stack to fit in place; this is most easily done by placing the stack down on the table oring up and then using your palm (while standing) to force the closure over the stack.
Finally, take the forward main oring, lube it similar to the delay oring, and then stretch it over the forward closure as shown in the 6th image. If it is loose, it is probably the wrong oring; consult the assembly diagram and the oring identification chart.
Set this assembly aside for the time being.
Step 4: Front End Assembly and Liner Insertion
Take the case and the liner, and see if the liner fits neatly in the case. The liner should slide easily in and out. If it does not, it needs to be sanded, or if it doesn't fit at all, then the outer layer of paper wrap needs to be removed entirely. Once the liner fits, it should be thoroughly greased on the outside, and slid into the case as shown, leaving about half an inch exposed (wipe up excess grease).
NOTE: if your motor has a black hard phenolic liner, it should not be greased.
Place one of the black hard washers (insulator discs) on top of the liner, as shown in the first image. pushing on the washer, slide the liner the rest of the way in, so that the threads inside the case are exposed. Then, take the assembled forward closure (including the last oring) from the previous step, and screw the forward closure onto the case-make sure not to drop the oring, though that shouldn't be a problem. The insulator should sit up against the oring on the forward closure. The excess grease from the liner insertion should protect the threads.
The far end of the case should look like the second image, shown next to the stacked propellant grains we will use in the next step.
Step 5: Grain Insertion
Stand the motor case on the forward closure. Using one or two fingers, pull the liner out about half of an inch from the inside of the case, as shown in the first image. Then, handling only the paper sides of the casting tubes, drop the grains down into the motor case inside the liner. This method makes it impossible to accidentally get grease on the grain faces. From above, it should look like the second image. With the liner still protruding, set down the remaining hard black insulator washer on the liner, and slide it down into the case until it stops., as shown in the third image.
Step 6: Aft End Assembly
This step differs in appearance and order for different size motors, but the gist of it is assemble the remainder of the motor. Place the nozzle and greased oring in the appropriate places, and then screw on the aft closure. The illustration is for 38mm, where the oring goes around the nozzle, so the oring goes in first. In 29mm and 54mm, the nozzle goes in first, followed by the oring that fits into a gland on the nozzle casting.
Step 7: Final Assembly
Use a knife to clip the corner off of the plastic cap that fits over the nozzle, and slip it over the nozzle as shown for storage. Tape the ignitor to the side of the motor case, labeled with the type of motor within, and put the large red cap over the forward closure (only a little bit-it's a pain to take off. Black powder should be stored separately and added in the field.