This video details the complete build and static test fire of "Super 8," a large, 120-pound solid rocket motor intended for a crowdfunded space shot. The process begins with meticulously mixing the "Risky Batman" propellant formula, a composite of HTPB, aluminum powder, and ammonium perchlorate. The fabrication of the motor's hardware, including the aluminum case, a multi-part nozzle made of graphite and phenolic, and the forward closure, is also shown. A critical error occurs during the three-batch casting process when equipment limitations lead to scraping the mixing bowl, introducing uncured propellant at the boundaries between the cast segments. This flaw creates significant risks of unpredictable performance and potential motor failure. Despite these "uncomfy unknowns," the motor is test-fired. The test is a success, with performance closely matching simulations that accounted for the flaws. A post-fire cross-section analysis confirms the motor liner survived, validating the design and marking the completion of the propulsion development phase for the project.
Building and Firing a Flawed 120-Pound Rocket Motor
0:26 Propellant Formulation: The motor is loaded with 120 lbs of "Risky Batman," a solid rocket propellant. The formula consists of an HTPB rubber binder, aluminum powder as fuel (12%), and ammonium perchlorate as an oxidizer (69%), along with other additives to control burn rate and ensure proper bonding.
5:05 Custom Hardware Fabrication: The video details the creation of the motor's main components: an 8-inch diameter, 68.5-inch long 6061 aluminum case; a multi-material nozzle with an isomolded graphite throat for heat resistance; and a 6061 aluminum forward closure that houses the igniter and a pressure transducer port.
10:00 Complex Casting Process: The propellant is cast into the motor casing in three separate 40 lb batches. Before casting, the inside of the case is coated with a protective liner ("mystery goo") using a custom-built spin caster. A central mandrel is used to form the propellant's core geometry, which dictates the motor's thrust profile.
11:47 The Chemistry of Curing: The propellant transitions from a liquid slurry to a solid rubber through a chemical reaction where a curative (MDI) cross-links the long chains of the HTPB binder. An improper or uneven mix of the curative results in soft, uncured sections.
14:06 A Critical Casting Error: A crane used to pour the propellant could not lift the mixing bowl high enough to tilt it fully. This forced the creator to manually scrape the remaining propellant from the bowl, inadvertently introducing improperly mixed, uncured material from the bowl's sides into the motor at the boundary of each of the three batches.
16:22 Discovering and Analyzing the Flaw: After the propellant cured, the removal of the mandrel revealed soft, uncured propellant at the junctions between the three cast segments. This flaw was highly dangerous, as the soft sections would be ejected at ignition, creating voids that drastically increase the burn surface area and could lead to over-pressurization and catastrophic failure.
18:14 Risk Assessment: An internal inspection with an endoscope and new simulations in Open Motor software suggested that despite the flaws, the motor would likely not explode. A major remaining concern was whether the motor's thin liner could withstand direct flame exposure for the full 12-second burn in the areas where the propellant was flawed.
20:39 Successful 12-Second Static Fire: Despite the significant risks, the motor was fired. It completed a full 12-second burn, producing 112,000 Newton-seconds of total impulse and a peak thrust of nearly 15 kilonewtons, performing close to the revised simulations.
21:40 Post-Fire Analysis Validates the Design: The motor was cut into cross-sections for inspection. The analysis revealed that the liner, while shockingly thin in some spots (1-2 mm), did not burn through. Discoloration patterns confirmed that the flawed sections burned to the casing wall first, but the hardware survived, validating the robustness of the design.
22:39 Propulsion Development Complete: The successful test of the Super 8 motor marks the end of a two-year propulsion development effort. The project can now confidently move on to other challenges like flight software, avionics, and recovery systems for the space shot.
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