Demonstration of Polyethylene Nitrous Oxide Catalytic Decomposition Hybrid Thruster with Dual-Catalyst Bed Preheated by Hydrogen Peroxide

Abstract

Although various studies on nitrous oxide as a prospective green propellant have been recently explored, a polyethylene nitrous oxide catalytic decomposition hybrid thruster was barely demonstrated due to an inordinately high catalyst preheating time of a heater, which led to the destruction of components. Therefore, hydrogen peroxide was used as a preheatant, a substance to preheat, with a dual-catalyst bed. The thruster with polyethylene (PE) as a fuel, N₂O as an oxidizer, H₂O₂ as the preheatant, Ru/Al₂O₃ as a catalyst for the oxidizer, and Pt/Al₂O₃ as a catalyst for the preheatant was arranged. A preheatant supply time of 10 s with a maximum catalyst bed temperature of more than 500 °C and without combustion and an oxidizer supply time of 20 s with a burning time of approximately 15 s were decided. Because the catalyst bed upstream part for decomposing the preheatant was far from the post-combustion chamber, the post-combustion chamber pressure increased and the preheatant mass flow rate decreased after a hard start during the preheatant supply time. Moreover, because the catalyst bed upstream part primarily contributed to preheating, the maximum catalyst bed temperature was less than the decomposition temperature of the preheatant during the preheatant supply time. Additionally, because the catalyst bed downstream part for decomposing the oxidizer was far from the post-combustion chamber, the post-combustion chamber pressure decreased and then increased during a transient state in the oxidizer supply time.

1. Introduction

Nitrous oxide is a promising green propellant that has advantages such as low toxicity, a moderate specific impulse, high applicability, self-pressurization, and catalytic decomposition as well as disadvantages such as the necessity to preheat a catalyst [1,2,3]. Recently, thrusters with nitrous oxide as a monopropellant and an oxidizer of a bipropellant have been investigated worldwide. For example, Giuseppe Gallo et al. [4,5] experimentally investigated N₂O decomposition with a Pd/Al₂O₃ cylindrical pellet catalyst and predicted the fuel regression rate in an HDPE single-port hybrid rocket fed by liquid nitrous oxide, Ryota Okuda et al. [6] investigated the fuel regression characteristics of an axial-injection end-burning hybrid rocket using nitrous oxide, Jincheol Kim et al. [7] studied the endurance and performance of an impregnated ruthenium catalyst for a thruster system, Vadim Zakirov et al. [8] investigated a model for the operation of nitrous oxide monopropellant, Raffaele D’Elia et al. [9] experimentally analyzed SiC-based refractory concrete in hybrid rocket nozzles, and Shinichiro Tokudome et al. [10] experimentally studied a nitrous oxide/ethanol (NOEL) propulsion system. However, a polyethylene nitrous oxide catalytic decomposition hybrid thruster was scarcely demonstrated, though the catalytic decomposition for a hybrid thruster was proposed by Eun Sang Jung et al. [11] who investigated autoignitable and restartable hybrid rockets using catalytic decomposition of an oxidizer, and a counterflow burner rather than the hybrid thruster was demonstrated by Coit T. Hendley, IV et al. [12] who investigated catalytic decomposition of nitrous oxide for use in hybrid rocket motors. As shown in Figure 1, a major problem in the demonstration of the polyethylene nitrous oxide catalytic decomposition hybrid thruster with a heater for preheating the catalyst is that melted PE blocked a catalyst bed void, distributor holes, and a fuel port because of an excessively high catalyst preheating time. In order to settle the problem, a method for preheating the catalyst with a low catalyst preheating time is required. Meanwhile, the catalytic decomposition of H₂O₂ was introduced to preheat the catalyst by Doyun Lee et al. [13], who investigated a high-performance microthruster with ammonium-dinitramide-based monopropellant. Furthermore, a dual-catalyst bed with two different catalysts was introduced to reduce the pressure drop across a catalyst bed by Seonuk Heo et al. [14], who investigated the effect of a dual-catalytic bed using two different catalyst sizes for hydrogen peroxide thruster. Because of its low-temperature startability, it is feasible to use H₂O₂ with an additional catalyst as a “preheatant”, which is defined as a substance to preheat. Hence, in this research, a polyethylene nitrous oxide catalytic decomposition hybrid thruster with a dual-catalyst bed preheated by hydrogen peroxide was demonstrated as a novel solution to mitigate the problem concerning the failure of components caused by the heater.

