1. Introduction
Hybrid rockets generally use solid fuels and liquid or gaseous oxidizers. Combustion is heterogeneous and usually takes the form of diffusion combustion, wherein the boundary layer formed on the solid fuel surface has a dominant influence. The fuel regression rate in boundary-layer combustion is related to many physical and chemical processes, such as the boundary-layer thickness, heat transfer from the flame, and heat of melting and vaporization of the solid fuel [
1]. The fuel regression rate in hybrid rocket engines is less than the burning rate of solid propellants, such as ammonium perchlorate (AP)/hydroxyl-terminated polybutadiene (HTPB)/Al composite propellants, which makes it difficult to realize a large thrust with the same propellant volume as that of solids. This is a major factor delaying the practical use of hybrid rocket engines.
In addition to the different phases of the fuel and oxidizer before combustion, diffusion combustion is less prone to explosive events, both in propellant storage and during combustion. Shimada et al. referred to hybrid rocket propulsion as an essentially non-explosive propulsion system [
2]; these characteristics are difficult to realize in rockets that utilize solid or liquid propellants. One advantage of hybrid rocket engines is their safety. Virgin Galactic has continuously used hybrid rocket propulsion systems in its manned spacecraft SpaceShipTwo owing to their safety [
3]. Hybrid rocket engines are also used globally for sounding rockets, student rocket projects, etc. [
4,
5,
6], but there have been a few major accidents during their launch and handling.
Owing to the high safety features of hybrid rocket engines and the low cost resulting from the aforementioned safety features, the research and development of small hybrid rockets conducted by venture companies have been active in meeting the demand for launching nanosatellites [
7,
8,
9,
10,
11]. Gilmour Space, TiSpace, and HyImpulse are developing small launch vehicles that use hybrid rocket engines to launch small satellites into low-Earth orbits. Mjolnir SpaceWorks specializes in the sale of CAMUI-type hybrid rocket motors for small launch vehicles. Letara is a venture company that developed a modular propulsion system for small satellites and spacecraft developers [
12]. A more comprehensive status of hybrid rocket R&D and market observations on the use of hybrid rockets for nanosatellites and microsatellites was summarized by Mazzetti et al. [
13].
Although hybrid rocket engines are characterized by safety and low cost, increasing the fuel regression rate remains an inherently important issue. Methods for increasing the fuel regression rate include the use of oxidizer injection or grain geometry to create a flow field [
14,
15,
16] and the use of cryogenic fuels such as frozen pentane [
17]. Several other methods have also been proposed. In recent years, liquefied fuels, particularly paraffin wax, have attracted considerable attention. In this case, a fuel with a low melting point that is easily liquefied is used to obtain a high fuel regression rate owing to the entrainment effect [
18,
19]. The effect of viscosity on the entrainment mechanism was also investigated [
20,
21].
The main drawback of paraffin wax fuel is its mechanical properties, and research is being conducted to improve these properties. One method of improving the mechanical properties of paraffin wax is to mix it with additive materials. The mechanical and combustion properties of paraffin-blended fuels have been investigated for various materials such as polyethylene (PE), polypropylene (PP), ethylene vinyl acetate copolymer, and stearic acid [
22,
23,
24,
25]. The effects of a wide range of additives, including metal particles, on the combustion and mechanical properties of paraffin wax were summarized in a review by Veale et al. [
26]. The addition of small amounts of additives increases the strength and ductility of paraffin wax but increases the viscosity coefficient, which often results in lower fuel regression rates. Another method comprises the reinforcement of the mechanical properties of paraffin wax using additive manufacturing [
27,
28,
29,
30]. The mechanical and combustion properties of paraffin wax fuel are balanced by the reinforcement structure. Paraffin wax (FT-0070, Nippon Seiro Co. Ltd., Tokyo, Japan) was used by the authors; if a shallow crack remained during the grain molding, the fuel receded significantly from the crack during combustion, and the burned area changed irregularly [
31]. These cracks were caused by the volume change of the molten paraffin wax as it solidified.
The LT fuel is a paraffin-based, low-melting-point thermoplastic fuel with excellent mechanical properties. Several LT fuels of different compositions have been developed, as listed in
Table 1 [
32]. All of these have slightly higher melting points than paraffin wax, and it has been confirmed that they have the same entrainment mechanism during combustion as paraffin wax [
33]. LT fuels have a higher fuel regression rate than other polymer fuels, such as PE, PP, and polymethyl methacrylate (PMMA) [
32,
34]. The disadvantage of LT fuel is that it has a lower theoretical specific impulse than the majority of HR fuels, as shown in
Figure 1. The theoretical specific impulse was calculated using the NASA program Chemical Equilibrium with Applications (CEA) [
35]. The chemical composition of LT421 is C
7.08132H
13.41038O
0.08358, based on the analytical results of Wada et al. [
36]. The molar mass of LT421 is 99.96 g/mol, and the heat of formation is
1088 kJ/mol.
Table 2 lists the maximum specific impulse and O/F values at combustion chamber pressure
Pc = 4 MPa. The lower specific impulse of the LT fuel was due to the lower adiabatic flame temperature of the LT fuel and the higher molar mass of the burned gas. The thermal analysis of LT fuel is currently being conducted to understand its combustion mechanism [
37]. The authors also conducted firing tests using LT fuel in a swirling-oxidizer-flow-type (SOFT) engine and found that cracks did not remain in the grain after the molding in the LT fuel, the grain was almost uniform, and no cracks occurred in the grain during combustion.
