How to assemble an AeroTech 24/40 hobby RMS reload

by Rick Dickinson, NAR#73975

Introduction

Maybe you've been using Estes and Quest blackpowder engines in your model rockets for a while now, and you're looking to move up the power scale slightly. Or, maybe you're getting tired of the poor quality control on the ejection charges in Estes D engines with A or B date codes, and want to move to a more reliable technology. Or, maybe you just like the sound, smoke, and propellant choices available when moving to composite motors.

In any case, you have in your hands your very first AeroTech Reloadable Motor System (RMS) casing, the "24/40" hobby motor. This is a black aluminum tube, 24mm in diameter (the same size as an Estes D engine) with a brass aft closure and a black anodized aluminum forward closure.

There are a number of reload kits available for this casing. They are the D9W, D15T, E11J, E18W, E28T, F12J, F24W and F39T. Each is available in several delay times, just like the black powder motors you are familiar with. Most AeroTech delays are pretty close to accurate, as opposed to the Estes delays which tend to be slightly fast. The letter at the end indicates propellant type: J is BlackJack, a low-thrust, longer burning, highly smoky propellant; W is White Lightning, an average thrust, medium burn time motor with a white or light grey smoke trail; T indicates Blue Thunder, which is very fast burning, easy to light, has a blue or purple-tinged flame, and produces relatively little smoke, and the most thrust of any of the three propellant types.

For your first RMS flight, I suggest using a rocket that you usually fly on an Estes D12 engine, and flying it on a D15 reload. If you normally use a D12-3 or D12-5, use the D15-4T reload. If you usually use a D12-7, you should probably use the D15-7T, although lightweight minimum or near-minimum diameter birds, like a Longshot, might need a D15-10T. An Estes Silver Comet flys very well on a D15-4T.

Make sure that your shock cord is in good shape, and nice and long. This is good advice anytime, but especially when you are experimenting to find the perfect delay and motor combination for a particular rocket.

In addition, because the forward closure sticks out past the end of the engine, if you are using an Estes engine hook, clip or file off about half of the 1/8" projection at the front end of the clip to prevent it from scraping the side of the closure when you put the reload casing in your rocket. This is easiest to do before assembly, but can be done after the fact, if you are patient with a file, or use a dremel-type tool on an extension wand to reach into the motor mount.

Now, let's get ready to assemble your first reload. Gather together the bag containing your reload kits, your casing, some paper towels or rags, and your grease. AeroTech supplies a sample of Permatex lubricant with the casing, which should last you through a couple of packs of reloads. When it runs out, Vaseline works well, as does the tube of "Super Lube" sold at Radio Shack, which is the exact same Permatex lubricant AeroTech gave you a sample of.

Getting Started

Your bag contains parts for three reloads. Dump them out carefully on a clean table, and seperate them into three equal piles, with the same parts in each pile. Now, put two of the piles away, and let's work with the pile you have left in front of you.

First step -- lightly grease the O-rings. There should be three of them. One will be small (the delay O-ring), and two will be larger, with one thicker than the other. Place a *small* bead of lubricant on your fingertip, pick up all three O-rings, and rub them between your fingers to evenly spread a film of grease on all three. They should be shiny, but have no globs of grease. Set them to the side, and lightly grease the threads on the casing and closures, as well. A very light coat of grease inside the casing can help with cleanup, later. Set everything down, and go wash your hands.

Delay Assembly

Next, assemble the delay grain in the delay liner. Depending on the exact delay of your reload, there may also be a spacer that fits in the delay liner. This is a tight fit. Rolling the edge of the delay grain on the tabletop to "push" a bit of bevel into the leading edge, and avoid "frayed" cardboard helps, as does "deburring" the inside of the end of the delay liner with the side of a pencil or pen or your fingernail. I usually place the liner on the table, deburred side up, and push the grain (deburred side down) firmly straight into the liner, followed by the spacer, if needed.

Flip the delay assembly over and make sure that the delay element (and the spacer, if included) are flush on both ends of the liner. If one end has to stick out a tiny bit (1/32" or so), make it the "spacer" end.

Propellant Assembly

Next, assemble the propellant grain in the propellant liner. If this is a D or E reload, there will also be a spacer tube included (there are no spacers for the F reloads, as the grain fills the entire casing). Take the propellent grain, and place a piece of 3/4" wide masking tape over one end, running vertically just like the slot. The ends of the tape should extend about 1/2" over each side, and fold down onto the sides. Insert this end of the propellant grain into the propellant liner. Insert the propellent spacer, if any, into the liner from the other side, and push it in until it is flush against the end of the liner. This will push the grain about 1/16" out of the other end. This is normal and correct.

