# Solid Propellant Research > A reusable threaded-closure motor hardware built as a platform for novel propellant research. Nitrate based propellants that calculate 40% to 75% above KNSB in specific impulse were made. - Role: Propulsion Lead - Year: 2025 - Page: https://madhavsonkusare.com/work/solid-prop-research/ ## Achievements - +40% to +75% Isp over KNSB, calculated - Ø73 × 220 mm aluminium casing - Proof-tested at 1.5 × MEOP ## Inside the research motor ### Forward closure A knurled aluminium closure that threads straight into the casing, so the motor can be taken apart for the next propellant. A port through it takes the chamber pressure transducer, and a carbon-fibre disc shields its face from the flame. - Thread: Metric, threaded in - Engagement: 10 mm - Port: Pressure transducer ### Propellant grain One grain, 150 mm long, cast in an oven straight into a carbon-fibre liner round a coring rod. The liner is the casting mould and then the heat shield for the casing. To try another formulation, a new lined grain goes into the same hardware. - Grain: 150 mm long - Liner: Carbon fibre - Baseline: KNSB · 65 : 35 ### Casing A 220 mm tube of aluminium, sized by the ASME pressure-vessel code. By calculation it bursts at 18.98 MPa, and the propellants were expected to reach 5 MPa at most. Before the firing it held water at 1.5 times its maximum expected operating pressure. - Material: Aluminium - Size: Ø73 × 220 mm - Wall: 5.2 mm ### Nozzle The nozzle is a separate stainless steel insert. A second knurled closure, the retainer, threads into the aft end of the casing and holds it in, sealed by a silicone O-ring. - Insert: Stainless steel - Seal: Silicone O-ring - Retainer: Threads into casing ## The problem Every motor the team had fired before this used KNSB: potassium nitrate and sorbitol in a 65 to 35 ratio. It is simple to make and proven in earlier tests, but it calculates to only 120 s of specific impulse. We wanted to try nitrate formulations that promise more, on one reusable motor instead of a new motor for each. Two new formulations came out of the calculations, Formula 1 and Formula 2. For the same hardware and the same mass of propellant they give 167.9 s and 209.5 s, which is 40% and 75% more than KNSB, and J-class total impulse of 792 and 980 N·s against 565 N·s. The question was whether either could be cast, lit and fired. ## Design The motor is a 220 mm tube of aluminium, 73 mm across with a 5.2 mm wall, closed at both ends by knurled closures that thread straight into the casing. Silicone O-rings seal them. To change propellant, unscrew both closures, clean the parts and load a new grain. Sizing followed ASME BPVC Section VIII Division 1, UG-27. By calculation the casing bursts at 18.98 MPa, with a design pressure of 12.65 MPa and a maximum expected operating pressure (MEOP) of 8.44 MPa. The highest chamber pressure predicted for the new formulations is 5 MPa, a factor of safety of 2.53. Thread engagement is 10 mm against 3.07 mm required, a factor of 3.25. The nozzle is a separate stainless steel insert, and the forward closure has a port for a chamber pressure transducer. A transient thermal analysis using the Bartz film coefficient, and a structural study at the 5 MPa design pressure, both in ANSYS, checked the casing and closures against the pressure and heat of the burn. I drew the assembly and wrote the assembly guide. - Figure: Motor assembly drawing: 1 forward closure, 2 nozzle retainer, 3 nozzle insert, 4 casing - Figure: Nozzle retainer (left) and forward closure with its carbon headliner (right) ## Casting Grains were cast in an oven. Ours was an Inalsa 46 litre, 2000 W oven, checked with an infrared gun to within 5 °C. The mixtures melted in 30 to 35 minutes at 120 °C, which worked best and was the only setting the new formulations could be made at. Grains went into carbon-fibre liners, were centred with tooling, compacted, and checked for voids and density. Small test grains found the problems early. KNSB lost 30 to 40% of its volume as it melted, and cracked as the coring rod was pressed out. Silicone grease eased the release but left small voids. Neither new formulation lit reliably: Formula 1 refused three igniters, 4F black powder and copper thermite at 2 g and 3 g, and Formula 2 would not keep burning. The test notes list what to try next: sieve the chemicals, preheat the oven, release wax and nylon coring rods, a tapered rod, a 3D-printed funnel and compacting tool, filling by mass, and a different igniter. - Figure: A grain in its carbon-fibre liner under a spring-loaded plate, 20 March 2025 - Figure: The bore of a cast grain, 21 March 2025, rough in places ## Testing Before firing, the casing was filled with water and pressurised to 1.5 times its maximum expected operating pressure, with a pressure transducer in one closure and a pump line in the other. The test checked that the threaded closures sealed, and afterwards the casing was inspected for leaks and deformation. The first static fire was on 26 March 2025. The motor stood upright on a load cell in a steel-mesh cage, with a pressure transducer in the forward closure, temperature strips on the casing and a camera above the stand. Nothing failed structurally. The motor was fired with Formula 1. The igniter delivered too little heat, so ignition was slow, but the burn that followed was smooth. The calculated thrust times are only 1.8 to 3.6 s. Char and a crust of residue were left in the forward closure. - Figure: Proof test set-up: pressure transducer on the left, pump connection on the right - Figure: The upright test stand in its cage, 26 March 2025 - Figure: Forward closure face after the firing, 26 March 2025 - Figure: Casing and the liner beside it, after the firing ## Gallery - Preparing the O-rings on a closure before assembly - A cast grain in its carbon-fibre liner - A lined grain and the aluminium casing - Motor on the Test Stand, 20 March 2025 - Levelling the test stand - End face of a cast grain, 21 March 2025 - The pressure transducer and forward closure, 26 March 2025 - The casing, 26 March 2025 ## Films - [Static fire, 26 March 2025 · Slow-motion](https://madhavsonkusare.com/videos/solid-prop-research-static-fire.mp4)