The paper provides a review of additive manufacturing in hybrid rocket technology, focusing on current developments and the main methods utilised.
Before delving into additive manufacturing technology itself, the authors also provide an overview of hybrid rocket technology. HRMs (hybrid rocket motors) have both liquid and solid components (as oxidizer and fuel or vice versa), are simplified in use in comparison to other propulsion methods, and a lot safer to handle due to less volatility and lower environmental impact threats. In addition to the advantages, there are limitations as well -- the most important one being low regression rates (one of the most important parameters for propulsion).
There are four main methods of additive manufacturing that the paper describes. All in all, additive manufacturing is a good method for rapid prototyping of complex geometries -- such that are not achievable by traditional manufacturing. AM also allows for very different materials to be used, without compromising the structural integrity of the finished objects. The variation comes from the methods, particularly the technique used to extract material.
The methods are classified as:
1) Material Extrusion
The material filament is extruded through a nozzle. Main materials used are ABS, PLA, Nylon and PVA at about 50-200 micrometer resolution. Although cheap and easy, the low resolution results in low surface quality and anisotropy. This is a highly popular method for fabricating hybrid rocket fuel grains with ABS being a particularly interesting polymer. Apart from fuel grains, thrust systems and their components can also be printed used material extrusion; moreover, ME can be used simply to create a more efficient volume for hybrid rocket propulsion systems.
2) Vat Photopolymerization
Photoreactive resin is cured by a laser. The materials used are UV-curable photopolymers at around 10-100 micrometer resolution. With the surface quality being much higher than with filament printing, the smoothness and high detail of components is a clear advantage -- however, it is expensive, needs post-curing, is rather low-yield and with low mechanical properties for the manufactured objects.
3) Powder Bed Fusion
Material in powder form is fused by a laser. Main materials used are metals and thermoplastics at 20-100 micrometers resolution. The printed objects have good strength and surface quality, but the method is expensive and vulnerable to oxidation.
4) Direct Ink Writing
Viscous ink is extruded through a nozzle. Main materials printed are thermosets at 4-100 micrometers. Even though it's a straightforward method that allows inks to be printed, it requires post-curing and adequate nozzles.
Conclusion
- Additive manufacturing provides design freedom to complex geometries, reducing waste compared to conventional systems
- AM thus can benefit hybrid rocket efficiency
- Out of the four methods discussed, material extrusion is the most interesting possibility – particularly mixed with conventional methods, e.g casting a material into an additive-manufactured structure
- Current challenges are mainly to do with issues with standardisation and sufficient replication of results.
Source:
Oztan, C., & Coverstone, V. (2021). Utilization of additive manufacturing in hybrid rocket technology: A review. Acta Astronautica, 180, 130–140. https://doi.org/10.1016/j.actaastro.2020.11.024