Innovations made on NASA initiator, 3D printing of propellants, cool gas generators
By JOHN F. ZEVENBERGEN|December 2019
The Energetic Components and Systems Technical Committee provides a forum for the dissemination of information about propellant and explosive-based systems for applications ranging from aircraft to space vehicles.
Changes to the production process for NASA standard initiators or NSIs, are never made lightly. NSIs provide the initial pulse to begin critical thermal and mechanical processes in both human-rated and uncrewed missions. For example, the “seven minutes of terror” required for landing a rover on Mars depends on the NSIs functioning perfectly.
The first NSI production lot utilizing a new automated manufacturing process was acceptance-tested and delivered to NASA’s Johnson Space Center in Houston in September by Chemring Energetic Devices of Illinois. CED completed the qualification program in late 2018 to become the only certified NSI producer in the world.
The company’s qualification included implementation and verification of new manufacturing technology. Robotics were employed for bridgewire welding and energetic printing, which eliminated the traditionally operator-dependent processes and improved manufacturing safety. Test results showed the new process significantly increased the manufacturing uniformity and reduced the performance standard deviation, especially in the actuation timing at minus 251 degrees Celsius. Turning to additive manufacturing for solid propellant fabrication, these techniques enable structural and compositional gradients that can improve combustion performance. However, until this year, additive technologies were only compatible with materials whose oxidizer mass fractions were less than 60 volume %. Research by Purdue University this year led to the development of a new additive technique called vibration assisted printing or VAP. This direct-write technique has removed prior material limitations. The Purdue group demonstrated in January the additive manufacturing of solid propellants with oxidizer volume fractions in excess of 76 vol.%. Furthermore, these material systems were 3D-printed at high rates with fine layer resolution (approximately 100 microns) and tailorable combustion characteristics. The adopted approach eliminates many of the hurdles commonplace in the industry and can be a transformative fabrication tool, while also supporting new research in materials development.
The holy grail for metal particle combustion would be to harness the enormous chemical potential energy stored within metal fuel particles at time scales relevant to a detonation event. Diffusion controlled reactivity limits aluminum’s energy release potential but with the development of nanoparticles, time scales for diffusion-controlled kinetics could approach detonation. Nanoparticles cannot achieve reactivity at detonation time scales by themselves. This year, researchers at Texas Tech University and the U.S. Army’s Aberdeen Proving Ground in Maryland continued improving the detonation time scales. They did this by capitalizing on a 2018 process breakthrough in which they transformed the inert alumina passivation shell surrounding the aluminum core into a highly oxidizer-rich salt called aluminum iodate hexahydrate, or AIH, through surface chemistry. In May, the Texas Tech and Aberdeen team prepared the aluminum particle surface with plasma surface treatments prior to AIH surface chemistry. The plasma etches the surface layer so that AIH formation and concentration are more controlled and repeatable. Plasma surface treatment improves the particles’ safe handling and stability under ambient conditions by producing a more consistent and repeatable coating. AIH-coated Al particles improve the detonation velocity of TNT by 30%.
In July, ExxFire of the Netherlands finalized installation of 105 of its fire-suppression systems at Zee Media in New Delhi — the largest shipment ever and a clear vote of confidence from the market. Zee Media is the largest streaming media company in India and part of the Essel Group. The ExxFire systems use cool-gas generator technology from the Netherlands Organisation for Applied Scientific Research, or TNO, for suppressing fires in closed volumes such as server racks and switching cabinets in production facilities. The extinguishers produce nitrogen gas that lowers the oxygen level and suffocates the fire. While today’s high-pressure cylinders require maintenance, ExxFire’s cool-gas generators store nitrogen gas in solid form in unpressurized cylinders requiring no maintenance.
Contributors: Steve Son, Emre Gunduz, Michelle Pantoya, Hobin Lee and Harm Botter