Tubes, Washers, O-Rings
Casting Tubes/Motor Liners
I have made my own casting tubes on a number of occasions but more and more, because of the time involved in preparing them, I find myself simply purchasing the tubes I use. The two main sources that I use are RCS and Always Ready Rocketry. These outlets are also the source of of the motor liners that I use. The casting tubes sold by both vendors are virtually identical. RCS sells the black, highly cured phenolic tubing at a very high price for their motor liners. ARR sells a tan, less highly cured phenolic at a much better price. I find that the ARR tubes work very well for the sugar motors that I fly. Even long burn moon burner configuration motors show minimal heat distress with these motor liners.
I find that the most fascinating aspect of this hobby is in experimenting with propellant formulations. Because of this, I do a large number of static test stand evaluations. My favorite configuration to do this is using a 2 grain, 38mm motor using Aerotech type hardware. The typical Aerotech 38mm propellant grain is 46mm long with a 3/8" central core. As I was preparing so many of these grains I found that the time involved in making the casting tubes for these motors too time consuming. I contacted several paper tube makers that were prominent in their advertising on the internet and asked them to give me a quote on the preparation of some thin walled, kraft paper tubes of the dimensions that RCS uses/sells for their 38mm casting tube stock. The best quote required a minimum order of $150 and I bit the bullet and had them make $150 worth of 46mm long casting tubes. This turned out to be over 2500 such tubes and wound up costing me six cents a piece (the same sized tube from RCS stock costs 25 cents a tube). I have not regretted my investment. These tubes were purchased from Jonesville Paper Tube. The next time that I find myself needing to order more of these tubes I will most likely shorten the tube to 45mm so that I will have enough space in the motor to include o-ring spacers between the grains.
Another important component used in motor construction of Aerotech type motors are fiber washers used to face the forward end (and sometimes the aft end as well) of the grain stack used in the motor. These fiber washers are used once and then thrown away. They are quite expensive when purchased from RCS. They are also quite expensive if purchased direct from vendors as well. I have found that steel "shim" washers work just as well as fiber washers. I purchase these washers from McMaster-Carr. They can be purchased in sizes that fit very well into the standard aluminum motor cases used in HPR although some times I have had to file them down just slightly to get them to slide easily into the case. These washers are inexpensive, have a large central hole, and best of all, are reusuable.
I use a lot of o-rings in my motors. Again, since they can be purchased much less expensivly from vendors that manufacture their product, I buy my o-rings in large quantities. The vendor that I use is Marco Rubber. They have excellent prices and will sell their standard size Buna-N O-rings at around 5 cents a piece when purchased in lots of ~300 to 500. Not only do I use these washers in the traditional areas where hot propellant gas sealing is necessary but also as propellant grain face separators. They provide an easy method to separate grain faces so that each face surface can ignite more easily, thus providing for a more neutral burn in a motor. The o-rings used as grain separators are 0.070" thick and are chosen in size to match, as closely as possible, the I.D. of the motor liner into which the grains are inserted. Silicone O-rings can also be purchased in the same sizes from this vendor for use with graphite or steel nozzles which get hotter than composite type nozzles.