Jeongsub Lee

Chugging Instability of H2O2 Monopropellant Thrusters with Reactor Aspect Ratio and Pressures

Among the three types of instabilities, the low-frequency instability (chugging instability) was experimentally investigated with respect to the chamber pressure and aspect ratio (L=D) of catalytic reactors in a monopropellant thruster. ThreeH2O2 thrusterswere used, and two parameters were found to be the dominant factors that generated a chugging instability of the order of several tens of hertz. Ahigh chamber pressure and lowL=D values (lowpressure drop across the catalyst bed) were preferable for reducing pressure oscillation inside the reaction chamber. In addition, it was found that these two parameters were not independent but coupled; therefore, the point where chugging instability occurred varied slightly depending on the interaction between these parameters.

Comparison of Catalyst Support Between Monolith and Pellet in Hydrogen Peroxide Thrusters

The effect of catalyst support on the performance of monopropellant thrusters was investigated. In the present study, two support materials (monolith honeycombs and alumina pellets) were tested and their relative performances were compared. A reference catalyst (Na 0.2 MnO 2 ) was coated on both catalyst supports, and 90 wt% hydrogen peroxide was used as the monopropellant. Two test thrusters of different sizes were fabricated, and the performance of each thruster when using monolith honeycomb and alumina pellets as the catalyst bed was evaluated by measuring the product-gas temperature at the rear end of the catalyst bed and the pressure of the gas at the front and rear ends of the catalyst bed; during these measurements, the feed pressure of the propellant was fixed. Under the given test conditions, the performance of the thrusters was better when using alumina pellets as the catalyst support than when using monolith honeycomb. Since the monolith support was less reactive than the pellets, pressure buildup in the former case was relatively small; consequently, the chamber pressure and temperature were lower when using the monolith support than when using the pellet support. The pressure drop across the catalyst bed was moderate in both cases (0.02-0.1 bar in the case of a monolith and 0.3-0.7 bar in the case of a pellet catalyst).

Development of a Liquid Propellant Rocket utilizing Hydrogen Peroxide as a Monopropellant

This work was supported by the Korea Science and Engineering Foundation(KOSEF) grant funded by the Korea government(MEST) thourgh NRL(No. R0A-2007-000-20065-0)

Fabrication of Catalyst-Insertion-Type Microelectromechanical Systems Monopropellant Thruster

Amicroelectromechanical systemsmonopropellant thruster having a design thrust of 100mNwas fabricated and tested. Hydrogen peroxide (90 wt%), which is a green propellant, was selected as the monopropellant. Platinumwas used to decompose the hydrogen peroxide, and it was coated on Al2O3 via impregnation. Three catalyst loading conditions were tested and experimentally optimized. The optimized catalyst was directly inserted into the thruster chamber to enhance decomposition performance because its surface-to-mass ratio significantly affected its performance. The microelectromechanical systems thruster was composed of eight photosensitive cylindrical-type glasses. An additional reduction process was carried out after the thermal bonding procedure because performance degradation of the catalyst was detected at this stage. The microelectromechanical systems thruster decomposed hydrogen peroxide well. However, chamber instability was observed. This instability resulted from an insufficient pressure drop at the injector. Therefore, it is necessary to increase the feeding pressure and reinforce the injector wafer to decrease chamber instability.

Optimum Nozzle Angle of a Micro Solid-Propellant Thruster

The characteristics of supersonic flow field and thrust for a micronozzle used in a microthruster were analyzed numerically to determine the optimum nozzle geometry. Typical parameter values are 300 μm throat diameter, 10 bar and 1000 K for stagnation conditions, and 1–100 ms for combustion completion. The effects of the boundary layer, heat loss, and unsteady stagnation conditions were considered. Thrust was much more dependent on the nozzle expansion angle than the area ratio or length. Optimum nozzle expansion angle for maximum thrust was close to 30° for a planar 2D nozzle and 25° for a 2D axisymmetric nozzle.

Pulse Response Times of Hydrogen Peroxide Monopropellant Thrusters

The transient behavior of a monopropellant thruster was investigated. Throughout the study, MnO2/Al2O3 was used as the catalyst bed in order to eliminate the influence of the catalyst bed on the transient behavior. Three 50 Newton level test thrusters with different injectors, ullage volumes, and bed sizes were built. H2O2 (90 wt%) was used as the monopropellant in the thrusters and experiments were carried out using these thrusters. The transient characteristics of the thrusters—the ignition delay and the time taken for pressure rise and pressure decay—were determined. Among the injectors considered, the transient characteristics of the shower-head injector are better than those of the spray injector The shower-head injector showed the best performance when a catalyst bed with a small volume was used: the ignition delay was 14 ms; the pressure rise, 108 ms, and the pressure decay, 94 ms.