The propellant liner will hold the masking tape in place, as will the spacer, if included.

[Side note: I like to pre-assemble the delay assembly and propellent assembly of each of my reloads as soon as I get them, to save time later (and save space in my range box). I write the reload designation, such as D15-4T, on both assemblies, in case they get mixed up. The O-rings and fibre washers are the same in all of the 24mm reloads. The delays and propellents are, of course, different, as are the nozzles. Keep careful track of these.]

Final Assembly

Place the small O-ring into the delay cavity in the forward closure, and make sure it's all the way in and lying flat. Slide the delay assembly (with the "flush" end of the delay going in first) all the way in to the cavity. It will stick out a bit, this is normal and correct.

Take the complete propellant assembly, and slide it into the casing so that it is centered between the ends. The side with the spacer, if any, is the "forward" end. If there's no spacer (F engines), then the "flush" end is the forward end. Place the fibre washer (the dark grey thing that looks like a washer) against the forward end of the propellant assembly, followed by the thin O-ring. Screw the forward closure (containing the delay assembly) into this end fully.

Make sure that the nozzle isn't clogged. If necessary, you can poke out any mold "flashing" blocking the nozzle with the tip of a pencil. Place the nozzle into the aft end of the motor, against the propellant grain. Place the thick o-ring into the groove around the nozzle, and gently push it into place with a popsicle stick or small screwdriver, being careful not to scratch the casing threads.

Finally, thread the aft closure into the aft end of the casing, over the nozzle. You may have to push firmly to compress the O-rings and get the threads to engage while turning. A piece of thick metal with a U-shaped notch cut out of it (to fit over the nozzle and engage the notches on either side of the aft closure) makes a handy tool for tightening the aft closure, or you can simply use a rag or leather glove to protect your hand while tightening.

Igniter and Ejection Charge Installation

Contrary to popular belief, Copperhead igniters can work very reliably *IF* they are handled carefully. contrary to the instructions, don't bother to use the red cap to hold the igniter in place. Instead, carefully insert a nice, straight, uncracked Copperhead igniter through the nozzle, gently probing for the slot in the grain, and down the slot of the grain until you hit the masking tape. Wrap another loop of masking tape around the end of the nozzle, and pinch it together to hold the copperhead in place. Then, use the point of a pencil to poke a hole through the tape covering the nozzle, to allow pressure to vent.

If you are using some other type of igniter than Copperheads, you may need to insert the igniter before putting the nozzle in place, as the head of the igniter may have trouble clearing the nozzle in certain reloads (particularly black jack and white lightning -- blue thunder reloads tend to use bigger nozzles, I've noticed).

Before you fly, don't forget to put the ejection charge in place! Holding the red cap containing the black powder ejection charge like a cup so as not to spill the powder, place the engine, forward closure down, into place, and snap the cap onto the forward closure. There are two caps -- one of them is made to fit on the nozzle, and the other one is the one for the ejection charge. Sometimes, Aerotech puts the charge in the wrong one -- test fit to make sure you have the right one. Turn the engine right side up (nozzle-end down), and gently shake it a bit to make sure some black powder falls through the hole in the forward closure and makes contact with the delay grain.

Some Final Advice

I usually assemble the "first reloads of the day" the night before a launch. If you do the same, don't put the ejection charges in place until you are ready to put the reload in your rocket. The red caps have a tendency to get knocked off in your range box. Also, if you decide to disassemble the reload for any reason (for example, if you decided that another reload would be a better choice for what you want to fly), you'll get black powder all over everything, including the O-rings, which will need to be cleaned off.

Once the ejection charge has been installed, if you have to loosen any of the closures for any reason (such as removing the aft closure and nozzle to install an igniter that's too big to fit through the nozzle), you run the risk of getting black powder under the O-ring in the ejection well, which can casuse premature ejection and other problems. If you loosen the closures, it's a good idea to desassemble the motor, clean and grease the O-rings and casing, and then reassemble.

Clean your reloads as soon as they are cool enough to handle, using soapy water (I use dish soap and water in a small plastic bowl), paper towels or rags, a small nylon brush like an old toothbrush, and/or baby wipes. They are *much* easier to clean when still warm, before the crud has time to harden.

If you have any more questions, feel free to ask.

- Rick "Whoosh generator specialist" Dickinson ( rtd@notesguy.com )