An ablative liner was made in order to shield the section of motor casing surrounding the injector from the intense heat produced during firing of the rocket motor. Initially I considered graphite for this section, however, several drawbacks became apparent with this material. Graphite has a low shear strength and may crack under the high mechanical loads exerted during machining and firing considering the small thickness needed. The liner must operate in a highly oxidizer rich environment. At high temperatures, graphite is easily oxidized into carbon dioxide, leading to substantial erosion in the presented application. New liners would have to be machined prior to each firing, raising issues with cost and labor time.
A temperature resistant graphite epoxy ablative liner was selected instead. New liners are easily made in bulk and are replaced after each firing. Graphite powder was mixed with West Systems epoxy and microballoons in order to produce an opaque, thermally resistant layer 4 mm thick that will be consumed at a significantly lower rate than the fuel grain itself. A "doughnut" of APCP propellant with substantial ferric or cupric oxide content will be mounted inside the liner with high temperature RTV rubber as a pre-heater.
After the ablative coating was mixed, the volume needed to produce the necessary thickness over a length substantial enough for several liner sections was measured out. The coating was poured into a paper propellant liner tube with ends sealed with waxed paper. Next, the tubing was centered and spun axially at a high rate on my lathe in order to deposit an even 4 mm thick coat on the inside of the liner tube.
The length of coated liner tubing was cut into five even 2.25" lengths which will serve as ablative liners for individual trials. Interestingly, the microballoons separated from the graphite mixture during the centrifugation due to a difference in density. Appearing at the surface of the ablative coating, the microballoons may actually slow the rate of liner consumption due to their properties as insulators and light color, demonstrating low thermal absorption. Also, bubbles of air are forced out of the epoxy and towards surface by this process. I don't expect this to present any significant challenges with regard to bulk ablative characteristics as the bubbles only exist at the surface and don't penetrate more than 1 mm into the liner itself.
The following images demonstrate the orientation of the ablative liner with respect to the injector/forward closure. The ablative liner along with the fuel grains exist within a continuous phenolic liner tube capped at both ends with the nozzle and the forward closure.
Note that the pressure tap in the forward closure is not obscured by the ablative liner.
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| Injector and pressure tap positioned within the ablative liner |
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