Defect-Free Paraffin Fuel Manufacturing

A paraffin-based hybrid fuel formulation for low-temperature cycling has been developed. Cracked or flawed paraffin fuel grains can pose a safety risk during testing and are unuseable. Cracking of the fuel grain typically occurs during the cool-down process. The paraffin-based fuel grain contracts by as much as 15% while cooling from a liquid at 230°F to a solid state and eventually room temperature, not only causing visible cracking, but residual stresses that can damage the grain throughout processing. The new process is an improvement to others that were tried and found unworkable including spin casting and some additive manufacturing techniques. To remedy the issues with previous manufacturing efforts, the innovators at MSFC developed an oven program mimicking the intrinsic cooldown process of paraffin wax for use in monolithic casting. Innovators monitored paraffin wax cooling in a stainless steel vessel with four thermocouples to develop the program. Rather than cooling quickly, the oven program cools the grain incrementally, allowing the temperature to equilibrate along the entire grain before cooling further, resulting in greater temperature uniformity. This process produces intact full-scale paraffin fuel grains (11" diameter by 35" length) capable of surviving cold temperatures for use in propulsion systems with hybrid engines.