PROPELLANT MIXING

Once you have developed your propellant formula and designed your motor requirements as to the number and type(s) of grains, you are ready to begin mixing propellant. SAFETY IS PRIME at all times from this point on. Please check our document on BASIC SAFETY, which by no means is comprehensive. At all times keep the safety of your environment, others who may be present and of yourself. Mixing propellant is not a walk in the park. In all honesty once you have made your first batch you may sit back and say, "Well that was easy." It is easy BUT this does not mean the hazards are minor. The slightest lax of attention to what you are doing, the casual inter mixing of tools and/or chemicals, sloppy house keeping or just plain confidence of what you are doing can create major disaster. Pay attention and be safe.

To begin a clean work area is mandatory. Plenty of light and ventilation is needed, as well as open space to move around in. A cluttered work area is a path to problems. Place newspaper over your work table. This will make clean up easier and insure all pieces, parts and crumbles of propellant, and spilled chemicals, are confined to one area.

Be sure your mixer is clean and its surrounding area is free of junk as well, not like this example!. A cluttered work area, again, can be disastrous. Things that do not belong here should not be stored here.

Set out the chemicals you will be using first, usually the liquid items such as R45, DOA. etc. Try to organize the work area to permit plenty of work area. It will get crowded as the process advances.

Prep your grain liners before you begin mixing anything. Once mixing starts you will basically want to concentrate on that alone. Different people use different methods to handle their grains from wooden grain boards, which hold all of the grains at one time, or to individual grain set ups as shown to the left. The individual set ups are preferred as they can be worked and moved about much easier.

Make sure all grain set ups are well lubricated, meaning the bases and the mandrels, otherwise you may have problems releasing them. We use Mann's Mold Release, which comes in an easy to use spray can.

Once you have your work area secured, and your equipment ready, and your chemicals ready and you have your grain liners set up then you must make sure you have your recipe. Make sure you have decided how large a batch you wish to make, many times experience is needed, and make sure each ingredient is broken down to proper required weights per percentages of your batch size.

BEGIN

When mixing propellant the second most important thing, after safety, is always mix your liquids first. Place your mixing bowl on the scale and tar the scale. This is the process of zeroing out the weight of the bowl so all you will be measuring is what you place in the bowl. Usually the first ingredient is always the rubber fuel, in our case R45M.

Pay attention to the scale and be as precise as you can when you measure out each ingredient. Small batches require very exact measurements, while larger batches offer a little leeway, but this does not mean you needn't pay attention. Attention to everything is always critical or you will be in for a very unwanted surprise.

Add each liquid ingredient as you work down your recipe list. Do NOT add the curative at this time! If your formula calls for Tepanol, now is the time to add it. Turn the mixer on to begin blending your liquids while you begin the next phase.

Solids are next. Do NOT add the AP at this time. You want to add your "metal" first such as Aluminum, Magnesium, Strontium, Copper Oxide and whatever other chemical solids the recipe calls for. This process is extremely dangerous because of the inherent dust factor. All of these solids, alone, may be completely safe and non-flammable, however in dust form they all - no exceptions, can be deadly. This may sound over serious AND IT IS! Dust will be your most dangerous factor.

Carefully measure these out in cardboard bowls. Do not use plastic bowls or plastic tools. Start with your lightest colored chemicals first, sometimes it aids in knowing where you are in the recipe. Stop the mixer and place the first individual chemical into the liquid and hand stir with a rubber spatula. Be attentive NOT to create any dust. Turn the mixer back on and move to the next item in the recipe while the first is being blended in the mixture.

Repeat for each "metal" item to be added. Be precise, make no dust and stop the mixer to add it. Never add chemicals of any kind into a running mixer.

