**1. Introduction**

In the last decade, interest and investment in space exploration has increased dramatically. This has led to greater demand for satellites, and subsequently, more companies are investing significant resources into lowering the cost of space launches. Previously, the manufacture of a Hybrid Rocket Motor (HRM) was costly and labor-intensive, and the traditional 'cast and cure' method limited the geometry of internal combustion ports [1,2]. In an effort to overcome the manufacturing limitations of hybrid rocket fuel grains, researchers have looked to emerging technology and novel techniques to produce innovative solutions that have previously been unattainable. The most notable of these has been the impressive advancement in rapid prototyping, especially fused deposition manufacturing (FDM), commonly known as 3D printing [3]. FDM has enabled designers to incorporate complex combustion ports into HRMs and has opened up an entirely new set of materials for the fabrication thereof.

The process of the 3D printing involves the laying down of successive layers of material on top of each other until a 3D shape has been created [4]. The material is generally supplied in the form a thin strand, or filament, that is wound onto spools to be fed into a heating reservoir and extrusion nozzle assembly. The nozzle assembly is controlled by a computer numerically controlled (CNC) mechanism and has the ability to travel both horizontally and vertically. This means that the only limitations placed on the size of the object being printing is the size of the print area of the particular printer being used. However, some companies have already created printers that can essentially print large parts, such as a house, in a short period of time (less than a day) [3]. This is achieved by printing the part length-ways on a conveyor belt, with each new layer being added on a 45◦ angle, allowing a full-sized hybrid rocket fuel grain to be completed in a single print. Other companies, such as Gilmour Space Technologies, have created both a proprietary printing method and material [5].

Traditionally, the materials used in 3D printing are generally polymers, such as acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), acrylonitrile styrene acrylate (ASA), or polypropylene (PP), but can include anything from metals, ceramics, food, and concrete to living stem cells [3,6,7]. More recently, the commercial market has seen a rapid increase in the number of filaments with metal particles in its composition, such as brass, copper and aluminum. Although it is possible to 3D print metallic materials [8], the filaments discussed herein have a PLA binding structure, with a fine powder of the desired metal finish added during the manufacturing process.

Favorable comparison of ABS against hydroxyl-terminated polybutadiene (HTPB) as a hybrid rocket fuel suggests that FDM has the potential to revolutionize the fabrication of hybrid rocket fuel grains, and has led a groundbreaking push for the development and enhancement of 3D-printed fuel grains at the Utah State University [9]. This study, carried out at the Utah State University, demonstrated that not only did ABS have an acceptable (although slightly reduced) performance when compared to HTPB, but that it also showed much higher burn-to-burn consistency which was attributed to the FDM process [9]. Similar work has been carried out at Purdue University and at the University of Tennessee [10,11].

The introduction of FDM into the manufacturing process of HRM fuel grains has also allowed for the creation of far more complex combustion port geometries. The traditional method of 'cast and cure,' which is used for materials such as HTPB and para ffin, requires a mold and internal tooling that can be quite complex depending on the desired geometry of the fuel grain [12]. Although recent work has been done on the use of disposable tooling structures, it is generally accepted that once the fuel mixture has cured, the internal tooling must be removed before the motor is ready for use. This has limited the complexity of the design of HRM grain ports, in so far as they must be able to allow for the removal of the tooling after the manufacturing process, generally resulting in straight ports that run the length of the motor. In contrast to this, FDM lays down each successive layer of material on top of the last, which, in components like HRM fuel grains, rarely requires the use of internal support. Even if support structure is required, this is added in the same manner as the fuel and is manufactured from water soluble material that is easily removed after manufacture [13].

Several recent studies have explored the use of rapid prototyping in the design and fabrication of HRM fuel grains. The bulk of research in this field has been carried out at the Utah State University and has centered around the exploration of the e ffects of a helical fuel grain on the performance of the HRM regression rate [12,14–17]. It has been shown that the helical combustion port significantly increased the regression rate when compared to straight cylindrical ports. It has been posited that this is a result of both an increase in the local skin friction, and the suppression of the radial wall blowing effect is due to the centrifugal forces introduced by the helical flow [16]. These centrifugal forces cause compression of the boundary layer, forcing the flame front closer to the fuel port wall and significantly increasing the convective heat transfer. The use of aluminum particles to increase performance of solid and HRMs has long been studied [18–20]. It has been shown that the addition of aluminum particles can increase specific impulse, volumetric heat of oxidation, and radiative heat transfer [21], as well as increase regression rates in HTBP rocket motors by up to 40% [22]. It is generally understood that this increase in regression rate is due to improvement of radiative heat flux from the di ffusion flame zone to the fuel surface area via the radiating metallic particles and higher gas-phase temperature [23,24].

However, it has also been demonstrated that the addition of aluminum particles does not necessarily always result in an increase in regression rates [25]. Regression rate improvement can also be a result of the release of energy during metal oxidation, but the higher temperatures required to melt the aluminum oxide layer often mean that the fuel binder is melted and burnt before the

aluminum [21,24]. The result is that the aluminum particles are ejected from the motor without significantly contributing to the regression rate increase [26], and can form slag build-up on the nozzle wall, as well as drastically increase nozzle throat ablation [20]. A key contributing factor to the performance enhancement abilities of the added aluminum is the size of the particles. It has been shown that nanosized particles perform better than micro-sized particles, resulting in higher combustion efficiencies, better heat transfer, and reduced burn time [23].

This research project conducted a series of small-scale static fire tests of FDM HRMs that were designed and 3D-printed to explore the performance of a variety of commonly available FDM materials. These materials included ABS, ASA, PLA, PP, PETG (Polyethylene terephthalate glycol), Nylon, and AL (PLA with aluminum particles). The main objective of this research study was to analyze and compare the performance of these materials in terms of the mass flux and regression rate.
