This section outlines the steps taken in the prototype project: selection of materials; thermal analysis of the grain; combustion simulation and geometry determination.
Selection of Materials
The selection of the oxidizer physical state can be done attempting to the complexity inserted by this decision. A swirling two phase flow is avoided when the oxidizer is gaseous, meaning that more accurate and resistant hydraulic valves, which are considered necessary to operate in cryogenic temperatures, are not needed; this choice decreases cost and engine weight and simplifies the systems design.
Gaseous oxygen was used for its good performance, ease of handling and cost. The desired fuel must present low burning rates, in such a way that small changes in regression rates can be easily determined, in other words, such attribute make comparisons between miscellaneous performance results readily feasible. Most polymers present high viscosity and high surface tension, which fulfills the features required. Polyethylene was found to be rather appealing, once it is cheap, easy to find in the market and to conform in the desired form.
Two branches of polyethylene were tested for fuel grains, Ultra High Molecular Weight (UHMW) and High-Density Polyethylene (HDPE). The first tests evidenced that UHMW has better combustion stability and did not produce excessive carbon black, see Figure 1a. Meanwhile, tests conducted with HDPE showed high concentration of carbon black deposited all over the grain surface, Figure 1b.
Figure 1. A: Ultra High Molecular Weight fuel grain after test. B: High Density Polyethylene fuel grain after testing
The first nozzle used was made of stainless steel. But it could not handle a 15 s test without substantial throat erosion, see Figure 2. Therefore, graphite (20 µm grain length) was used. Graphite could endure each test with a maximum throat expansion of 0.5 mm, Figure 3.
Figure 2. Stainless steel nozzle test (after 15s test)
Long tests were performed to address nozzle endurance in high temperature conditions. As a result it was observed that the stainless steel around the nozzle suffered melting before problems could be noticed on the graphite nozzle, which confirms the high quality of the graphite used, Figure 3.
Figure 3. Graphite nozzles. A: Before test. B: After test, the nozzle is surrounded by melted stainless steel, 60s test