2. Materials and Methods

Based on Reference [15], the thruster, catalysts, and an experimental setup of the polyethylene nitrous oxide catalytic decomposition hybrid thruster with the dual-catalyst bed preheated by hydrogen peroxide were designed, and differences in details are described as follows:

As shown in Equations (1) and (2),

Table 1. Specifications of PE N₂O catalytic decomposition hybrid thruster with dual-catalyst bed preheated by H₂O₂.

Parameter	Value
Fuel type	PE
Oxidizer type	N2O
Preheatant type	H2O2 (+H2O)
Preheatant concentration	90 wt.%
Oxidizer to fuel mass ratio	6
Combustion chamber pressure	10 bar
Specific impulse (Sea level)	2070.4 m/s, 211.1 s
Characteristic velocity	1633.3 m/s
Nozzle expansion ratio	2.3
Thrust	10 N
Fuel mass flow rate	0.69 g/s
Oxidizer mass flow rate	4.14 g/s
Characteristic velocity efficiency	0.95
Nozzle throat diameter	3.1 mm
Nozzle exit diameter	4.7 mm
Nozzle type	Cone
Nozzle convergence half-angle	45 deg
Nozzle divergence half-angle	15 deg
Catalyst bed diameter	25.4 mm
Catalyst bed length	100 mm
Preheatant catalyst bed decomposition capacity	1.27 g/s/cm3
Preheatant catalyst bed length to diameter ratio	1
Preheatant catalyst bed length	25.4 mm
Maximum decomposable preheatant mass flow rate	16.3 g/s
Nozzle throat to fuel port total area ratio	0.5
Fuel port quantity	1
Fuel port diameter	4.4 mm
Combustion chamber diameter	25.4 mm
Average fuel regression rate	0.7 mm/s
Burning time	15 s
Fuel density	974.5 kg/m3
Fuel port length	66.9 mm

, and Figure 2, the thruster was designed. PE, N₂O (Shincheon Total Gas Co., (Daejeon, Republic of Korea)), and H₂O₂ were chosen as the fuel, the oxidizer, and the preheatant. A catalyst bed diameter and a combustion chamber diameter were established as an MFC fitting diameter. The catalyst bed length was established as a value suitable for the experimental setup. The fuel port quantity was established as a minimum to exclude a multiport. A pressure sensor of a pressure transducer with an accuracy of ±0.15% was positioned at the post-combustion chamber. Temperature sensors of thermocouples (Hankook Electric Heater (Daejeon, Republic of Korea), Type K thermocouple) with an accuracy of ±0.19% were positioned at points of 20, 40, 60, and 80 mm from the front of the catalyst bed. As shown in Figure 3, an injector manifold was devised to harmonize an oxidizer supply line, a preheatant supply line, and a purge line. A spray nozzle (Spraying Systems Co., (Glendale Heights, IL, USA), B1/8HH-316SS1) was employed to supply the preheatant.

$$Z_{cb,ph}=\dot{m_{ph}}/V_{cb,ph}, V_{cb,ph}=\left(\pi D_{cb}^2/4\right)L_{cb,ph}, {LD}_{cb,ph}=L_{cb,ph}/D_{cb}$$

(1)

$$D_{cc}=D_{po,fu}+2\overline{\dot{r_{fu}}}{t}_b$$

(2)

As shown in

Table 2. Specifications of catalysts for preheatant and oxidizer.