As mentioned above, although the LT fuel is inferior to many hybrid rocket fuels in terms of specific impulse, it has a high fuel regression rate and excellent mechanical properties to compensate for a specific impulse. However, a series of studies conducted by Wada, Kawabata, and Banno et al. concentrated on one of several types of LT fuel, LT460, while research on other compositions is thus still lacking [
32,
33,
34,
36,
37]. In particular, LT421, which has a lower melting point and viscosity than LT460, as shown in
Figure 2 [
38], may have a higher fuel regression rate based on the aforementioned previous studies [
18,
20,
21]. It is also interesting to observe the fuel regression rate that can be obtained when using a SOFT engine, which the authors have been working on. Kawabata et al. also studied the effect of swirling combustion on the regression rate of LT460 and demonstrated that the swirling flow increased the fuel regression rate by a factor of nearly three compared to the non-swirling case [
34]. The maximum fuel regression rate obtained in their study was 3 mm/s at an oxidizer mass flux of 70 kg/(m
2s). A higher fuel regression rate is expected for LT421 with a low viscosity.
The objective of this study was to clarify the fuel regression rate of an LT421-oxygen propellant when a swirling flow is applied. This is our first study on the application of swirl flow to LT421, and the combustion conditions were studied in detail to obtain fundamental data. First, firing tests were conducted by varying the oxygen mass flow rate, and the relationship between the obtained fuel regression rate and oxidizer mass flux is discussed. In this study, phenomena such as ignition delay of the LT fuel and grain separation due to the shear force of the oxygen-swirling flow were observed. Firing tests were conducted by varying the burn time to examine whether the fuel regression rate could be organized in terms of the oxidizer mass flux. In tests with burn time as a parameter, the temperatures of the nozzle and engine components increased as the burn time increased. The erosion and heat resistance of the components were also investigated.
4. Conclusions
The fuel regression rate of LT421, a low-melting-point thermoplastic fuel combined with a swirling-flow-type engine, was investigated to obtain a high fuel regression rate. Gaseous oxygen was used as the oxidizer, and Sg = 19.4 was set as the swirl strength. The oxygen mass flow rate and burn time were varied as the parameters of the oxidizer mass flux. In addition, the heat resistance of the engine components, which is necessary for the practical use of the engine, was investigated via firing tests with different burn times. In particular, the presence of erosion in the post-combustion chamber and the nozzle fabricated using graphite are discussed. The findings of this study are as follows.
When the LT fuel was applied to the SOFT engine, an ignition delay of the fuel grain occurred. This did not occur in the case of the PP fuel in a similar engine. A transient combustion state, wherein the combustion chamber pressure and thrust fluctuated, was observed for up to approximately 2 s after ignition. After this state, the combustion stabilized, and the combustion pressure tended to increase or remain constant over time. This trend with respect to the burn time did not change until 13 s, when the fuel burned out, thus suggesting that the fuel regression rate was strongly dependent on the oxygen mass flow rate rather than the oxidizer mass flux.
The LT fuel was molded into a cylindrical PMMA case for the firing tests. When the oxygen mass flow rate was 100 g/s or greater, the shear force generated by the swirling oxygen flow caused the fuel grains to separate from the PMMA case. Fuel separation occurred immediately after the start of combustion, and the separated fuel passed through the nozzle throat and was exhausted outside the engine because of its high flexibility.
By attaching fins to the inner surface of the cylindrical PMMA case, the LT fuel was burned without separation, and the highest fuel regression rate of 4.88 mm/s was achieved at an oxygen mass flow rate of 190 g/s, which corresponded to Gox = 72.4 kg/(m2s). Although the fins mainly comprised acrylic resin and the fuel regression rate was lower than that of the LT fuel, the change in the flow field due to the fins was small. The pressure and thrust profiles did not differ from those of the case without fins.
At the same oxidizer mass flux, the fuel regression rate of LT421 was approximately four times greater than that of PP when using the same engine. The fuel regression rate of LT421 was even higher than that of LT460, owing to its lower viscosity coefficient. The correlation between the fuel regression rate and oxidizer mass flux obtained with the oxygen mass flow rate as a parameter was , which had a higher mass flux exponent than those of PP and LT460. When the fuel regression rate was organized in terms of the grain leading-edge parameter, the data could be organized with the same exponent as the correlation obtained for the same engine using PP fuel. Based on the time history of the combustion-chamber pressure, it was more appropriate to organize the fuel regression rate for this engine as a parameter based on the oxygen mass flow rate. However, the results of the firing tests with the burn time as a parameter could not be organized well using only this parameter.
The fuel grain, which was 30 mm thick, burned out after a burn time of 16 s. In contrast, the molten fuel remained at the bottom of the combustion chamber after combustion. LT421 was considered to be the fuel that was liquefied during combustion, it remained in the combustion chamber for a long time owing to the swirling flow.
The roughness of the nozzle inlet surface became more noticeable as the number of uses increased and the wall temperature gradient during combustion increased. The erosion of the nozzle throat occurred when the oxygen mass flow rate increased from 140 to 190 g/s. The increase in the wall temperature of the post-combustion chamber was significantly affected by the condition of the Bakelite installed outside the chamber. The post-combustion chamber reached a higher wall temperature than the nozzle owing to its larger surface area; however, no erosion was observed.