Endurance and failure of an alumina-based monopropellant microthruster with integrated heater, catalytic bed and temperature sensors

Abstract

Monopropellant ceramic microthrusters with an integrated heater, catalytic bed and two temperature sensors, but of various designs, were manufactured by milling a fluidic channel and chamber, and a nozzle, and screen printing platinum patterns on green tapes of alumina that were stacked and laminated before sintering. In order to increase the surface area of the catalytic bed, the platinum paste was mixed with a sacrificial paste that disappeared during sintering, to leave behind a porous and rough layer. As an early development level in manufacturing robust and high-temperature tolerant microthrusters, the influence of design on the temperature gradients and dry temperature tolerance of the devices was studied. On average, the small reaction chambers showed a more than 1.5 times higher dry temperature tolerance (in centigrade) compared to devices with larger chambers, independent of the heater and device size. However, for a given temperature, big devices consumed on average 2.9 times more power than the small ones. It was also found that over the same area and under the same heating conditions, devices with small chambers were subjected to approximately 40% smaller temperature differences. A pressure test done on two small devices with small chambers revealed that pressures of at least 26.3 bar could be tolerated. Above this pressure, the interfaces failed but the devices were not damaged. To investigate the cooling effect of the micropropellant, the endurance of a full thruster was also studied under wet testing where it was fed with 31 wt.% hydrogen peroxide. The thruster demonstrated complete evaporation and/or full decomposition at a power above 3.7 W for a propellant flow of 50 µl min−1. At this power, the catalytic bed locally reached a temperature of 147 °C. The component was successfully heated to an operating temperature of 307 °C, where it cracked. Under these firing conditions, and assuming complete decomposition, calculations give a thrust and specific impulse of 0.96 mN and 106 s, respectively. In the case of evaporation, the corresponding values are calculated to be 0.84 mN and 92 s.

Systematic variation of design aspects for a significant increase in thermal fracture resistance of alumina microthrusters

“…Apart from the high temperatures reached, the rapid heating towards these temperatures should also be noted. To avoid thermal fracture, the temperature has previously been increased very slowly, requiring 25 min to reach the final temperature [16]. In this study, final temperatures during tests of thrusters without tube were reached after 90 s.…”

“…The processed cavity part along with the top and bottom sheets were stacked in a fixture with alignment pins [16]. This five-sheet tape stack, along with the fixture, was vacuum packaged, and the tapes were joined together through a 10 min lamination at 70 • C and 3 MPa.…”

“…In [15], the thermal fracture resistance of an HTCC/LTCC mesoscale combustor was improved by increasing the thickness of the orifice plate. Furthermore, in [16], the effects on thermal performance of catalytic chamber, component and heater sizes in HTCC microthrusters were investigated.…”

Recent Advances in MEMS-Based Microthrusters

“…Under these conditions, a thrust and specific impulse of 0.96 mN and 106 s were obtained by calculations, respectively. Khaji et al [70,71] designed a liquid monopropellant alumina microthruster with an integrated heater. The microthruster is comprised of four ceramic tapes stacked on each other as shown in Figure 27.…”

Methodology for Geometric Optimization and Sizing for Subnewton Monopropellant Catalyst Beds

“…At these scales it is challenging to create these features, and more complex geometries required to fully instrument the internal flow on experimental or breadboard thrusters is unfeasible. Most sub-newton systems developed within research institutions favor 2D planar designs, manufactured using various etching [3][4][5] and ceramic sintering [6][7][8] techniques. These methods produce planar designs on thin wafers of material, which are then stacked and bonded to create the sealed thruster cavity.…”