Fabrication of a liquid monopropellant microthruster with built-in regenerative micro-cooling channels

Highlights

• The use of micro-cooling channels to cool a liquid propellant microthruster is suggested.

• A liquid propellant microthruster with micro-cooling channels was fabricated with a photosensitive glass MEMS process.

• The feasibility of using micro-cooling channels was successfully validated through the design, fabrication and testing of a liquid propellant microthruster.

Abstract

This paper reports a feasibility study of regenerative micro-cooling channels in a liquid microthruster composed of thermally fragile materials. Glass, which is among the most thermally insulating materials, has been used as microthruster fabrication material to suppress excessive heat loss in micro scale thruster. However, the frangibility of glass has remained a challenge to be solved. To thermally manage the fragile structure, the use of regenerative micro-cooling channels in a microthruster is suggested in this work, and the feasibility was tested through design, fabrication and experimental performance of a glass microthruster with microchannels. Nine photosensitive glass layers were wet etched and integrated to fabricate the microthruster. Before integration of the layers, a fabricated Pt/Al₂O₃ catalyst was inserted into the chamber of the microthruster for propellant decomposition. Hydrogen peroxide (90 wt%) was used as a monopropellant and served as the working fluid for regenerative cooling. A liquid microthruster with micro-cooling channels was successfully fabricated with a photosensitive glass MEMS process. Experimental performance tests were conducted while measuring the microthruster chamber pressure, chamber temperature, and surface temperatures. The test results showed normal operation of the microthruster, which had an estimated thrust of approximately 48 mN and temperature efficiency of approximately 41%. The decreasing surface temperatures of the microthruster during thruster operation successfully validated the cooling effect of the micro-cooling channels and demonstrated their practicality for the regenerative cooling of liquid microthrusters.

Introduction

MEMS technology has resulted in many imaginary suggestions becoming reality. This is also valid for space technology. Nanosatellites that weigh 1–10 kg have been proposed [1], [2], [3], [4], [5], [6], [7], [8] and provide significant advantages, including extreme cost effectiveness, as compared with traditional satellites, which have the drawback of high costs of launch vehicles. Limitations on the functionality of small-scale satellites, owing to their restricted mass and volume, can be supplemented through constellation group operations involving several small-scale satellites. It is expected that operating constellations of small satellites will result in higher reliability for entire satellite systems, improved revisit times, and the possibility for more versatile mission operations. Those operations are feasible only after meeting the prerequisites associated with down-sized thruster development for attitude control, drag compensation, and orbital transfer. The required thrust depends on the satellite weight and mission profile [9], [10].

Electrical and chemical propulsion are possible for micropropulsion. Electric propulsion has a high specific impulse but requires high electric power, which may be an excessive burden on small-scale satellites. Chemical propulsion has the advantage of high energy density, and it consumes little electrical power. From these choices, many microthruster studies have examined chemical propulsion by using monopropellants [11], [12], [13], [14], [15], [16], [17], [18], [19], bipropellants [20], [21], [22], solid propellants [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], and cold gases [40], [41], [42].

Monopropellant thrusters are among the most appropriate micropropulsion options because they are simpler than bipropellant thrusters and possess re-ignition and throttling abilities, which are difficult to implement in solid propellant thrusters. Monopropellant thrusters also have higher specific impulses than cold gas thrusters.

Most microthrusters have been fabricated using silicon [29], [30], [31], [32], [33], [34], [35], [36]. However, some studies have attempted to use low thermal conductivity materials, such as glasses [37], [38], [39] and ceramics [11], [12], [20], to prevent heat energy losses stemming from the large surface-to-volume ratios of small-scale thrusters. Cheah et al. [43] have fabricated a vaporizing liquid microthruster using a high-temperature co-fired ceramic (HTCC) and have found that using HTCC as the structural material is more efficient and consumes less electrical power than silicon. Wu et al. [11] have successfully manufactured a monopropellant microthruster based on low-temperature co-fired ceramic (LTCC) tape technology and have demonstrated that the thruster operates normally. However, under certain thermal stresses, the combustor wall cracks, and the level of thrust decreases. In [44], a monopropellant microthruster fabricated using glass and a decomposed propellant using a catalyst coated on a spherical support is described, but cracking has been observed near the catalyst chamber wall of the microthruster, where the highest thermal stresses occur. Low thermal conductivity materials have been good choices for conserving the heat energy of thermal devices. However, the thermal frangibility and brittleness of some materials have remained challenges to solve.

In this work, the use of regenerative cooling channels added to a liquid monopropellant microthruster is suggested as an alternative for thermal management of a microthruster structure fabricated by using a thermally fragile material. Glass, as one of the fragile and best low-conduction materials, was selected as a micro-fabrication material for this study. The suggested microthruster was fabricated using a photosensitive glass MEMS fabrication process. The feasibility of regenerative cooling using micro-channels was evaluated through the design, fabrication and performance testing of a glass microthruster with built-in regenerative micro channels.