Tip 1: Acquire as much knowledge as possible.
That includes reading books, scouring the internet and most importantly, find an experienced mentor to help. The knowledge and experiences of an experienced motor builder will be invaluable. Don't take one book, or one web site (mine included) or even one persons advice as the gospel truth. Take a long look at all the information as a whole, and make informed decisions from that.
First, let me start by saying there is one thing that has really caught my attention. No two motor builders do it exactly the same. Casting sugar propellant is perhaps as much an art as it is a science. Some of that has to do with what the builder has available to work with and some is personal preference. I've also noticed it's difficult for one builder to exactly duplicate anothers results. Let's explore the reasons for this.
First of all the chemicals used. KNO3 comes in a variety of flavors; prilled, granular, powdered, anti-cake added/no anti-cake, greenhouse grade, fertilizer grade, USP grade, you get the idea. Then there is environmental conditions such as ambient humidity and temperature. Was the KNO3 dried first or used as obtained? How was it stored? If the KNO3 was ground, how was it ground? For how long? Was it ball milled? Now you can go through the same with the sugar, perhaps not to that degree, but it all makes a difference.
Next we look at the process used to make the propellant. Everyone seems to have their own way of doing that too. Some add water, corn syrup, glycerin, or in my case propylene glycol. The melting pot you use makes a difference too. If your pot is 20 degrees hotter than mine, you boil off all the propylene glycol before it has a chance to work. Maybe your melting pot has a couple of hot spots and some cold spots.
Tip 2: Keep detailed records.
The point I'm trying to make is this, everything you do makes a difference. The only way for you to reproduce your results is to make detailed notes of all conditions, and then the results. Try not to make a bunch of changes at once. If you only change one parameter at time, your confidence level of the results of the change will be much higher.
Tip 3: Use an accurate scale.
I use a triple beam lab scale accurate to 1/10 gram. I know a lot of people use inexpensive digital scales now, but I'm not sure I really trust a $50 digital scale to be that accurate. If you must use a cheap digital scale, at least buy a set of accurate weights so you can test the scale.
Tip 4: Avoid creating dust clouds with any materials, oxidizers or fuels.
A dust cloud of any chemical presents the added risk of a dust explosion. We don't usually use powdered metals with sugar propellant, but they present the greatest risk.
Tip 5: Ground yourself before working with chemicals.
A dedicated ground strap is the best, Radio Shack sells them. The idea is to prevent static discharge which could ignite propellant or chemicals. If you work with powdered metals, they are generally more sensitive to high levels of humidity than sugar propellants, and require a fairly low humidity level, be aware the risk of static discharge is much greater at low humidity levels. With standard sugar propellants, i.e., KNO3 and a sugar, a mid range humidity is generally the safest to work in, such as 40 to 60% relative humidity.
Tip 6: Use a thermostatically controlled embedded element heating vessel.
Casting sugar propellant involves heating the KNO3 and sugar. Providing it is done using a heating pot that has embedded heating elements and has a thermostat control it's a fairly safe operation. Under no conditions do I feel it would be safe to heat over a gas stove or open electric element. I don't care how thick the pan or pot is I'm using. I don't think heating the propellant in a double boiler (wax or oil bath), is really safe either. The heating elements still need to be embedded, and now you have the increased risk of an oil or wax spill causing a burn. I use a Presto Multi cooker, it's $20 from most any department store and has a large capacity. I ALWAYS, always, always preheat the pot before adding the chemicals. That keeps the propellant away from those initial hot spots as the cooker heats up.
Tip 7: Don't cast in your house.
Don't cast propellant in an area you don't mind burning down. Serious now. You don't need the wife and kids living in the streets because you burned the house down. Do your casting outside if possible, or in an outbuilding of some sort. Regardless, keep a fire extinguisher and a five gallon bucket of water on hand at all times.
Tip 8: Wear safety clothing.
I'd recommend a leather welders apron, long sleeve cotton shirt, leather gloves and a face shield. Eye protection and gloves at the absolute minimum. One drop of melted propellant on your skin and I guarantee you'll wear protection next time.
Tip 9: Start small, work up incrementally.
One of the scariest things I see, is people wanting to build a "K" or "O" class motor right away. Their theory is they don't want to "waste their time with the little stuff". Oh, my! I've seen it many times and it still makes me shudder. Small PVC cased motors are a great way to get started. They are inexpensive, easy to build and much safer if they cato.
