One of the main problems most of us have making motors is eliminating air bubbles which can cause a motor to over pressurize and create havoc. This is especially true in large diameter motors where propellant is placed in pieces at a time to fill a casting tube. Despite rumor, motor propellant is not poured it is mostly "placed". This process can create voids in the propellant despite the best efforts to evacuate the propellant before casting it.

A complete working Vacuum Casting Machine capable of filling a 6" diameter Casting Tube is simple and workable.

A discussion of NASSA members took place at a BALLS launch to figure out a better way to cast larger 6" diameter grains. Present at the discussion was John Rakonnan, long time Amateur Rocket person and former Thiokol Propellant Master. He told us of a way to cast propellant under vacuum and assured us that this technique was proven and worth the effort. Under such a recommendation NASSA members began the development of a true Vacuum Casting System.

Les Derkovitz head up the development team, we should say "was" the development team. With assistance from Jerry McKinlay and Oliver Schubert an apparatus was designed and fabricated that, with minor technical adjustments, became a working Vacuum Casting Machine.

Developing vacuum casting in NASSA, both the hardware and the production techniques, was sort of a natural. We had thought and talked about it for a long time, and after we had a couple of disappointing casting sessions pouring a motor for BALLS when voids were coming up on the surfaces against both the mandrel and the casting base, the time was right to pursue vacuum casting. Because in NASSA we pour motors in many different sizes and configurations, the challenge was to develop a casting chamber that could accommodate all the sizes we deal with in a convenient “one motor at a time” production session.

Wooden base with the 6" aluminum chamb- er (not shown in position) and an air board sander explained in the text.

The concept is to have a chamber that contains the casting tube, is sealed and can be drawn down to a vacuum with our pump.  Into that chamber the propellant has to be drawn in and injected into the casting tube.  The "tube" of the chamber is a 30" inch long piece of the tube that we can make 6 inch motor grains with.  We went with 30 inches because it is doubtful that we will cast a single grain any longer than 25 inches.  The remaining 5 inches is needed to align the "spigot" from the ball valve to the casting tube via use of a hose.

A base plate was fashioned from MDF and plexiglas. Click on the photo to the left to see how the plexiglas sheet is sandwiched between the two pieces of MDF. A slot was routed out in the plexiglas. to allow for the silicon gasket that would seal the bottom of the chamber. A set of 4 bolts pulls the vacuum chamber tube down against the gasket to preload it so that the chamber can pull a vacuum. The air board sander is attached to the base to vibrate the unit to settle propellant.

The heart of the whole operation is the top plate. Just about self explanatory click on the image for a larger version.

The top of the chamber is another plexiglas plate with a silicon gasket and holds the mechanism to attach the vacuum pump plus introduce the propellant into the casting chamber and into the motor casting tube itself. These are made up of brass fittings allowing the connection of the vacuum pump, along with a vacuum gauge to monitor the degree of vacuum, plus a small ball valve to seal off the chamber if the pump is disconnected. This allows for the vacuum hose to be disconnected from the chamber and still maintain a vacuum inside. The idea here is to be able to use only one pump to degas the propellant in the usual way and use the same pump to draw the vacuum on the chamber. It saves time not having to draw a vacuum from scratch in the chamber when the pump is changed from the mixing bowl to the chamber. The gauge can show if any vacuum was lost.

The main challenge was developing a system that could introduce the propellant to the inside of the chamber without loosing the vacuum. It was decided to go with a one inch brass ball valve having an aluminum funnel, used as a loading hopper, attached to the top. The theory being that the chamber could be drawn down with the ball valve closed and then the propellant could be added to the hopper. Once the funnel is full of propellant and the valve is opened, the propellant provides the seal as it is being drawn down into the chamber. Before all the propellant has been drawn out of the hopper, the valve is closed again and more propellant added to the hopper and the process repeated again till the grain is full of propellant.

A piece of flexible tubing is attached to the chamber side of the ball valve fitting and is used to guide the propellant to the casting tube inside the chamber. A special "Do-Dad" was designed to break up the propellant while introducing it into the casting tube. The tube can be manipulated, to a point, for targeting various size grains.

The Do-Dad is designed in two pieces for cleaning purposes and possible future modifications.

The Do-Dad assembled. Note the "flange" on the end to help hold it inside the flexible tubing.

The Do-Dad attached to the end of the flexible tubing. A hose clamp (not shown) would assure it staying on during vacuum.

Assembled flexible hose which is inside the vacuum chamber. Note the flange on the underside of the plexiglas. which attaches the feeder hose to the ball valve topside.

Last item is that air board sander attached to the base of the unit. While casting propellant the air sander is turned on and the vibration shakes the unit to help settle propellant as it is dumped into the casting tube. A little extra something to assure any possible trapped air is released. Under vacuum this possible trapped air is virtually non-existant.

After reformulating the NASSA propellants to be more “pourable”, and casting up a couple of motors using the newly developed hardware, some problems were exposed and corrected that the original engineering did not foresee. After the refinements were in place, casting under vacuum can be done with relative ease however, clean up of all the additional hardware that comes in contact with propellant takes quite a bit longer. Also, even though the propellants are now more “pourable”, one must work as fast as possible because it takes a long time to draw all the propellant through the one inch ball valve and any delays will give the curative time to “kick” off the propellant enough to make it almost impossible to be drawn into the chamber. We did go with a room temperature curative that gives us as long of a working time as possible.

As expected, grains cast under vacuum show no voids at all. When inspecting grains that have burned part way through and then extinguished, they show a very smooth burn surface. The surface does not look like that of the moon with the craters we are all used to seeing from motors not cast under vacuum. Now that the bugs have been worked out of the production techniques and the chamber has proven it’s worth, we expect to cast most of our large NASSA motors under vacuum.