Once all the solids are blended in the liquid mixture you are ready for the AP. Turn off the mixer and remove the bowl. Carefully measure out your coarsest AP. Pay attention to the scale AND dust. Once you have the AP measured place it into the mixing bowl with the liquid. Do this softly as to not make dust. You would be surprised how much dust could develop with even 400AP. With the AP in the bowl hand mix it to work the AP into the liquid. Don't mix it completely by hand, that's what you have a mixer for. If you are using additional AP meshes, measure them and add them in the same way. Always place your coarsest AP first and your finest last. Always hand mix your potion before placing on the mixer. This is a Safe Practice and while not required it does make common sense and common sense is always right.

MIXING
Now you are ready to begin mixing. Place the bowel on the mixer. Make sure it is secured. Turn on the mixer. Depending on your formulation it is best to mix for at least 60 minutes. This time may vary from person to person, formula to formula. 60 minutes will assure a good mix. During the mixing you should clean up your work area. Its amazing how cluttered things can get! In between cleaning and prepping your liners you should stop the mixing process and use a rubber spatula to clean down the sides of the mixing bowel and paddle. You can do this at least every 10 to 15 minutes during the mix.

At this point in the process you can stop mixing and do other chores or even leave the mixture sit overnight, protected of course. The mixture of liquids and solids will not do anything but sit and wait for the next procedure - adding the curative.

Here is where time begins to run. Adding the curative to the mixture begins the catylization process to turn the liquid mixture into a solid rubber matter. Make sure your liners are ready, work area is clean and the vacuum pump is ready. Measure out your curative and pour it into the mixture. Turn on the mixer and let it run for 10 to 15 minutes. Again this time is critical and may vary from formula to formula. You may want to stop and clean down the bowel sides and paddle but don't take forever and your mixture is kicking off and the clock is running.

Some people will vacuum their mixture before adding the curative but it is not needed. At the end of the curative mixing time turn off the mixer and connect the bowel to your vacuuming apparatus.

The first time you vacuum you may be taken by surprise by the process. Under vacuum the mixture expand rapidly rising up and if not caught could cover the nice plexiglas vacuum lid you made. Be prepared to shut the vacuum off quickly if it rises too fast. Turning the vacuum off and on and off and on will soon bring the mixture under control where the swelling is not as radical or fast. Rotating the vacuum bowel will also release trapped air within the mixture. In a short time you will master the vacuum process and when the mixture seems to be stable you can turn off the vacuum. Do not spend an excessive amount of time vacuuming, remember the clock is running.

POURING
Once your propellant is vacuumed the "pouring" takes place. This word is used although in most cases you're mixture is so thick, much like cookie dough, you will have to scoop it out and place it in the liner. Wear gloves, which you should have been wearing all along. Remove some mixture and roll it into cigar shaped pieces and then place them into the liner. Using a wooden tamping stick tamp the propellant down into the liner. Tamping will remove any air pockets created when you place the pieces of propellant into the liner. This is not a hard process and you will quickly see what is happening.

Take your time, even though the clock is running, to assure your propellant is placed solidly in the liners, no air pockets exist and you keep the outside of the liners clean. Don't be messy, it always seems clean looking liners make great rocket motors. Your propellant should fill the liner completely. As the propellant cures it will rise slightly forming a small biscuit head above the liner top. This is good.

By this time any unplanned hazards are past. The propellant is in the liners and you should place them in a safe stable place to finish their cure.

The job isn't done yet. Clean up everything. Make sure the mixing bowel and paddle, as well as all the tools you used are cleaned and put away. Do not let any mixture remain on any surfaces. This is flammable material now and will add to any possibilities for hazard in unpredictable situations. Collect all paper and dispose of it properly. Any left over propellant, pieces, ingredients and whatnot should be disposed of by burning in a safe place away, far away, from your work area. Collecting unused and scrap propellant in the time honored burn bag is not a good practice. All unused propellant should be safely burned as soon as possible. As long as you pay attention to all aspects of your endeavor making Rocket Motors is easy and fun but never safe.

 

REMEMBER TO CLEAN UP