Tip 10: Derive your own burn rate data for your propellant.
I see a lot of numbers floating around, the a and n values, burn rates and such on different sugar propellants. Don't trust the numbers someone else came up with. Because of the differences mentioned at the start of this page, you're propellant will likely not follow the same numbers. I don't like the idea of gauging a propellant by its open air burn rates. What I've found is that how a propellant burns in open air has little to do with how it burns under pressure.
Tip 11: Understand the Kn ratio and how it affects the motors burn.
I think most people use the Kn (Kn is the ratio of propellant burning surface area to nozzle throat area) numbers when building a new motor. You can also use propellant flow rates when designing a new motor. Either way works fine, but Kn ratios seem a bit easier to work with. I have a Kn calculator program that I wrote in my Software page, you don't have to use mine (in fact there are flashier ones out there), but mine is accurate and I use it all the time. Understanding the Kn is vital to understanding how the motor performs.
Tip 12: Use good inhibitors and perform tests.
A Bates grain allows a propellant grain to burn on its outside ends and the core, the outer surface of the grain is inhibited from burning by applying an insulator or ablative material of some sort. Two things are absolutely imperative with a Bates grain: 1) The inhibitor must be securely bonded to the propellant so no burning can take place between the inhibitor and the propellant surface. 2) The inhibitor layer must be able to withstand the heat of the entire motor burn. It really depends on how long the motor burns, and the design of the motor. In most cases, casting propellant into cardboard, or on larger motors a phenolic tube is adequate. Of course testing is always in order.
Tip 13: Make certain your sugar propellant grains are free standing and not case bonded.
There are those that have had success case bonding (case bonding is casting propellant directly into the motor casing), but that is only after using additives to increase the flexibility of the propellant. All the propellants I use are too brittle for case bonding. The propellant grains must be sized to allow the combustion gases and pressure to flow around the grains, that way there is only compressive forces on the grains, which they can easily handle. If the grains are too large in diameter, they can seal the inhibitor to the casing, causing the grain to fracture. If the grains are too long, the grains may seal at the top and bottom of the motor, again, causing the grains to fracture from overpressurizing.
Tip 14: Use spacers between Bates grains.
The reason for spacers is to prevent the grains from sticking together, (end to end). This really concerns me, and I'll not sure how other people address this issue, or if they even do. If the grains all stuck together, you'd have one long grain, that would create a low Kn at the start of the burn, and a very high Kn at the end of the burn. The result would likely be an overpressure of the motor near the end of the burn.
I use PVC pipe large enough to touch the outside wall of the motor chamber, the segments vary in width depending on the motor size. 1.5" motors are generally about 1/8" thick, larger motors up to 1/4" thick. I then cut out a section of the ring so it forms a "C" shape. The reason for cutting out that section is to allow gases around and between each of the grains.
I'm not certain how long these spacers have to support the grains, but keep in mind the loading that will occur on the grains and spacers during liftoff of a rocket. Presumably, during a static test the spacers would only support the weight of the grains themselves. I also assume that once all the end surfaces are burning the gas layer that forms would help support and keep the grains apart, though I have no data to back that up.
Tip 15: Inspect grains for cracks.
With an outside inhibited grain it's not easy to see cracks in the grains. But it is vital to inspect the grains carefully. Most cracks are visible on the end surfaces of the grains, and extend into the grain.
Tip 16: Clean up the burning surfaces of the grains before using them.
I usually do this with erythritol and xylitol grains, I use a wire brush to clean the inside surfaces of the core, and I shave the ends of the grains with a razor blade. This removes any residue of wax paper, and exposes a clean layer of propellant. I wonder if a layer of only sugar forms too, on those outside surfaces. So cleaning them up a bit may help to expose a layer of propellant with a balanced ratio of KNO3 and sugar.
Tip 17: Do a density check on grains before testing.
Once the grain is trimmed to length (if needed), I carefully weigh and measure each grain. Using my Density/Converter software I input the numbers to see where the density is. Density will vary between propellants, but after a few tests you'll get the feel for what the density should be. My xylitol and erythritol grains run in the .061 to .063 pounds per cubic inch range. Sucrose grains run .064 to .065 pounds per cubic inch. If I need to cut or trim a grain, I usually use my band saw. The band saw cuts a very fine kerf and allows me to see some of the inside surfaces and check for voids or small bubbles.