Platinum/Barium Hexaaluminate Catalyst to Small-Scale Hydroxylamine Nitrate Thrusters Application

M OSTmonopropellant systems use hydrazine as the propellant. Hydrazine is, however, a very strong carcinogen and also flammable [1]. Considering current environmental concerns, studies have been conducted to discover an alternative propellant. Among potential green propellants, the blend of hydroxylamine nitrate (HAN) propellants and fuel has the advantages of higher specific impulse and higher density than hydrazine, and it is also environmentally friendly [2–5]. It is important to maintain a relatively large specific surface area of a catalyst support at high temperatures. This is because HAN-based propellants have very high adiabatic decomposition temperatures [2]. However, gamma alumina-based catalysts have been reported to suffer from sintering and deactivation at high temperatures, such as the decomposition temperatures of HAN-based propellants [6]. A barium hexaaluminate (BHA) catalyst support exhibits excellent thermal stability in high-temperature application, such as methane combustion [7]. The performance of BHA, which has better thermal stability than the conventional gamma alumina support, when using platinum as the active material was tested on a small-scale HAN thruster. The performance was compared with that of alpha alumina, which was used as a control.

Reduction of Catalyst Volume by Using Metal Mesh in Small-Scale H2O2 Thrusters

P ROPELLANTatomization is one of the most important aspects in monopropellant thruster systems. The atomization of propellants is closely related to the response time and catalyst bed volume. The performance of the thruster and perfect decomposition is affected by the catalyst, but the response time can be improved, and the catalyst bed volume can be reduced by fine atomization and fast vaporization of the propellant. The atomization of propellants occurs at the injector of the thruster in most cases. There are many types of injectors, and it is difficult to determine which one is the most suitable, because it is difficult to simply compare different types of injectors. The atomization and dispersion of propellants can be improved by using an impinging-type injector, but it is difficult to fabricate, and the technical expertise required for fabrication is relatively high for a small-scale thruster. The spray-type injector has improvedmixing and atomization characteristics, but there should be enough space between the injector and catalyst where the propellant can spread; in addition, the spray-type injector has a relatively longer response time than the showerhead injector [1]. Therefore, it can be said that the showerhead-type injector is more appropriate for application to small-scale thrusters. Nevertheless, atomization of the showerhead-type injector is much poorer than that of other injector types. Therefore, the atomization characteristics of the showerheadtype injector should be improved. In this Note, a simple but effective method is introduced to improve the atomization characteristics of the showerhead-type injector applied to a small-scale hydrogen peroxide thruster.

Fabrication of ethanol blended hydrogen peroxide 50 mN class MEMS thruster

MEMS thruster with blended propellant was fabricated and experimentally tested in order to improve specific impulse of micro scale monopropellant thruster and to improve stability of thrust to be better. 90 wt. % H2O2 blended with 25 O/F ratio ethanol was used as propellant of thruster and platinum on alumina support was used as catalyst for decomposition of propellant. Thruster was made by five layers of photosensitive glasses. Four layers were integrated by thermal bonding method and catalyst was directly inserted into chamber before UV bonding process for the last layer bonding. Results of experimental tests showed ethanol blended hydrogen peroxide had higher specific impulse than unblended hydrogen peroxide. Expected improvement of thrust stability due to the blended propellant was found only in the transient state of thrust. Also, unlike the thrust instability of vertical type thruster of previous research, improvement of thrust stability was found owe to horizontal type thruster pattern on glass, despite aspect ratio limitation of glass fabrication with wet etching process. During the experimental test, combustion phenomena of ethanol with decomposed hydrogen peroxide were observed through glass layer and it made fracture on structure of thruster.

EFFECT OF UNSTEADINESS AND NOZZLE ASYMMETRY ON THRUST OF A MICROTHRUSTER

Due to the small size and short propulsion pulses of solid propellant microthrusters, unsteady characteristics and/or fabrication errors may cause appreciable effects on the total thrust. The present work simulates a 2D planar and conical nozzle of 200 μm throat diameter using the traditional momentum equations and turbulence closure model. Unsteady characteristics were found to be insensitive to the inlet mass flux but sensitive to the chamber size. The characteristic time of transient momentum variations in the nozzle was found to be on the order of 0.1 ms, and that for decay was smaller than that for development by about 25%. This work confirmed that the magnitude of thrust was not sensitive to distortions in the nozzle geometry. A mismatch of the nozzle throat changed the position of sonic plane, but its effect on thrust was almost negligible.