The preheater grain is required for proper operation of the motor and encourages combustion of the fuel grain. The materials are still produced by Aerotech but you must know what to look for on the RCS website. It is made from a section of Blue Thunder propellant 1 inch long with a 3/8 hole bored through it and uses either a fiberglass or phenolic case with an outer diameter of 1.125. There are two options for the insulator. I went with the 12 inch long 1.125 OD x 1.005 ID which would give me 12 insulators under ideal conditions.
The alternative is the phenolic motor casing that has a 1.130 OD and a 1.005 ID. I just did not need 48 inches at this time. Once I get everything satisfactory I will probably order one.
I ordered two of the Blue Thunder propellant grains, the way HAZMAT shipping works I figured why not. But ordering 12 inch lengths of the grain is very economical. The correct grain is shown below.
The fist step is to mark them off in 1 inch increments for both the grain and insulator. Instead of using the razor saws I used a larger Japanese saw, it cuts on the pull stroke and is available from Amazon. I use them a lot in my home improvement and other wood working projects. I also used a larger aluminum miter box that would accommodate the saw.
The actual propellant grain cuts very easy, the motor casing take a little longer.
Once the grain and insulator is cut you must drill out the core because it comes from RCS as a solid tube of material. The use of a center finder makes it quite easy.
The preheater has a 0.375 or 3/8 hole through it, just drill it slow. I believe RCS states a max RPM of 500 to keep the heat down for safety reasons. I used a drill press, those that can drill a straight hole with hand drill could probably attempt it as well.
After using a center finder to mark the hole I got the idea to 3D print a drill guide to simplify the process. I designed it Fusion360 and it worked great. All that is needed to to be done now is to place the guide on top of the grain and drill the hole all the way through. Much neater and speeds up the process.
Fits just like it is supposed to.
The complete stack up for the forward closure is below.
From left to right: internal snap ring, injector and o-ring, nitrous o-ring, 0.010 teflon sealing disc, pyrovalve, 2 x 0.032 stainless washers, allen plug, forward insulator, preheater grain and insulator, aft forward insulator. A little bit more preparation than the standard Aerotech motor but not too complicated. The forward closure is now complete. The forward and aft preheater grain insulators are still available from Seastrom but for a single use product they are rather pricey at 3.35 each. I suspect they are made of fish paper, an electrical insulator, and could probably be made out of sheets of fish paper and cut out with a laser cutter if you have access to one. I have already made the SVG file and have access to a glowforge so I will try and cut some out.
Once again thank you to all have reached out and have given me tips as I undertake this endeavor.
Patrick Carroll 