Tip 18: Find a remote area to test your motors.
If you live in a city, you absolutely must find a wide open area to test your motors. I would suggest at least 1/2 mile and preferably more from anyone or anything of value. Even then, the motor should be surrounded by a berm or protective barrier of some sort. Digging a hole in the ground and performing the test with the motor underground is one of the easiest ways to add protection while static testing.
Tip 19: Keep yourself protected during a motor test.
Distance is fine, but the potential exists for a cato to throw motor parts for thousands of feet. Distance really only helps in the fact that if parts are thrown your direction, there is less of a chance of being hit by one. Let's say for example you are located 100' from the test motor during a cato. If your body is two feet wide, and the circumference of a 100' radius is 628, you have a one in 314 chance of being hit if a piece flies off on your plane. Now let's increase that to 200' distance. Now the circumference is 1256', and your chance of being hit falls to one in 628. So distance increases your odds, but is no guarantee. (Note: I'm just using a 2 dimensional plane as an example, in reality, the third dimension would make it even safer the farther back you move.)
With that in mind, there are other things to protect yourself. Building a bunker is nice, but not practical for most of us. Again, testing the motor with a protective barrier around it increases the odds in our favor. I would not recommend watching a motor test with the naked eye. Use a mirror from behind something such as a bunker wall, hill, berm, or big heavy car or truck. You should be video taping the event for data anyway, so you can watch it over and over once it's recorded. If you're feeling lucky, and had good distance, watching the test through both rear and front windows of a vehicle would provide some protection. As would a dedicated blast shield with a thick plastic window of some sort.
Tip 20: Have a fail proof safety switch on your ignition control box.
My launch control box has a key switch, I can take the key with me when setting the motor on the test stand. Here's the way I set up for a static test.
Once at my remote test site I check for any people in the area while I set up the test stand. Then I set up the data acquisition power supply and wiring. The launch control wire is then laid to the test stand, but is not hooked up at either end. I use regular extension cords, and have a whip end with alligator leads to go to the igniter that plugs into the extension cord. The motor is placed in the test stand, the key to the launch box is in MY pocket, the launch control box is not yet set up. I get a single igniter from my igniter case, (it's leads are shunted) and remove the nozzle seal on the motor. I unshunt the leads and insert the igniter all the way to the forward bulkhead of the motor. I grab the alligator clips leads from the still unconnected ignition wire, then short the alligator clip leads together. This is to make sure there is no power on the wire, there shouldn't be, as the launch box isn't hooked up yet, and also to discharge any possible static charge in the wire. The alligator clips are attached to the igniter, and I quickly make my way back to my safe area for performing the ignition.
Data acquisition and video recording is now started. The ignition lead wire from the test stand is only now inserted into the receptacle on the launch control box. Next, the power supply leads to the launch control box are connected to the battery. One last check to make sure all is clear. The key is inserted into the launch control box and turned to the power on position. The main power switch for the launch control box is now turned on. Next, announcement of the impending countdown, and to take cover. The continuity button is pressed to check continuity to the igniter. Good continuity is announced. Then, "Countdown, 5,4,3,2,1, ignition."
Tip 21: Wait at least 5 minutes before approaching a motor that failed to ignite.
This should be common sense. I've had motors that looked like they failed to ignite, without even visible signs of smoke coming from the motor, then after some time they took off. After a failed ignition, turn the power off, disconnect the battery, remove the safety key, unplug the ignition lead wire from the launch control box and wait 5 minutes before approaching the motor.
Tip 22: Keep extra people in the safe area all the time.
If you must have spectators, don't let them in the danger area at any point before, during or after the test.
Tip 23: It's best for only one person to prep, set up and conduct the test.
If you have other people performing some of the tasks, you can never be certain of what they did. This is a case of, "If you want it done properly, do it yourself."
Tip 24: Use a checklist.
To make sure you don't forget anything, and do everything in the proper sequence. It's only natural to get a little excited about the impending test, in your excitement it's easy to forget procedures.
Tip 25: Keep a well stocked first aid kit in your launch box and know how to use it.
Tip 26: Be over cautious.
You can never be too careful. But one time being under cautious can lead to disaster.