Parameter	Value
Catalyst type	Pt/Al2O3	Ru/Al2O3
Catalyst target	Preheatant, H2O2	Oxidizer, N2O
Catalyst volume fraction	25 vol.%	75 vol.%
Catalyst form	Pellet	Pellet
Catalyst size	10–16 mesh, 1.19–2.00 mm	10–16 mesh, 1.19–2.00 mm
Catalyst active material mass fraction	20.4 wt.%	21.1 wt.%

and Figure 4, catalysts were categorized into Pt/Al₂O₃ for the preheatant and Ru/Al₂O₃ for the oxidizer and were composed of Pt and Ru as active materials and Al₂O₃ as a support. H₂PtCl₆·6H₂O (HanTech PMC Co., Ltd. (Anyang-si, Gyeonggi-do, Republic of Korea)) and RuCl₃·3H₂O (HanTech PMC Co., Ltd.) were employed as active material precursors. In the fabrication by impregnation, rinsing was replaced with a reduction with a mixture (H₂ 4 vol.% + N₂ 96 vol.% (Shincheon Total Gas Co.)) and the furnace (500 °C, 5 h).

As shown in Figure 5, the experimental setup was designed. N₂ was chosen as a pressurant. MFC (Teledyne Hastings Instruments (Hampton, VA, USA), HFC-D-307B) with an accuracy of ±0.20% was employed to control an oxidizer mass flow rate, PLC to activate solenoid valves, and DAQ to measure pressures with a rate of 1000 Hz and temperatures with a rate of 80 Hz.

Before a combustion test, a real propellant test for the preheatant was implemented to assess the discharge coefficient of MFM and inspect the spray nozzle. As shown in

Table 3. Conditions of combustion tests.

Parameter	Value
Combustion test name	P1	P2	P3	O1	O2-1	O2-2
Preheatant supply time (s)	2	5	10	10	10	10
Purge time (s)	5	5	5	0	0	0
Oxidizer supply time (s)	0	0	0	15	20	20

and Figure 6, six combustion tests were implemented with a preheatant mass flow rate of approximately 5 g/s by measuring the MFM upstream pressure, the MFM downstream pressure, the post-combustion chamber pressure, four catalyst bed temperatures, and the oxidizer mass flow rate. Catalysts were replaced with new ones after each combustion test to avoid deactivation. In order to establish a preheatant supply time such that the maximum catalyst bed temperature is more than 500 °C and combustion does not occur, combustion tests P1 through P3 were conducted by increasing the preheatant supply time. In order to establish an oxidizer supply time such that the burning time is approximately 15 s, combustion tests O1 through O2-2 were conducted by increasing the oxidizer supply time. Combustion tests O2-1 and O2-2 were replicates under the final test condition for assessing repeatability.

3. Results and Discussion

As shown in Equations (3)–(5) and

Table 4. Results of combustion tests P1 through P3.

Parameter	Value
Combustion test name	P1	P2	P3
Preheatant mass flow rate (g/s)	5.16	4.22	4.24
Maximum catalyst bed temperature during preheatant supply time (°C)	166 ± 0.32	273 ± 0.52	655 ± 1.24
Combustion during preheatant supply time	No	No	No

and

Table 5. Results of combustion tests O1 through O2-2.

Parameter	Value
Combustion test name	O1	O2-1	O2-2
Preheatant mass flow rate (g/s)	4.57	4.63	4.27
Maximum catalyst bed temperature during preheatant supply time (°C)	557 ± 1.06	583 ± 1.11	521 ± 0.99
Combustion during preheatant supply time	No	No	No
Burning time (s)	10.3	15.7	15.8
Post-combustion chamber pressure 1 (bar)	8.99 ± 0.01	8.88 ± 0.01	10.80 ± 0.02
Post-combustion chamber pressure fluctuation 1	0.01	0.05	0.03
Oxidizer mass flow rate 2 (g/s)	3.97 ± 0.01	4.05 ± 0.01	4.68 ± 0.01
Fuel mass flow rate 2 (g/s)	0.80	0.94	1.09
Average fuel regression rate 2 (mm/s)	0.44	0.42	0.46
Characteristic velocity efficiency	0.87	0.82	0.86

¹ During steady state. ² During burning time.

, the thruster with appropriate preheatant and oxidizer supply times was successfully demonstrated in combustion tests. Preheatant mass flow rates were computed by MFM. Combustions during the preheatant supply time were not confirmed. This is because flames were not observed during the preheatant supply time of every combustion test and the fuel masses before and after combustion tests P1 through P3 were not changed. Post-combustion chamber pressure fluctuations were computed by Equation (3). Since every post-combustion chamber pressure fluctuation was below 0.10, every post-combustion chamber pressure was stable. Oxidizer mass flow rates were computed by MFC. Average fuel regression rates were computed by Equation (4) with a fuel density [16]. Characteristic velocity efficiencies were computed by Equation (5) with post-combustion chamber pressures as combustion chamber pressures. Since every characteristic velocity efficiency was above 0.80, every combustion was complete. In combustion test P3, the preheatant supply time of 10 s was decided to achieve the maximum catalyst bed temperature of more than 500 °C and no combustion. In combustion tests O2-1 and O2-2, the oxidizer supply time of 20 s was decided to achieve the burning time of around 15 s.

$$Y_{P_{cc,post}}=(P_{cc,post,max}-P_{cc,post,min})/\overline{P_{cc,post}}$$

(3)

$$\dot{m_{fu}}{t}_b=N_{po,fu}{\rho }_{fu}(\pi ({(D_{po,fu}+2\overline{\dot{r_{fu}}}{t}_b)}^2-D_{po,fu}^2)/4)L_{po,fu}$$

(4)

$${\eta }_{C^{\ast }}=C_{exp}^{\ast }/C_{the}^{\ast }=(P_{cc}{A}_{nt}/\dot{m_{pr}})/C^{\ast },A_{nt}=\pi D_{nt}^2/4$$

(5)

Combustion test O2-1 was chosen as an example because combustion tests O1 through O2-2 had a resemblance. As shown in Figure 7, the flame was observed during the oxidizer supply time, not the preheatant supply time. As shown in Figure 8, the decrease in fuel port length was negligible. As shown in Figure 9 and Figure 10, there were three phenomena. First, during the preheatant supply time, the post-combustion chamber pressure increased and the preheatant mass flow rate decreased after a hard start. This is because the catalyst bed upstream part for decomposing the preheatant and the post-combustion chamber were far apart. Second, during the preheatant supply time, the maximum catalyst bed temperature was less than the decomposition temperature of the preheatant of approximately 741 °C [17,18]. This is because the catalyst bed upstream part, not the downstream part for decomposing the oxidizer, primarily participated in the preheating of the catalyst bed. Third, during a transient state in the oxidizer supply time, the post-combustion chamber pressure decreased and then increased. This is because the catalyst bed downstream part and the post-combustion chamber were far apart.

4. Conclusions

A dual-catalyst bed preheated by hydrogen peroxide was applied to demonstrate the polyethylene nitrous oxide catalytic decomposition hybrid thruster. In the case of PE as the fuel, N₂O as the oxidizer, H₂O₂ as the preheatant, Ru/Al₂O₃ as the catalyst for the oxidizer, Pt/Al₂O₃ as the catalyst for the preheatant, 10 s as the preheatant supply time, and 20 s as the oxidizer supply time, the thruster was satisfactorily demonstrated with the maximum catalyst bed temperature during the preheatant supply time of more than 500 °C, no combustion during the preheatant supply time, and the burning time of approximately 15 s. An increase in the post-combustion chamber pressure and a decrease in the preheatant mass flow rate after the hard start during the preheatant supply time were due to a high distance between the catalyst bed upstream part and the post-combustion chamber. The maximum catalyst bed temperature during the preheatant supply time being less than the decomposition temperature of the preheatant was due to the low participation of the catalyst bed downstream part in the preheating of the catalyst bed. An increase after a decrease in the post-combustion chamber pressure during the transient state in the oxidizer supply time was due to a high distance between the catalyst bed downstream part and the post-combustion chamber. For future research, an improvement of the thruster by supplementing a pre-combustion chamber and a scale effect, both of which are absent in the current design, is planned. The pre-combustion chamber and the scale effect were studied in References [19,20,21] and References [22,23,24,25,26,27], respectively.